



Free  Pascal

Reference  guide

==============================================================================================================================

                                                   Reference guide for Free Pascal, version 3.0.0

                                                                                         Document version 3.0

                                                                                                   November 2015



Micha"el Van Canneyt
______________________________________________________________________________________________________________________________




Contents



1    Pascal Tokens                                                                                                        11

     1.1    Symbols       .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  .  .    11

     1.2    Comments        .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  .  .    12

     1.3    Reserved words         .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  .    13

            1.3.1     Turbo Pascal reserved words                .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.    13

            1.3.2     Free Pascal reserved words              .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.    14

            1.3.3     Object Pascal reserved words               .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.    14

            1.3.4     Modifiers       .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  .    14

     1.4    Identifiers     .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  .  .    15

     1.5    Hint directives        .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  .    16

     1.6    Numbers       .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  .  .    17

     1.7    Labels     .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  .  .    18

     1.8    Character strings         .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  .    18


2    Constants                                                                                                            20

     2.1    Ordinary constants          .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  .    20

     2.2    Typed constants           .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  .    21

     2.3    Resource strings          .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  .    22


3    Types                                                                                                                23

     3.1    Base types      .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  .  .    23

            3.1.1     Ordinal types        .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  .    24

            3.1.2     Real types      .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  .    28

     3.2    Character types        .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  .    29

            3.2.1     Char or AnsiChar            .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .    29

            3.2.2     WideChar        .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  .    29

            3.2.3     Other character types            .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .    29

            3.2.4     Single-byte String types           .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .    29

            3.2.5     Multi-byte String types            .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .    35

            3.2.6     Constant strings          .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .    35

            3.2.7     PChar - Null terminated strings                 .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *    36



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           3.2.8     String sizes       .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  .    37

    3.3    Structured Types          .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  .    37

           3.3.1     Arrays     .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  .    39

           3.3.2     Record types         .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .    45

           3.3.3     Set types       .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  .    48

           3.3.4     File types      .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  .    49

    3.4    Pointers      .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  .  .    49

    3.5    Forward type declarations             .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .    51

    3.6    Procedural types          .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  .    52

    3.7    Variant types        .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  .    55

           3.7.1     Definition      .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  .    55

           3.7.2     Variants in assignments and expressions                   .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .   *
 * 56

           3.7.3     Variants and interfaces          .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .    57

    3.8    Type aliases       .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .  .    58


4    Variables                                                                                                            60

    4.1    Definition      .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .  .    60

    4.2    Declaration        .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .  .    60

    4.3    Scope       .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  .  .    62

    4.4    Initialized variables        .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  .    62

    4.5    Initializing variables using default            .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.    63

    4.6    Thread Variables          .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  .    64

    4.7    Properties      .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .  .    64


5    Objects                                                                                                              68

    5.1    Declaration        .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .  .    68

    5.2    Fields      .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  .  .    69

    5.3    Class or Static fields         .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .    70

    5.4    Constructors and destructors               .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .    71

    5.5    Methods       .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  .  .    72

           5.5.1     Declaration        .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  .    73

           5.5.2     Method invocation           .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .    73

    5.6    Visibility      .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .  .    78


6    Classes                                                                                                              79

    6.1    Class definitions         .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  .    79

    6.2    Normal and static fields            .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .    83

           6.2.1     Normal fields/variables             .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .    83

           6.2.2     Class fields/variables         .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .    83

    6.3    Class instantiation          .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  .    84

    6.4    Class destruction         .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  .    85



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    6.5    Methods       .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  .  .    86

           6.5.1     Declaration        .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  .    86

           6.5.2     invocation      .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  .    86

           6.5.3     Virtual methods           .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .    86

           6.5.4     Class methods          .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .    87

           6.5.5     Class constructors and destructors                .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *  88

           6.5.6     Static class methods           .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .    90

           6.5.7     Message methods           .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .    92

           6.5.8     Using inherited        .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .    93

    6.6    Properties      .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .  .    94

           6.6.1     Definition      .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  .    94

           6.6.2     Indexed properties          .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .    96

           6.6.3     Array properties          .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .    97

           6.6.4     Default properties          .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .    98

           6.6.5     Published properties           .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .    98

           6.6.6     Storage information            .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .    99

           6.6.7     Overriding properties            .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .    99

    6.7    Class properties        .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  .  101

    6.8    Nested types, constants and variables                .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  101


7    Interfaces                                                                                                         103

    7.1    Definition      .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .  .  103

    7.2    Interface identification:  A GUID               .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  104

    7.3    Interface implementations             .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  105

    7.4    Interface delegation         .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  .  106

    7.5    Interfaces and COM             .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .  108

    7.6    CORBA and other Interfaces                 .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  108

    7.7    Reference counting           .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  .  108


8    Generics                                                                                                           110

    8.1    Introduction       .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .  .  110

    8.2    Generic type definition          .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  110

    8.3    Generic type specialization              .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  113

    8.4    Generic type restrictions           .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  114

    8.5    Delphi compatibility           .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .  116

           8.5.1     Syntax elements           .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  116

           8.5.2     Record type restrictions            .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  117

           8.5.3     Type overloads         .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  117

           8.5.4     Name space considerations                .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  117

           8.5.5     Scope considerations           .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  118

    8.6    Type compatibility           .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  .  118



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    8.7    Using the default intrinsic           .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  121

    8.8    A word about scope             .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .  121

    8.9    Operator overloading and generics                  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  123


9    Extended records                                                                                                   126

    9.1    Definition      .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .  .  126

    9.2    Extended record enumerators                .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  128


10   Class, Record and Type helpers                                                                                     131

    10.1   Definition      .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .  .  131

    10.2   Restrictions on class helpers            .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  132

    10.3   Restrictions on record helpers             .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  133

    10.4   Considerations for (simple) type helpers                  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * 134

    10.5   A note on scope and lifetime for record and type helpers                        .  .  .  .  .  .  .  .  .  .  .  .  135

    10.6   Inheritance        .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .  .  137

    10.7   Usage       .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  .  .  137


11   Objective-Pascal Classes                                                                                           141

    11.1   Introduction       .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .  .  141

    11.2   Objective-Pascal class declarations                .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  141

    11.3   Formal declaration           .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  .  144

    11.4   Allocating and de-allocating Instances                 .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * 145

    11.5   Protocol definitions         .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  .  146

    11.6   Categories      .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .  .  147

    11.7   Name scope and Identifiers               .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  148

    11.8   Selectors     .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  .  .  149

    11.9   The id type        .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .  .  149

    11.10  Enumeration in Objective-C classes                 .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  150


12   Expressions                                                                                                        151

    12.1   Expression syntax            .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  .  152

    12.2   Function calls       .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  .  153

    12.3   Set constructors        .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  .  155

    12.4   Value typecasts         .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  .  156

    12.5   Variable typecasts           .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  .  156

    12.6   Unaligned typecasts            .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .  157

    12.7   The @ operator          .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  .  158

    12.8   Operators       .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .  .  159

           12.8.1    Arithmetic operators           .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  159

           12.8.2    Logical operators         .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  160

           12.8.3    Boolean operators           .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  160



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           12.8.4    String operators          .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  161

           12.8.5    Set operators        .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .  161

           12.8.6    Relational operators           .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  163

           12.8.7    Class operators        .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  163


13   Statements                                                                                                         167

    13.1   Simple statements            .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  .  167

           13.1.1    Assignments          .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .  167

           13.1.2    Procedure statements             .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  168

           13.1.3    Goto statements           .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  169

    13.2   Structured statements            .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  170

           13.2.1    Compound statements              .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  170

           13.2.2    The Case statement             .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  171

           13.2.3    The If..then..else statement                    .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * 172

           13.2.4    The For..to/downto..do statement                     .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *174

           13.2.5    The For..in..do statement                  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  175

           13.2.6    The Repeat..until statement                  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * 182

           13.2.7    The While..do statement               .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  183

           13.2.8    The With statement             .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  183

           13.2.9    Exception Statements             .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  185

    13.3   Assembler statements             .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  185


14   Using functions and procedures                                                                                     187

    14.1   Procedure declaration            .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  187

    14.2   Function declaration           .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .  188

    14.3   Function results        .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  .  188

    14.4   Parameter lists         .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  .  189

           14.4.1    Value parameters          .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  189

           14.4.2    Variable parameters            .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  190

           14.4.3    Out parameters         .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  191

           14.4.4    Constant parameters            .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  192

           14.4.5    Open array parameters               .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  193

           14.4.6    Array of const         .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  194

    14.5   Managed types and reference counts                   .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  196

    14.6   Function overloading           .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .  199

    14.7   Forward declared functions               .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  199

    14.8   External functions           .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  .  200

    14.9   Assembler functions            .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .  201

    14.10  Modifiers       .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .  .  202

           14.10.1   alias    .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .  .  202

           14.10.2   cdecl    .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .  .  203



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           14.10.3   export     .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  .  204

           14.10.4   inline     .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  .  204

           14.10.5   interrupt       .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  .  204

           14.10.6   iocheck       .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  .  204

           14.10.7   local    .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .  .  204

           14.10.8   noreturn      .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  .  205

           14.10.9   nostackframe         .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .  205

           14.10.10  overload      .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  .  205

           14.10.11  pascal     .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  .  206

           14.10.12  public     .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  .  207

           14.10.13  register      .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  .  207

           14.10.14  safecall      .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  .  207

           14.10.15  saveregisters      .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  .  208

           14.10.16  softfloat     .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  .  208

           14.10.17  stdcall    .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  .  208

           14.10.18  varargs       .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  .  208

    14.11  Unsupported Turbo Pascal modifiers                   .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  208


15   Operator overloading                                                                                               210

    15.1   Introduction       .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .  .  210

    15.2   Operator declarations            .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  210

    15.3   Assignment operators             .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  211

    15.4   Arithmetic operators           .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .  215

    15.5   Comparison operator            .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .  216

    15.6   In operator        .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .  .  217


16   Programs, units, blocks                                                                                            219

    16.1   Programs        .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .  .  219

    16.2   Units    .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  .  .  220

    16.3   Namespaces:  Dotted Units             .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  222

    16.4   Unit dependencies            .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  .  225

    16.5   Blocks      .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  .  .  225

    16.6   Scope       .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  .  .  227

           16.6.1    Block scope        .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  .  227

           16.6.2    Record scope         .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .  227

           16.6.3    Class scope        .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  .  227

           16.6.4    Unit scope         .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  .  228

    16.7   Libraries     .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  .  .  228


17   Exceptions                                                                                                         230

    17.1   The raise statement            .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .  230



                                                                  6

____________________________________________________________________________________________________________________CONTENTS_______*
 *___



    17.2   The try...except statement            .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  232

    17.3   The try...finally statement           .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  233

    17.4   Exception handling nesting               .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  234

    17.5   Exception classes         .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  .  234


18   Using assembler                                                                                                    236

    18.1   Assembler statements             .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  236

    18.2   Assembler procedures and functions                   .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  236



                                                                  7




List   of   Tables



     3.1    Predefined integer types            .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .    24

     3.2    Predefined integer types            .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .    25

     3.3    Boolean types        .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .  .    26

     3.4    Supported Real types             .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .    28

     3.5    PChar pointer arithmetic            .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .    37

     3.6    String memory sizes            .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  .    37


     12.1   Precedence of operators             .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .  151

     12.2   Binary arithmetic operators              .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  159

     12.3   Unary arithmetic operators               .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  159

     12.4   Logical operators         .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  .  160

     12.5   Boolean operators           .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  .  160

     12.6   Set operators        .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .  .  161

     12.7   Relational operators           .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .*
 *  .  .  163

     12.8   Class operators        .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  .  .  163


     13.1   Allowed C constructs in Free Pascal                  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  *
 *.  168


     14.1   Unsupported modifiers            .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  .  . *
 * .  .  208



                                                              8

  ___________________________________________________________________________________________________________LIST_OF_TABLES________*
 *_____



  About  this  guide


  This document serves as the reference for the Pascal language as implemented by the Free
  Pascal compiler.  It describes all Pascal constructs supported by Free Pascal, and lists all sup-
  ported data types.  It does not, however, give a detailed explanation of the Pascal language: it
  is not a tutorial.  The aim is to list which Pascal constructs are supported, and to show where
  the Free Pascal implementation differs from the Turbo Pascal or Delphi implementations.

  The  Turbo  Pascal  and  Delphi  Pascal  compilers  introduced  various  features  in  the  Pascal
  language.  The Free Pascal compiler emulates these compilers in the appropriate mode of the
  compiler:  certain features are available only if the compiler is switched to the appropriate
  mode.  When required for a certain feature, the use of the -M command-line switch or {$MODE
  } directive will be indicated in the text.  More information about the various modes can be
  found in the user's manual and the programmer's manual.

  Earlier versions of this document also contained the reference documentation of the system
  unit and objpas unit.  This has been moved to the RTL reference guide.



  Notations

  Throughout this document, we will refer to functions, types and variables with typewriter
  font.  Files are referred to with a sans font:  filename.



  Syntax  diagrams

  All elements of the Pascal language are explained in syntax diagrams.  Syntax diagrams are
  like flow charts.  Reading a syntax diagram means getting from the left side to the right side,
  following the arrows.  When the right side of a syntax diagram is reached, and it ends with
  a single arrow, this means the syntax diagram is continued on the next line.  If the line ends
  on 2 arrows pointing to each other, then the diagram is ended.

  Syntactical elements are written like this

--  ___ syntactical elements are like this __     _____________________________________________________________________________-oe


  Keywords which must be typed exactly as in the diagram:

--  ___ keywords are like this __     ________________________________________________________________________________________-oe


  When something can be repeated, there is an arrow around it:

--  _____  _ this can be repeated __   _________________________________________________________________________________________-oe
         6||_____________________________|__|


  When there are different possibilities, they are listed in rows:

--  _____|___ First possibility __ ___|____________________________________________________________________________________________*
 *-oe
         |_ Second possibility __   _|


  Note, that one of the possibilities can be empty:

--  _____|____________________________|____________________________________________________________________________________________*
 *-oe

         |___|First_possibility __ ___|
                     Second possibility __   _|

  This means that both the first or second possibility are optional.  Of course, all these elements
  can be combined and nested.



                                                                    9

___________________________________________________________________________________________________________LIST_OF_TABLES__________*
 *___



About  the  Pascal  language


The language Pascal was originally designed by Niklaus Wirth around 1970.  It has evolved
significantly since that day, with a lot of contributions by the various compiler constructors
(Notably:  Borland).  The basic elements have been kept throughout the years:


     o Easy syntax, rather verbose, yet easy to read.  Ideal for teaching.

     o Strongly typed.

     o Procedural.

     o Case insensitive.

     o Allows nested procedures.

     o Easy input/output routines built-in.


The  Turbo  Pascal  and  Delphi  Pascal  compilers  introduced  various  features  in  the  Pascal
language,  most  notably  easier  string  handling  and  object  orientedness.   The  Free  Pascal
compiler  initially  emulated  most  of  Turbo  Pascal  and  later  on  Delphi.   It  emulates  these
compilers in the appropriate mode of the compiler:  certain features are available only if the
compiler is switched to the appropriate mode.  When required for a certain feature, the use
of  the  -M  command-line  switch  or  {$MODE  }  directive  will  be  indicated  in  the  text.  More
information about the various modes can be found in the user's manual and the programmer's
manual.



                                                                 10




Chapter   1



Pascal   Tokens



Tokens  are  the  basic  lexical  building  blocks  of  source  code:   they  are  the  'words'  of  the
language:  characters  are  combined  into  tokens  according  to  the  rules  of  the  programming
language.  There are five classes of tokens:


reserved words           These are words which have a fixed meaning in the language.  They cannot
        be changed or redefined.

identifiers      These are names of symbols that the programmer defines.  They can be changed
        and re-used.  They are subject to the scope rules of the language.

operators        These are usually symbols for mathematical or other operations:  +, -, * and so
        on.

separators        This is usually white-space.

constants        Numerical or character constants are used to denote actual values in the source
        code, such as 1 (integer constant) or 2.3 (float constant) or 'String constant' (a string:
        a piece of text).


In  this  chapter  we  describe  all  the  Pascal  reserved  words,  as  well  as  the  various  ways  to
denote strings, numbers, identifiers etc.



1.1         Symbols


Free Pascal allows all characters, digits and some special character symbols in a Pascal source
file.


        |______________________________________________________________________________________________________________|
        Recognised symbols



      --  ___ letter ____|_ A...Z __ _|_________________________________________________________________________________-oe
                         |__ a...z ____|

      --  ___ digit __ 0...9 ________________________________________________________________________________________-oe


      --  ___ hex digit __ __|__ 0...9 _____|___________________________________________________________________________-oe

                             |_|A...F_____|
                                          a...f  ____|



                                                              11

_____________________________________________________________________________________CHAPTER_1.___PASCAL_TOKENS____________________*
 *___



       |_______________________________________________________________________________________________________________|


The following characters have a special meaning:


 +  -  *  /  =  <  >  [  ]  .  ,  (  )  :  ^  @  {  }  $  #  &  %


and the following character pairs too:


<<  >>  **  <>  ><  <=  >=  :=  +=  -=  *=  /=  (*  *)  (.  .)  //


When used in a range specifier, the character pair (.  is equivalent to the left square bracket
[.  Likewise, the character pair .)  is equivalent to the right square bracket ].  When used for
comment delimiters, the character pair (* is equivalent to the left brace { and the character
pair *) is equivalent to the right brace }.  These character pairs retain their normal meaning
in string expressions.



1.2         Comments


Comments  are  pieces  of  the  source  code  which  are  completely  discarded  by  the  compiler.
They exist only for the benefit of the programmer, so he can explain certain pieces of code.
For the compiler, it is as if the comments were not present.

The following piece of code demonstrates a comment:



(*  My  beautiful  function  returns  an  interesting  result  *)
Function  Beautiful  :  Integer;



The  use  of  (*  and  *)  as  comment  delimiters  dates  from  the  very  first  days  of  the  Pascal
language.  It has been replaced mostly by the use of  { and } as comment delimiters, as in
the following example:



{  My  beautiful  function  returns  an  interesting  result  }
Function  Beautiful  :  Integer;



The comment can also span multiple lines:



{
     My  beautiful  function  returns  an  interesting  result,
     but  only  if  the  argument  A  is  less  than  B.
}
Function  Beautiful  (A,B  :  Integer):  Integer;


Single line comments can also be made with the // delimiter:



//  My  beautiful  function  returns  an  interesting  result
Function  Beautiful  :  Integer;



                                                                 12

               _____________________________________________________________________________________CHAPTER_1.___PASCAL_TOKENS_____*
 *__________________



               The comment extends from the // character till the end of the line.  This kind of comment
               was introduced by Borland in the Delphi Pascal compiler.

               Free Pascal supports the use of nested comments.  The following constructs are valid com-
               ments:


               (*  This  is  an  old  style  comment  *)
               {    This  is  a  Turbo  Pascal  comment  }
               //  This  is  a  Delphi  comment.  All  is  ignored  till  the  end  of  the  line.


               The following are valid ways of nesting comments:


               {  Comment  1  (*  comment  2  *)  }
               (*  Comment  1  {  comment  2  }  *)
               {  comment  1  //  Comment  2  }
               (*  comment  1  //  Comment  2  *)
               //  comment  1  (*  comment  2  *)
               //  comment  1  {  comment  2  }


               The last two comments must be on one line.  The following two will give errors:


                //  Valid  comment  {  No  longer  valid  comment  !!
                     }


               and


                //  Valid  comment  (*  No  longer  valid  comment  !!
                     *)


               The compiler will react with a 'invalid character' error when it encounters such constructs,
               regardless of the -Mtp switch.

Remark:        In TP and Delphi mode, nested comments are not allowed, for maximum compatibility with
               existing code for those compilers.



               1.3         Reserved  words


               Reserved  words  are  part  of  the  Pascal  language,  and  as  such,  cannot  be  redefined  by  the
               programmer.  Throughout the syntax diagrams they will be denoted using a bold typeface.
               Pascal is not case sensitive so the compiler will accept any combination of upper or lower
               case letters for reserved words.

               We make a distinction between Turbo Pascal and Delphi reserved words.  In TP mode, only
               the Turbo Pascal reserved words are recognised, but the Delphi ones can be redefined.  By
               default, Free Pascal recognises the Delphi reserved words.



               1.3.1        Turbo  Pascal  reserved  words

               The following keywords exist in Turbo Pascal mode


               absolute                         begin                           destructor                      else
               and                              case                            div                             end
               array                            const                           do                              file
               asm                              constructor                     downto                          for



                                                                                13

_____________________________________________________________________________________CHAPTER_1.___PASCAL_TOKENS____________________*
 *___



function                        nil                             reintroduce                     unit
goto                            not                             repeat                          until
if                              object                          self                            uses
implementation                  of                              set                             var
in                              operator                        shl                             while
inherited                       or                              shr                             with
inline                          packed                          string                          xor
interface                       procedure                       then
label                           program                         to
mod                             record                          type



1.3.2        Free  Pascal  reserved  words

On  top  of  the  Turbo  Pascal  reserved  words,  Free  Pascal  also  considers  the  following  as
reserved words:


dispose                          false                           true
exit                             new



1.3.3        Object  Pascal  reserved  words

The reserved words of Object Pascal (used in Delphi or Objfpc mode) are the same as the
Turbo Pascal ones, with the following additional keywords:


as                               finalization                    library                         raise
class                            finally                         on                              resourcestring
dispinterface                    initialization                  out                             threadvar
except                           inline                          packed                          try
exports                          is                              property



1.3.4        Modifiers

The following is a list of all modifiers.  They are not exactly reserved words in the sense that
they can be used as identifiers,  but in specific places,  they have a special meaning for the
compiler, i.e., the compiler considers them as part of the Pascal language.


absolute                         export                          nodefault                       reintroduce
abstract                         external                        noreturn                        result
alias                            far                             nostackframe                    safecall
assembler                        far16                           oldfpccall                      saveregisters
bitpacked                        forward                         otherwise                       softfloat
break                            generic                         overload                        specialize
cdecl                            helper                          override                        static
continue                         implements                      pascal                          stdcall
cppdecl                          index                           platform                        stored
cvar                             interrupt                       private                         strict
default                          iochecks                        protected                       unaligned
deprecated                       local                           public                          unimplemented
dynamic                          message                         published                       varargs
enumerator                       name                            read                            virtual
experimental                     near                            register                        write



                                                                 14

               _____________________________________________________________________________________CHAPTER_1.___PASCAL_TOKENS_____*
 *__________________



Remark:         Predefined  types  such  as  Byte,  Boolean  and  constants  such  as  maxint  are  not  reserved
               words.  They are identifiers, declared in the system unit.  This means that these types can
               be redefined in other units.  The programmer is however not encouraged to do this, as it will
               cause a lot of confusion.

Remark:         As of version 2.5.1 it is possible to use reserved words as identifiers by escaping them with
               a & sign.  This means that the following is possible


               var
                  &var  :  integer;


               begin
                  &var:=1;
                  Writeln(&var);
               end.


               however, it is not recommended to use this feature in new code, as it makes code less readable.
               It is mainly intended to fix old code when the list of reserved words changes and encompasses
               a word that was not yet reserved (See also section 1.4 , page 15 ).



               1.4         Identifiers


               Identifiers denote programmer defined names for specific constants, types, variables, proce-
               dures and functions, units, and programs.  All programmer defined names in the source code
               -excluding reserved words- are designated as identifiers.

               Identifiers consist of between 1 and 127 significant characters (letters, digits and the under-
               score character), of which the first must be a letter (a-z or A-Z), or an underscore (__ ).  The
               following diagram gives the basic syntax for identifiers.


                       |___________________________________________________________________________________________________________*
 *___|
                       Identifiers



                     --  ___ identifier ____|_ letter ___|____|__________________|_________________________________________________*
 *__________-oe
                                            |___ __  _____|   6|___|_ letter ___|__|

                                                                   |__|digit_____|_
                                                                                    __  _____|

                       |___________________________________________________________________________________________________________*
 *___|


               Like Pascal reserved words, identifiers are case insensitive, that is, both


                   myprocedure;


               and


                 MyProcedure;


               refer to the same procedure.

Remark:         As of version 2.5.1 it is possible to specify a reserved word as an identifier by prepending it
               with an ampersand (&).  This means that the following is possible:


               program  testdo;



                                                                                15

_____________________________________________________________________________________CHAPTER_1.___PASCAL_TOKENS____________________*
 *___



procedure  &do;


begin
end;


begin
   &do;
end.


The reserved word do is used as an identifier for the declaration as well as the invocation of
the procedure 'do'.



1.5         Hint  directives


Most  identifiers  (constants,  variables,  functions  or  methods,  properties)  can  have  a  hint
directive appended to their definition:


        |______________________________________________________________________________________________________________|
        Hint directives



      --  ___ hint directive __ __|________________________________________________|______________________________________-oe
                                  |_ Deprecated __     __ __________________________|
                                  |                      ||_                 __   |_||
                                  |                          string constant        |
                                  |_____________ Experimental __     _____________|
                                  |________________|Platform_____ ________________|
                                                                  Unimplemented __       ____________|

        |______________________________________________________________________________________________________________|


Whenever  an  identifier  marked  with  a  hint  directive  is  later  encountered  by  the  compiler,
then a warning will be displayed, corresponding to the specified hint.


deprecated         The use of this identifier is deprecated, use an alternative instead.  The depre-
        cated keyword can be followed by a string constant with a message.  The compiler will
        show this message whenever the identifier is encountered.

experimental          The use of this identifier is experimental: this can be used to flag new features
        that should be used with caution.

platform       This is a platform-dependent identifier:  it may not be defined on all platforms.

unimplemented             This should be used on functions and procedures only.  It should be used
        to signal that a particular feature has not yet been implemented.


The following are examples:


Const
    AConst  =  12  deprecated;


var
    p  :  integer  platform;


Function  Something  :  Integer;  experimental;



                                                                 16

_____________________________________________________________________________________CHAPTER_1.___PASCAL_TOKENS____________________*
 *___



begin
   Something:=P+AConst;
end;


begin
   Something;
end.


This would result in the following output:


testhd.pp(11,15)  Warning:  Symbol  "p"  is  not  portable
testhd.pp(11,22)  Warning:  Symbol  "AConst"  is  deprecated
testhd.pp(15,3)  Warning:  Symbol  "Something"  is  experimental


Hint directives can follow all kinds of identifiers:  units, constants, types, variables, functions,
procedures and methods.



1.6         Numbers


Numbers are by default denoted in decimal notation.  Real (or decimal) numbers are written
using engineering or scientific notation (e.g.  0.314E1).

For integer type constants, Free Pascal supports 4 formats:


    1.  Normal, decimal format (base 10).  This is the standard format.

    2.  Hexadecimal  format  (base  16),  in  the  same  way  as  Turbo  Pascal  does.   To  specify
        a  constant  value  in  hexadecimal  format,  prepend  it  with  a  dollar  sign  ($).   Thus,
        the  hexadecimal  $FF  equals  255  decimal.  Note  that  case  is  insignificant  when  using
        hexadecimal constants.

    3.  As of version 1.0.7, Octal format (base 8) is also supported.  To specify a constant in
        octal format, prepend it with an ampersand (&).  For instance 15 is specified in octal
        notation as &17.

    4.  Binary  notation  (base  2).  A  binary  number  can  be  specified  by  preceding  it  with  a
        percent sign (%).  Thus, 255 can be specified in binary notation as %11111111.


The following diagrams show the syntax for numbers.


        |______________________________________________________________________________________________________________|
        Numbers



      --  ___ hex digit sequence __    __  _ hex digit ________________________________________________________________-oe
                                         6||_______________|_|


      --  ___ octal digit sequence __    __  _ octal digit ____________________________________________________________-oe
                                           6||_________________|_|


      --  ___ bin digit sequence __   __  ___ _ 1 ________________________________________________________________________-oe
                                        6||  ||_ 0 __|_|||
                                        |____________|


      --  ___ digit sequence __   __  _ digit ___________________________________________________________________________-oe
                                    6||__________|_|



                                                                 17

               _____________________________________________________________________________________CHAPTER_1.___PASCAL_TOKENS_____*
 *__________________



                    - - ___ unsigned integer __   __|________ digit sequence __   ________|________________________________________*
 *_____-oe

                                                    |___ $ __ hex digit sequence __    ___|
                                                    |_|&____ octal digit sequence __    _|
                                                             % __   bin digit sequence __   __|

                    - - ___sign__|_ + __ _|________________________________________________________________________________________*
 *__-oe
                                 |__ - ____|


                    - - ___ unsigned real __   digit sequence __   __|____________________________|____|____________________|______*
 *_________-oe
                                                                     |_ . __ digit sequence __  _|     |_ scale factor __ _|


                    - - ___ scale factor __ __|_ E ____|___|____________|__ digit sequence __   ___________________________________*
 *_________-oe
                                              |__ e ____|  |_ sign ___|


                    - - ___ unsigned number __     __|___ unsigned real __   ___|__________________________________________________*
 *____-oe
                                                     |_ unsigned integer __   _|


                    - - ___ signed number __    __|____________|__ unsigned number __     _________________________________________*
 *_____-oe
                                                  |_ sign ___|


                      |____________________________________________________________________________________________________________*
 *___|


Remark:         Octal and Binary notation are not supported in TP or Delphi compatibility mode.



               1.7         Labels


               A label is a name for a location in the source code to which can be jumped to from another
               location with a goto statement.  A Label is a standard identifier or a digit sequence.


                       |___________________________________________________________________________________________________________*
 *___|
                       Label



                     --  ___ label ____|_ digit sequence __  __|___________________________________________________________________*
 *___-oe
                                       |_____ identifier _______|

                       |___________________________________________________________________________________________________________*
 *___|


Remark:          The  -Sg  or  -Mtp  switches  must  be  specified  before  labels  can  be  used.  By  default,  Free
               Pascal doesn't support label and goto statements.  The {$GOTO  ON} directive can also be
               used to allow use of labels and the goto statement.

               The following are examples of valid labels:


               Label
                   123,
                   abc;



               1.8         Character  strings


               A character string (or string for short) is a sequence of zero or more characters (byte sized),
               enclosed in single quotes, and on a single line of the program source code:  no literal carriage
               return or linefeed characters can appear in the string.

               A character set with nothing between the quotes ('') is an empty string.



                                                                                18

_____________________________________________________________________________________CHAPTER_1.___PASCAL_TOKENS____________________*
 *___



       |_______________________________________________________________________________________________________________|
       Character strings



     - - ___ character string __   __  ___ |_ quoted string __ ___|________________________________________________________-oe
                                     6||  ||_ control string __  _||||
                                     |__________________________|


     - - ___ quoted string __    ' ____  _ string character __ ___' ___________________________________________________-oe
                                       6||________________________|_|


     - - ___ string character __   __|__   Any character except ' or CR        ___|____________________________________-oe
                                     |___________________ " _____________________|


     - - ___ control string __  __  _ # __  unsigned integer __  _____________________________________________________-oe
                                  6||_______________________________|_|


       |_______________________________________________________________________________________________________________|


The string consists of standard, 8-bit ASCII characters or Unicode (normally UTF-8 encoded)
characters.  The control  string can be used to specify characters which cannot be typed
on a keyboard, such as #27 for the escape character.

The single quote character can be embedded in the string by typing it twice.  The C construct
of escaping characters in the string (using a backslash) is not supported in Pascal.

The following are valid string constants:


   'This  is  a  pascal  string'
   ''
   'a'
   'A  tabulator  character:  '#9'  is  easy  to  embed'


The following is an invalid string:


   'the  string  starts  here
    and  continues  here'


The above string must be typed as:


   'the  string  starts  here'#13#10'     and  continues  here'


or


   'the  string  starts  here'#10'     and  continues  here'


on unices (including Mac OS X), and as


   'the  string  starts  here'#13'     and  continues  here'


on a classic Mac-like operating system.

It is possible to use other character sets in strings:  in that case the codepage of the source file
must be specified with the {$CODEPAGE  XXX} directive or with the -Fc command line option
for  the  compiler.  In  that  case  the  characters  in  a  string  will  be  interpreted  as  characters
from the specified codepage.



                                                                 19




Chapter   2



Constants



Just as in Turbo Pascal, Free Pascal supports both ordinary and typed constants.  They are
declared in a constant declaration block in a unit, program or class, function or procedure
declaration (section 16.5  , page 225  ).



2.1         Ordinary  constants


Ordinary  constants  declarations  are  constructed  using  an  identifier  name  followed  by  an
"=" token, and followed by an optional expression consisting of legal combinations of num-
bers, characters, boolean values or enumerated values as appropriate.  The following syntax
diagram shows how to construct a legal declaration of an ordinary constant.


        |______________________________________________________________________________________________________________|
        Constant declaration



      --  ___ constant declaration __    __  _ identifier __ = __  expression __   hintdirectives __  ; _________________-oe
                                           6||_____________________________________________________________|_|

        |______________________________________________________________________________________________________________|


The compiler must be able to evaluate the expression in a constant declaration at compile
time.   This  means  that  most  of  the  functions  in  the  Run-Time  library  cannot  be  used  in
a constant declaration.  Operators such as +,  -,  *,  /,  not,  and,  or,  div,  mod,  ord,
chr,  sizeof,  pi,  int,  trunc,  round,  frac,  odd  can  be  used,  however.  For  more  in-
formation on expressions, see chapter 12 , page 151  .

Only constants of the following types can be declared:


     o  Ordinal types

     o  Set types

     o  Pointer types (but the only allowed value is Nil).

     o  Real types

     o  Char,

     o  String



                                                              20

               ____________________________________________________________________________________________CHAPTER_2.___CONSTANTS__*
 *__________________



               The following are all valid constant declarations:


               Const
                  e  =  2.7182818;    {  Real  type  constant.  }
                  a  =  2;                 {  Ordinal  (Integer)  type  constant.  }
                  c  =  '4';             {  Character  type  constant.  }
                  s  =  'This  is  a  constant  string';  {String  type  constant.}
                  sc  =  chr(32)
                  ls  =  SizeOf(Longint);
                  P  =  Nil;
                  Ss  =  [1,2];


               Assigning a value to an ordinary constant is not permitted.  Thus, given the previous decla-
               ration, the following will result in a compiler error:


                  s  :=  'some  other  string';


               For  string  constants,  the  type  of  the  string  is  dependent  on  some  compiler  switches.  If  a
               specific type is desired, a typed constant should be used, as explained in the following section.

               Prior to version 1.9,  Free Pascal did not correctly support 64-bit constants.  As of version
               1.9, 64-bit constants can be specified.



               2.2         Typed  constants


               Sometimes  it  is  necessary  to  specify  the  type  of  a  constant,  for  instance  for  constants  of
               complex structures (defined later in the manual).  Their definition is quite simple.


                       |___________________________________________________________________________________________________________*
 *___|
                       Typed constant declaration


                     --   typed constant declaration __     __  _ identifier __ : __ type __ = __  typed constant __     hintdirect*
 *ive __  ; ____-
                                                              6||__________________________________________________________________*
 *_____________|_|
                     -  ___________________________________________________________________________________________________________*
 *_-oe

                     --  ___ typed constant __    __|________ constant __  ________|_______________________________________________*
 *______-oe

                                                    |___ address constant __    ___|
                                                    |_____ array constant __   _____|
                                                    |____|record_constant __   ____|
                                                                  procedural constant __     _|

                       |___________________________________________________________________________________________________________*
 *___|


               Contrary to ordinary constants, a value can be assigned to them at run-time.  This is an old
               concept from Turbo Pascal, which has been replaced with support for initialized variables:
               For a detailed description, see section 4.4 , page 62 .

               Support for assigning values to typed constants is controlled by the {$J} directive:  it can be
               switched off, but is on by default (for Turbo Pascal compatibility).  Initialized variables are
               always allowed.

Remark:         It should be stressed that typed constants are automatically initialized at program start.
               This is also true for local  typed constants and initialized variables.  Local typed constants
               are also initialized at program start.  If their value was changed during previous invocations
               of the function, they will retain their changed value, i.e.  they are not initialized each time
               the function is invoked.



                                                                                21

               ____________________________________________________________________________________________CHAPTER_2.___CONSTANTS__*
 *__________________



               2.3         Resource  strings


               A special kind of constant declaration block is the Resourcestring block.  Resourcestring
               declarations  are  much  like  constant  string  declarations:   resource  strings  act  as  constant
               strings, but they can be localized by means of a set of special routines in the objpas unit.  A
               resource string declaration block is only allowed in the Delphi or Objfpc modes.

               The following is an example of a resourcestring definition:


               Resourcestring


                   FileMenu  =  '&File...';
                   EditMenu  =  '&Edit...';


               All string constants defined in the resourcestring section are stored in special tables.  The
               strings in these tables can be manipulated at runtime with some special mechanisms in the
               objpas unit.

               Semantically, the strings act like ordinary constants; It is not allowed to assign values to them
               (except through the special mechanisms in the objpas unit).  However, they can be used in
               assignments or expressions as ordinary string constants.  The main use of the resourcestring
               section is to provide an easy means of internationalization.

               More on the subject of resourcestrings can be found in the Programmer's Guide           , and in the
               objpas unit reference.

Remark:         Note that a resource string which is given as an expression will not change if the parts of
               the expression are changed:


               resourcestring
                   Part1  =  'First  part  of  a  long  string.';
                   Part2  =  'Second  part  of  a  long  string.';
                   Sentence  =  Part1+'  '+Part2;


               If  the  localization  routines  translate  Part1  and  Part2,  the  Sentence  constant  will  not  be
               translated  automatically:  it  has  a  separate  entry  in  the  resource  string  tables,  and  must
               therefor be translated separately.  The above construct simply says that the initial value of
               Sentence equals Part1+'  '+Part2.

Remark:          Likewise,  when  using  resource  strings  in  a  constant  array,  only  the  initial  values  of  the
               resource strings will be used in the array:  when the individual constants are translated, the
               elements in the array will retain their original value.


               resourcestring
                   Yes  =  'Yes.';
                   No  =  'No.';


               Var
                   YesNo  :  Array[Boolean]  of  string  =  (No,Yes);
                   B  :  Boolean;


               begin
                   Writeln(YesNo[B]);
               end.


               This will print 'Yes.'  or 'No.'  depending on the value of B, even if the constants Yes and No
               have been localized by some localization mechanism.



                                                                                22




Chapter   3



Types



All variables have a type.  Free Pascal supports the same basic types as Turbo Pascal, with
some extra types from Delphi as well as some of its own.

The programmer can declare his own types,  which is in essence defining an identifier that
can be used to denote this custom type when declaring variables further in the source code..
Declaring a type happens in a Type block (section 16.5  , page 225  ), which is a collection of
type declarations, separated by semicolons:


        |______________________________________________________________________________________________________________|
        Type declaration



      --  ___ type declaration __    identifier __ = __  type __ __|________________________|__ ; _________________________-oe
                                                                   |_ hint directives __  _|

        |______________________________________________________________________________________________________________|


There are 8 major kinds of types :


        |______________________________________________________________________________________________________________|
        Types



      --  ___ type __ __|____ simple type __  ____|______________________________________________________________________-oe

                        |_____ string type __  _____|
                        |__ structured type __   __|
                        |____ pointer type __  ____|
                        ||_ procedural type __   _||
                        |____ generic type __  ____|
                        |_|specialized_type___   _|
                                             type alias __ _____|

        |______________________________________________________________________________________________________________|


Each of these cases will be examined separately.



3.1         Base  types


The  base  or  simple  types  of  Free  Pascal  are  the  Delphi  types.  We  will  discuss  each  type
separately.



                                                              23

_____________________________________________________________________________________________________CHAPTER_3.___TYPES____________*
 *___



       |_______________________________________________________________________________________________________________|
       Simple types



     - - ___ simple type __  __|_ ordinal type __ __|________________________________________________________________-oe
                               |___ real type __ ___|

     - - ___ real type __  real type identifier __  _________________________________________________________________-oe


       |_______________________________________________________________________________________________________________|



3.1.1        Ordinal  types

With the exception of int64, qword and Real types, all base types are ordinal types.  Ordinal
types have the following characteristics:


    1.  Ordinal  types  are  countable  and  ordered,  i.e.   it  is,  in  principle,  possible  to  start
        counting them one by one, in a specified order.  This property allows the operation of
        functions as Inc, Ord, Dec on ordinal types to be defined.

    2.  Ordinal values have a smallest possible value.  Trying to apply the Pred function on the
        smallest possible value will generate a range check error if range checking is enabled.

    3.  Ordinal values have a largest possible value.  Trying to apply the Succ function on the
        largest possible value will generate a range check error if range checking is enabled.



Integers


A list of pre-defined integer types is presented in table (3.1 ).



                                       Table 3.1:  Predefined integer types


                                                      __Name____________
                                                        Integer
                                                        Shortint
                                                        SmallInt
                                                        Longint
                                                        Longword
                                                        Int64
                                                        Byte
                                                        Word
                                                        Cardinal
                                                        QWord
                                                        Boolean
                                                        ByteBool
                                                        WordBool
                                                        LongBool
                                                      __Char____________



The integer types, and their ranges and sizes, that are predefined in Free Pascal are listed
in table (3.2 ).  Please note that the qword and int64 types are not true ordinals, so some
Pascal constructs will not work with these two integer types.



                                                                 24

               _____________________________________________________________________________________________________CHAPTER_3.___TY*
 *PES_______________



                                                      Table 3.2:  Predefined integer types


                         _Type__________________________________________Range___________________________________Size_in_bytes______*
 *__
                           Byte                                         0 ..  255                                                1
                           Shortint                                   -128 ..  127                                               1
                           Smallint                               -32768 ..  32767                                               2
                           Word                                       0 ..  65535                                                2
                           Integer                          either smallint or longint                              size 2 or 4
                           Cardinal                                    longword                                                  4
                           Longint                         -2147483648 ..  2147483647                                            4
                           Longword                               0 ..  4294967295                                               4
                           Int64            -9223372036854775808 ..  9223372036854775807                                         8
                         __QWord__________________________0_..__18446744073709551615_____________________________________________8_*
 *__



               The integer type maps to the smallint type in the default Free Pascal mode.  It maps to
               either a longint in either Delphi or ObjFPC mode.  The cardinal type is currently always
               mapped to the longword type.

Remark:        All decimal constants which do no fit within the -2147483648..2147483647 range are silently
               and  automatically  parsed  as  64-bit  integer  constants  as  of  version  1.9.0.   Earlier  versions
               would convert it to a real-typed constant.

               As a pascal compiler, Free Pascal does automatic type conversion and upgrading in expres-
               sions where different kinds of integer types are used:


                   1.  Every platform has a "native" integer size, depending on whether the platform is 8-bit,
                       16-bit, 32-bit or 64-bit.  e.g.  On AVR this is 8-bit.

                   2.  Every  integer  smaller  than  the  "native"  size  is  promoted  to  a  signed  version  of  the
                       "native" size.  Integers equal to the "native" size keep their signedness.

                   3.  The result of binary arithmetic operators (+, -, *, etc.)  is determined in the following
                       way:

                         (a)  If at least one of the operands is larger than the native integer size, the result is
                              chosen to be the smallest type that encompasses the ranges of the types of both
                              operands.  This  means  that  mixing  an  unsigned  with  a  smaller  or  equal  in  size
                              signed will produce a signed type that is larger than both of them.

                        (b)   If both operands have the same signedness, the result is the same type as them.
                              The only exception is subtracting (-):  in the case of unsigned-unsigned subtracting
                              produces a signed result in FPC (as in Delphi, but not in TP7).

                         (c)  Mixing signed and unsigned operands of the "native" int size produces a larger
                              signed result.  This means that mixing longint and longword on 32-bit platforms
                              will  produce  an  int64.   Similarly,  mixing  byte  and  shortint  on  8-bit  platforms
                              (AVR) will produce a smallint.



               Boolean types


               Free  Pascal  supports  the  Boolean  type,  with  its  two  pre-defined  possible  values  True  and
               False.  These are the only two values that can be assigned to a Boolean type.  Of course,
               any expression that resolves to a boolean value, can also be assigned to a boolean type.



                                                                                25

               _____________________________________________________________________________________________________CHAPTER_3.___TY*
 *PES_______________



                                                             Table 3.3:  Boolean types


                                                  __Name______________Size_____Ord(True)___________________
                                                    Boolean           1        1
                                                    ByteBool          1        Any nonzero value
                                                    WordBool          2        Any nonzero value
                                                  __LongBool__________4________Any_nonzero_value___________



               Free Pascal also supports the ByteBool, WordBool and LongBool types.  These are of type
               Byte, Word or Longint, but are assignment compatible with a Boolean:  the value False is
               equivalent to 0 (zero) and any nonzero value is considered True when converting to a boolean
               value.  A boolean value of  True is converted to -1 in case it is assigned to a variable of type
               LongBool.

               Assuming B to be of type Boolean, the following are valid assignments:


                B  :=  True;
                B  :=  False;
                B  :=  1<>2;    {  Results  in  B  :=  True  }


               Boolean expressions are also used in conditions.

Remark:        In Free Pascal, boolean expressions are by default always evaluated in such a way that when
               the  result  is  known,  the  rest  of  the  expression  will  no  longer  be  evaluated:  this  is  called
               short-cut boolean evaluation.

               In  the  following  example,  the  function  Func  will  never  be  called,  which  may  have  strange
               side-effects.


                ...
                B  :=  False;
                A  :=  B  and  Func;


               Here Func is a function which returns a Boolean type.

               This behaviour is controllable by the {$B  } compiler directive.



               Enumeration types


               Enumeration types are supported in Free Pascal.  On top of the Turbo Pascal implementation,
               Free Pascal allows also a C-style extension of the enumeration type, where a value is assigned
               to a particular element of the enumeration list.


                      |____________________________________________________________________________________________________________*
 *___|
                      Enumerated types



                    - - ___ enumerated type __      ( ____  ___ _____ identifier list ___________ ) _______________________________*
 *_______-oe
                                                          6||  ||_ assigned enum list __    _||||
                                                          |______________ , ________________|


                    - - ___ identifier list ____  _ identifier ____________________________________________________________________*
 *____-oe
                                                6||______ , ______|_|


                    - - ___ assigned enum list __    __  _ identifier __ := __  expression ________________________________________*
 *__-oe
                                                       6||__________________ , _________________|__|



                                                                                26

_____________________________________________________________________________________________________CHAPTER_3.___TYPES____________*
 *___



       |_______________________________________________________________________________________________________________|


(see chapter 12 , page 151   for how to use expressions) When using assigned enumerated types,
the assigned elements must be in ascending numerical order in the list, or the compiler will
complain.  The expressions used in assigned enumerated elements must be known at compile
time.  So the following is a correct enumerated type declaration:


Type
   Direction  =  (  North,  East,  South,  West  );


A C-style enumeration type looks as follows:


Type
   EnumType  =  (one,  two,  three,  forty  :=  40,fortyone);


As a result, the ordinal number of forty is 40, and not 3, as it would be when the ':=  40'
wasn't present.  The ordinal value of fortyone is then 41, and not 4, as it would be when the
assignment  wasn't  present.  After  an  assignment  in  an  enumerated  definition  the  compiler
adds 1 to the assigned value to assign to the next enumerated value.

When specifying such an enumeration type,  it is important to keep in mind that the enu-
merated elements should be kept in ascending order.  The following will produce a compiler
error:


Type
   EnumType  =  (one,  two,  three,  forty  :=  40,  thirty  :=  30);


It  is  necessary  to  keep  forty  and  thirty  in  the  correct  order.  When  using  enumeration
types it is important to keep the following points in mind:


    1.  The Pred and Succ functions cannot be used on this kind of enumeration types.  Trying
        to do this anyhow will result in a compiler error.

    2.  Enumeration types are stored using a default, independent of the actual number of val-
        ues:  the compiler does not try to optimize for space.  This behaviour can be changed
        with the {$PACKENUM  n} compiler directive, which tells the compiler the minimal num-
        ber of bytes to be used for enumeration types.  For instance


        Type
        {$PACKENUM  4}
           LargeEnum  =  (  BigOne,  BigTwo,  BigThree  );
        {$PACKENUM  1}
           SmallEnum  =  (  one,  two,  three  );
        Var  S  :  SmallEnum;
               L  :  LargeEnum;
        begin
           WriteLn  ('Small  enum  :  ',SizeOf(S));
           WriteLn  ('Large  enum  :  ',SizeOf(L));
        end.


        will, when run, print the following:


        Small  enum  :  1
        Large  enum  :  4


More information can be found in the Programmer's Guide           , in the compiler directives section.



                                                                 27

_____________________________________________________________________________________________________CHAPTER_3.___TYPES____________*
 *___



Subrange types


A  subrange  type  is  a  range  of  values  from  an  ordinal  type  (the  host  type).   To  define  a
subrange type, one must specify its limiting values:  the highest and lowest value of the type.


       |_______________________________________________________________________________________________________________|
       Subrange types



     - - ___ subrange type __    constant __   .. __ constant __ ___________________________________________________-oe


       |_______________________________________________________________________________________________________________|


Some of the predefined integer types are defined as subrange types:


Type
   Longint    =  $80000000..$7fffffff;
   Integer    =  -32768..32767;
   shortint  =  -128..127;
   byte        =  0..255;
   Word        =  0..65535;


Subrange types of enumeration types can also be defined:


Type
   Days  =  (monday,tuesday,wednesday,thursday,friday,
                saturday,sunday);
   WorkDays  =  monday  ..  friday;
   WeekEnd  =  Saturday  ..  Sunday;



3.1.2        Real  types

Free Pascal uses the math coprocessor (or emulation) for all its floating-point calculations.
The Real native type is processor dependent, but it is either Single or Double.  Only the IEEE
floating point types are supported, and these depend on the target processor and emulation
options.  The true Turbo Pascal compatible types are listed in table (3.4 ).



                                         Table 3.4:  Supported Real types



__Type__________________________________________Range____________________________________Significant_digits____________Size___
  Real                                 platform dependant                                          ???              4 or 8
  Single                                1.5E-45 ..  3.4E38                                         7-8                     4
  Double                               5.0E-324 ..  1.7E308                                      15-16                     8
  Extended                           1.9E-4932 ..  1.1E4932                                      19-20                   10
  Comp                                 -2E64+1 ..  2E63-1                                        19-20                     8
  Currency         -922337203685477.5808 ..  922337203685477.5807                                19-20                     8



The Comp type is, in effect, a 64-bit integer and is not available on all target platforms.  To get
more information on the supported types for each platform, refer to the Programmer's Guide           .

The currency type is a fixed-point real data type which is internally used as an 64-bit integer
type (automatically scaled with a factor 10000), this minimalizes rounding errors.



                                                                 28

_____________________________________________________________________________________________________CHAPTER_3.___TYPES____________*
 *___



3.2         Character  types



3.2.1        Char  or  AnsiChar

Free Pascal supports the type Char.  A Char is exactly 1 byte in size, and contains one ASCII
character.

A character constant can be specified by enclosing the character in single quotes, as follows
:  'a' or 'A' are both character constants.

A  character  can  also  be  specified  by  its  character  value  (commonly  an  ASCII  code),  by
preceding the ordinal value with the number symbol (#).  For example specifying #65 would
be the same as 'A'.

Also, the caret character (^) can be used in combination with a letter to specify a character
with ASCII value less than 27.  Thus ^G equals #7 - G is the seventh letter in the alphabet.
The compiler is rather sloppy about the characters it allows after the caret, but in general
one should assume only letters.

When the single quote character must be represented, it should be typed two times succes-
sively, thus '''' represents the single quote character.

To distinguish Char from WideChar, the system unit also defines the AnsiChar type, which
is the same as the char type.  In future versions of FPC, the Char type may become an alias
for either WideChar or AnsiChar.



3.2.2        WideChar

Free Pascal supports the type WideChar.  A WideChar is exactly 2 bytes in size, and contains
one UNICODE character in UTF-16 encoding.

A unicode character can be specified by its character value (an UTF-16 code), by preceding
the ordinal value with the number symbol (#).

A normal ansi (1-byte) character literal can also be used for a widechar, the compiler will
automatically convert it to a 2-byte UTF-16 character.

The following defines some greek characters (phi, omega):


Const
    C3  :  widechar  =  #$03A8;
    C4  :  widechar  =  #$03A9;


The same can be accomplished by typecasting a word to widechar:


Const
    C3  :  widechar  =  widechar($03A8);
    C4  :  widechar  =  widechar($03A9);



3.2.3        Other  character  types

Free Pascal defines some other character types in the system unit such as UCS2Char, UCS4Char,
UniCodeChar.  However, no special support for these character types exists, they have been
defined for Delphi compatibility only.



3.2.4        Single-byte  String  types

Free Pascal supports the String type as it is defined in Turbo Pascal:  a sequence of single-
byte characters with an optional size specification.  It also supports ansistrings (with unlim-



                                                                 29

_____________________________________________________________________________________________________CHAPTER_3.___TYPES____________*
 *___



ited length) and codepage information1  as in Delphi.

To declare a variable as a string, use the following type specification:


       |_______________________________________________________________________________________________________________|
       String Type



     - - ___ string type __  __ _________ string __ __ _____________________________________________________________________-oe
                               |                      ||_   __                  __      ___||        |
                               ||_                        [    unsigned integer       ]              |
                                   type __ __|____ string __ ____|_ ( __ unsigned integer __    ) ___|
                                             |_ ansistring __  _|

       |_______________________________________________________________________________________________________________|


If there is a size specifier (using square brackets), then its maximum value - indicating the
maximum size of the string - is 255.  If there is a codepage specifier, (using round brackets)
it indicates an ansistring with associated code page information.

The  meaning  of  a  string  declaration  statement  without  size  and  code  page  indication  is
interpreted differently depending on the {$H} switch:


var
   A  :  String;


If no size and code page indication indication is present, the above declaration can declare
an ansistring or a short string.

Whatever  the  actual  type,  single  byte  strings  can  be  used  interchangeably.  The  compiler
always  takes  care  of  the  necessary  type  conversions.  Note,  however,  that  the  result  of  an
expression that contains ansistrings and short strings will always be an ansistring.



Short strings


A string declaration declares a short string in the following cases:


    1.  If  the  $H  switch  is  off:   {$H-},  the  string  declaration  will  always  be  a  short  string
        declaration.

    2.  If  the  switch  is  on  {$H+},  and  there  is  a  maximum  length  (the  size)  specifier,  the
        declaration is a short string declaration.


Short strings are always assumed to use the system code page.  The predefined type ShortString
is defined as a string of size 255:


  ShortString  =  String[255];


If the size of the string is not specified, 255 is taken as a default.  The actual length of the
string can be obtained with the Length standard runtime routine.  For example in


{$H-}


Type
    NameString  =  String[10];
    StreetString  =  String;
___________________________________________________1
     As of version 3.0 of Free Pascal



                                                                 30

               _____________________________________________________________________________________________________CHAPTER_3.___TY*
 *PES_______________



               NameString can contain a maximum of 10 characters.  While StreetString can contain up
               to 255 characters.

Remark:        Short strings have a maximum length of 255 characters:  when specifying a maximum length,
               the maximum length may not exceed 255.  If a length larger than 255 is attempted, then the
               compiler will give an error message:


               Error:  string  length  must  be  a  value  from  1  to  255


               For short strings,  the length is stored in the character at index 0.  Old Turbo Pascal code
               relies on this, and it is implemented similarly in Free Pascal.

               Despite  this,  to  write  portable  code,  it  is  best  to  set  the  length  of  a  shortstring  with  the
               SetLength call, and to retrieve it with the Length call.  These functions will always work,
               whatever the internal representation of the shortstrings or other strings in use:  this allows
               easy switching between the various string types.



               Ansistrings


               Ansistrings  are  strings  that  have  no  length  limit,  and  have  a  code  page  associated  with
               them2 .  They are reference counted and are guaranteed to be null terminated.

               Internally, an ansistring is treated as a pointer:  the actual content of the string is stored on
               the heap, as much memory as needed to store the string content is allocated.

               If no codepage is given in the declaration, the system codepage is assumed.  What codepage
               this is, is determined by the DefaultSystemCodePage constant in the system unit.

               This is all handled transparently, i.e.  they can be manipulated as a normal short string.  An-
               sistrings can be defined using the predefined AnsiString type or using the string keyword
               in mode {$H+}.

Remark:         The null-termination does not mean that null characters (char(0) or #0) cannot be used:
               the null-termination is not used internally,  but is there for convenience when dealing with
               external routines that expect a null-terminated string (as most C routines do).

               If  the  {$H}  switch  is  on,  then  a  string  definition  using  the  regular  String  keyword  that
               doesn't  contain  a  length  specifier,  will  be  regarded  as  an  ansistring  as  well.   If  a  length
               specifier is present, a short string will be used, regardless of the {$H} setting.

               If the string is empty (''), then the internal pointer representation of the string pointer is
               Nil.  If the string is not empty, then the pointer points to a structure in heap memory.

               The internal representation as a pointer, and the automatic null-termination make it possible
               to typecast an ansistring to a pchar.  If the string is empty (so the pointer is Nil) then the
               compiler makes sure that the typecasted pchar will point to a null byte.

               Assigning one ansistring to another doesn't involve moving the actual string.  A statement


                  S2:=S1;


               results  in  the  reference  count  of  S2  being  decreased  with  1,  The  reference  count  of  S1  is
               increased by 1, and finally S1 (as a pointer) is copied to S2.  This is a significant speed-up
               in the code.

               If the reference count of a string reaches zero,  then the memory occupied by the string is
               deallocated automatically, and the pointer is set to Nil, so no memory leaks arise.

               When an ansistring is declared, the Free Pascal compiler initially allocates just memory for a
               pointer, not more.  This pointer is guaranteed to be Nil, meaning that the string is initially
               ___________________________________________________2
                    codepage was introduced in version 3.0 of Free Pascal



                                                                                31

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empty.  This is true for local and global ansistrings or ansistrings that are part of a structure
(arrays, records or objects).

This does introduce an overhead.  For instance, declaring


Var
   A  :  Array[1..100000]  of  string;


Will copy the value Nil 100,000 times into A. When A goes out of scope, then the reference
count of the 100,000 strings will be decreased by 1 for each of these strings.  All this happens
invisible to the programmer, but when considering performance issues, this is important.

Memory for the string content will be allocated only when the string is assigned a value.  If
the string goes out of scope, then its reference count is automatically decreased by 1.  If the
reference count reaches zero, the memory reserved for the string is released.

If  a  value  is  assigned  to  a  character  of  a  string  that  has  a  reference  count  greater  than  1,
such as in the following statements:


   S:=T;    {  reference  count  for  S  and  T  is  now  2  }
   S[I]:='@';


then  a  copy  of  the  string  is  created  before  the  assignment.   This  is  known  as  copy-on-
write semantics.  It is possible to force a string to have reference count equal to 1 with the
UniqueString call:


   S:=T;
   R:=T;  //  Reference  count  of  T  is  at  least  3
   UniqueString(T);
   //  Reference  count  of  T  is  quaranteed  1


It's recommended to do this e.g.  when typecasting an ansistring to a PChar var and passing
it to a C routine that modifies the string.

The Length function must be used to get the length of an ansistring:  the length is not stored
at character 0 of the ansistring.  The construct


 L:=ord(S[0]);


which  was  valid  for  Turbo  Pascal  shortstrings,  is  no  longer  correct  for  Ansistrings.   The
compiler will warn if such a construct is encountered.

To set the length of an ansistring, the SetLength function must be used.  Constant ansistrings
have a reference count of -1 and are treated specially, The same remark as for Length must
be given:  The construct


   L:=12;
   S[0]:=Char(L);


which  was  valid  for  Turbo  Pascal  shortstrings,  is  no  longer  correct  for  Ansistrings.   The
compiler will warn if such a construct is encountered.

Ansistrings are converted to short strings by the compiler if needed, this means that the use
of ansistrings and short strings can be mixed without problems.

Ansistrings can be typecasted to PChar or Pointer types:


Var  P  :  Pointer;
      PC  :  PChar;



                                                                 32

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      S  :  AnsiString;


begin
   S  :='This  is  an  ansistring';
   PC:=Pchar(S);
   P  :=Pointer(S);


There is a difference between the two typecasts.  When an empty ansistring is typecasted to
a pointer, the pointer will be Nil.  If an empty ansistring is typecasted to a PChar, then the
result will be a pointer to a zero byte (an empty string).

The result of such a typecast must be used with care.  In general, it is best to consider the
result of such a typecast as read-only, i.e.  only suitable for passing to a procedure that needs
a constant pchar argument.

It is therefore not advisable to typecast one of the following:


    1.  Expressions.

    2.  Strings that have reference count larger than 1.  In this case you should call Uniquestring
        to ensure the string has reference count 1.



Code page conversions


Since strings have code page information associated with them, it is important to know which
code page a string uses:


     o  Short strings always use the system code page.

     o  Plain ansistrings use the system code page.

     o  Single byte strings with declared code page use that code page.

     o  The RawBytestring type has no code page information associated with it.


The compiler will convert the code page of strings as needed:  When assigning a string, the
actual codepage of the source string will always be converted to the declared code page of
the target string.

This means that in the following code:


Type
    TString1  =  Type  String(1252);
    TString2  =  Type  String(1251);


Var
    A  :  TString1;
    B  :  TString2;


begin
    A:='123'+'345'+intToStr(123);
    B:=A;
    Writeln('B  :  ',StringRefCount(B));
    Writeln('A  :  ',StringRefCount(A));
end.



                                                                 33

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 *PES_______________



               The compiler will convert the contents in string B to the codepage of string A. Note that if
               a code page conversion takes place, the reference count mechanism is not used:  a new string
               will be allocated.

               This automated conversion of code pages can slow down the code seriously, so care must be
               taken to see to it that the code page conversions are limited to a minimum.

               The code page of a string can be set explicitly using the SetCodePage routine of the system
               unit.  Calling this routine will convert the value of a string to the requested code page.

Remark:        Code page conversions can result in loss of data:  if a certain character cannot be represented
               in the target code page, the output for that character is undefined.

Remark:         Code page support requires quite some helper routines, these are implemented in the uni-
               codestring  manager.  On  windows,  the  system  routines  are  used  for  this.  On  Unices,  the
               cwstring unit can be used to link to the C library and use the C library conversion sup-
               port.  Alternatively, the fpwidestring unit contains a unicodestring manager implemented
               natively in Object Pascal.



               RawByteString


               The pre-defined RawByteString type is an ansistring string type without codepage informa-
               tion (CP__ NONE):


               Type
                  RawByteString  =  type  ansistring(CP_NONE);


               It  is  treated  specially  in  the  sense  that  if  the  conversion  routines  encounter  CP__ NONE  in  a
               target  string,  no  code  page  conversion  is  performed,  the  code  page  of  the  source  string  is
               preserved.

               For  this  reason,  most  single-byte  string  routines  in  the  system  and  sysutils  units  use  the
               RawByteString type.



               UTF8String


               Single-byte  code  page  strings  can  only  store  the  characters  available  in  that  code  page.
               Characters that are not present in the code page, cannot be represented by that string.  The
               UTF-8 unicode encoding is an encoding that can be used with single-byte strings:  The ASCII
               characters (ordinal value <128) in this encoding map exactly to the CP__ ACP encoding.  This
               fact is used to define a single byte string type that can contain all characters:


               Type
                  UTF8String  =  type  AnsiString(CP_UTF8);


               The  UTF8string  string  type  can  be  used  to  represent  all  Unicode  characters.  This  power
               comes as a price, though.  Since a unicode character may require several bytes to be repre-
               sented in the UTF-8 encoding, there are 2 points to take care of when using UTF8String:


                   1.  The character index - which retrieves a byte-sized char at a certain position - must
                       be used with care:  the expression S[i] will not necessarily be a valid character for a
                       string S of type UTF8String.

                   2.  The byte length of the string is not equal to the number of characters in the string.  The
                       standard  function  length  cannot  be  used  to  get  the  character  length,  it  will  always
                       return the byte length.



                                                                                34

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For all other code pages, the number of characters in a single-byte code page string is equal
to the byte length of the string.



3.2.5        Multi-byte  String  types

For multi-byte string types, the basic character has a size of at least 2.  This means it can
be used to store a unicode character in UTF16 or UCS2 encoding.



UnicodeStrings


Unicodestrings (used to represent unicode character strings) are implemented in much the
same  way  as  ansistrings:  reference  counted,  null-terminated  arrays,  only  they  are  imple-
mented as arrays of  WideChars instead of regular Chars.  A WideChar is a two-byte charac-
ter (an element of a DBCS: Double Byte Character Set).  Mostly the same rules apply for
UnicodeStrings as for AnsiStrings.  The compiler transparently converts UnicodeStrings
to AnsiStrings and vice versa.

Similarly to the typecast of an Ansistring to a PChar null-terminated   array of characters,
a UnicodeString can be converted to a PUnicodeChar null-terminated array of characters.
Note that the PUnicodeChar array is terminated by 2 null bytes instead of 1, so a typecast
to a pchar is not automatic.

The compiler itself provides no support for any conversion from Unicode to ansistrings or
vice  versa.  The  system  unit  has  a  unicodestring  manager  record,  which  can  be  initialized
with some OS-specific unicode handling routines.  For more information, see the system unit
reference.

A unicode string literal can be constructed in a similar manner as a widechar:


Const
    ws2:  unicodestring  =  'phi  omega  :  '#$03A8'  '#$03A9;



WideStrings


Widestrings  (used  to  represent  unicode  character  strings  in  COM  applications)  are  imple-
mented  in  much  the  same  way  as  unicodestrings.   Unlike  the  latter,  they  are  not  refer-
ence counted,  and on Windows,  they are allocated with a special windows function which
allows  them  to  be  used  for  OLE  automation.   This  means  they  are  implemented  as  null-
terminated  arrays  of  WideChars  instead  of  regular  Chars.   Mostly  the  same  rules  apply
for WideStrings as for AnsiStrings.  Similar to unicodestrings, the compiler transparenty
converts WideStrings to AnsiStrings and vice versa.

For typecasting and conversion, the same rules apply as for the unicodestring type.



3.2.6        Constant  strings

To specify a constant string, it must be enclosed in single-quotes, just as a Char type, only
now more than one character is allowed.  Given that S is of type String, the following are
valid assignments:


S  :=  'This  is  a  string.';
S  :=  'One'+',  Two'+',  Three';
S  :=  'This  isn''t  difficult  !';
S  :=  'This  is  a  weird  character  :  '#145'  !';



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As can be seen, the single quote character is represented by 2 single-quote characters next to
each other.  Strange characters can be specified by their character value (usually an ASCII
code).  The example shows also that two strings can be added.  The resulting string is just the
concatenation of the first with the second string, without spaces in between them.  Strings
can not be subtracted, however.

Whether  the  constant  string  is  stored  as  an  ansistring  or  a  short  string  depends  on  the
settings of the {$H} switch.



3.2.7        PChar  -  Null  terminated  strings

Free Pascal supports the Delphi implementation of the PChar type.  PChar is defined as a
pointer to a Char type, but allows additional operations.  The PChar type can be understood
best as the Pascal equivalent of a C-style null-terminated string, i.e.  a variable of type PChar
is a pointer that points to an array of type Char, which is ended by a null-character (#0).  Free
Pascal supports initializing of  PChar typed constants, or a direct assignment.  For example,
the following pieces of code are equivalent:


program  one;
var  P  :  PChar;
begin
    P  :=  'This  is  a  null-terminated  string.';
    WriteLn  (P);
end.


Results in the same as


program  two;
const  P  :  PChar  =  'This  is  a  null-terminated  string.';
begin
    WriteLn  (P);
end.


These examples also show that it is possible to write the  contents of the string to a file of
type Text.  The strings   unit contains procedures and functions that manipulate the PChar
type as in the standard C library.  Since it is equivalent to a pointer to a type Char variable,
it is also possible to do the following:


Program  three;
Var  S  :  String[30];
       P  :  PChar;
begin
    S  :=  'This  is  a  null-terminated  string.'#0;
    P  :=  @S[1];
    WriteLn  (P);
end.


This will have the same result as the previous two examples.  Null-terminated strings cannot
be added as normal Pascal strings.  If two PChar strings must be concatenated; the functions
from the unit strings   must be used.

However, it is possible to do some pointer arithmetic.  The operators + and - can be used to
do operations on PChar pointers.  In table (3.5 ), P and Q are of type PChar, and I is of type
Longint.



                                                                 36

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                                       Table 3.5:  PChar pointer arithmetic


              __Operation_____________________________________________________________________________Result_____
                P  +  I                                   Adds I to the address pointed to by P.
                I  +  P                                   Adds I to the address pointed to by P.
                P  -  I                        Substracts I from the address pointed to by P.
                P  -  Q           Returns, as an integer, the distance between 2 addresses
              _________________________________(or_the_number_of_characters_between_P_and_Q)_____________________



3.2.8        String  sizes

The memory occupied by a string depends on the string type.  Some string types allocate
the string data in memory on the heap, others have the string data on the stack.  Table table
(3.6 ) summarizes the memory usage of the various string types for the various string types.
In the table, the following symbolic constants are used:


    1.  L is the actual length of the string.

    2.  HS depends on the version of Free Pascal, but is 16 bytes as of Free Pascal 2.7.1.

    3.  WHS size is 4 bytes for all versions of Free Pascal.

    4.  UHS size is 8 bytes for all versions of Free Pascal.



                                          Table 3.6:  String memory sizes


                     __String_type____________Stack_size______________________heap_size___________________
                       Shortstring            Declared length + 2             0
                       Ansistring             Pointer size                    L + 1 + HS
                       Widestring             Pointer size                    2*(L + 1) + WHS
                       UnicodeString          Pointer size                    2*(L + 1) + UHS
                     __Pchar__________________Pointer_size____________________L+1_________________________



3.3         Structured  Types


A structured type is a type that can hold multiple values in one variable.  Structured types
can be nested to unlimited levels.


        |______________________________________________________________________________________________________________|
        Structured Types



      --  ___ structured type __   __|_______ array type __  _______|_____________________________________________________-oe

                                     |______ record type __  ______|
                                     |_______ object type __  _______|
                                     |_______ class type __ _______|
                                     |_ class reference type __   _|
                                     |_____ interface type __  _____|
                                     |________|set_type____________|
                                                            file type __________|



                                                                 37

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       |_______________________________________________________________________________________________________________|


Unlike Delphi, Free Pascal does not support the keyword Packed for all structured types.  In
the following sections each of the possible structured types is discussed.  It will be mentioned
when a type supports the packed keyword.



Packed structured types


When  a  structured  type  is  declared,  no  assumptions  should  be  made  about  the  internal
position of the elements in the type.  The compiler will lay out the elements of the structure
in  memory  as  it  thinks  will  be  most  suitable.   That  is,  the  order  of  the  elements  will  be
kept, but the location of the elements are not guaranteed, and is partially governed by the
$PACKRECORDS directive (this directive is explained in the Programmer's Guide           ).

However, Free Pascal allows controlling the layout with the Packed and Bitpacked keywords.
The meaning of these words depends on the context:


Bitpacked        In this case, the compiler will attempt to align ordinal types on bit boundaries,
       as explained below.

Packed      The meaning of the Packed keyword depends on the situation:

           1.  In MACPAS mode, it is equivalent to the Bitpacked keyword.

           2.  In other modes, with the $BITPACKING directive set to ON, it is also equivalent to
               the Bitpacked keyword.

           3.  In  other  modes,  with  the  $BITPACKING  directive  set  to  OFF,  it  signifies  normal
               packing on byte boundaries.

        Packing on byte boundaries means that each new element of a structured type starts
        on a byte boundary.


The byte packing mechanism is simple:  the compiler aligns each element of the structure on
the first available byte boundary, even if the size of the previous element (small enumerated
types, subrange types) is less than a byte.

When using the bit packing mechanism, the compiler calculates for each ordinal type how
many bits are needed to store it.  The next ordinal type is then stored on the next free bit.
Non-ordinal types - which include but are not limited to - sets, floats, strings, (bitpacked)
records,  (bitpacked) arrays,  pointers,  classes,  objects,  and procedural variables,  are stored
on the first available byte boundary.

Note that the internals of the bitpacking are opaque:  they can change at any time in the
future.  What is more:  the internal packing depends on the endianness of the platform for
which the compilation is done, and no conversion between platforms are possible.  This makes
bitpacked structures unsuitable for storing on disk or transport over networks.  The format
is however the same as the one used by the GNU Pascal Compiler, and the Free Pascal team
aims to retain this compatibility in the future.

There are some more restrictions to elements of bitpacked structures:


     o  The address cannot be retrieved, unless the bit size is a multiple of 8 and the element
        happens to be stored on a byte boundary.

     o  An element of a bitpacked structure cannot be used as a var parameter, unless the bit
        size is a multiple of 8 and the element happens to be stored on a byte boundary.



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To determine the size of an element in a bitpacked structure, there is the BitSizeOf function.
It returns the size - in bits - of the element.  For other types or elements of structures which
are not bitpacked, this will simply return the size in bytes multiplied by 8, i.e., the return
value is then the same as 8*SizeOf.

The size of bitpacked records and arrays is limited:


     o On 32 bit systems the maximal size is 229  bytes (512 MB).

     o On 64 bit systems the maximal size is 261  bytes.


The reason is that the offset of an element must be calculated with the maximum integer
size of the system.



3.3.1        Arrays

Free Pascal supports arrays as in Turbo Pascal.  Multi-dimensional arrays and (bit)packed
arrays are also supported, as well as the dynamic arrays of Delphi:


        |______________________________________________________________________________________________________________|
        Array types



      --  ___ array type __  __|__________________|__ array __ __|__________________________________|__ of  __ type __________-oe

                               |___|packed___   ___|             |_ [ ____|_ ordinal type __ __|_ ] ___|
                                            bitpacked __   _|             6|________ , __________|

        |______________________________________________________________________________________________________________|



Static arrays


When the range of the array is included in the array definition,  it is called a static array.
Trying to access an element with an index that is outside the declared range will generate
a  run-time  error  (if  range  checking  is  on).   The  following  is  an  example  of  a  valid  array
declaration:


Type
    RealArray  =  Array  [1..100]  of  Real;


Valid indexes for accessing an element of the array are between 1 and 100, where the borders
1 and 100 are included.  As in Turbo Pascal, if the array component type is in itself an array,
it  is  possible  to  combine  the  two  arrays  into  one  multi-dimensional  array.   The  following
declaration:


Type
     APoints  =  array[1..100]  of  Array[1..3]  of  Real;


is equivalent to the declaration:


Type
     APoints  =  array[1..100,1..3]  of  Real;


The functions High and Low return the high and low bounds of the leftmost index type of the
array.  In the above case, this would be 100 and 1.  You should use them whenever possible,
since it improves maintainability of your code.  The use of both functions is just as efficient
as using constants, because they are evaluated at compile time.



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When static array-type variables are assigned to each other, the contents of the whole array
is copied.  This is also true for multi-dimensional arrays:


program  testarray1;


Type
   TA  =  Array[0..9,0..9]  of  Integer;


var
   A,B  :  TA;
   I,J  :  Integer;
begin
   For  I:=0  to  9  do
      For  J:=0  to  9  do
         A[I,J]:=I*J;
   For  I:=0  to  9  do
      begin
      For  J:=0  to  9  do
         Write(A[I,J]:2,'  ');
      Writeln;
      end;
   B:=A;
   Writeln;
   For  I:=0  to  9  do
      For  J:=0  to  9  do
         A[9-I,9-J]:=I*J;
   For  I:=0  to  9  do
      begin
      For  J:=0  to  9  do
         Write(B[I,J]:2,'  ');
      Writeln;
      end;
end.


The output of this program will be 2 identical matrices.



Dynamic arrays


As  of  version  1.1,  Free  Pascal  also  knows  dynamic  arrays:  In  that  case  the  array  range  is
omitted, as in the following example:


Type
   TByteArray  =  Array  of  Byte;


When declaring a variable of a dynamic array type,  the initial length of the array is zero.
The  actual  length  of  the  array  must  be  set  with  the  standard  SetLength  function,  which
will allocate the necessary memory to contain the array elements on the heap.  The following
example will set the length to 1000:


Var
   A  :  TByteArray;


begin
   SetLength(A,1000);



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After a call to SetLength, valid array indexes are 0 to 999:  the array index is always zero-
based.

Note  that  the  length  of  the  array  is  set  in  elements,  not  in  bytes  of  allocated  memory
(although these may be the same).  The amount of memory allocated is the size of the array
multiplied by the size of 1 element in the array.  The memory will be disposed of at the exit
of the current procedure or function.

It is also possible to resize the array:  in that case, as much of the elements in the array as
will fit in the new size, will be kept.  The array can be resized to zero, which effectively resets
the variable.

At all times, trying to access an element of the array with an index that is not in the current
length of the array will generate a run-time error.

Dynamic  arrays  are  reference  counted:  assignment  of  one  dynamic  array-type  variable  to
another will let both variables point to the same array.  Contrary to ansistrings, an assignment
to an element of one array will be reflected in the other:  there is no copy-on-write.  Consider
the following example:


Var
   A,B  :  TByteArray;


begin
   SetLength(A,10);
   A[0]:=33;
   B:=A;
   A[0]:=31;


After the second assignment, the first element in B will also contain 31.

It can also be seen from the output of the following example:


program  testarray1;


Type
   TA  =  Array  of  array  of  Integer;


var
   A,B  :  TA;
   I,J  :  Integer;
begin
   Setlength(A,10,10);
   For  I:=0  to  9  do
      For  J:=0  to  9  do
         A[I,J]:=I*J;
   For  I:=0  to  9  do
      begin
      For  J:=0  to  9  do
         Write(A[I,J]:2,'  ');
      Writeln;
      end;
   B:=A;
   Writeln;
   For  I:=0  to  9  do
      For  J:=0  to  9  do
         A[9-I,9-J]:=I*J;



                                                                 41

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   For  I:=0  to  9  do
      begin
      For  J:=0  to  9  do
         Write(B[I,J]:2,'  ');
      Writeln;
      end;
end.


The output of this program will be a matrix of numbers, and then the same matrix, mirrored.

As remarked earlier, dynamic arrays are reference counted:  if in one of the previous examples
A goes out of scope and B does not, then the array is not yet disposed of:  the reference count
of A (and B) is decreased with 1.  As soon as the reference count reaches zero the memory,
allocated for the contents of the array, is disposed of.

The SetLength call will make sure the reference count of the returned array is 1, that it, if
2 dynamic array variables were pointing to the same memory they will no longer do so after
the setlength call:


program  testunique;


Type
   TA  =  array  of  Integer;


var
   A,B  :  TA;
   I  :  Integer;


begin
   Setlength(A,10);
   For  I:=0  to  9  do
      A[I]:=I;
   B:=A;
   SetLength(B,6);
   A[0]:=123;
   For  I:=0  to  5  do
      Writeln(B[I]);
end.


It is also possible to copy and/or resize the array with the standard Copy function,  which
acts as the copy function for strings:


program  testarray3;


Type
   TA  =  array  of  Integer;


var
   A,B  :  TA;
   I  :  Integer;


begin
   Setlength(A,10);
   For  I:=0  to  9  do
         A[I]:=I;



                                                                 42

_____________________________________________________________________________________________________CHAPTER_3.___TYPES____________*
 *___



   B:=Copy(A,3,6);
   For  I:=0  to  5  do
      Writeln(B[I]);
end.


The Copy function will copy 6 elements of the array to a new array.  Starting at the element
at index 3 (i.e.  the fourth element) of the array.

The Length function will return the number of elements in the array.  The Low function on
a dynamic array will always return 0, and the High function will return the value Length-1,
i.e., the value of the highest allowed array index.



Dynamic array Type compatibility


Object Pascal is a strictly typed language.  2 technically distinct types are sometimes consid-
ered assignment compatible (i.e.  a value of one type can be assigned to a variable of another
type) under certain circumstances.  Dynamic arrays are considered assignment compatible
when they use the same element type.  That means that the following will compile:


{$mode  objfpc}


Type
   TA  =  Array  of  Integer;
   TB  =  Array  of  Integer;


Var
   A  :  TA;
   B  :  TB;


begin
   SetLength(A,1);
   A[0]:=1;
   B:=A;
end.


But the following will not, even though the integer and word types are assignement compat-
ible:


{$mode  objfpc}


Type
   TA  =  Array  of  Word;
   TB  =  Array  of  Integer;


Var
   A  :  TA;
   B  :  TB;


begin
   SetLength(A,1);
   A[0]:=1;
   B:=A;
end.



                                                                 43

_____________________________________________________________________________________________________CHAPTER_3.___TYPES____________*
 *___



Dynamic array constructor


As of version 3.0 of Free Pascal, Dynamic array types have a constructor.  This is intrinsic,
the compiler provides it.  Up to version 2.6.4, the only way to initialize a dynamic array was
as follows:


Type
   TIntegerArray  =  Array  of  Integer;


var
   A  :  TIntegerArray;


begin
   SetLength(A,3);
   A[0]:=1;
   A[1]:=2;
   A[3]:=3;
   Writeln(Length(A));
end.


As of version 3.0 of Free Pascal, an dynamic array can be initialized using a constructor-like
syntax.  The constructor is called Create, and accepts as parameters a variable number of
parameters of the element type of the array type.  This means the above initialization can
now be done as:


Type
   TIntegerArray  =  Array  of  Integer;


var
   A  :  TIntegerArray;


begin
   A:=TIntegerArray.Create(1,2,3);
   Writeln(Length(A));
end.


Note that this will not work for dynamic arrays for which no type was created.  That is, the
following will not work:


var
   A  :  Array  of  Integer;


begin
   A:=Array  of  Integer.Create(1,2,3);
   Writeln(Length(A));
end.


This approach also works recursively for multi-dimensional arrays:


Type
   TIntegerArray  =  Array  of  Integer;
   TIntegerArrayArray  =  Array  of  TIntegerArray;


var



                                                                 44

_____________________________________________________________________________________________________CHAPTER_3.___TYPES____________*
 *___



   A  :  TIntegerArrayArray;


begin
   A:=TIntegerArrayArray.Create(TIntegerArray.Create(1,2,3),
                                                   TIntegerArray.Create(4,5,6),
                                                   TIntegerArray.Create(7,8,9));
   Writeln('Length  ',length(A));
end.


However, since it is a constructor (code is run at run-time) it is not possible to use this in
an initialized variable syntax.  That is, the following will not work:


Type
   TIntegerArray  =  Array  of  Integer;


var
   A  :  TIntegerArray  =  TIntegerArray.Create(1,2,3);


begin
   Writeln('Length  ',length(A));
end.



Packing and unpacking an array


Arrays  can  be  packed  and  bitpacked.  2  array  types  which  have  the  same  index  type  and
element type, but which are differently packed are not assignment compatible.

However, it is possible to convert a normal array to a bitpacked array with the pack routine.
The reverse operation is possible as well; a bitpacked array can be converted to a normally
packed array using the unpack routine, as in the following example:


Var
   foo  :  array  [  'a'..'f'  ]  of  Boolean
      =  (  false,  false,  true,  false,  false,  false  );
   bar  :  bitpacked  array  [  42..47  ]  of  Boolean;
   baz  :  array  [  '0'..'5'  ]  of  Boolean;


begin
   pack(foo,'a',bar);
   unpack(bar,baz,'0');
end.


More  information  about  the  pack  and  unpack  routines  can  be  found  in  the  system  unit
reference.



3.3.2        Record  types

Free Pascal supports fixed records and records with variant parts.  The syntax diagram for
a record type is


        |______________________________________________________________________________________________________________|
        Record types



                                                                 45

               _____________________________________________________________________________________________________CHAPTER_3.___TY*
 *PES_______________



                    - - ___ record type __  __|__________________|__ record __  __|________________|__ end __ _____________________*
 *________-oe

                                              |___|packed___   ___|               |_ field list ___|
                                                           bitpacked __   _|

                    - - ___ field list ____|_______________ fixed fields _________________|____|________|__________________________*
 *___________-oe
                                           |___|________________________|_ variant part __  _| |_ ; ___|
                                               |_ fixed fields __ ; ___|


                    - - ___ fixed fields ____  _ identifier list __ : __type ______________________________________________________*
 *__-oe
                                             6||_______________ ; _______________|_|


                    - - ___ variant part __   case __ __|______________________|__ ordinal type identifier __    of  ____|_ variant*
 * ____|___-oe
                                                        |_ identifier __ : ___|                                          6|_____ ; *
 *_______|


                    - - ___ variant __ __  _ constant __   , _____ : __ ( ____ __________________ ) _______________________________*
 *_________-oe
                                         6||____________________|_|           ||_ field list __|_|


                      |____________________________________________________________________________________________________________*
 *___|


               So the following are valid record type declarations:


               Type
                  Point  =  Record
                               X,Y,Z  :  Real;
                               end;
                  RPoint  =  Record
                               Case  Boolean  of
                               False  :  (X,Y,Z  :  Real);
                               True  :  (R,theta,phi  :  Real);
                               end;
                  BetterRPoint  =  Record
                               Case  UsePolar  :  Boolean  of
                               False  :  (X,Y,Z  :  Real);
                               True  :  (R,theta,phi  :  Real);
                               end;


               The variant part must be last in the record.  The optional identifier in the case statement
               serves to access the tag field value, which otherwise would be invisible to the programmer.
               It can be used to see which variant is active at a certain time3 .  In effect, it introduces a new
               field in the record.

Remark:         It is possible to nest variant parts, as in:


               Type
                  MyRec  =  Record
                               X  :  Longint;
                               Case  byte  of
                                  2  :  (Y  :  Longint;
                                           case  byte  of
                                           3  :  (Z  :  Longint);
                                           );
                               end;
               ___________________________________________________3
                    However, it is up to the programmer to maintain this field.



                                                                                46

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 *___



Record layout and size


The layout and size of a record is influenced by five aspects:


     o The size of its fields.

     o The alignment requirements of the types of the fields, which are platform-dependent.
       Note that the alignment requirements of a type inside a record may be different from
       those of a separate variable of that type.  Additionally, the location of a field inside a
       record may also influence its type's alignment requirements.

     o The  currently  active  {$ALIGN  N}  or  {$PACKRECORDS  N}  setting  (these  settings  over-
       ride each other,  so the last one specified is the active one;  note that these directives
       to  not  accept  exactly  the  same  arguments,  see  the  programmer's  manual  for  more
       information).

     o The currently active {$CODEALIGN  RECORDMIN=X} setting.

     o The currently active {$CODEALIGN  RECORDMAX=X} setting.


The layout and size of variant parts in records is determined by replacing them with a field
whose type is a record with as first element a field of the tag field type if an identifier was
declared for this tag field, followed by the elements of the biggest variant.

Field F2's offset in a record is equal to the sum of the previous field F1's offset and F1's size,
rounded up to a multiple of  F2's required alignment.  This required alignment is calculated
as follows:


     o The  required  alignment  is  set  to  the  default  alignment  of  the  field's  type,  possibly
       adjusted based on the fact that this type occurs in a record and on the field's location
       in the record.

     o If the required alignment is smaller than the currently active {$CODEALIGN  RECORDMIN=X}
       setting, it is changed to this X value.

     o If the currently active {$ALIGN  N} or {$PACKRECORDS  N} setting is

           -  a numerical value:  if the required alignment is larger than N, it is changed to N.
              I.e., if  N is 1, all fields will be placed right after each other.

           -  RESET or DEFAULT: the resulting required alignment is target dependent.

           -  C: the required alignment is adjusted according to the rules specified in the official
              ABI for the current platform.

           -  POWER/POWERPC, MAC68K: the alignment value's adjustment is determined by fol-
              lowing the official ABI rules for resp. the (Classic) Macintosh PowerPC or Mac-
              intosh 680x0 platforms.


The size of a record is equal to the sum of the record's last field's offset and this field's size,
rounded up to a multiple of the record's required alignment.  The record's required alignment
is calculated as follows:


     o The  required  alignment  is  set  to  the  alignment  of  the  record's  field  with  the  largest
       alignment, as determined while laying out the record.

     o If the current {$ALIGN  N} or {$PACKRECORDS  N} setting is different from C and the re-
       quired alignment is larger than than the currently active {$CODEALIGN  RECORDMAX=X},
       the required alignment is changed to X.

     o If  the  current  {$ALIGN  N}  or  {$PACKRECORDS  N}  setting  is  equal  to  C,  the  required
       alignment is determined by following the official ABI rules.



                                                                 47

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 *___



Remarks and examples


Free  Pascal  also  supports  a  'packed  record',  which  is  a  record  where  all  the  elements  are
byte-aligned.  As a result, the two following declarations are equivalent:


        {$PackRecords  1}
        Trec2  =  Record
           A  :  Byte;
           B  :  Word;
           end;
        {$PackRecords  default}


and


        Trec2  =  Packed  Record
           A  :  Byte;
           B  :  Word;
           end;


Note the {$PackRecords  Default} after the first declaration to restore the default setting!

Given the platform-dependent nature of how records are laid out in memory, the only way
to ensure a compatible layout across platforms (assuming that all fields are declared using a
type with the same meaning across these same platforms) is by using {$PACKRECORDS  1}.

In  particular,  if  a  typed  file  with  records,  produced  by  a  Turbo  Pascal  program,  must  be
read, then chances are that attempting to read that file correctly will fail.  The reason is that
Free  Pascal's  default  {$PACKRECORDS  N}  setting  is  not  necessarily  compatible  with  Turbo
Pascal's.  It can be changed to {$PACKRECORDS  1} or {$PACKRECORDS  2} depending on the
setting used in the Turbo Pascal program that create the file (although it may still fail with
{$PACKRECORDS  2} due to different type alignment requirements between 16 bit MSDOS and
your current platform).

The same remark goes for Delphi:  exchanging data is only guaranteed to be possible if both
the producer and consumer use a packed record, or if they are on the same platform and use
the same {$PACKRECORDS  X} setting.



3.3.3        Set  types

Free Pascal supports the set types as in Turbo Pascal.  The prototype of a set declaration is:


        |______________________________________________________________________________________________________________|
        Set Types


      --  ___ set type __  set __  of  __ordinal type __  ___________________________________________________________-oe


        |______________________________________________________________________________________________________________|


Each  of  the  elements  of  SetType  must  be  of  type  TargetType.   TargetType  can  be  any
ordinal type with a range between 0 and 255.  A set can contain at most 255 elements.  The
following are valid set declaration:


Type
    Junk  =  Set  of  Char;


    Days  =  (Mon,  Tue,  Wed,  Thu,  Fri,  Sat,  Sun);
    WorkDays  :  Set  of  days;



                                                                 48

_____________________________________________________________________________________________________CHAPTER_3.___TYPES____________*
 *___



Given these declarations, the following assignment is legal:


WorkDays  :=  [Mon,  Tue,  Wed,  Thu,  Fri];


The compiler stores small sets (less than 32 elements) in a Longint, if the type range allows
it.  This allows for faster processing and decreases program size.  Otherwise, sets are stored
in 32 bytes.

Several  operations  can  be  done  on  sets:  taking  unions  or  differences,  adding  or  removing
elements, comparisons.  These are documented in section 12.8.5  , page 161



3.3.4        File  types

File types are types that store a sequence of some base type, which can be any type except
another  file  type.  It  can  contain  (in  principle)  an  infinite  number  of  elements.  File  types
are used commonly to store data on disk.  However, nothing prevents the programmer, from
writing a file driver that stores its data for instance in memory.

Here is the type declaration for a file type:


        |______________________________________________________________________________________________________________|
        File types



      --  ___ file type __ file ____|__________________|__________________________________________________________________-oe
                                    |_ of  __ type ___|

        |______________________________________________________________________________________________________________|


If no type identifier is given, then the file is an untyped file; it can be considered as equivalent
to a file of bytes.  Untyped files require special commands to act on them (see Blockread,
Blockwrite).  The following declaration declares a file of records:


Type
    Point  =  Record
       X,Y,Z  :  real;
       end;
    PointFile  =  File  of  Point;


Internally, files are represented by the FileRec record, which is declared in the Dos or SysUtils
units.

A special file type is the Text file type, represented by the TextRec record.  A file of type
Text uses special input-output routines.  The default Input, Output and StdErr file types
are defined in the system unit:  they are all of type Text, and are opened by the system unit
initialization code.



3.4         Pointers


Free Pascal supports the use of pointers.  A variable of the pointer type contains an address
in memory, where the data of another variable may be stored.  A pointer type can be defined
as follows:


        |______________________________________________________________________________________________________________|
        Pointer types



                                                                 49

               _____________________________________________________________________________________________________CHAPTER_3.___TY*
 *PES_______________



                    - - ___ pointer type __   ^ __ type identifier __  ____________________________________________________________*
 *_-oe


                      |____________________________________________________________________________________________________________*
 *___|


               As  can  be  seen  from  this  diagram,  pointers  are  typed,  which  means  that  they  point  to  a
               particular kind of data.  The type of this data must be known at compile time.

               Dereferencing the pointer (denoted by adding ^ after the variable name) behaves then like
               a variable.  This variable has the type declared in the pointer declaration, and the variable
               is stored in the address that is pointed to by the pointer variable.  Consider the following
               example:


               Program  pointers;
               type
                  Buffer  =  String[255];
                  BufPtr  =  ^Buffer;
               Var  B    :  Buffer;
                     BP  :  BufPtr;
                     PP  :  Pointer;
               etc..


               In this example, BP is  a  pointer  to a Buffer type;  while B is a variable of type Buffer.  B
               takes 256 bytes memory,  and BP only takes 4 (or 8) bytes of memory:  enough memory to
               store an address.

               The expression


                BP^


               is known as the dereferencing of  BP. The result is of type Buffer, so


                BP^[23]


               Denotes the 23-rd character in the string pointed to by BP.

Remark:         Free Pascal treats pointers much the same way as C does.  This means that a pointer to
               some type can be treated as being an array of this type.

               From this point of view, the pointer then points to the zeroeth element of this array.  Thus
               the following pointer declaration


               Var  p  :  ^Longint;


               can be considered equivalent to the following array declaration:


               Var  p  :  array[0..Infinity]  of  Longint;


               The difference is that the former declaration allocates memory for the pointer only (not for
               the array), and the second declaration allocates memory for the entire array.  If the former
               is used, the memory must be allocated manually, using the Getmem function.  The reference
               P^ is then the same as p[0].  The following program illustrates this maybe more clear:


               program  PointerArray;
               var  i  :  Longint;
                     p  :  ^Longint;
                     pp  :  array[0..100]  of  Longint;
               begin



                                                                                50

_____________________________________________________________________________________________________CHAPTER_3.___TYPES____________*
 *___



   for  i  :=  0  to  100  do  pp[i]  :=  i;  {  Fill  array  }
   p  :=  @pp[0];                                   {  Let  p  point  to  pp  }
   for  i  :=  0  to  100  do
      if  p[i]<>pp[i]  then
         WriteLn  ('Ohoh,  problem  !')
end.


Free Pascal supports pointer arithmetic as C does.  This means that, if P is a typed pointer,
the instructions


Inc(P);
Dec(P);


Will  increase,  respectively  decrease  the  address  the  pointer  points  to  with  the  size  of  the
type P is a pointer to.  For example


Var  P  :  ^Longint;
...
 Inc  (p);


will increase P with 4, because 4 is the size of a longint.  If the pointer is untyped, a size of
1 byte is assumed (i.e.  as if the pointer were a pointer to a byte:  ^byte.)

Normal  arithmetic  operators  on  pointers  can  also  be  used,  that  is,  the  following  are  valid
pointer arithmetic operations:


var    p1,p2  :  ^Longint;
        L  :  Longint;
begin
   P1  :=  @P2;
   P2  :=  @L;
   L  :=  P1-P2;
   P1  :=  P1-4;
   P2  :=  P2+4;
end.


Here, the value that is added or subtracted is  multiplied by the size of the type the pointer
points  to.   In  the  previous  example  P1  will  be  decremented  by  16  bytes,  and  P2  will  be
incremented by 16.



3.5         Forward  type  declarations


Programs often need to maintain a linked list of records.  Each record then contains a pointer
to the next record (and possibly to the previous record as well).  For type safety, it is best to
define this pointer as a typed pointer, so the next record can be allocated on the heap using
the New call.  In order to do so, the record should be defined something like this:


Type
    TListItem  =  Record
       Data  :  Integer;
       Next  :  ^TListItem;
    end;



                                                                 51

_____________________________________________________________________________________________________CHAPTER_3.___TYPES____________*
 *___



When trying to compile this, the compiler will complain that the TListItem type is not yet
defined  when  it  encounters  the  Next  declaration:  This  is  correct,  as  the  definition  is  still
being parsed.

To be able to have the Next element as a typed pointer, a 'Forward type declaration' must
be introduced:


Type
   PListItem  =  ^TListItem;
   TListItem  =  Record
      Data  :  Integer;
      Next  :  PTListItem;
   end;


When the compiler encounters a typed pointer declaration where the referenced type is not
yet known, it postpones resolving the reference till later.  The pointer definition is a 'Forward
type declaration'.

The referenced type should be introduced later in the same Type block.  No other block may
come between the definition of the pointer type and the referenced type.  Indeed, even the
word Type itself may not re-appear:  in effect it would start a new type-block, causing the
compiler to resolve all pending declarations in the current block.

In most cases, the definition of the referenced type will follow immediately after the definition
of the pointer type, as shown in the above listing.  The forward defined type can be used in
any type definition following its declaration.

Note that a forward type declaration is only possible with pointer types and classes, not with
other types.



3.6         Procedural  types


Free  Pascal  has  support  for  procedural  types,  although  it  differs  a  little  from  the  Turbo
Pascal or Delphi implementation of them.  The type declaration remains the same,  as can
be seen in the following syntax diagram:


        |______________________________________________________________________________________________________________|
        Procedural types


      --  ___ procedural type __   __|__ function header __   __|____|____________________|____|__________________________|_____-oe
                                     |_ procedure header __    _|    |_ of  __ object __ _|    |_ ; __ call modifiers __ _|
                                                                     |__ is __ nested __  __|

      --  ___ function header __     function __   formal parameter list __    : __ result type __ __________________-oe

      --  ___ procedure header __     procedure __     formal parameter list __   ___________________________________-oe

      --  ___ call modifiers __  __|_ register __ __|___________________________________________________________________-oe

                                   |___ cdecl __ ___|
                                   |__ pascal __  __|
                                   |__ stdcall __ __|
                                   |__|safecall______|
                                                    inline __ ___|

        |______________________________________________________________________________________________________________|


For  a  description  of  formal  parameter  lists,  see  chapter  14 ,  page  187  .   The  two  following
examples are valid type declarations:



                                                                 52

               _____________________________________________________________________________________________________CHAPTER_3.___TY*
 *PES_______________



               Type  TOneArg  =  Procedure  (Var  X  :  integer);
                       TNoArg  =  Function  :  Real;
               var  proc  :  TOneArg;
                     func  :  TNoArg;


               One can assign the following values to a procedural type variable:


                   1.  Nil, for both normal procedure pointers and method pointers.

                   2.  A variable reference of a procedural type, i.e.  another variable of the same type.

                   3.  A global procedure or function address, with matching function or procedure header
                       and calling convention.

                   4.  A method address.


               Given these declarations, the following assignments are valid:


               Procedure  printit  (Var  X  :  Integer);
               begin
                   WriteLn  (x);
               end;
               ...
               Proc  :=  @printit;
               Func  :=  @Pi;


               From this example, the difference with Turbo Pascal is clear:  In Turbo Pascal it isn't nec-
               essary to use the address operator (@) when assigning a procedural type variable,  whereas
               in Free Pascal it is required.  In case the -MDelphi or -MTP switches are used, the address
               operator can be dropped.

Remark:         The modifiers concerning the calling conventions must be the same as the declaration; i.e.
               the following code would give an error:


               Type  TOneArgCcall  =  Procedure  (Var  X  :  integer);cdecl;
               var  proc  :  TOneArgCcall;
               Procedure  printit  (Var  X  :  Integer);
               begin
                   WriteLn  (x);
               end;
               begin
               Proc  :=  @printit;
               end.


               Because the TOneArgCcall type is a procedure that uses the cdecl calling convention.

               In  case  the  is  nested  modified  is  added,  then  the  procedural  variable  can  be  used  with
               nested procedures.  This requires that the sources be compiled in macpas or ISO mode, or
               that the nestedprocvars modeswitch be activated:


               {$modeswitch  nestedprocvars}
               program  tmaclocalprocparam3;


               type
                   tnestedprocvar  =  procedure  is  nested;



                                                                                53

_____________________________________________________________________________________________________CHAPTER_3.___TYPES____________*
 *___



var
   tempp:  tnestedprocvar;


procedure  p1(  pp:  tnestedprocvar);
begin
   tempp:=pp;
   tempp
end;


procedure  p2(  pp:  tnestedprocvar);
var
   localpp:  tnestedprocvar;
begin
   localpp:=pp;
   p1(  localpp)
end;


procedure  n;
begin
   writeln(  'calling  through  n')
end;


procedure  q;


var  qi:  longint;


   procedure  r;
   begin
      if  qi  =  1  then
         writeln(  'success  for  r')
      else
         begin
         writeln(  'fail');
         halt(  1)
      end
   end;


begin
   qi:=  1;
   p1(  @r);
   p2(  @r);
   p1(  @n);
   p2(  @n);
end;


begin
   q;
end.


In case one wishes to assign methods of a class to a variable of procedural type, the procedural
type must be declared with the of  object modifier.

The two following examples are valid type declarations for method procedural variables (also
known as event handlers because of their use in GUI design):



                                                                 54

_____________________________________________________________________________________________________CHAPTER_3.___TYPES____________*
 *___



Type  TOneArg  =  Procedure  (Var  X  :  integer)  of  object;
        TNoArg  =  Function  :  Real  of  object;
var
   oproc  :  TOneArg;
   ofunc  :  TNoArg;


A method of the correct signature can be assigned to these functions.  When called,  Self
will be pointing to the instance of the object that was used to assign the method procedure.

The following object methods can be assigned to oproc and ofunc:


Type
   TMyObject  =  Class(TObject)
      Procedure  DoX  (Var  X  :  integer);
      Function  DoY:  Real;
   end;


Var
   M  :  TMyObject;


begin
   oproc:=@M.DoX;
   ofunc:=@M.DOY;
end;


When calling oproc and ofunc, Self will equal M.

This mechanism is sometimes called Delegation.



3.7         Variant  types



3.7.1        Definition

As of version 1.1, FPC has support for variants.  For maximum variant support it is recom-
mended to add the variants unit to the uses clause of every unit that uses variants in some
way:  the variants unit contains support for examining and transforming variants other than
the default support offered by the System or ObjPas units.

The type of a value stored in a variant is only determined at runtime:  it depends what has
been assigned to the variant.  Almost any simple type can be assigned to variants:  ordinal
types, string types, int64 types.

Structured types such as sets, records, arrays, files, objects and classes are not assignment-
compatible with a variant, as well as pointers.  Interfaces and COM or CORBA objects can
be assigned to a variant (basically because they are simply a pointer).

This means that the following assignments are valid:


Type
    TMyEnum  =  (One,Two,Three);


Var
    V  :  Variant;
    I  :  Integer;
    B  :  Byte;
    W  :  Word;



                                                                 55

_____________________________________________________________________________________________________CHAPTER_3.___TYPES____________*
 *___



   Q  :  Int64;
   E  :  Extended;
   D  :  Double;
   En  :  TMyEnum;
   AS  :  AnsiString;
   WS  :  WideString;


begin
   V:=I;
   V:=B;
   V:=W;
   V:=Q;
   V:=E;
   V:=En;
   V:=D:
   V:=AS;
   V:=WS;
end;


And of course vice-versa as well.

A variant can hold an array of values:  All elements in the array have the same type (but
can be of type 'variant').  For a variant that contains an array, the variant can be indexed:


Program  testv;


uses  variants;


Var
   A  :  Variant;
   I  :  integer;


begin
   A:=VarArrayCreate([1,10],varInteger);
   For  I:=1  to  10  do
      A[I]:=I;
end.


For the explanation of  VarArrayCreate, see Unit Reference       .

Note that when the array contains a string, this is not considered an 'array of characters',
and  so  the  variant  cannot  be  indexed  to  retrieve  a  character  at  a  certain  position  in  the
string.



3.7.2        Variants  in  assignments  and  expressions

As can be seen from the definition above, most simple types can be assigned to a variant.
Likewise,  a  variant  can  be  assigned  to  a  simple  type:  If  possible,  the  value  of  the  variant
will be converted to the type that is being assigned to.  This may fail:  Assigning a variant
containing a string to an integer will fail unless the string represents a valid integer.  In the
following example, the first assignment will work, the second will fail:


program  testv3;


uses  Variants;



                                                                 56

               _____________________________________________________________________________________________________CHAPTER_3.___TY*
 *PES_______________



               Var
                  V  :  Variant;
                  I  :  Integer;


               begin
                  V:='100';
                  I:=V;
                  Writeln('I  :  ',I);
                  V:='Something  else';
                  I:=V;
                  Writeln('I  :  ',I);
               end.


               The  first  assignment  will  work,  but  the  second  will  not,  as  Something  else  cannot  be
               converted to a valid integer value.  An EConvertError exception will be the result.

               The result of an expression involving a variant will be of type variant again, but this can be
               assigned to a variable of a different type - if the result can be converted to a variable of this
               type.

               Note that expressions involving variants take more time to be evaluated, and should therefore
               be used with caution.  If a lot of calculations need to be made, it is best to avoid the use of
               variants.

               When considering implicit type conversions (e.g.  byte to integer, integer to double, char to
               string) the compiler will ignore variants unless a variant appears explicitly in the expression.



               3.7.3        Variants  and  interfaces

Remark:         Dispatch interface support for variants is currently broken in the compiler.

               Variants  can  contain  a  reference  to  an  interface  -  a  normal  interface  (descending  from
               IInterface)  or  a  dispatchinterface  (descending  from  IDispatch).   Variants  containing  a
               reference to a dispatch interface can be used to control the object behind it:  the compiler
               will use late binding to perform the call to the dispatch interface:  there will be no run-time
               checking  of  the  function  names  and  parameters  or  arguments  given  to  the  functions.  The
               result type is also not checked.  The compiler will simply insert code to make the dispatch
               call and retrieve the result.

               This means basically, that you can do the following on Windows:


               Var
                   W  :  Variant;
                   V  :  String;


               begin
                   W:=CreateOleObject('Word.Application');
                   V:=W.Application.Version;
                   Writeln('Installed  version  of  MS  Word  is  :  ',V);
               end;


               The line


                   V:=W.Application.Version;


               is  executed  by  inserting  the  necessary  code  to  query  the  dispatch  interface  stored  in  the
               variant W, and execute the call if the needed dispatch information is found.



                                                                                57

_____________________________________________________________________________________________________CHAPTER_3.___TYPES____________*
 *___



3.8         Type  aliases


Type aliases are a way to give another name to a type, but can also be used to create real
new types.  Which of the 2 depends on the way the type alias is defined:


        |______________________________________________________________________________________________________________|
        Type aliases


      --  ___ type alias __ __|____________|__ identifier ________________________________________________________________-oe
                              |_ type __ _|

        |______________________________________________________________________________________________________________|


The first case is just a means to give another name to a type:


Type
    MyInteger  =  Integer;


This creates a new name to refer to the Integer type,  but does not create an actual new
type.  That is, 2 variables:


Var
    A  :  MyInteger;
    B  :  Integer;


Will actually have the same type from the point of view of the compiler (namely:  Integer).

The  above  presents  a  way  to  make  types  platform  independent,  by  only  using  the  alias
types, and then defining these types for each platform individually.  Any programmer who
then  uses  these  custom  types  doesn't  have  to  worry  about  the  underlying  type  size:  it  is
opaque  to  him.   It  also  allows  to  use  shortcut  names  for  fully  qualified  type  names.   e.g.
define system.longint as Olongint and then redefine longint.

The alias is frequently seen to re-expose a type:


Unit  A;


Interface


Uses  B;


Type
    MyType  =  B.MyType;


This construction is often seen after some refactoring, when moving some declarations from
unit A to unit B, to preserve backwards compatibility of the interface of unit A.

The second case is slightly more subtle:


Type
    MyInteger  =  Type  Integer;


This not only creates a new name to refer to the Integer type, but actually creates a new
type.  That is, 2 variables:


Var
    A  :  MyInteger;
    B  :    Integer;



                                                                 58

_____________________________________________________________________________________________________CHAPTER_3.___TYPES____________*
 *___



Will not have the same type from the point of view of the compiler.  However, these 2 types
will be assignment compatible.  That means that an assignment


   A:=B;


will work.

The difference can be seen when examining type information:


If  TypeInfo(MyInteger)<>TypeInfo(Integer)  then
   Writeln('MyInteger  and  Integer  are  different  types');


The  compiler  function  TypeInfo  returns  a  pointer  to  the  type  information  in  the  binary.
Since  the  2  types  MyInteger  and  Integer  are  different,  they  will  generate  different  type
information blocks, and the pointers will differ.

There are 3 consequences of having different types:


    1.  That they have different typeinfo, hence different RTTI (Run-Time Type Information).

    2.  They can be used in function overloads, that is


        Procedure  MyProc(A  :  MyInteger);  overload;
        Procedure  MyProc(A  :  Integer);  overload;


        will work.  This will not work with a simple type alias.

    3.  They can be used in operator overloads, that is


        Operator  +(A,B  :  MyInteger)  :  MyInteger;


        will work too.



                                                                 59




Chapter   4



Variables



4.1         Definition


Variables are explicitly named memory locations with a certain type.  When assigning values
to  variables,  the  Free  Pascal  compiler  generates  machine  code  to  move  the  value  to  the
memory location reserved for this variable.  Where this variable is stored depends on where
it is declared:


     o  Global variables are variables declared in a unit or program, but not inside a procedure
        or function.  They are stored in fixed memory locations, and are available during the
        whole execution time of the program.

     o  Local variables are declared inside a procedure or function.  Their value is stored on
        the program stack, i.e.  not at fixed locations.


The  Free  Pascal  compiler  handles  the  allocation  of  these  memory  locations  transparently,
although this location can be influenced in the declaration.

The  Free  Pascal  compiler  also  handles  reading  values  from  or  writing  values  to  the  vari-
ables transparently.  But even this can be explicitly handled by the programmer when using
properties.

Variables must be explicitly declared when they are needed.  No memory is allocated unless
a variable is declared.  Using a variable identifier (for instance, a loop variable) which is not
declared first, is an error which will be reported by the compiler.



4.2         Declaration


The variables must be declared in a variable declaration block of a unit or a procedure or
function (section 16.5  , page 225  ).  It looks as follows:


        |______________________________________________________________________________________________________________|
        Variable declaration



      --  ___ variable declaration __    identifier __ : __type __ __|________________________|___-
                                                                     |_ = __  expression __  _|
      -  ______|____________________________|__ hint directive __   ; _____________________________________________________-oe
               |_ variable modifiers __   _|



                                                              60

______________________________________________________________________________________________CHAPTER_4.___VARIABLES_______________*
 *___



     - -  variable modifiers __   __|___ ___________________ absolute __   __ _ integer expression __   _______________________|_-
                                    6|  |                                    ||_______ identifier _______|_|                 | |
                                    |   |__________________________________          __  __________________________________  | |
                                    |   |                                   ; export                                       |   |
                                    |    ___________________________________ ; cvar __ ___________________________________     |
                                    |   |_            __  __ ___________________________ _________________________________|____|
                                    |   |  ; external       |                        |  |                                    | ||
                                    |   |                   |_ string constant __   _|  |_ name __    string constant __  _|   ||
                                    |   ||_______________________________                __ _______________________________|   ||
                                    ||____________________________________hint_directive_______________________________________|___*
 *__|
     -  ____________________________________________________________________________________________________________-oe


       |_______________________________________________________________________________________________________________|


This means that the following are valid variable declarations:


Var
   curterm1  :  integer;


   curterm2  :  integer;  cvar;
   curterm3  :  integer;  cvar;  external;


   curterm4  :  integer;  external  name  'curterm3';
   curterm5  :  integer;  external  'libc'  name  'curterm9';


   curterm6  :  integer  absolute  curterm1;


   curterm7  :  integer;  cvar;    export;
   curterm8  :  integer;  cvar;    public;
   curterm9  :  integer;  export  name  'me';
   curterm10  :  integer;  public  name  'ma';


   curterm11  :  integer  =  1  ;


The difference between these declarations is as follows:


    1.  The first form (curterm1) defines a regular variable.  The compiler manages everything
        by itself.

    2.  The  second  form  (curterm2)  declares  also  a  regular  variable,  but  specifies  that  the
        assembler  name  for  this  variable  equals  the  name  of  the  variable  as  written  in  the
        source.

    3.  The third form (curterm3) declares a variable which is located externally:  the compiler
        will assume memory is located elsewhere, and that the assembler name of this location
        is specified by the name of the variable, as written in the source.  The name may not
        be specified.

    4.  The fourth form is completely equivalent to the third, it declares a variable which is
        stored externally, and explicitly gives the assembler name of the location.  If  cvar is
        not used, the name must be specified.

    5.  The fifth form is a variant of the fourth form, only the name of the library in which
        the memory is reserved is specified as well.

    6.  The sixth form declares a variable (curterm6), and tells the compiler that it is stored
        in the same location as another variable (curterm1).



                                                                 61

______________________________________________________________________________________________CHAPTER_4.___VARIABLES_______________*
 *___



    7.  The seventh form declares a variable (curterm7), and tells the compiler that the as-
        sembler label of this variable should be the name of the variable (case sensitive) and
        must be made public.  i.e.  it can be referenced from other object files.

    8.  The eighth form (curterm8) is equivalent to the seventh: 'public' is an alias for 'export'.

    9.  The  ninth  and  tenth  form  are  equivalent:   they  specify  the  assembler  name  of  the
        variable.

   10.  the elevents form declares a variable (curterm11) and initializes it with a value (1 in
        the above case).


Note that assembler names must be unique.  It's not possible to declare or export 2 variables
with the same assembler name.



4.3         Scope


Variables,  just  as  any  identifier,  obey  the  general  rules  of  scope.   In  addition,  initialized
variables are initialized when they enter scope:


     o  Global initialized variables are initialized once, when the program starts.

     o  Local initialized variables are initialized each time the procedure is entered.


Note  that  the  behaviour  for  local  initialized  variables  is  different  from  the  one  of  a  local
typed constant.  A local typed constant behaves like a global initialized variable.



4.4         Initialized  variables


By  default,  variables  in  Pascal  are  not  initialized  after  their  declaration.  Any  assumption
that they contain 0 or any other default value is erroneous:  They can contain rubbish.  To
remedy this, the concept of initialized variables exists.  The difference with normal variables
is that their declaration includes an initial value, as can be seen in the diagram in the previous
section.

Given the declaration:


Var
    S  :  String  =  'This  is  an  initialized  string';


The value of the variable following will be initialized with the provided value.  The following
is an even better way of doing this:


Const
    SDefault  =  'This  is  an  initialized  string';


Var
    S  :  String  =  SDefault;


Initialization is often used to initialize arrays and records.  For arrays, the initialized elements
must be specified, surrounded by round brackets, and separated by commas.  The number of
initialized elements must be exactly the same as the number of elements in the declaration
of the type.  As an example:



                                                                 62

               ______________________________________________________________________________________________CHAPTER_4.___VARIABLES*
 *__________________



               Var
                  tt  :  array  [1..3]  of  string[20]  =  ('ikke',  'gij',  'hij');
                  ti  :  array  [1..3]  of  Longint  =  (1,2,3);


               For  constant  records,  each  element  of  the  record  should  be  specified,  in  the  form  Field:
               Value, separated by semicolons, and surrounded by round brackets.  As an example:


               Type
                  Point  =  record
                     X,Y  :  Real
                     end;
               Var
                  Origin  :  Point  =  (X:0.0;  Y:0.0);


               The order of the fields in a constant record needs to be the same as in the type declaration,
               otherwise a compile-time error will occur.

Remark:         It should be stressed that initialized variables are initialized when they come into scope, in
               difference with typed constants, which are initialized at program start.  This is also true for
               local initialized variables.  Local initialized variables are initialized whenever the routine is
               called.  Any changes that occurred in the previous invocation of the routine will be undone,
               because they are again initialized.



               4.5         Initializing  variables  using  default


               Some variables must be initialized because they contain managed types.  For variables that
               are declared in the var section of a function or in the main program, this happens automat-
               ically.  For variables that are allocated on the heap, this is not necessarily the case.

               For this, the compiler contains the Default intrinsic.  This function accepts a type identifier
               as the argument, and will return a correctly initialized variable of that type.  in essence, it
               will zero out the whole variable.

               The following gives an example of its use:


               type
                   TRecord  =  record
                      i:  LongInt;
                      s:  AnsiString;
                   end;


               var
                   i:  LongInt;
                   o:  TObject;
                   r:  TRecord;
               begin
                   i  :=  Default(LongInt);  //  0
                   o  :=  Default(TObject);  //  Nil
                   r  :=  Default(TRecord);  //  (  i:  0;  s:  '')
               end.


               The case where a variable is allocated on the heap, is more interesting:


               type
                   TRecord  =  record



                                                                                63

               ______________________________________________________________________________________________CHAPTER_4.___VARIABLES*
 *__________________



                     i:  LongInt;
                     s:  AnsiString;
                  end;


               var
                  i:  ^LongInt;
                  o:  ^TObject;
                  r:  ^TRecord;
               begin
                  i:=GetMem(SizeOf(Longint));
                  i^  :=  Default(LongInt);  //  0
                  o:=GetMem(SizeOf(TObject));
                  o^  :=  Default(TObject);  //  Nil
                  r:=GetMem(SizeOf(TRecord));
                  r^  :=  Default(TRecord);  //  (  i:  0;  s:  '')
               end.


               It works for all types, except the various file types (or complex types containing a file type).

Remark:         For generics, the use of  Default is especially useful, since the type of a variable may not
               be known during the declaration of a generic.  For more information section 8.7 , page 121  .



               4.6         Thread  Variables


               For a program which uses threads,  the variables can be really global,  i.e.  the same for all
               threads,  or  thread-local:  this  means  that  each  thread  gets  a  copy  of  the  variable.   Local
               variables (defined inside a procedure) are always thread-local.  Global variables are normally
               the same for all threads.  A global variable can be declared thread-local by replacing the var
               keyword at the start of the variable declaration block with Threadvar:


               Threadvar
                   IOResult  :  Integer;


               If no threads are used, the variable behaves as an ordinary variable.  If threads are used then
               a copy is made for each thread (including the main thread).  Note that the copy is made with
               the original value of the variable, not with the value of the variable at the time the thread
               is started.

               Threadvars  should  be  used  sparingly:  There  is  an  overhead  for  retrieving  or  setting  the
               variable's value.  If possible at all, consider using local variables; they are always faster than
               thread variables.

               Threads are not enabled by default.  For more information about programming threads, see
               the chapter on threads in the Programmer's Guide           .



               4.7         Properties


               A global block can declare properties, just as they could be defined in a class.  The difference
               is that the global property does not need a class instance:  there is only 1 instance of this
               property.  Other than that, a global property behaves like a class property.  The read/write
               specifiers for the global property must also be regular procedures, not methods.

               The  concept  of  a  global  property  is  specific  to  Free  Pascal,  and  does  not  exist  in  Delphi.
               ObjFPC mode is required to work with properties.



                                                                                64

______________________________________________________________________________________________CHAPTER_4.___VARIABLES_______________*
 *___



The concept of a global property can be used to 'hide' the location of the value, or to calculate
the value on the fly, or to check the values which are written to the property.

The declaration is as follows:


       |_______________________________________________________________________________________________________________|
       Properties



     - - ___ property definition __    identifier ____|____________________________|__ property specifiers __   __________-oe
                                                      |_ property interface __   _|


     - - ___ property interface __   __|__________________________________|__ : __ type identifier __-_
                                       |_ property parameter list __    _|
     -  ______|____________________________________|______________________________________________________________________-oe
              |_ index __   integerconstant __   _|


     - - ___ property parameter list __     [ ____  _ parameter declaration __   ___] ________________________________-oe
                                                  6||______________ ; ______________|_|


     - - ___ property specifiers __   __|______________________|____|________________________|____|__________________________|_____*
 *-oe
                                        |_ read specifier __  _|    |_ write specifier __ _|      |_ default specifier __  _|

     - - ___ read specifier __   read __  field or function __ _____________________________________________________-oe


     - - ___ write specifier __   write __  field or procedure __  _________________________________________________-oe


     - - ___ default specifier __  __ _ default __  __ __________________________________________________________________-oe
                                     |                ||_          __  |_||
                                     ||_________          constant       |
                                                 nodefault __   _________|

     - - ___ field or procedure __   __|_____ field identifier _______|__________________________________________________-oe
                                       |_ procedure identifier __   _|


     - - ___ field or function __  __|____ field identifier ______|______________________________________________________-oe
                                     |_ function identifier __  _|


       |_______________________________________________________________________________________________________________|


The following is an example:


{$mode  objfpc}
unit  testprop;


Interface


Function  GetMyInt  :  Integer;
Procedure  SetMyInt(Value  :  Integer);


Property
   MyProp  :  Integer  Read  GetMyInt  Write  SetMyInt;


Implementation


Uses  sysutils;


Var
   FMyInt  :  Integer;



                                                                 65

______________________________________________________________________________________________CHAPTER_4.___VARIABLES_______________*
 *___



Function  GetMyInt  :  Integer;


begin
   Result:=FMyInt;
end;


Procedure  SetMyInt(Value  :  Integer);


begin
   If  ((Value  mod  2)=1)  then
      Raise  Exception.Create('MyProp  can  only  contain  even  value');
   FMyInt:=Value;
end;


end.


The read/write specifiers can be hidden by declaring them in another unit which must be
in the uses clause of the unit.  This can be used to hide the read/write access specifiers for
programmers, just as if they were in a private section of a class (discussed below).  For the
previous example, this could look as follows:


{$mode  objfpc}
unit  testrw;


Interface


Function  GetMyInt  :  Integer;
Procedure  SetMyInt(Value  :  Integer);


Implementation


Uses  sysutils;


Var
   FMyInt  :  Integer;


Function  GetMyInt  :  Integer;


begin
   Result:=FMyInt;
end;


Procedure  SetMyInt(Value  :  Integer);


begin
   If  ((Value  mod  2)=1)  then
      Raise  Exception.Create('Only  even  values  are  allowed');
   FMyInt:=Value;
end;


end.


The unit testprop would then look like:



                                                                 66

______________________________________________________________________________________________CHAPTER_4.___VARIABLES_______________*
 *___



{$mode  objfpc}
unit  testprop;


Interface


uses  testrw;


Property
   MyProp  :  Integer  Read  GetMyInt  Write  SetMyInt;


Implementation


end.


More information about properties can be found in chapter 6 , page 79 .



                                                                 67




Chapter   5



Ob jects



5.1         Declaration


Free Pascal supports object oriented programming.  In fact, most of the compiler is written
using objects.  Here we present some technical questions regarding object oriented program-
ming in Free Pascal.

Objects should be treated as a special kind of record.  The record contains all the fields that
are declared in the objects definition, and pointers to the methods that are associated to the
objects' type.

An object is declared just as a record would be declared; except that now, procedures and
functions can be declared as if they were part of the record.  Objects can "inherit" fields and
methods from "parent" objects.  This means that these fields and methods can be used as if
they were included in the objects declared as a "child" object.

Furthermore,  a concept of visibility is introduced:  fields,  procedures and functions can be
declared as public, protected or private.  By default, fields and methods are public, and
are exported outside the current unit.

Fields or methods that are declared private are only accessible in the current unit:  their
scope is limited to the implementation of the current unit.

The prototype declaration of an object is as follows:


        |______________________________________________________________________________________________________________|
        object types



      --  _____|________________|__ object __  __|____________|____|_ component list __   __|_ end __ ____________________________-*
 *oe
               |_ packed __   _|                 |_heritage_|      6|________________________|


      --  ___ heritage __  ( __ object type identifier __    ) ______________________________________________________-oe


      --  ___ component list __    __|____________________________________|____|____________________________|___-
                                     |_ object visibility specifier __  _|     |___  _ field definition _____|
                                                                                   6||______________________|_|
      -  ______|________________________________|_________________________________________________________________________-oe
               |___  _ method definition __  ____|
                   6||__________________________|_|


      --  ___ field definition __  identifier list __: __ type __ ; ____|______________|_________________________________-oe
                                                                        |_ static; __ _|



                                                              68

               _________________________________________________________________________________________________CHAPTER_5.___OBJECT*
 *S_________________



                    - - ___ object visibility specifier __  __|___ private __  ___|________________________________________________*
 *_____-oe

                                                              |_|protected____  _|
                                                                               public __ ____|

                      |____________________________________________________________________________________________________________*
 *___|


               As can be seen, as many private and public blocks as needed can be declared.

               The following is a valid definition of an object:


               Type
                  TObj  =  object
                  Private
                     Caption  :  ShortString;
                  Public
                     Constructor  init;
                     Destructor  done;
                     Procedure  SetCaption  (AValue  :  String);
                     Function  GetCaption  :  String;
                  end;


               It  contains  a  constructor/destructor  pair,  and  a  method  to  get  and  set  a  caption.   The
               Caption field is private to the object:  it cannot be accessed outside the unit in which TObj
               is declared.

Remark:         In MacPas mode, the Object keyword is replaced by the class keyword for compatibility
               with other pascal compilers available on the Mac.  That means that objects cannot be used
               in MacPas mode.

Remark:         Free  Pascal  also  supports  the  packed  object.   This  is  the  same  as  an  object,  only  the
               elements (fields) of the object are byte-aligned, just as in the packed record.  The declaration
               of a packed object is similar to the declaration of a packed record :


               Type
                  TObj  =  packed  object
                   Constructor  init;
                   ...
                   end;
                  Pobj  =  ^TObj;
               Var  PP  :  Pobj;


               Similarly, the {$PackRecords  } directive acts on objects as well.



               5.2         Fields


               Object  Fields  are  like  record  fields.   They  are  accessed  in  the  same  way  as  a  record  field
               would be accessed :  by using a qualified identifier.  Given the following declaration:


               Type  TAnObject  =  Object
                           AField  :  Longint;
                           Procedure  AMethod;
                           end;
               Var  AnObject  :  TAnObject;


               then the following would be a valid assignment:



                                                                                69

_________________________________________________________________________________________________CHAPTER_5.___OBJECTS______________*
 *___



   AnObject.AField  :=  0;


Inside methods, fields can be accessed using the short identifier:


Procedure  TAnObject.AMethod;
begin
   ...
   AField  :=  0;
   ...
end;


Or, one can use the self identifier.  The self identifier refers to the current instance of the
object:


Procedure  TAnObject.AMethod;
begin
   ...
   Self.AField  :=  0;
   ...
end;


One cannot access fields that are in a private or protected sections of an object from outside
the  objects'  methods.   If  this  is  attempted  anyway,  the  compiler  will  complain  about  an
unknown identifier.

It is also possible to use the with statement with an object instance, just as with a record:


With  AnObject  do
   begin
   Afield  :=  12;
   AMethod;
   end;


In  this  example,  between  the  begin  and  end,  it  is  as  if  AnObject  was  prepended  to  the
Afield and Amethod identifiers.  More about this in section 13.2.8  , page 183  .



5.3         Class  or  Static  fields


An object can contain class or static fields:  these fields are global to the object type, and act
like global variables, but are known only in the scope of the object.  The difference between
static and class variables is purely the mode in which they work:  The static keyword will
always work, the class keyword will need {$MODE  DELPHI}.

They can be referenced from within the objects methods,  but can also be referenced from
outside the object by providing the fully qualified name.

For instance, the output of the following program:


{$mode  delphi}
{$static  on}
type
    cl=object
       l  :  longint;  static;
       class  var  v  :  integer;
    end;



                                                                 70

_________________________________________________________________________________________________CHAPTER_5.___OBJECTS______________*
 *___



var
   cl1,cl2  :  cl;


begin
   Writeln('Static');
   cl1.l:=2;
   writeln(cl2.l);
   cl2.l:=3;
   writeln(cl1.l);
   Writeln(cl.l);
   Writeln('Class');
   cl1.v:=4;
   writeln(cl2.v);
   cl2.v:=5;
   writeln(cl1.v);
   Writeln(cl.v);
end.


will be the following


Static
2
3
3
Class
4
5
5


Note that the last line of code references the object type itself (cl), and not an instance of
the object (cl1 or cl2).



5.4         Constructors  and  destructors


As can be seen in the syntax diagram for an object declaration, Free Pascal supports con-
structors and destructors.  The programmer is responsible for calling the constructor and the
destructor explicitly when using objects.

The declaration of a constructor or destructor is as follows:


        |______________________________________________________________________________________________________________|
        Constructors and destructors


      --  ___ constructor declaration __     constructor header __     ; __ subroutine block __   ___________________-oe

      --  ___ destructor declaration __     destructor header __    ; __ subroutine block __   ______________________-oe

      --  ___ constructor header __     constructor __   __|____________ identifier ______________|_-
                                                           |_ qualified method identifier __    _|
      -  ______ formal parameter list __    ___________________________________________________________________________-oe

      --  ___ destructor header __     destructor __  __|____________ identifier ______________|_-
                                                        |_ qualified method identifier __    _|
      -  ______ formal parameter list __    ___________________________________________________________________________-oe



                                                                 71

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 *___



       |_______________________________________________________________________________________________________________|


A constructor/destructor pair is required if the object uses virtual methods.  The reason is
that  for  an  object  with  virtual  methods,  some  internal  housekeeping  must  be  done:  this
housekeeping is done by the constructor1 .

In the declaration of the object type, a simple identifier should be used for the name of the
constructor or destructor.  When the constructor or destructor is implemented, a qualified
method identifier should be used, i.e.  an identifier of the form objectidentifier.methodidentifier.

Free  Pascal  supports  also  the  extended  syntax  of  the  New  and  Dispose  procedures.   In
case a dynamic variable of an object type must be allocated the constructor's name can be
specified in the call to New.  The New is implemented as a function which returns a pointer
to the instantiated object.  Consider the following declarations:


Type
   TObj  =  object;
    Constructor  init;
    ...
    end;
   Pobj  =  ^TObj;
Var  PP  :  Pobj;


Then the following 3 calls are equivalent:


 pp  :=  new  (Pobj,Init);


and


   new(pp,init);


and also


   new  (pp);
   pp^.init;


In  the  last  case,  the  compiler  will  issue  a  warning  that  the  extended  syntax  of  new  and
dispose must be used to generate instances of an object.  It is possible to ignore this warning,
but it's better programming practice to use the extended syntax to create instances of an
object.  Similarly, the Dispose procedure accepts the name of a destructor.  The destructor
will then be called, before removing the object from the heap.

In view of the compiler warning remark, the following chapter presents the Delphi approach to
object-oriented programming, and may be considered a more natural way of object-oriented
programming.



5.5         Methods


Object methods are just like ordinary procedures or functions,  only they have an implicit
extra parameter :  self.  Self points to the object with which the method was invoked.  When
implementing  methods,  the  fully  qualified  identifier  must  be  given  in  the  function  header.
When declaring methods, a normal identifier must be given.
___________________________________________________1
     A pointer to the VMT must be set up.



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 *___



5.5.1        Declaration

The declaration of a method is much like a normal function or procedure declaration, with
some additional specifiers, as can be seen from the following diagram, which is part of the
object declaration:


        |______________________________________________________________________________________________________________|
        methods



      --  ___ method definition __    __|___ function header __   ___|__ ; __ method directives __    ____________________-oe

                                        |__ procedure header __    __|
                                        |_|constructor_header___    _|
                                                               destructor header __    __|

      --  ___ method directives __    __|__________________________________________|____|__________________________|_________-oe
                                        |_ virtual __  ; ___|______________________|__| |_ call modifiers __   ; ___|
                                                            |_ abstract __   ; ___|


        |______________________________________________________________________________________________________________|


from the point of view of declarations, Method  definitions are normal function or proce-
dure declarations.  Contrary to TP and Delphi, fields can be declared after methods in the
same block, i.e.  the following will generate an error when compiling with Delphi or Turbo
Pascal, but not with FPC:


Type
    MyObj  =  Object
       Field  :  Longint;
       Procedure  Doit;
    end;



5.5.2        Method  invocation

Methods are called just as normal procedures are called, only they have an object instance
identifier prepended to them (see also chapter 13 , page 167  ).  To determine which method is
called, it is necessary to know the type of the method.  We treat the different types in what
follows.



Normal static methods


Normal  (static)  methods  are  methods  that  have  been  declared  without  a  abstract  or
virtual keyword.  When calling a static method, the declared (i.e.  compile time) method
of the object is used.  For example, consider the following declarations:


Type
    TParent  =  Object
       ...
       procedure  Doit;
       ...
       end;
    PParent  =  ^TParent;
    TChild  =  Object(TParent)
       ...
       procedure  Doit;



                                                                 73

_________________________________________________________________________________________________CHAPTER_5.___OBJECTS______________*
 *___



      ...
      end;
   PChild  =  ^TChild;


As it is visible, both the parent and child objects have a method called Doit.  Consider now
the following declarations and calls:


Var
   ParentA,ParentB  :  PParent;
   Child                  :  PChild;


begin
    ParentA  :=  New(PParent,Init);
    ParentB  :=  New(PChild,Init);
    Child  :=  New(PChild,Init);
    ParentA^.Doit;
    ParentB^.Doit;
    Child^.Doit;


Of  the  three  invocations  of  Doit,  only  the  last  one  will  call  TChild.Doit,  the  other  two
calls  will  call  TParent.Doit.  This  is  because  for  static  methods,  the  compiler  determines
at  compile  time  which  method  should  be  called.   Since  ParentB  is  of  type  TParent,  the
compiler decides that it must be called with TParent.Doit, even though it will be created
as a TChild.  There may be times when the method that is actually called should depend on
the actual type of the object at run-time.  If so, the method cannot be a static method, but
must be a virtual method.



Virtual methods


To remedy the situation in the previous section, virtual methods are created.  This is simply
done  by  appending  the  method  declaration  with  the  virtual  modifier.   The  descendent
object can then override the method with a new implementation by re-declaring the method
(with the same parameter list) using the virtual keyword.

Going back to the previous example, consider the following alternative declaration:


Type
   TParent  =  Object
      ...
      procedure  Doit;virtual;
      ...
      end;
   PParent  =  ^TParent;
   TChild  =  Object(TParent)
      ...
      procedure  Doit;virtual;
      ...
      end;
   PChild  =  ^TChild;


As it is visible, both the parent and child objects have a method called Doit.  Consider now
the following declarations and calls :


Var



                                                                 74

_________________________________________________________________________________________________CHAPTER_5.___OBJECTS______________*
 *___



   ParentA,ParentB  :  PParent;
   Child                  :  PChild;


begin
    ParentA  :=  New(PParent,Init);
    ParentB  :=  New(PChild,Init);
    Child  :=  New(PChild,Init);
    ParentA^.Doit;
    ParentB^.Doit;
    Child^.Doit;


Now, different methods will be called, depending on the actual run-time type of the object.
For  ParentA,  nothing  changes,  since  it  is  created  as  a  TParent  instance.   For  Child,  the
situation also doesn't change:  it is again created as an instance of  TChild.

For ParentB however, the situation does change:  Even though it was declared as a TParent,
it is created as an instance of  TChild.  Now,  when the program runs,  before calling  Doit,
the program checks what the actual type of ParentB is, and only then decides which method
must  be  called.  Seeing  that  ParentB  is  of  type  TChild,  TChild.Doit  will  be  called.  The
code for this run-time checking of the actual type of an object is inserted by the compiler at
compile time.

The  TChild.Doit  is  said  to  override  the  TParent.Doit.         It  is  possible  to  acces  the
TParent.Doit from within the varTChild.Doit, with the inherited keyword:


Procedure  TChild.Doit;
begin
   inherited  Doit;
   ...
end;


In the above example, when TChild.Doit is called, the first thing it does is call TParent.Doit.
The inherited keyword cannot be used in static methods, only on virtual methods.

To be able to do this, the compiler keeps - per object type - a table with virtual methods:
the  VMT  (Virtual  Method  Table).   This  is  simply  a  table  with  pointers  to  each  of  the
virtual  methods:  each  virtual  method  has  its  fixed  location  in  this  table  (an  index).  The
compiler uses this table to look up the actual method that must be used.  When a descendent
object overrides a method, the entry of the parent method is overwritten in the VMT. More
information about the VMT can be found in Programmer's Guide           .

As remarked earlier, objects that have a VMT must be initialized with a constructor:  the
object variable must be initialized with a pointer to the VMT of the actual type that it was
created with.



Abstract methods


An abstract method is a special kind of virtual method.  A method that is declared abstract
does not have an implementation for this method.  It is up to inherited objects to override
and implement this method.

From this it follows that a method can not be abstract if it is not virtual (this can be seen
from the syntax diagram).  A second consequence is that an instance of an object that has
an abstract method cannot be created directly.

The reason is obvious:  there is no method where the compiler could jump to !  A method
that  is  declared  abstract  does  not  have  an  implementation  for  this  method.   It  is  up  to



                                                                 75

               _________________________________________________________________________________________________CHAPTER_5.___OBJECT*
 *S_________________



               inherited objects to override and implement this method.  Continuing our example, take a
               look at this:


               Type
                  TParent  =  Object
                     ...
                     procedure  Doit;virtual;abstract;
                     ...
                     end;
                  PParent=^TParent;
                  TChild  =  Object(TParent)
                     ...
                     procedure  Doit;virtual;
                     ...
                     end;
                  PChild  =  ^TChild;


               As it is visible, both the parent and child objects have a method called Doit.  Consider now
               the following declarations and calls :


               Var
                  ParentA,ParentB  :  PParent;
                  Child                  :  PChild;


               begin
                   ParentA  :=  New(PParent,Init);
                   ParentB  :=  New(PChild,Init);
                   Child  :=  New(PChild,Init);
                   ParentA^.Doit;
                   ParentB^.Doit;
                   Child^.Doit;


               First of all, Line 3 will generate a compiler error, stating that one cannot generate instances
               of  objects  with  abstract  methods:  The  compiler  has  detected  that  PParent  points  to  an
               object which has an abstract method.  Commenting line 3 would allow compilation of the
               program.

Remark:         If an abstract method is overridden, the parent method cannot be called with inherited,
               since there is no parent method; The compiler will detect this, and complain about it, like
               this:


               testo.pp(32,3)  Error:  Abstract  methods  can't  be  called  directly


               If, through some mechanism, an abstract method is called at run-time, then a run-time error
               will occur.  (run-time error 211, to be precise)



               Class or static methods


               Class methods or methods declared with the static directive are methods that are global
               to the object type.  When called, the implicit 'self' pointer is not available.  This means that
               normal methods cannot be called, and none of the fields of an object can be accessed.  Class
               variables can be used, however.

               Class or static methods are regular methods, they can be assigned to a procedural variable.

               The following program demonstrates all this.  The commented-out statements will not com-
               pile.



                                                                                76

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 *___



{$APPTYPE  CONSOLE}
{$IFDEF  FPC}{$MODE  DELPHI}{$H+}{$ENDIF}
type
   TTest  =  object
      const  Epsylon  =  100;
      var  f  :  integer;
      class  var  cv1,cv2:integer;
      procedure  myproc;
      class  procedure  testproc;
      class  procedure  testproc2;static;
      procedure  testproc3;  static;
   end;


   procedure  TTest.myproc;
   begin
      cv1:=0;
      f:=1;
   end;


   class  procedure  TTest.Testproc;
   begin
      cv1:=1;
      //  f:=1;
   end;


   class  procedure  TTest.Testproc2;
   begin
      cv1:=2;
      //  f:=1;
   end;


   procedure  TTest.Testproc3;
   begin
      cv1:=3;
      //  f:=1;
   end;



Var
   P  :  Procedure;


begin
   P:=@TTest.Myproc;
   P:=@TTest.Testproc;
   P:=@TTest.Testproc2;
   P:=@TTest.Testproc3;
end.


Uncommenting one of the commented statements and trying to compile the resulting code
will result in a compiler error:


ocv.pp(32,6)  Error:  Only  class  methods,  class  properties  and
   class  variables  can  be  accessed  in  class  methods



                                                                 77

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 *___



5.6         Visibility


For  objects,  3  visibility  specifiers  exist  :  private,  protected  and  public.   If  a  visibility
specifier is not specified, public is assumed.  Both methods and fields can be hidden from a
programmer by putting them in a private section.  The exact visibility rule is as follows:


Private        All fields and methods that are in a private block, can only be accessed in the
        module (i.e.  unit or program) that contains the object definition.  They can be accessed
        from inside the object's methods or from outside them e.g.  from other objects' methods,
        or global functions.

Protected          Is the same as Private, except that the members of a Protected section are
        also accessible to descendent types, even if they are implemented in other modules.

Public       fields and methods are always accessible, from everywhere.  Fields and methods in
        a public section behave as though they were part of an ordinary record type.



                                                                 78




               Chapter   6



               Classes



               In  the  Delphi  approach  to  Object  Oriented  Programming,  everything  revolves  around  the
               concept of 'Classes'.  A class can be seen as a pointer to an object, or a pointer to a record,
               with methods associated with it.

               The difference between objects and classes is mainly that an object is allocated on the stack,
               as an ordinary record would be, and that classes are always allocated on the heap.  In the
               following example:


               Var
                   A  :  TSomeObject;  //  an  Object
                   B  :  TSomeClass;    //  a  Class


               The main difference is that the variable A will take up as much space on the stack as the size
               of the object (TSomeObject).  The variable B, on the other hand, will always take just the
               size of a pointer on the stack.  The actual class data is on the heap.

               From  this,  a  second  difference  follows:  a  class  must  always  be  initialized  through  its  con-
               structor, whereas for an object, this is not necessary.  Calling the constructor allocates the
               necessary memory on the heap for the class instance data.

Remark:         In earlier versions of Free Pascal it was necessary, in order to use classes, to put the objpas
               unit in the uses clause of a unit or program.  This is no longer needed as of version 0.99.12.
               As  of  this  version,  the  unit  will  be  loaded  automatically  when  the  -MObjfpc  or  -MDelphi
               options are specified, or their corresponding directives are used:


               {$mode  objfpc}
               {$mode  delphi}


               In fact, the compiler will give a warning if it encounters the objpas unit in a uses clause.



               6.1         Class  definitions


               The prototype declaration of a class is as follows:


                       |___________________________________________________________________________________________________________*
 *___|
                       Class types


                     --  ___ class type __ __|_ class forward definition __   _|___________________________________________________*
 *____-oe
                                             |______ class definition __ ______|



                                                                             79

               __________________________________________________________________________________________________CHAPTER_6.___CLASS*
 *ES________________



                    - - ___ class forward definition __    class __ _______________________________________________________________*
 *-oe


                    - - ___ class definition __ __|________________|__ class __ __|____________|____|______________________________*
 *______|______-oe
                                                  |_ packed __   _|               |_heritage_|      |___  _ component list __  ___e*
 *nd __ _|
                                                                                                        6||_______________________|*
 *_|


                    - - ___ heritage __  ( __ class type identifier __  __|__________________________________|__ ) ________________*
 *_____-oe
                                                                          |_ implemented interfaces __      _|


                    - - ___ implemented interfaces __      __  _ , __ interface identifier ________________________________________*
 *__-oe
                                                             6||_______________________________|_|


                    - - ___ component list __    __|__________________________|____|____________________________|___-
                                                   |_ visibility specifier ___|    |___  _ field definition _____|
                                                                                       6||______________________|_|
                    -  ______|__________________________________________________|__________________________________________________*
 *____-oe
                             |___ ___ ______ const declaration part __    __________|
                                 6|| |_______ type declaration part __    ______|_||
                                 |   |_____ variable declaration part __    _____ ||
                                 |   |_                                 __    _  ||
                                 |   |  class variable declaration part        |  |
                                 |    _________ method definition __    _________ |
                                 |   ||________                     __   ________|||
                                 ||_____________property_definition_______________|________|


                    - - ___ class variable declaration part __     class __  variable declaration part __    ______________________*
 *_-oe


                    - - ___ field definition __  identifier list __: __ type __ ; ____|______________|_____________________________*
 *____-oe
                                                                                      |_ static; __ _|


                    - - ___ method definition __    __|____________|____|___ function header __   ___|__ ; ___-
                                                      |_ class __ _|    |__ procedure header __    __|

                                                                        |_|constructor_header___    _|
                                                                                               desctuctor header __    __|
                    -  ______|______________________________________________________|____|__________________________|______________*
 *_________-oe
                             |___ |___ ___ virtual __ ______ ________________________|_ ;|___|_ call modifiers __   ; ___|
                                 |    ||_         __   _||  ||_   __          __  _|| |
                                 |        dynamic               ;    abstract         |
                                 |_______________|override___  _______________|
                                                            message __    __|_ integer constant __   _|__|
                                                                            |__ string constant __   __|


                    - - ___ class visibility specifier __ __ ___ _________________ ___ private ____________________________________*
 *_______-oe
                                                            |   ||_        __ _|| ||_           __   _|||
                                                            |       strict            protected         |
                                                            |_____________|public_________________|_
                                                                                                     published __   ___________|

                      |____________________________________________________________________________________________________________*
 *___|


Remark:         In MacPas mode, the Object keyword is replaced by the class keyword for compatibility
               with other pascal compilers available on the Mac.  That means that in MacPas mode,  the
               reserved word 'class' in the above diagram may be replaced by the reserved word 'object'.

               In a class declaration, as many private, protected, published and public blocks as needed
               can be used:  the various blocks can be repeated, and there is no special order in which they
               must appear.

               Methods are normal function or procedure declarations.  As can be seen, the declaration of a
               class is almost identical to the declaration of an object.  The real difference between objects
               and classes is in the way they are created (see further in this chapter).

               The visibility of the different sections is as follows:



                                                                                80

               __________________________________________________________________________________________________CHAPTER_6.___CLASS*
 *ES________________



               Private       All fields and methods that are in a private block, can only be accessed in the
                      module (i.e.  unit) that contains the class definition.  They can be accessed from inside
                      the classes' methods or from outside them (e.g.  from other classes' methods)

               Strict Private          All fields and methods that are in a strict  private block, can only be
                      accessed from methods of the class itself.  Other classes or descendent classes (even in
                      the same unit) cannot access strict private members.

               Protected         Is the same as Private, except that the members of a Protected section are
                      also accessible to descendent types, even if they are implemented in other modules.

               Strict Protected            Is  the  same  as  Protected,  except  that  the  members  of  a  Protected
                      section  are  also  accessible  to  other  classes  implemented  in  the  same  unit.   Strict
                      protected members are only visible to descendent classes, not to other classes in the
                      same unit.

               Public      sections are always accessible.

               Published         From  a  language  perspective,  this  is  the  same  as  a  Public  section,  but  the
                      compiler  generates  also  type  information  that  is  needed  for  automatic  streaming  of
                      these  classes  if  the  compiler  is  in  the  {$M+}  state.   Fields  defined  in  a  published
                      section must be of class type.  Array properties cannot be in a published section.


               In  the  syntax  diagram,  it  can  be  seen  that  a  class  can  list  implemented  interfaces.   This
               feature will be discussed in the next chapter.

               Classes can contain Class methods:  these are functions that do not require an instance.  The
               Self identifier is valid in such methods, but refers to the class pointer (the VMT).

Remark:         Like with functions and pointer types, sometimes a forward definition of a class is needed.
               A class forward definition is simply the name of the class, with the keyword Class, as in the
               following example:


               Type
                  TClassB  =  Class;
                  TClassA  =  Class
                     B  :  TClassB;
                  end;


                  TClassB  =  Class
                   A  :  TClassA;
                  end;


               When  using  a  class  forward  definition,  the  class  must  be  defined  in  the  same  unit,  in  the
               same section (interface/implementation).  It must not necessarily be defined in the same type
               section.

               It is also possible to define class reference types:


                      |____________________________________________________________________________________________________________*
 *___|
                      Class reference type



                    - - ___ class of  __  classtype __ ____________________________________________________________________________*
 *-oe


                      |____________________________________________________________________________________________________________*
 *___|


               Class reference types are used to create instances of a certain class, which is not yet known
               at compile time, but which is specified at run time.  Essentially, a variable of a class reference



                                                                                81

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 *___



type contains a pointer to the definition of the speficied class.  This can be used to construct
an instance of the class corresponding to the definition, or to check inheritance.  The following
example shows how it works:


Type
   TComponentClass  =  Class  of  TComponent;


Function  CreateComponent(AClass:  TComponentClass;
                                         AOwner:  TComponent):  TComponent;


begin
   //  ...
   Result:=AClass.Create(AOwner);
   //  ...
end;


This function can be passed a class reference of any class that descends from TComponent.
The following is a valid call:


Var
   C  :  TComponent;


begin
   C:=CreateComponent(TEdit,Form1);
end;


On return of the CreateComponent function, C will contain an instance of the class TEdit.
Note that the following call will fail to compile:


Var
   C  :  TComponent;


begin
   C:=CreateComponent(TStream,Form1);
end;


because  TStream  does  not  descend  from  TComponent,  and  AClass  refers  to  a  TComponent
class.  The compiler can (and will) check this at compile time, and will produce an error.

References to classes can also be used to check inheritance:


   TMinClass  =  Class  of  TMyClass;
   TMaxClass  =  Class  of  TMyClassChild;


Function  CheckObjectBetween(Instance  :  TObject)  :  boolean;


begin
   If  not  (Instance  is  TMinClass)
        or  ((Instance  is  TMaxClass)
                and  (Instance.ClassType<>TMaxClass))  then
      Raise  Exception.Create(SomeError)
end;


The  above  example  will  raise  an  exception  if  the  passed  instance  is  not  a  descendent  of
TMinClass or a descendent of  TMaxClass.

More about instantiating a class can be found in section 6.3 , page 84 .



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               6.2         Normal  and  static  fields


               .  Classes can have fields.  Depending on how they are defined, fields hold data specific to an
               instance of a class or to the class as a whole.  Whatever the way they were defined,  fields
               observe the rules of visibilities just like any other member of the class.



               6.2.1        Normal  fields/variables

               There are 2 ways to declare a normal field.  The first one is the classical way,  similar to a
               definition in an object:


               {$mode  objfpc}
               type
                   cl=class
                      l  :  longint;
                   end;
               var
                   cl1,cl2  :  cl;
               begin
                   cl1:=cl.create;
                   cl2:=cl.create;
                   cl1.l:=2;
                   writeln(cl1.l);
                   writeln(cl2.l);
               end.


               will be the following


               2
               0


               The example demonstrates that values of fields are initialized with zero (or the equivalent of
               zero for non ordinal types:  empty string, empty array and so on).

               The second way to declare a field (only available in more recent versions of Free Pascal) is
               using a var block:


               {$mode  objfpc}
               type
                   cl=class
                   var
                      l  :  longint;
                   end;


               This definition is completely equivalent to the previous definition.

Remark:         As of version 3.0 of the compiler, the compiler can re-order the fields in memory if this leads
               to better alignment and smaller instances.  That means that in an instance, the fields do not
               necessarily appear in the same order as in the declaration.  RTTI generated for a class will
               reflect this change.



               6.2.2        Class  fields/variables

               Similar to objects, a class can contain static fields or class variables:  these fields or variables
               are global to the class, and act like global variables, but are known only as part of the class.



                                                                                83

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 *___



They can be referenced from within the classes' methods, but can also be referenced from
outside the class by providing the fully qualified name.

Again, there are 2 ways to define class variables.  The first one is equivalent to the way it is
done in objects, using a static modifier:

For instance, the output of the following program is the same as the output for the version
using an object:


{$mode  objfpc}
type
   cl=class
      l  :  longint;static;
   end;
var
   cl1,cl2  :  cl;
begin
   cl1:=cl.create;
   cl2:=cl.create;
   cl1.l:=2;
   writeln(cl2.l);
   cl2.l:=3;
   writeln(cl1.l);
   Writeln(cl.l);
end.


The output of this will be the following:


2
3
3


Note that the last line of code references the class type itself (cl),  and not an instance of
the class (cl1 or cl2).

In addition to the static field approach, in classes, a Class  Var can be used.  Similar to the
way a field can be defined in a variable block, a class variable can be declared in a class var
block:


{$mode  objfpc}
type
   cl=class
   class  var
      l  :  longint;
   end;


This definition is equivalent to the previous one.



6.3         Class  instantiation


Classes must be created using one of their constructors (there can be multiple constructors).
Remember that a class is a pointer to an object on the heap.  When a variable of some class
is  declared,  the  compiler  just  allocates  room  for  this  pointer,  not  the  entire  object.   The
constructor of a class returns a pointer to an initialized instance of the object on the heap.
So, to initialize an instance of some class, one would do the following :



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                  ClassVar  :=  ClassType.ConstructorName;


               The extended syntax of  new and dispose can not be used to instantiate and destroy class
               instances.  That construct is reserved for use with objects only.  Calling the constructor will
               provoke  a  call  to  getmem,  to  allocate  enough  space  to  hold  the  class  instance  data.  After
               that, the constructor's code is executed.  The constructor has a pointer to its data, in Self.

Remark:


                    o The {$PackRecords  } directive also affects classes,  i.e.  the alignment in memory of
                      the different fields depends on the value of the {$PackRecords  } directive.

                    o Just as for objects and records, a packed class can be declared.  This has the same effect
                      as on an object, or record, namely that the elements are aligned on 1-byte boundaries,
                      i.e.  as close as possible.

                    o SizeOf(class) will return the same as SizeOf(Pointer), since a class is a pointer to
                      an object.  To get the size of the class instance data, use the TObject.InstanceSize
                      method.



               6.4         Class  destruction


               Class instances must be destroyed using the destructor.  In difference with the constructor,
               there is no choice in destructors: the destructor must have the name Destroy, it must override
               the  Destroy  destructor  declared  in  TObject,  cannot  have  arguments,  and  the  inherited
               destructor must always be called.

               To avoid calling a destructor on a Nil instance, it is best to call the Free method of TObject.
               This method will check if  Self is not Nil, and if so, then it calls Destroy.  If  Self equals
               Nil, it will just exit.

               Destroying an instance does not free a reference to an instance:


               Var
                   A  :  TComponent;


               begin
                   A:=TComponent.Create;
                   A.Name:='MyComponent';
                   A.Free;
                   Writeln('A  is  still  assigned:  ',Assigned(A));
               end.


               After the call to Free, the variable A will not be Nil, the output of this program will be:


               A  is  still  assigned:  TRUE


               To  make  sure  that  the  variable  A  is  cleared  after  the  destructor  was  called,  the  function
               FreeAndNil from the SysUtils unit can be used.  It will call Free and will then write Nil in
               the object pointer (A in the above example):


               Var
                   A  :  TComponent;


               begin



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   A:=TComponent.Create;
   A.Name:='MyComponent';
   FreeAndNil(A);
   Writeln('A  is  still  assigned:  ',Assigned(A));
end.


After  the  call  to  FreeAndNil,  the  variable  A  will  contain  Nil,  the  output  of  this  program
will be:


A  is  still  assigned:  FALSE



6.5         Methods



6.5.1        Declaration

Declaration of methods in classes follows the same rules as method declarations in objects:


        |______________________________________________________________________________________________________________|
        methods



      --  ___ method definition __    __|___ function header __   ___|__ ; __ method directives __    ____________________-oe

                                        |__ procedure header __    __|
                                        |_|constructor_header___    _|
                                                               destructor header __    __|

      --  ___ method directives __    __|____________________________________________|____|__________________________|_______-oe
                                        |___ ___ virtual __ ______ ________________________||_ call modifiers __   ; ___|
                                        |   ||_         __   _||  ||_   __          __  _|||
                                        |       dynamic               ;    abstract        |
                                        |___________ reintroduce __     ; _____________|
                                        |_____________|override|___  ; _______________|
                                                                    message __     constant expression __    _||

        |______________________________________________________________________________________________________________|


The only differences are the override, reintroduce and message directives.



6.5.2        invocation

Method invocation for classes is no different than for objects.  The following is a valid method
invocation:


Var    AnObject  :  TAnObject;
begin
    AnObject  :=  TAnObject.Create;
    ANobject.AMethod;



6.5.3        Virtual  methods

Classes have virtual methods, just as objects do.  There is however a difference between the
two.  For objects, it is sufficient to redeclare the same method in a descendent object with
the keyword virtual to override it.  For classes, the situation is different:  virtual methods
must  be  overridden  with  the  override  keyword.  Failing  to  do  so,  will  start  a  new  batch



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               of virtual methods, hiding the previous one.  The Inherited keyword will not jump to the
               inherited method, if  Virtual was used.

               The following code is wrong:


               Type
                  ObjParent  =  Class
                     Procedure  MyProc;  virtual;
                  end;
                  ObjChild    =  Class(ObjPArent)
                     Procedure  MyProc;  virtual;
                  end;


               The compiler will produce a warning:


               Warning:  An  inherited  method  is  hidden  by  OBJCHILD.MYPROC


               The compiler will compile it, but using Inherited can produce strange effects.

               The correct declaration is as follows:


               Type
                  ObjParent  =  Class
                     Procedure  MyProc;  virtual;
                  end;
                  ObjChild    =  Class(ObjPArent)
                     Procedure  MyProc;  override;
                  end;


               This will compile and run without warnings or errors.

               If the virtual method should really be replaced with a method with the same name, then the
               reintroduce keyword can be used:


               Type
                  ObjParent  =  Class
                     Procedure  MyProc;  virtual;
                  end;
                  ObjChild    =  Class(ObjPArent)
                     Procedure  MyProc;  reintroduce;
                  end;


               This new method is no longer virtual.

               To  be  able  to  do  this,  the  compiler  keeps  -  per  class  type  -  a  table  with  virtual  methods:
               the  VMT  (Virtual  Method  Table).   This  is  simply  a  table  with  pointers  to  each  of  the
               virtual  methods:  each  virtual  method  has  its  fixed  location  in  this  table  (an  index).  The
               compiler uses this table to look up the actual method that must be used at runtime.  When
               a  descendent  object  overrides  a  method,  the  entry  of  the  parent  method  is  overwritten  in
               the VMT. More information about the VMT can be found in Programmer's Guide           .

Remark:        The keyword 'virtual' can be replaced with the 'dynamic' keyword: dynamic methods behave
               the  same  as  virtual  methods.   Unlike  in  Delphi,  in  FPC  the  implementation  of  dynamic
               methods is equal to the implementation of virtual methods.



               6.5.4        Class  methods

               Class  methods  are  identified  by  the  keyword  Class  in  front  of  the  procedure  or  function
               declaration, as in the following example:



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   Class  Function  ClassName  :  String;


Class methods are methods that do not have an instance (i.e.  Self does not point to a class
instance) but which follow the scoping and inheritance rules of a class.  They can be used
to return information about the current class, for instance for registration or use in a class
factory.  Since no instance is available, no information available in instances can be used.

Class methods can be called from inside a regular method,  but can also be called using a
class identifier:


Var
   AClass  :  TClass;  //  AClass  is  of  type  "type  of  class"


begin
   ..
   if  CompareText(AClass.ClassName,'TCOMPONENT')=0  then
   ...



But calling them from an instance is also possible:


Var
   MyClass  :  TObject;


begin
   ..
   if  MyClass.ClassNameis('TCOMPONENT')  then
   ...


The reverse is not possible:  Inside a class method,  the Self identifier points to the VMT
table of the class.  No fields, properties or regular methods are available inside a class method.
Accessing a regular property or method will result in a compiler error.

Note that class methods can be virtual, and can be overridden.

Class methods can be used as read or write specifiers for a regular property, but naturally,
this property will have the same value for all instances of the class, since there is no instance
available in the class method.



6.5.5        Class  constructors  and  destructors

A class constructor or destructor can also be created.  They serve to instantiate some class
variables  or  class  properties  which  must  be  initialized  before  a  class  can  be  used.   These
constructors are called automatically at program startup: The constructor is called before the
initialization section of the unit it is declared in, the destructor is called after the finalisation
section of the unit it is declared in.

There are some caveats when using class destructors/constructors:


     o  The constructor must be called Create and can have no parameters.

     o  The destructor must be called Destroy and can have no parameters.

     o  Neither constructor nor destructor can be virtual.

     o  The class constructor/destructor is called irrespective of the use of the class:  even if a
        class is never used, the constructor and destructor are called anyway.



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     o There is no guaranteed order in which the class constructors or destructors are called.
       For nested classes, the only guaranteed order is that the constructors of nested classes
       are called after the constructor of the encompassing class is called, and for the destruc-
       tors the opposite order is used.


The following program:


{$mode  objfpc}
{$h+}


Type
   TA  =  Class(TObject)
   Private
      Function  GetA  :  Integer;
      Procedure  SetA(AValue  :  integer);


   public
      Class  Constructor  create;
      Class  Destructor  destroy;
      Property  A  :  Integer  Read  GetA  Write  SetA;
   end;


{Class}  Function  TA.GetA  :  Integer;


begin
   Result:=-1;
end;


{Class}  Procedure  TA.SetA(AValue  :  integer);


begin
   //
end;


Class  Constructor  TA.Create;


begin
   Writeln('Class  constructor  TA');
end;


Class  Destructor  TA.Destroy;


begin
   Writeln('Class  destructor  TA');


end;


Var
   A  :  TA;


begin
end.


Will, when run, output the following:



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Class  constructor  TA
Class  destructor  TA



6.5.6        Static  class  methods

FPC knows static class methods in classes:  these are class methods that have the Static
keyword at the end.  These methods behave completely like regular procedures or functions.
This means that:


     o  They  do  not  have  a  Self  parameter.   As  a  result,  they  cannot  access  properties  or
        fields or regular methods.

     o  They cannot be virtual.

     o  They can be assigned to regular procedural variables.


Their  use  is  mainly  to  include  the  method  in  the  namespace  of  the  class  as  opposed  to
having  the  procedure  in  the  namespace  of  the  unit.  Note  that  they  do  have  access  to  all
class variables, types etc, meaning something like this is possible:


  {$mode  objfpc}
{$h+}


Type
    TA  =  Class(TObject)
    Private
       class  var  myprivatea  :  integer;
    public
       class  Function  GetA  :  Integer;    static;
       class  Procedure  SetA(AValue  :  Integer);  static;
    end;


Class  Function  TA.GetA  :  Integer;


begin
    Result:=myprivateA;
end;


Class  Procedure  TA.SetA(AValue  :  integer);


begin
    myprivateA:=AValue;
end;



begin
    TA.SetA(123);
    Writeln(TA.MyPrivateA);
end.


Which will output 123, when run.

In  the  implementation  of  a  static  class  method,  the  Self  identifier  is  not  available.   The
method  behaves  as  if  Self  is  hardcoded  to  the  declared  class,  not  the  actual  class  with
which it was called.  In regular class methods, Self contains the Actual class for which the
method was called.  The following example makes this clear:



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Type
   TA  =  Class
      Class  procedure  DoIt;  virtual;
      Class  Procedure  DoitStatic;  static;
   end;


   TB  =  CLass(TA)
      Class  procedure  DoIt;  override;
   end;



Class  procedure  TA.DOit;


begin
   Writeln('TA.Doit  :  ',Self.ClassName);
end;


Class  procedure  TA.DOitStatic;


begin
   Doit;
   Writeln('TA.DoitStatic  :  ',ClassName);
end;


Class  procedure  TB.DoIt;


begin
   Inherited;
   Writeln('TB.Doit  :  ',Self.ClassName);
end;


begin
   Writeln('Through  static  method:');
   TB.DoItStatic;
   Writeln('Through  class  method:');
   TB.Doit;
end.


When run, this example will print:


Through  static  method:
TA.Doit  :  TA
TA.DoitStatic  :  TA
Through  class  method:
TA.Doit  :  TB
TB.Doit  :  TB


For the static class method, even though it was called using TB, the class (Self, if it were
available) is set to TA, the class in which the static method was defined.  For the class method,
the class is set to the actual class used to call the method (TB).



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6.5.7        Message  methods

New in classes are message methods.  Pointers to message methods are stored in a special
table, together with the integer or string constant that they were declared with.  They are
primarily intended to ease programming of callback functions in several GUI toolkits, such
as  Win32  or  GTK.  In  difference  with  Delphi,  Free  Pascal  also  accepts  strings  as  message
identifiers.  Message methods are always virtual.

As  can  be  seen  in  the  class  declaration  diagram,  message  methods  are  declared  with  a
Message keyword, followed by an integer constant expression.

Additionally, they can take only one var argument (typed or not):


  Procedure  TMyObject.MyHandler(Var  Msg);  Message  1;


The method implementation of a message function is not different from an ordinary method.
It is also possible to call a message method directly, but this should not be done.  Instead,
the TObject.Dispatch method should be used.  Message methods are automatically virtual,
i.e.  they can be overridden in descendent classes.

The TObject.Dispatch method can be used to call a message handler.  It is declared in the
system unit and will accept a var parameter which must have at the first position a cardinal
with the message ID that should be called.  For example:


Type
    TMsg  =  Record
       MSGID  :  Cardinal;
       Data  :  Pointer;
Var
    Msg  :  TMSg;


MyObject.Dispatch  (Msg);


In this example, the Dispatch method will look at the object and all its ancestors (starting
at the object, and searching up the inheritance class tree), to see if a message method with
message MSGID has been declared.  If such a method is found, it is called, and passed the Msg
parameter.

If no such method is found, DefaultHandler is called.  DefaultHandler is a virtual method
of TObject that doesn't do anything, but which can be overridden to provide any processing
that might be needed.  DefaultHandler is declared as follows:


     procedure  DefaultHandler(var  message);virtual;


In  addition  to  the  message  method  with  a  Integer  identifier,  Free  Pascal  also  supports  a
message method with a string identifier:


  Procedure  TMyObject.MyStrHandler(Var  Msg);  Message  'OnClick';


The  working  of  the  string  message  handler  is  the  same  as  the  ordinary  integer  message
handler:

The TObject.DispatchStr method can be used to call a message handler.  It is declared in
the system unit and will accept one parameter which must have at the first position a short
string with the message ID that should be called.  For example:


Type
    TMsg  =  Record



                                                                 92

               __________________________________________________________________________________________________CHAPTER_6.___CLASS*
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                     MsgStr  :  String[10];  //  Arbitrary  length  up  to  255  characters.
                     Data  :  Pointer;
               Var
                  Msg  :  TMSg;


               MyObject.DispatchStr  (Msg);


               In this example, the DispatchStr method will look at the object and all its ancestors (start-
               ing at the object, and searching up the inheritance class tree), to see if a message method
               with message MsgStr has been declared.  If such a method is found, it is called, and passed
               the Msg parameter.

               If no such method is found, DefaultHandlerStr is called.  DefaultHandlerStr is a virtual
               method of  TObject that doesn't do anything, but which can be overridden to provide any
               processing that might be needed.  DefaultHandlerStr is declared as follows:


                   procedure  DefaultHandlerStr(var  message);virtual;


               In addition to this mechanism, a string message method accepts a self parameter:


               Procedure  StrMsgHandler(Data:  Pointer;
                                                      Self:  TMyObject);  Message  'OnClick';


               When  encountering  such  a  method,  the  compiler  will  generate  code  that  loads  the  Self
               parameter into the object instance pointer.  The result of this is that it is possible to pass
               Self as a parameter to such a method.

Remark:        The type of the Self parameter must be of the same class as the class the method is defined
               in.



               6.5.8        Using  inherited

               In an overridden virtual method, it is often necessary to call the parent class' implementa-
               tion of the virtual method.  This can be done with the inherited keyword.  Likewise, the
               inherited keyword can be used to call any method of the parent class.

               The first case is the simplest:


               Type
                   TMyClass  =  Class(TComponent)
                      Constructor  Create(AOwner  :  TComponent);  override;
                   end;


               Constructor  TMyClass.Create(AOwner  :  TComponent);


               begin
                   Inherited;
                   //  Do  more  things
               end;


               In the above example, the Inherited statement will call Create of  TComponent, passing it
               AOwner as a parameter:  the same parameters that were passed to the current method will
               be passed to the parent's method.  They must not be specified again:  if none are specified,
               the compiler will pass the same arguments as the ones received.

               The second case is slightly more complicated:



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Type
   TMyClass  =  Class(TComponent)
      Constructor  Create(AOwner  :  TComponent);  override;
      Constructor  CreateNew(AOwner  :  TComponent;  DoExtra  :  Boolean);
   end;


Constructor  TMyClass.Create(AOwner  :  TComponent);
begin
   Inherited;
end;


Constructor  TMyClass.CreateNew(AOwner  :  TComponent;  DoExtra  :  Boolean);
begin
   Inherited  Create(AOwner);
   //  Do  stuff
end;


The  CreateNew  method  will  first  call  TComponent.Create  and  will  pass  it  AOwner  as  a
parameter.  It will not call TMyClass.Create.

Although the examples were given using constructors, the use of inherited is not restricted
to constructors, it can be used for any procedure or function or destructor as well.



6.6         Properties



6.6.1        Definition

Classes can contain properties as part of their fields list.  A property acts like a normal field,
i.e.  its value can be retrieved or set, but it allows to redirect the access of the field through
functions and procedures.  They provide a means to associate an action with an assignment
of  or  a  reading  from  a  class  'field'.   This  allows  e.g.   checking  that  a  value  is  valid  when
assigning, or, when reading, it allows to construct the value on the fly.  Moreover, properties
can be read-only or write only.  The prototype declaration of a property is as follows:


        |______________________________________________________________________________________________________________|
        Properties



      --  ___ property definition __   __|_ class ____|_ property __    identifier ____|____________________________|_-
                                         |____________|                                |_ property interface __   _|
      -  ______ property specifiers __    hintdirective __ ____________________________________________________________-oe


      --  ___ property interface __   __|__________________________________|__ : __ type identifier __-_
                                        |_ property parameter list __    _|
      -  ______|____________________________________|______________________________________________________________________-oe
               |_ index __   integerconstant __   _|


      --  ___ property parameter list __     [ ____  _ parameter declaration __   ___] ________________________________-oe
                                                   6||______________ ; ______________|_|


      --  ___ property specifiers __   __|______________________|____|______________________________|___-
                                         |_ read specifier __  _|    |_____ write specifier __ _____|
                                                                     |_ implements specifier __    _|
      -  ______|__________________________|____|________________________|____|________________________________________|____________*
 *-oe
               |_ default specifier __  _|     |_ stored specifier __  _|    |_ defaultarraypropertyspecifier __     _|



                                                                 94

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     - - ___ read specifier __   read __  field or method __   _____________________________________________________-oe


     - - ___ write specifier __   write __  field or method __  ____________________________________________________-oe


     - - ___ implements specifier __     implements __      identifier ______________________________________________-oe


     - - ___ default specifier __  __ _ default __  __ __________________________________________________________________-oe
                                     |                ||_          __  |_||
                                     ||_________          constant       |
                                                 nodefault __   _________|

     - - ___ stored specifier __   stored __ __|__ constant __ ___|____________________________________________________-oe
                                               |_ identifier ___|


     - - ___ field or method __   __|____ field identifier ______|_______________________________________________________-oe
                                    |_ method identifier __   _|

     - - ___ defaultarraypropertyspecifier __      ; __default __  _________________________________________________-oe


       |_______________________________________________________________________________________________________________|


A read  specifier is either the name of a field that contains the property, or the name of a
method function that has the same return type as the property type.  In the case of a simple
type, this function must not accept an argument.  In case of an array property, the function
must accept a single argument of the same type as the index.  In case of an indexed property,
it must accept a integer as an argument.

A  read  specifier  is  optional,  making  the  property  write-only.  Note  that  class  methods
cannot be used as read specifiers.

A write  specifier is optional:  If there is no write  specifier, the property is read-only.
A write specifier is either the name of a field, or the name of a method procedure that accepts
as a sole argument a variable of the same type as the property.  In case of an array property,
the procedure must accept 2 arguments:  the first argument must have the same type as the
index, the second argument must be of the same type as the property.  Similarly, in case of
an indexed property, the first parameter must be an integer.

The section   (private,  published) in which the specified function or procedure resides is
irrelevant.  Usually, however, this will be a protected or private method.

For example, given the following declaration:


Type
   MyClass  =  Class
      Private
      Field1  :  Longint;
      Field2  :  Longint;
      Field3  :  Longint;
      Procedure    Sety  (value  :  Longint);
      Function  Gety  :  Longint;
      Function  Getz  :  Longint;
      Public
      Property  X  :  Longint  Read  Field1  write  Field2;
      Property  Y  :  Longint  Read  GetY  Write  Sety;
      Property  Z  :  Longint  Read  GetZ;
      end;


Var
   MyClass  :  TMyClass;



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The following are valid statements:


WriteLn  ('X  :  ',MyClass.X);
WriteLn  ('Y  :  ',MyClass.Y);
WriteLn  ('Z  :  ',MyClass.Z);
MyClass.X  :=  0;
MyClass.Y  :=  0;


But the following would generate an error:


MyClass.Z  :=  0;


because Z is a read-only property.

What happens in the above statements is that when a value needs to be read, the compiler
inserts a call to the various getNNN methods of the object, and the result of this call is used.
When  an  assignment  is  made,  the  compiler  passes  the  value  that  must  be  assigned  as  a
parameter to the various setNNN methods.

Because of this mechanism, properties cannot be passed as var arguments to a function or
procedure, since there is no known address of the property (at least, not always).



6.6.2        Indexed  properties

If  the  property  definition  contains  an  index,  then  the  read  and  write  specifiers  must  be  a
function and a procedure.  Moreover, these functions require an additional parameter :  An
integer parameter.  This allows to read or write several properties with the same function.
For this, the properties must have the same type.  The following is an example of a property
with an index:


{$mode  objfpc}
Type
    TPoint  =  Class(TObject)
    Private
       FX,FY  :  Longint;
       Function  GetCoord  (Index  :  Integer):  Longint;
       Procedure  SetCoord  (Index  :  Integer;  Value  :  longint);
    Public
       Property  X  :  Longint  index  1  read  GetCoord  Write  SetCoord;
       Property  Y  :  Longint  index  2  read  GetCoord  Write  SetCoord;
       Property  Coords[Index  :  Integer]:Longint  Read  GetCoord;
    end;


Procedure  TPoint.SetCoord  (Index  :  Integer;  Value  :  Longint);
begin
    Case  Index  of
     1  :  FX  :=  Value;
     2  :  FY  :=  Value;
    end;
end;


Function  TPoint.GetCoord  (INdex  :  Integer)  :  Longint;
begin
    Case  Index  of
     1  :  Result  :=  FX;



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    2  :  Result  :=  FY;
   end;
end;


Var
   P  :  TPoint;


begin
   P  :=  TPoint.create;
   P.X  :=  2;
   P.Y  :=  3;
   With  P  do
      WriteLn  ('X=',X,'  Y=',Y);
end.


When  the  compiler  encounters  an  assignment  to  X,  then  SetCoord  is  called  with  as  first
parameter the index (1 in the above case) and with as a second parameter the value to be
set.  Conversely, when reading the value of X, the compiler calls GetCoord and passes it index
1.  Indexes can only be integer values.



6.6.3        Array  properties

Array properties also exist.  These are properties that accept an index, just as an array does.
The index can be one-dimensional, or multi-dimensional.  In difference with normal (static
or dynamic) arrays, the index of an array property doesn't have to be an ordinal type, but
can be any type.

A read  specifier for an array property is the name method function that has the same
return type as the property type.  The function must accept as a sole arguent a variable of
the same type as the index type.  For an array property, one cannot specify fields as read
specifiers.

A write  specifier for an array property is the name of a method procedure that accepts
two arguments:  the first argument has the same type as the index, and the second argument
is  a  parameter  of  the  same  type  as  the  property  type.   As  an  example,  see  the  following
declaration:


Type
    TIntList  =  Class
    Private
       Function  GetInt  (I  :  Longint)  :  longint;
       Function  GetAsString  (A  :  String)  :  String;
       Procedure  SetInt  (I  :  Longint;  Value  :  Longint;);
       Procedure  SetAsString  (A  :  String;  Value  :  String);
    Public
       Property  Items  [i  :  Longint]  :  Longint  Read  GetInt
                                                                       Write  SetInt;
       Property  StrItems  [S  :  String]  :  String  Read  GetAsString
                                                                         Write  SetAsstring;
    end;


Var
    AIntList  :  TIntList;


Then the following statements would be valid:



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AIntList.Items[26]  :=  1;
AIntList.StrItems['twenty-five']  :=  'zero';
WriteLn  ('Item  26  :  ',AIntList.Items[26]);
WriteLn  ('Item  25  :  ',AIntList.StrItems['twenty-five']);


While the following statements would generate errors:


AIntList.Items['twenty-five']  :=  1;
AIntList.StrItems[26]  :=  'zero';


Because the index types are wrong.

Array properties can be multi-demensional:


Type
   TGrid  =  Class
   Private
      Function  GetCell  (I,J  :  Longint)  :  String;
      Procedure  SetCell  (I,J  :  Longint;  Value  :  String);
   Public
      Property  Cellcs  [Row,Col  :  Longint]  :  String  Read  GetCell
                                                                        Write  SetCell;
   end;


If there are N dimensions, then the types of the first N arguments of the getter and setter
must correspond to the types of the N index specifiers in the array property definition.



6.6.4        Default  properties

Array properties can be declared as default properties.  This means that it is not necessary
to specify the property name when assigning or reading it.  In the previous example, if the
definition of the items property would have been


  Property  Items[i  :  Longint]:  Longint  Read  GetInt
                                                               Write  SetInt;  Default;


Then the assignment


AIntList.Items[26]  :=  1;


Would be equivalent to the following abbreviation.


AIntList[26]  :=  1;


Only  one  default  property  per  class  is  allowed,  but  descendent  classes  can  redeclare  the
default property.



6.6.5        Published  properties

Classes compiled in the {$M+} state (such as TPersistent from the classes unit) can have a
published section.  For methods, fields and properties in the Published section, the compiler
generates RTTI information (Run Time Type Information), which can be used to query the
defined methods, fields and properties in the published section(s).  The typinfo unit contains



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the necessary routines to query this information, and this unit is used in the streaming system
in FPC in the classes unit.

The RTTI is generated regardless of what the read and write specifiers are:  fields, function-
s/procedures or indexed functions/procedures.

Only class-typed fields can be published.  For properties, any simple property whose size is
less  than  or  equal  to  a  pointer,  can  be  declared  published:  floats,  integers,  sets  (with  less
than 32 distinct elements), enumerateds, classes or dynamic arrays (not array properties).

Although run-time type information is available for other types, these types cannot be used
for a property or field definition in a published section.  The information is present to describe
f.e.  arguments of procedures or functions.



6.6.6        Storage  information

The  stored  specifier  should  be  either  a  boolean  constant,  a  boolean  field  of  the  class,  or  a
parameterless function which returns a boolean result.  This specifier has no result on the
class behaviour.  It is an aid for the streaming system:  the stored specifier is specified in the
RTTI generated for a class (it can only be streamed if RTTI is generated), and is used to
determine whether a property should be streamed or not:  it saves space in a stream.  It is
not possible to specify the 'Stored' directive for array properties.

The default specifier can be specified for ordinal types and sets.  It serves the same purpose
as the stored specifier:  properties that have as value their default value, will not be written
to  the  stream  by  the  streaming  system.   The  default  value  is  stored  in  the  RTTI  that  is
generated for the class.  Note that


    1.  When  the  class  is  instantiated,  the  default  value  is  not  automatically  applied  to  the
        property, it is the responsibility-g of the programmer to do this in the constructor of
        the class.

    2.  The  value  2147483648  cannot  be  used  as  a  default  value,  as  it  is  used  internally  to
        denote nodefault.

    3.  It is not possible to specify a default for array properties.


The nodefault specifier (nodefault) must be used to indicate that a property has no default
value.  The  effect  is  that  the  value  of  this  property  is  always  written  to  the  stream  when
streaming the property.



6.6.7        Overriding  properties

Properties can be overridden in descendent classes, just like methods.  The difference is that
for properties, the overriding can always be done:  properties should not be marked 'virtual'
so they can be overridden, they are always overridable (in this sense, properties are always
'virtual').  The type of the overridden property does not have to be the same as the parents
class property type.

Since they can be overridden, the keyword 'inherited' can also be used to refer to the parent
definition of the property.  For example consider the following code:


type
    TAncestor  =  class
    private
       FP1  :  Integer;
    public



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      property  P:  integer  Read  FP1  write  FP1;
   end;


   TClassA  =  class(TAncestor)
   private
      procedure  SetP(const  AValue:  char);
      function  getP  :  Char;
   public
      constructor  Create;
      property  P:  char  Read  GetP  write  SetP;
   end;


procedure  TClassA.SetP(const  AValue:  char);


begin
   Inherited  P:=Ord(AValue);
end;


procedure  TClassA.GetP  :  char;


begin
   Result:=Char((Inherited  P)  and  $FF);
end;


TClassA  redefines  P  as  a  character  property  instead  of  an  integer  property,  but  uses  the
parents P property to store the value.

Care must be taken when using virtual get/set routines for a property:  setting the inherited
property still observes the normal rules of inheritance for methods.  Consider the following
example:


type
   TAncestor  =  class
   private
      procedure  SetP1(const  AValue:  integer);  virtual;
   public
      property  P:  integer  write  SetP1;
   end;


   TClassA  =  class(TAncestor)
   private
      procedure  SetP1(const  AValue:  integer);  override;
      procedure  SetP2(const  AValue:  char);
   public
      constructor  Create;
      property  P:  char  write  SetP2;
   end;


constructor  TClassA.Create;
begin
   inherited  P:=3;
end;


In this case, when setting the inherited property P, the implementation TClassA.SetP1 will
be called, because the SetP1 method is overridden.



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If the parent class implementation of SetP1 must be called, then this must be called explicitly:


constructor  TClassA.Create;
begin
   inherited  SetP1(3);
end;



6.7         Class  properties


Class properties are very much like global property definitions.  They are associated with the
class, not with an instance of the class.

A consequence of this is that the storage for the property value must be a class var, not a
regular field or variable of the class:  normal fields or variables are stored in an instance of
the class.

Class properties can have a getter and setter method like regular properties, but these must
be static methods of the class.

That means that the following contains a valid class property definition:


TA  =  Class(TObject)
Private
    class  var  myprivatea  :  integer;
    class  Function  GetB  :  Integer;    static;
    class  Procedure  SetA(AValue  :  Integer);  static;
    class  Procedure  SetB(AValue  :  Integer);  static;
public
    Class  property  MyA  :  Integer  Read  MyPrivateA  Write  SetA;
    Class  property  MyA  :  Integer  Read  GetB  Write  SetB;
end;


The reason for the requirement is that a class property is associated to the particular class
in which it is defined, but not to descendent classes.  Since class methods can be virtual, this
would  allow  descendent  classes  to  override  the  method,  making  them  unsuitable  for  class
property access.



6.8         Nested  types,  constants  and  variables


A  class  definition  can  contain  a  type  section,  const  section  and  a  variable  section.   The
type and constant sections act as a regular type section as found in a unit or method/func-
tion/procedure implementation.  The variables act as regular fields of the class, unless they
are in a class  var section, in which case they act as if they were defined at the unit level,
within the namespace of the class (section 6.2 , page 83 ).

However,  the  visibility  of  these  sections  does  play  a  role:  private  and  protected  (strict  or
not) constants, types and variables can only be used as far as their visibility allows.

Public types can be used outside the class, by their full name:


type
    TA  =  Class(TObject)
    Public
       Type  TEnum  =  (a,b,c);
       Class  Function  DoSomething  :  TEnum;



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   end;


Class  Function  TA.DoSomething  :  TEnum;


begin
   Result:=a;
end;


var
   E  :  TA.TEnum;


begin
   E:=TA.DoSomething;
end.


Whereas


type
   TA  =  Class(TObject)
   Strict  Private
      Type  TEnum  =  (a,b,c);
   Public
      Class  Function  DoSomething  :  TEnum;
   end;


Class  Function  TA.DoSomething  :  TEnum;


begin
   Result:=a;
end;


var
   E  :  TA.TEnum;


begin
   E:=TA.DoSomething;
end.


Will not compile and will return an error:


tt.pp(20,10)  Error:  identifier  idents  no  member  "TEnum"



                                                                 102




Chapter   7



Interfaces



7.1         Definition


As of version 1.1, FPC supports interfaces.  Interfaces are an alternative to multiple inheri-
tance (where a class can have multiple parent classes) as implemented for instance in C++.
An interface is basically a named set of methods and properties:  a class that implements the
interface provides all the methods as they are enumerated in the Interface definition.  It is
not possible for a class to implement only part of the interface:  it is all or nothing.

Interfaces can also be ordered in a hierarchy, exactly as classes:  an interface definition that
inherits from another interface definition contains all the methods from the parent interface,
as well as the methods explicitly named in the interface definition.  A class implementing an
interface  must  then  implement  all  members  of  the  interface  as  well  as  the  methods  of  the
parent interface(s).

An interface can be uniquely identified by a GUID. GUID is an acronym for Globally Unique
Identifier, a 128-bit integer guaranteed always to be unique1 .  Especially on Windows systems,
the GUID of an interface can and must be used when using COM.

The definition of an Interface has the following form:


        |______________________________________________________________________________________________________________|
        Interface type


      --  ___ Interface __  __|____________|____|____________________|____|________________________|__ end __ ___________________-oe
                              |_heritage_|      |_ [' __GUID '] ___|      |_ component list __    _|

      --  ___ heritage __  ( __ interface type identifier __    ) ___________________________________________________-oe

      --  ___ component list __    __  ___ __ method definition __   _______________________________________________________-oe
                                     6||  ||_ property definition __   _||||
                                     |________________________________|

        |______________________________________________________________________________________________________________|


Along with this definition the following must be noted:


     o  Interfaces can only be used in DELPHI mode or in OBJFPC mode.

     o  There are no visibility specifiers.  All members are public (indeed, it would make little
        sense to make them private or protected).
___________________________________________________1
     In theory, of course.



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 *___



     o The properties declared in an interface can only have methods as read and write spec-
       ifiers.

     o There  are  no  constructors  or  destructors.   Instances  of  interfaces  cannot  be  created
       directly:  instead, an instance of a class implementing the interface must be created.

     o Only calling convention modifiers may be present in the definition of a method.  Mod-
       ifiers as virtual, abstract or dynamic, and hence also override cannot be present
       in the interface definition.


The following are examples of interfaces:


IUnknown  =  interface  ['{00000000-0000-0000-C000-000000000046}']
   function  QueryInterface(const  iid  :  tguid;out  obj)  :  longint;
   function  _AddRef  :  longint;
   function  _Release  :  longint;
end;
IInterface  =  IUnknown;


IMyInterface  =  Interface
   Function  MyFunc  :  Integer;
   Function  MySecondFunc  :  Integer;
end;


As can be seen, the GUID identifying the interface is optional.



7.2         Interface  identification:   A  GUID


An interface can be identified by a GUID. This is a 128-bit number, which is represented in
a text representation (a string literal):


['{HHHHHHHH-HHHH-HHHH-HHHH-HHHHHHHHHHHH}']


Each H character represents a hexadecimal number (0-9,A-F). The format contains 8-4-4-4-12
numbers.  A GUID can also be represented by the following record, defined in the objpas unit
(included automatically when in DELPHI or OBJFPC mode):


PGuid  =  ^TGuid;
TGuid  =  packed  record
     case  integer  of
          1  :  (
                  Data1  :  DWord;
                  Data2  :  word;
                  Data3  :  word;
                  Data4  :  array[0..7]  of  byte;
                 );
          2  :  (
                  D1  :  DWord;
                  D2  :  word;
                  D3  :  word;
                  D4  :  array[0..7]  of  byte;
                 );
          3  :  (  {  uuid  fields  according  to  RFC4122  }



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 *___



                  time_low  :  dword;
                  time_mid  :  word;
                  time_hi_and_version  :  word;
                  clock_seq_hi_and_reserved  :  byte;
                  clock_seq_low  :  byte;
                  node  :  array[0..5]  of  byte;
                  );
end;


A constant of type TGUID can be specified using a string literal:


{$mode  objfpc}
program  testuid;


Const
   MyGUID  :  TGUID  =  '{10101010-1010-0101-1001-110110110110}';


begin
end.


Normally, the GUIDs are only used in Windows, when using COM interfaces.  More on this
in the next section.



7.3         Interface  implementations


When a class implements an interface, it should implement all methods of the interface.  If a
method of an interface is not implemented, then the compiler will give an error.  For example:


Type
    IMyInterface  =  Interface
       Function  MyFunc  :  Integer;
       Function  MySecondFunc  :  Integer;
    end;


    TMyClass  =  Class(TInterfacedObject,IMyInterface)
       Function  MyFunc  :  Integer;
       Function  MyOtherFunc  :  Integer;
    end;


Function  TMyClass.MyFunc  :  Integer;


begin
    Result:=23;
end;


Function  TMyClass.MyOtherFunc  :  Integer;


begin
    Result:=24;
end;


will result in a compiler error:



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 *___



Error:  No  matching  implementation  for  interface  method
"IMyInterface.MySecondFunc:LongInt"  found


Normally, the names of the methods that implement an interface, must equal the names of
the methods in the interface definition.

However, it is possible to provide aliases for methods that make up an interface:  that is, the
compiler can be told that a method of an interface is implemented by an existing method
with a different name.  This is done as follows:


Type
   IMyInterface  =  Interface
      Function  MyFunc  :  Integer;
   end;


   TMyClass  =  Class(TInterfacedObject,IMyInterface)
      Function  MyOtherFunction  :  Integer;
      Function  IMyInterface.MyFunc  =  MyOtherFunction;
   end;


This declaration tells the compiler that the MyFunc method of the IMyInterface interface
is implemented in the MyOtherFunction method of the TMyClass class.



7.4         Interface  delegation


Sometimes,  the methods of an interface are implemented by a helper (or delegate) object,
or  the  class  instance  has  obtained  an  interface  pointer  for  this  interface  and  that  should
be  used.  This  can  be  for  instance  when  an  interface  must  be  added  to  a  series  of  totally
unrelated classes:  the needed interface functionality is added to a separate class, and each
of these classes uses an instance of the helper class to implement the functionality.

In such a case, it is possible to instruct the compiler that the interface is not implemented
by the object itself, but actually resides in a helper class or interface.  This can be done with
the implements property modifier.

If  the  class  has  a  pointer  to  the  desired  interface,  the  following  will  instruct  the  compiler
that when the IMyInterface interface is requested, it should use the reference in the field:


type
    IMyInterface  =  interface
       procedure  P1;
    end;


    TMyClass  =  class(TInterfacedObject,  IMyInterface)
    private
       FMyInterface:  IMyInterface;  //  interface  type
    public
       property  MyInterface:  IMyInterface
            read  FMyInterface  implements  IMyInterface;
    end;


The interface should not necessarily be in a field, any read identifier can be used.

If the interface is implemented by a delegate object, (a helper object that actually implements
the interface) then it can be used as well with the implements keyword:



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 *___



{$interfaces  corba}
type
   IMyInterface  =  interface
      procedure  P1;
   end;


   //  NOTE:  Interface  must  be  specified  here
   TDelegateClass  =  class(TObject,  IMyInterface)
   private
      procedure  P1;
   end;


   TMyClass  =  class(TInterfacedObject,  IMyInterface)
   private
      FMyInterface:  TDelegateClass;  //  class  type
      property  MyInterface:  TDelegateClass
         read  FMyInterface  implements  IMyInterface;
   end;


Note that in difference with Delphi, the delegate class must explicitly specify the interface:
the compiler will not search for the methods in the delegate class, it will simply check if the
delegate class implements the specified interface.

It is not possible to mix method resolution and interface delegation.  That means, it is not
possible to implement part of an interface through method resolution and implement part
of  the  interface  through  delegation.   The  following  attempts  to  implement  IMyInterface
partly  through  method  resolution  (P1),  and  partly  through  delegation.  The  compiler  will
not accept the following code:


{$interfaces  corba}
type
   IMyInterface  =  interface
      procedure  P1;
      procedure  P2;
   end;



   TMyClass  =  class(TInterfacedObject,  IMyInterface)
      FI  :  IMyInterface;
   protected
      procedure  IMyInterface.P1  =  MyP1;
      procedure  MyP1;
      property  MyInterface:  IMyInterface    read  FI  implements  IMyInterface;
   end;


The compiler will throw an error:


Error:  Interface  "IMyInterface"  can't  be  delegated  by  "TMyClass",
it  already  has  method  resolutions


However, it is possible to implement one interface through method resolution, and another
through delegation:


{$interfaces  corba}
type



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 *___



   IMyInterface  =  interface
      procedure  P1;
   end;


   IMyInterface2  =  interface
      procedure  P2;
   end;


   TMyClass  =  class(TInterfacedObject,
                               IMyInterface,  IMyInterface2)
      FI2  :  IMyInterface2;
   protected
      procedure  IMyInterface.P1  =  MyP1;
      procedure  MyP1;
   public
      property  MyInterface:  IMyInterface2
           read  FI2  implements  IMyInterface2;
   end;



7.5         Interfaces  and  COM


When using interfaces on Windows which should be available to the COM subsystem, the
calling convention should be stdcall - this is not the default Free Pascal calling convention,
so it should be specified explicitly.

COM  does  not  know  properties.   It  only  knows  methods.   So  when  specifying  property
definitions as part of an interface definition, be aware that the properties will only be known
in  the  Free  Pascal  compiled  program:  other  Windows  programs  will  not  be  aware  of  the
property definitions.



7.6         CORBA  and  other  Interfaces


COM is not the only architecture where interfaces are used.  CORBA knows interfaces, UNO
(the OpenOffice API) uses interfaces,  and Java as well.  These languages do not know the
IUnknown interface used as the basis of all interfaces in COM. It would therefore be a bad idea
if an interface automatically descended from IUnknown if no parent interface was specified.
Therefore,  a directive {$INTERFACES} was introduced in Free Pascal:  it specifies what the
parent  interface  is  of  an  interface,  declared  without  parent.  More  information  about  this
directive can be found in the Programmer's Guide           .

Note  that  COM  interfaces  are  by  default  reference  counted,  because  they  descend  from
IUnknown.

Corba  interfaces  are  identified  by  a  simple  string  so  they  are  assignment  compatible  with
strings and not with TGUID. The compiler does not do any automatic reference counting for
the CORBA interfaces, so the programmer is responsible for any reference bookkeeping.



7.7         Reference  counting


All COM interfaces use reference counting.  This means that whenever an interface is assigned
to a variable, it's reference count is updated.  Whenever the variable goes out of scope, the



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reference count is automatically decreased.  When the reference count reaches zero, usually
the instance of the class that implements the interface, is freed.

Care must be taken with this mechanism.  The compiler may or may not create temporary
variables when evaluating expressions, and assign the interface to a temporary variable, and
only then assign the temporary variable to the actual result variable.  No assumptions should
be made about the number of temporary variables or the time when they are finalized - this
may (and indeed does) differ from the way other compilers (e.g.  Delphi) handle expressions
with interfaces.  E.g.  a type cast is also an expression:


Var
   B  :  AClass;


begin
   //  ...
   AInterface(B.Intf).testproc;
   //  ...
end;


Assume  the  interface  intf  is  reference  counted.   When  the  compiler  evaluates  B.Intf,  it
creates a temporary variable.  This variable may be released only when the procedure exits:
it is therefor invalid to e.g.  free the instance B prior to the exit of the procedure, since when
the temporary variable is finalized, it will attempt to free B again.



                                                                 109




Chapter   8



Generics



8.1         Introduction


Generics are templates for generating other types.  These can be classes, objects, interfaces
and even functions, arrays, records.  It is a concept that comes from C++, where it is deeply
integrated  in  the  language.   As  of  version  2.2,  Free  Pascal  also  officially  has  support  for
templates  or  Generics.   They  are  implemented  as  a  kind  of  macro  which  is  stored  in  the
unit  files  that  the  compiler  generates,  and  which  is  replayed  as  soon  as  a  generic  class  is
specialized.

Creating and using generics is a 2-phase process.


    1.  The definition of the generic is defined as a new type:  this is a code template, a macro
        which can be replayed by the compiler at a later stage.

    2.  A generic type is specialized:  this defines a second type, which is a specific implemen-
        tation of the generic type:  the compiler replays the macro which was stored when the
        generic type was defined.



8.2         Generic  type  definition


A generic type definition is much like a type definition, with the exception that it contains
a list of placeholders for types, as can be seen in the following syntax diagram:


        |______________________________________________________________________________________________________________|
        Generic class types



      --  ___ generic type __   generic __   identifier __  <   template list __   >   = __  generic type __   ; _____-oe


      --  ___ template list __  __  _ identifier list ___________________________________________________________________-oe
                                  6||________ ; _________|_|


      --  ___ identifier list ____  _ identifier _______ ___________________________________________________________________-oe
                                  6||______ , ______|_| ||_ : ____ ___  _ type identifier ___|_____|
                                                                  |   6||___________ __________|_||
                                                                  |                 ,            |
                                                                  |_________ class __ _________|
                                                                  |______ interface __  ______|
                                                                  |________|object____ ________|
                                                                                       record __  ________|



                                                             110

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 *___



     - - ___ generic type __  __|_____ class type __ _____|_____________________________________________________________-oe

                                |____ object type __  ____|
                                |___ interface type __  ___|
                                |_ procedural type __   _|
                                |____|record_type___  ____|
                                                   array type __  _____|

       |_______________________________________________________________________________________________________________|


For  classes,  objects,  procedural  types  and  extended  records,  the  generic  type  declaration
should be followed by a type implementation.  It is the same as a normal class implementation
with a single exception, namely that any identifier with the same name as one of the template
identifiers must be a type identifier.

So, the generic type declaration is much like a normal type declaration, except that there is
an as yet unknown type present.  The unknown types are listed in the placeholder list, and
they are unknown until the class is specialized.

The following is a valid generic class definition:


Type
   generic  TList<_T>=class(TObject)
      type  public
           TCompareFunc  =  function(const  Item1,  Item2:  _T):  Integer;
      var  public
         data  :  _T;
      procedure  Add(item:  _T);
      procedure  Sort(compare:  TCompareFunc);
   end;


This class could be followed by an implementation as follows:


procedure  TList.Add(item:  _T);
begin
   data:=item;
end;


procedure  TList.Sort(compare:  TCompareFunc);
begin
   if  compare(data,  20)  <=  0  then
      halt(1);
end;


There are some noteworthy things about this declaration and implementation:


    1.  There is a single placeholder  __T. It will be substituted by a type identifier when the
        generic class is specialized.  The identifier  __T may not be used for anything else than
        a type placeholder.  This means that the following would be invalid:


        procedure  TList.Sort(compare:  TCompareFunc);


        Var
           _t  :  integer;


        begin
           //  do  something.
        end;



                                                                 111

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 *___



    2.  The local type block contains a single type TCompareFunc.  Note that the actual type
        is not yet known inside the generic class definition:  the definition contains a reference
        to the placeholder __ T. All other identifier references must be known when the generic
        class is defined, not when the generic class is specialized.

    3.  The local variable block is equivalent to the following:


           generic  TList<_T>=class(TObject)
               type  public
                    TCompareFunc  =  function(const  Item1,  Item2:  _T):  Integer;
           Public
               data  :  _T;
               procedure  Add(item:  _T);
               procedure  Sort(compare:  TCompareFunc);
           end;


Not only generic classes can be defined, but also other types:


{$mode  objfpc}
{$INTERFACES  CORBA}
type


    generic  PlanarCoordinate<t>  =  record
       x,y  :  t;
    end;


    TScreenCoordinate  =  specialize  PLanarCoordinate<word>;
    TDiscreteCoordinate  =  specialize  PlanarCoordinate<integer>;
    TRealCoordinate  =  specialize  PlanarCoordinate<extended>;


    generic  TDistanceFunction<t>  =  function  (x,y  :  t)  :  Extended  of  object;


    TScreenDistance  =  specialize  TDistanceFunction<word>;
    TDiscreteDistance  =  specialize  TDistanceFunction<integer>;
    TRealDistance  =  specialize  TDistanceFunction<Extended>;


    generic  TArray<t>  =  array  of  t;


    TMyIntegerArray  =  specialize  TArray<integer>;


    generic  IList<_T>  =  Interface
       Function  GetItem(AIndex  :  Integer)  :  _T;
       Procedure  SetItem(AIndex  :  Integer;  AValue  :  _T);
       Function  GetCount  :  Integer;
       Property  Items  [AIndex  :  Integer]  :  _T  Read  GetItem  Write  SetItem;
       Property  Count  :  Integer  Read  GetCount;
    end;


    generic  TList<_T>=class(TObject,  specialize  IList<_T>)
    public  type
       TCompareFunc  =  function(const  Item1,  Item2:  _T):  Integer;
       Function  GetItem(AIndex  :  Integer)  :  _T;
       Procedure  SetItem(AIndex  :  Integer;  AValue  :  _T);
       Function  GetCount  :  Integer;



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               _______________________________________________________________________________________________CHAPTER_8.___GENERICS*
 *__________________



                  Public
                       data  :  _T;
                       procedure  Add(item:  _T);
                       procedure  Sort(compare:  TCompareFunc);
                  end;


                  generic  TPointSet<t>  =  array  of  specialize  PlanarCoordinate<t>;


                  TScreenPointSet  =  specialize  TPointSet<word>;
                  TDiscretePointSet  =    specialize  TPointSet<integer>;
                  TRealPointSet  =  specialize  TPointSet<extended>;


Remark:         A  word  on  visibility:   the  template  types  T  or  __T  are  available  as  strict  private  types.
               That means that the types are not available in the descendent classes, unless they are made
               available through some protected or private mechanism, as in the following example:


                  generic  TList<_T>=class(TObject)
                  public  type
                     TItemType  =  _T;
                  end;



               8.3         Generic  type  specialization


               Once a generic type is defined, it can be used to generate other types:  this is like replaying the
               definition of the types, with the template placeholders filled in with actual type definitions.

               This can be done in any Type definition block.  The specialized type looks as follows:


                       |___________________________________________________________________________________________________________*
 *___|
                       Specialized type



                     --  ___ specialized type __    specialize __  identifier __  <   type identifier list __  >___________________*
 *_-oe


                     --  ___ type identifier list __ __  _ identifier _____________________________________________________________*
 *_____-oe
                                                       6||______ , ______|_|


                       |___________________________________________________________________________________________________________*
 *___|


               Which is a very simple definition.  Given the declaration of  TList in the previous section,
               the following would be a valid type definition:


               Type
                   TPointerList  =  specialize  TList<Pointer>;
                   TIntegerList  =  specialize  TList<Integer>;


               As  of  version  3.0  of  Free  Pascal,  the  specialize  keyword  can  also  be  used  in  a  variable
               declaration:


               Var
                   P  :  specialize  TList<Pointer>;


               The type in the specialize statement must be known, except in another generic type definition.
               Given the 2 generic class definitions:



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 *__________________



               type
                  Generic  TMyFirstType<T1>  =  Class(TMyObject);
                  Generic  TMySecondType<T2>  =  Class(TMyOtherObject);


               Then the following specialization is not valid:


               type
                  TMySpecialType  =  specialize  TMySecondType<TMyFirstType>;


               because the type TMyFirstType is a generic type, and thus not fully defined.  The compiler
               will complain:


               Error:  Generics  cannot  be  used  as  parameters  when  specializing  generics


               However, the following is allowed:


               type
                  TA  =  specialize  TMyFirstType<Atype>;
                  TB  =  specialize  TMySecondType<TA>;


               because TA is already fully defined when TB is specialized.

               However, the specialize keyword can be used in another generic type definition as shown
               in the example above:


                  generic  TList<_T>=class(TObject,  specialize  IList<_T>)


               and


                  generic  TPointSet<t>  =  array  of  specialize  PlanarCoordinate<t>;


               In  these  definitions,  the  specialization  is  only  performed  when  the  generic  type  itself  is
               specialized, and at that time, the types are known.

Remark:         As  of  version  3.0,  it  is  possible  to  make  a  forward  definition  of  a  generic  class.  In  prior
               versions the compiler would generate an error if a forward declaration of a class was later
               defined as a generic specialization.  This means that the following is now possible:


               {$mode  objfpc}
               Type
                  TMyClass  =  Class;


                  //  Other  declarations


                  TMyClass  =  specialize  TList<T>;



               8.4         Generic  type  restrictions


               The  diagram  in  section  8.1 ,  page  110    shows  that  the  type  template  list  can  have  extra
               specifiers for the types.  This is especially useful for object types:  if the template type must
               descend from a certain class, then this can be specified in the template list:


               {$mode  objfpc}
               {$h+}



                                                                                114

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 *___



uses  sysutils,  classes;


Type
   generic  TList<_T  :  TComponent>  =  class(TObject)
   public
      Type  TCompareFunc  =  function(const  Item1,  Item2:  _T):  Integer;
   Public
      data  :  _T;
      procedure  Add(item:  _T);
      procedure  Sort(compare:  TCompareFunc);
 end;


Given the above definition, the following will compile:


TPersistentList  =  specialize  TList<TComponent>;


But this will not compile


TPersistentList  =  specialize  TList<TPersistent>;


The compiler will return an error:


Error:  Incompatible  types:  got  "TPersistent"  expected  "TComponent"


Multiple types can be grouped together:


Type
   generic  TList<Key1,Key2  :  TComponent;  Value1  :  TObject>  =  class(TObject)


Additionally,  it  is  possible  to  specify  more  than  one  type  identifier  for  class  and  interface
type restrictions.  If a class is specified, then the type used for the template must be equal
to or descend from the indicated type:


Type
   generic  TList<T:  TComponent,  IEnumerable>  =  class(TObject)


A class used to specialize T must descend from TComponent and must implement IEnumerable.

If  an  interface  is  specified,  then  the  template  type  must  implement  at  least  this  interface,
but it can also be a descendent interface from this interface:


Type
   generic  TGenList<T:  IEnumerable>  =  class(TObject)


   IMyEnum  =  Interface  (IEnumerable)
      Procedure  DoMy;
   end;


   TList  =  specialize  TGenList<IMyEnum>;
   TSomeList  =  Specialize  TGenList<TList>;


Multiple  interfaces  can  be  specified,  in  that  case  the  class  type  must  implement  all  listed
interfaces:  It is possible to mix one class name with several interface names.

If  no  type  restrictions  are  in  effect,  the  compiler  will  assume  that  template  types  are  not
assignment compatible.

This is specially important when the generic class contains overloaded methods.  Given the
following generic type declaration:



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 *___



type
   generic  TTest<T1,  T2>  =  class
      procedure  Test(aArg:  LongInt);
      procedure  Test(aArg:  T1);
      procedure  Test(aArg:  T2);
   end;


Specializing the above will compile if T1 and T2 are of two different types and neither is also
LongInt.  The following will compile:


T1  =  specialize  TTest<String,  TObject>;


But the following 2 will not compile:


T2  =  specialize  TTest<String,  String>;


or


T2  =  specialize  TTest<String,  Longint>;



8.5         Delphi  compatibility


FPC's generics support is implemented somewhat different from Delphi.  In this section the
main differences are highlighted.



8.5.1        Syntax  elements

The  syntax  shown  in  the  syntax  diagrams  is  the  syntax  required  in  the  ObjFPC  mode  of
the compiler.  However, in Delphi mode, the specialize and generic keywords must not
be used, as shown in the following example:


Type
    TTest<T>  =  Class(TObject)
    Private
       FObj  :  T;
    Public
       Property  Obj  :  T  Read  FObj  Write  FObj;
    end;


    TIntegerTest  =  TTest<Integer>;


In  difference  with  Mode  Objfpc,  the  template  type  names  must  be  repeated  in  method
definitions.


Type
    TTest<T>  =  Class(TObject)
    Private
       FObj  :  T;
    Public
       Procedure  DoIt;
       Property  Obj  :  T  Read  FObj  Write  FObj;
    end;



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 *___



Procedure  TTest<T>.DoIt;


begin
end;


This requirement is directly related to the generic type overload capability mentioned in the
next section.



8.5.2        Record  type  restrictions

In Delphi mode, the record type restrictions will also allow the use of simple types:


Type
generic  TList<_T  :  record>  =  class(TObject)
    public
       Type  TCompareFunc  =  function(const  Item1,  Item2:  _T):  Integer;
    Public
       data  :  _T;
       procedure  Add(item:  _T);
       procedure  Sort(compare:  TCompareFunc);
  end;


TIntList  =  TList<Integer>;



8.5.3        Type  overloads

Delphi  mode  allows  generic  type  overloads.   This  means  that  it  is  possible  to  declare  the
same generic class with different template type lists.  The following declarations are therefor
possible:


Type
    TTest<T>  =  Class(TObject)
    Private
       FObj  :  T;
    Public
       Property  Obj  :  T  Read  FObj  Write  FObj;
    end;


    TTest<T,S>  =  Class(TObject)
    Private
       FObj1  :  T;
       FObj2  :  S;
    Public
       Property  Obj1  :  T  Read  FObj1  Write  FObj1;
       Property  Obj2  :  S  Read  FObj2  Write  FObj2;
    end;



8.5.4        Name  space  considerations

In Delphi mode, the generics do not interfere with the namespace for variables, this means
that the following will also compile:



                                                                 117

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 *___



Type
   TTest<T>  =  Class(TObject)
   Private
      FObj  :  T;
   Public
      Property  Obj  :  T  Read  FObj  Write  FObj;
   end;


Var
   TTest  :  Integer;


However, this does not work for constants and functions.



8.5.5        Scope  considerations

Delphi mode allows use a generic specialized type without explicitly declaring it.  That means
the following is possible:


var
    t:  TTest<LongInt>;
begin
    t  :=  TTest<LongInt>.Create;
end;


In objfpc mode, the type must be declared specifically.



8.6         Type  compatibility


Whenever a generic class is specialized, this results in a new, distinct type.  These types are
assignment compatible if the same template types are used.

Take the following generic definition:


{$mode  objfpc}
unit  ua;


interface


type
    Generic  TMyClass<T>  =  Class(TObject)
       Procedure  DoSomething(A  :  T;  B  :  INteger);
    end;


Implementation


Procedure  TMyClass.DoSomething(A  :  T;  B  :  Integer);


begin
    //  Some  code.
end;


end.



                                                                 118

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 *___



And the following specializations:


{$mode  objfpc}
unit  ub;


interface


uses  ua;


Type
   TB  =  Specialize  TMyClass<string>;


implementation


end.


the following specializations is identical, but appears in a different unit:


{$mode  objfpc}
unit  uc;


interface


uses  ua;


Type
   TB  =  Specialize  TMyClass<string>;


implementation


end.


The following will then compile:


{$mode  objfpc}
unit  ud;


interface


uses  ua,ub,uc;


Var
   B  :  ub.TB;
   C  :  uc.TB;


implementation


begin
   B:=C;
end.


The types ub.TB and uc.TB are assignment compatible.  It does not matter that the types
are defined in different units.  They could be defined in the same unit as well:


{$mode  objfpc}



                                                                 119

_______________________________________________________________________________________________CHAPTER_8.___GENERICS_______________*
 *___



unit  ue;


interface


uses  ua;


Type
   TB  =  Specialize  TMyClass<string>;
   TC  =  Specialize  TMyClass<string>;



Var
   B  :  TB;
   C  :  TC;


implementation


begin
   B:=C;
end.


Each specialization of a generic class with the same types as parameters is a new, distinct
type,  but  these  types  are  assignment  compatible  if  the  template  types  used  to  specialize
them are equal.

If  the  specialization  is  with  a  different  template  type,  the  types  are  still  distinct,  but  no
longer assignment compatible.  i.e.  the following will not compile:


{$mode  objfpc}
unit  uf;


interface


uses  ua;


Type
   TB  =  Specialize  TMyClass<string>;
   TC  =  Specialize  TMyClass<integer>;



Var
   B  :  TB;
   C  :  TC;


implementation


begin
   B:=C;
end.


When compiling, an error will result:


Error:  Incompatible  types:  got  "TMyClass<System.LongInt>"
                                            expected  "TMyClass<System.ShortString>"



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 *___



8.7         Using  the  default  intrinsic


When writing generic routines, sometimes a variable must be initialized whose type is not
known  during  the  declaration  of  the  generic.  This  is  where  the  Default  intrinsic  (section
4.5 , page 63 ) also comes into play.  Given the following generic declaration:


type
    generic  TTest<T>  =  class
       procedure  Test;
    end;


The following code will correctly initialize the variable myt during specialization:


procedure  TTest.Test;
var
    myt:  T;
begin
    //  will  have  the  correct  Default  if  class  is  specialized
    myt  :=  Default(T);
end;



8.8         A  word  about  scope


It  should  be  stressed  that  all  identifiers  other  than  the  template  placeholders  should  be
known when the generic class is declared.  This works in 2 ways.  First,  all types must be
known,  that  is,  a  type  identifier  with  the  same  name  must  exist.  The  following  unit  will
produce an error:


{$mode  objfpc}
unit  myunit;


interface


type
    Generic  TMyClass<T>  =  Class(TObject)
       Procedure  DoSomething(A  :  T;  B  :  TSomeType);
    end;


Type
    TSomeType  =  Integer;
    TSomeTypeClass  =  specialize  TMyClass<TSomeType>;


Implementation


Procedure  TMyClass.DoSomething(A  :  T;  B  :  TSomeType);


begin
    //  Some  code.
end;


end.


The above code will result in an error, because the type TSomeType is not known when the
declaration is parsed:



                                                                 121

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 *___



home:  >fpc  myunit.pp
myunit.pp(8,47)  Error:  Identifier  not  found  "TSomeType"
myunit.pp(11,1)  Fatal:  There  were  1  errors  compiling  module,  stopping


The second way in which this is visible, is the following.  Assume a unit


{$mode  objfpc}
unit  mya;


interface


type
   Generic  TMyClass<T>  =  Class(TObject)
      Procedure  DoSomething(A  :  T);
   end;



Implementation


Procedure  DoLocalThings;


begin
   Writeln('mya.DoLocalThings');
end;



Procedure  TMyClass.DoSomething(A  :  T);


begin
   DoLocalThings;
end;


end.


The compiler will not allow to compile this unit, since the DoLocalThings function will not
be visible when the generic type is specialized:


Error:  Global  Generic  template  references  static  symtable


Now,  if  the  unit  is  modified,  and  the  DoLocalThings  function  is  moved  to  the  interface
section, the unit will compile.  When using this generic in a program:


{$mode  objfpc}
program  myb;


uses  mya;


procedure  DoLocalThings;


begin
   Writeln('myb.DoLocalThings');
end;


Type



                                                                 122

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 *___



   TB  =  specialize  TMyClass<Integer>;


Var
   B  :  TB;


begin
   B:=TB.Create;
   B.DoSomething(1);
end.


Despite the fact that generics act as a macro which is replayed at specialization time,  the
reference to DoLocalThings is resolved when TMyClass is defined, not when TB is defined.
This means that the output of the program is:


home:  >fpc  -S2  myb.pp
home:  >myb
mya.DoLocalThings


This behaviour is dictated by safety and necessity:


    1.  A programmer specializing a class has no way of knowing which local procedures are
        used, so he cannot accidentally 'override' it.

    2.  A programmer specializing a class has no way of knowing which local procedures are
        used, so he cannot implement it either, since he does not know the parameters.

    3.  If  implementation  procedures  are  used  as  in  the  example  above,  they  cannot  be  ref-
        erenced  from  outside  the  unit.   They  could  be  in  another  unit  altogether,  and  the
        programmer  has  no  way  of  knowing  he  should  include  them  before  specializing  his
        class.



8.9         Operator  overloading  and  generics


Operator  overloading  (chapter  15 ,  page  210  )  and  generics  are  closely  related.   Imagine  a
generic class that has the following definition:


{$mode  objfpc}
unit  mya;


interface


type
    Generic  TMyClass<T>  =  Class(TObject)
       Function  Add(A,B  :  T)  :  T;
    end;


Implementation


Function  TMyClass.Add(A,B  :  T)  :  T;


begin
    Result:=A+B;
end;



                                                                 123

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 *___



end.


When the compiler replays the generics macro, the addition must be possible.  For a special-
ization like this:


TMyIntegerClass  =  specialize  TMyClass<integer>;


This is not a problem, as the Add method would become:


Procedure  TMyIntegerClass.Add(A,B  :  Integer)  :  Integer;


begin
   Result:=A+B;
end;


The compiler knows how to add 2 integers, so this code will compile without problems.  But
the following code:


Type
   TComplex  =  record
    Re,Im  :  Double;
   end;


Type
   TMyIntegerClass  =  specialize  TMyClass<TComplex>;


Will not compile, unless the addition of 2 TComplex types is defined.  This can be done using
record operators:


{$modeswitch  advancedrecords}
uses  mya;


Type
   TComplex  =  record
        Re,Im  :  Double;
        class  operator  +(a,b  :  TComplex)  :  TComplex;
   end;


class  operator  TComplex.+  (a,b  :  TComplex)  :  TComplex;


begin
   Result.re:=A.re+B.re;
   Result.im:=A.im+B.im;
end;



Type
   TMyComplexClass  =  specialize  TMyClass<TComplex>;


begin
   //  Code  here
end.



                                                                 124

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 *___



Currently, due to an implementation restriction, it will not work using a global operator, i.e.
the following does not work yet:


uses  mya;


Type
   TComplex  =  record
        Re,Im  :  Double;
   end;


operator  +  (a,b  :  TComplex)  :  TComplex;


begin
   Result.re:=A.re+B.re;
   Result.im:=A.im+B.im;
end;


Type
   TMyComplexClass  =  specialize  TMyClass<TComplex>;


begin
   //  Code  here
end.


Support for this construct is expected in a future version of Free Pascal.



                                                                 125




Chapter   9



Extended   records



9.1         Definition


Extended records are in many ways equivalent to objects and to a lesser extent to classes:
they  are  records  which  have  methods  associated  with  them,  and  properties.  Like  objects,
when  defined  as  a  variable  they  are  allocated  on  the  stack.   They  do  not  need  to  have  a
constructor.  Extended records have limitations over objects and classes in that they do not
allow  inheritance  and  polymorphism.   It  is  impossible  to  create  a  descendant  record  of  a
record1 .

Why then introduce extended records ?  They were introduced by Delphi 2005 to support
one  of  the  features  introduced  by  .NET.  Delphi  deprecated  the  old  TP  style  of  objects,
and re-introduced the features of .NET as extended records.  Free Pascal aims to be Delphi
compatible, so extended records are allowed in Free Pascal as well, but only in Delphi mode.

If extended records are desired in ObjFPC mode, then a mode switch must be used:


{$mode  objfpc}
{$modeswitch  advancedrecords}


Compatibility  is  not  the  only  reason  for  introducing  extended  records.   There  are  some
practical reasons for using methods or properties in records:


    1.  It  is  more  in  line  with  an  object-oriented  approach  to  programming:  the  type  also
        contains any methods that work on it.

    2.  In contrast with a procedural approach, putting all operations that work on a record
        in the record itself, allows an IDE to show the available methods on the record when
        it is displaying code completion options.


Defining an extended record is much as defining an object or class:


        |______________________________________________________________________________________________________________|
        extended record type



      --  _____|________________|__ record __  __|_ component list __   __|_ end __ __________________________________________-oe
               |_ packed __   _|                 6|________________________|
___________________________________________________1
     although it can be enhanced using record helpers, more about this in the chapter on record helpers.



                                                             126

_____________________________________________________________________________CHAPTER_9.___EXTENDED_RECORDS_________________________*
 *___



     - - ___ component list __    __|____________________________________|____|____________________________|___-
                                    |_ record visibility specifier __  _|     |___  _ field definition _____|
                                                                                  6||______________________|_|
     -  ______|________________________________________|____|________________________________|__ variant part __  ___________-oe
              |___  _ record method definition __    ____|  |___  _ property definition __ ____|
                  6||__________________________________|_|      6||___________________________|_|


     - - ___ field definition __  identifier list __: __ type __ ; _________________________________________________-oe


     - - ___ record visibility specifier __  __ ___ _______________ private _____________________________________________-oe
                                               |   ||_        __ _||             |
                                               |       strict                    |
                                               |_______|protected____  _______|
                                                                      public __ _________|

     - - ___ record method definition __     __|__ function header __   __|__ ; ____|__________________________|___________-oe
                                               |_ procedure header __    _|         |_ call modifiers __   ; ___|

       |_______________________________________________________________________________________________________________|


Some of the restrictions when compared to classes or objects are obvious from the syntax
diagram:


     o No inheritance of records.

     o No published section exists.

     o Constructors or destructors cannot be defined.

     o Class methods (if one can name them so) require the static keyword.

     o Methods cannot be virtual or abstract - this is a consequence of the fact that there is
       no inheritance.


Other than that the definition much resembles that of a class or object.

The following are few examples of valid extended record definitions:


TTest1  =  record
   a  :  integer;
   function  Test(aRecurse:  Boolean):  Integer;
end;


TTest2  =  record
private
   A,b  :  integer;
public
   procedure  setA(AValue  :  integer);
   property  SafeA  :  Integer  Read  A  Write  SetA;
end;


TTest3  =  packed  record
private
   fA,fb  :  byte;
   procedure  setA(AValue  :  Integer);
   function  geta  :  integer;
public
   property  A  :  Integer  Read  GetA  Write  SetA;
end;



                                                                 127

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 *___



TTest4  =  record
 private
    a  :  Integer;
 protected
    function  getp  :  integer;
 public
    b  :  string;
    procedure  setp  (aValue  :  integer);
    property  p  :  integer  read  Getp  Write  SetP;
 public
 case  x  :  integer  of
    1  :  (Q  :  string);
    2  :  (S  :  String);
 end;


Note that it is possible to specify a visibility for the members of the record.  This is partic-
ularly useful for example when creating an interface to a C library:  the actual fields can be
declared hidden,  and more 'pascal' like properties can be exposed which act as the actual
fields.  The TTest3 record definition shows that the packed directive can be used in extended
records.  Extended records have the same memory layout as their regular counterparts:  the
methods and properties are not part of the record structure in memory.

The TTest4 record definition in the above examples shows that the extended record still has
the ability to define a variant part.  As with the regular record, the variant part must come
last.  It cannot contain methods.



9.2         Extended  record  enumerators


Extended records can have an enumerator.  To this end, a function returning an enumerator
record must be defined in the extended record:


type
    TIntArray  =  array[0..3]  of  Integer;


    TEnumerator  =  record
    private
       FIndex:  Integer;
       FArray:  TIntArray;
       function  GetCurrent:  Integer;
    public
       function  MoveNext:  Boolean;
       property  Current:  Integer  read  GetCurrent;
    end;


    TMyArray  =  record
       F:  array[0..3]  of  Integer;
       function  GetEnumerator:  TEnumerator;
    end;


function  TEnumerator.MoveNext:  Boolean;
begin
    inc(FIndex);



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   Result  :=  FIndex  <  Length(FArray);
end;


function  TEnumerator.GetCurrent:  Integer;
begin
   Result  :=  FArray[FIndex];
end;


function  TMyArray.GetEnumerator:  TEnumerator;
begin
   Result.FArray  :=  F;
   Result.FIndex  :=  -1;
end;


After these definitions, the following code will compile and enumerate all elements in F:


var
   Arr:  TMyArray;
   I:  Integer;
begin
   for  I  in  Arr  do
      WriteLn(I);
end.


The same effect can be achieved with the enumerator operator:


type
   TIntArray  =  array[0..3]  of  Integer;


   TEnumerator  =  record
   private
      FIndex:  Integer;
      FArray:  TIntArray;
      function  GetCurrent:  Integer;
   public
      function  MoveNext:  Boolean;
      property  Current:  Integer  read  GetCurrent;
   end;


   TMyArray  =  record
      F:  array[0..3]  of  Integer;
   end;


function  TEnumerator.MoveNext:  Boolean;
begin
   inc(FIndex);
   Result  :=  FIndex  <  Length(FArray);
end;


function  TEnumerator.GetCurrent:  Integer;
begin
   Result  :=  FArray[FIndex];
end;



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operator  Enumerator(const  A:  TMyArray):  TEnumerator;
begin
   Result.FArray  :=  A.F;
   Result.FIndex  :=  -1;
end;


This will allow the code to run as well.



                                                                 130




Chapter   10



Class,   Record   and   Type   helpers



10.1          Definition


Class,  record and type helpers can be used to add methods to an existing class,  record or
simple type, without making a derivation of the class or re-declaring the record.

For a record or a simple type, the type helper acts as if record or the simple type is a class,
and  methods  are  declared  for  it.   Inside  the  methods,  Self  will  refer  to  the  value  of  the
record or simple type.

For  classes,  the  effect  is  like  inserting  a  method  in  the  method  table  of  the  class.   If  the
helper declaration is in the current scope of the code,  then the methods and properties of
the helper can be used as if they were part of the class declaration for the class or record
that the helper extends.

The syntax diagram for a class, record or type helper is presented below:

        |______________________________________________________________________________________________________________|
        Helper type


      --  ____|___ class ______|_ helper __  __|__________________________|__ for __Identifier__|_ helper component list __   -__|

              |_|record____ _|                 |_ ( __basehelper ) ___|                         6|________________________________|
                            type __ ___|
      -  ______ end __  hint modifiers __   ___________________________________________________________________________-oe

      --  ___ helper component list __     __|__ method definition __   ___|____________________________________________-oe
                                             |_ property definition __   _|

        |______________________________________________________________________________________________________________|


The diagram shows that a helper definition looks very much like a regular class definition.
It simply declares some extra constructors,  methods,  properties and fields for a class:  the
class, record or simple type for which the helper is an extension is indicated after the for
keyword.

Since an enumerator for a class is obtained through a regular method, class helpers can also
be used to override the enumerators for the class.

As can be seen from the syntax diagram, it is possible to create descendants of helpers:  the
helpers can form a hierarchy of their own, allowing to override methods of a parent helper.
They also have visibility specifiers, just like records and classes.

As in an instance of the class, the Self identifier in a method of a class helper refers to the
class instance (not the helper instance).  For a record, it refers to the record.



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               The  following  is  a  simple  class  helper  for  the  TObject  class,  which  provides  an  alternate
               version of the standard ToString method.


               TObjectHelper  =  class  helper  for  TObject
                  function  AsString(const  aFormat:  String):  String;
               end;


               function  TObjectHelper.AsString(const  aFormat:  String):  String;
               begin
                  Result  :=  Format(aFormat,  [ToString]);
               end;


               var
                  o:  TObject;
               begin
                  Writeln(o.AsString('The  object''s  name  is  %s'));
               end.


Remark:         The  helper  modifier  is  only  a  modifier  just  after  the  class  or  record  keywords.   That
               means that the first member of a class or record cannot be named helper.  A member of a
               class or record can be called helper, it just cannot be the first one, unless it is escaped with
               a &, as for all identifiers that match a keyword.



               10.2          Restrictions  on  class  helpers


               It is not possible to extend a class with any method or property.  There are some restrictions
               on the possibilities:


                    o  Destructors or class destructors are not allowed.

                    o  Class constructors are not allowed.

                    o  Class helpers cannot descend from record helpers, and cannot extend record types.

                    o  Field definitions are not allowed.  Neither are class fields.

                    o  Properties  that  refer  to  a  field  are  not  allowed.  This  is  in  fact  a  consequence  of  the
                       previous item.

                    o  Abstract methods are not allowed.

                    o  Virtual methods of the class cannot be overridden.  They can be hidden by giving them
                       the same name or they can be overloaded using the overload directive.

                    o  Unlike  for  regular  procedures  or  methods,  the  overload  specifier  must  be  explicitly
                       used when overloading methods from a class in a class helper.  If overload is not used,
                       the extended type's method is hidden by the helper method (as for regular classes).


               The following modifies the previous example by overloading the ToString method:


               TObjectHelper  =  class  helper  for  TObject
                   function  ToString(const  aFormat:  String):  String;  overload;
               end;


               function  TObjectHelper.ToString(const  aFormat:  String):  String;



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begin
   Result  :=  Format(aFormat,  [ToString]);
end;


var
   o:  TObject;
begin
   Writeln(o.ToString('The  object''s  name  is  %s'));
end.



10.3          Restrictions  on  record  helpers


Records  do  not  offer  the  same  possibilities  as  classes  do.  This  reflects  on  the  possibilities
when creating record helpers.  Below the restrictions on record helpers are enumerated:


     o  A record helper cannot be used to extend a class.  The following will fail:


        TTestHelper  =  record  helper  for  TObject
        end;


     o  Inside a helper's declaration the methods/fields of the extended record can't be accessed
        in e.g.  a property definition.  They can be accessed in the implementation, of course.
        This means that the following will not compile:


        TTest  =  record
           Test:  Integer;
        end;


        TTestHelper  =  record  helper  for  TTest
           property  AccessTest:  Integer  read  Test;
        end;


     o  Record helpers can only access public fields (in case an extended record with visibility
        specifiers is used).

     o  Inheritance of record helpers is only allowed in ObjFPC mode; In Delphi mode, it is
        not allowed.

     o  Record helpers can only descend from other record helpers, not from class helpers.

     o  Unlike class helpers, a descendent record helper must extend the same record type.

     o  In Delphi mode, it is not possible to call the extended record's method using inherited.
        It is possible to do so in ObjFPC mode.  The following code needs ObjFPC mode to
        compile:


        type
           TTest  =  record
               function  Test(aRecurse:  Boolean):  Integer;
           end;


           TTestHelper  =  record  helper  for  TTest
               function  Test(aRecurse:  Boolean):  Integer;
           end;



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       function  TTest.Test(aRecurse:  Boolean):  Integer;
       begin
           Result  :=  1;
       end;


       function  TTestHelper.Test(aRecurse:  Boolean):  Integer;
       begin
           if  aRecurse  then
              Result  :=  inherited  Test(False)
           else
              Result  :=  2;
       end;



10.4          Considerations  for  (simple)  type  helpers


For  simple  types,  the  rules  are  pretty  much  the  same  as  for  records,  plus  there  are  some
extra requirements:


     o  Support for type helpers needs to be activated using the modeswitch typehelpers:


        {$modeswitch  typehelpers}


        This modeswitch is enabled by default only in mode Delphi and DelphiUnicode.

     o  In  Delphi  (and  DelphiUnicode)  mode,  for  stricter  Delphi  compatibility,  the  record
        helpers must be used instead of a type helper.

     o  The  modes  ObjFPC  and  MacPas  use  type  helper,  but  the  modeswitch  TypeHelpers
        must be used.

     o  The following types are not supported:

            -  All file types (Text, file  of  ...)

            -  Procedural variables

            -  Types like records, classes, Objective C classes, C++ classes, objects and inter-
               faces are forbidden as well, the class helper must be used for classes.  That means
               that for instance, the following will fail:

               TTestHelper  =  type  helper  for  TObject
               end;

        This of course means that all other simple types are supported.

     o  Type helpers can implement constructors.

     o  Inheritance of record helpers is only allowed in ObjFPC mode; In Delphi mode, it is
        not allowed.

     o  Type helpers can only descend from other type helpers, not from class or record helpers.

     o  A descendent type helper must extend the same type.


The following gives an idea of the possibilities:



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 *___



{$mode  objpas}
{$modeswitch  typehelpers}


type
 TLongIntHelper  =  type  helper  for  LongInt
    constructor  create(AValue:  LongInt);
    class  procedure  Test;  static;
    procedure  DoPrint;
 end;


constructor  TLongIntHelper.create(AValue:  LongInt);


begin
   Self:=Avalue;
   DoPrint;
end;


class  procedure  TLongIntHelper.Test;


begin
    Writeln('Test');
end;


procedure  TLongIntHelper.DoPrint;


begin
    Writeln('Value  :',Self);
end;


var
   i:  LongInt;
begin
   I:=123;
   i.Test;
   $12345678.Test;
   LongInt.Test;
   I:=123;
   i.DoPrint;
   $12345678.DoPrint;
end.



10.5          A  note  on  scope  and  lifetime  for  record  and  type

              helpers


For classes, the lifetime of an instance of a class is explicitly managed by the programmer.
It is therefor clear what the Self parameter means and when it is valid.

Records and other simple types are allocated on the stack, which means they go out of scope
when the function, procedure or method in which they are used, exits.

Combined with the fact that helper methods are type compatible to class methods, and can
therefor be used as event handlers, this can lead to surprising situations:  The data pointer
in a helper method is set to the address of the variable.



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Consider the following example:


{$mode  objfpc}
{$modeswitch  typehelpers}
uses
   Classes;


type
   TInt32Helper  =  type  helper  for  Int32
      procedure  Foo(Sender:  TObject);
   end;


procedure  TInt32Helper.Foo(Sender:  TObject);
begin
   Writeln(Self);
end;


var
   i:  Int32  =  10;
   m:  TNotifyEvent;
begin
   m  :=  @i.Foo;
   WriteLn('Data  :  ',PtrUInt(TMethod(m).Data));
   m(nil);
end.


This will print something like (the actual value for data may differ):


Data  :  6848896
10


The variable i is still in scope when m is called.

But changing the code to


{$mode  objfpc}
{$modeswitch  typehelpers}
uses
   Classes;


type
   TInt32Helper  =  type  helper  for  Int32
      procedure  Foo(Sender:  TObject);
   end;


procedure  TInt32Helper.Foo(Sender:  TObject);
begin
   Writeln(Self);
end;


Function  GetHandler    :TNotifyEvent;


var
   i:  Int32  =  10;



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 *__________________



               begin
                  Result:=@i.foo;
               end;


               Var
                  m:  TNotifyEvent;
               begin
                  m  :=  GetHandler;
                  WriteLn(PtrUInt(TMethod(m).Data));
                  m(nil);
               end.

               The output will be:

               140727246638796
               0

               The  actual  output  will  depend  on  the  architecture,  but  the  point  is  that  i  is  no  longer
               in scope,  making the output of it's value meaningless,  and possibly even leading to access
               violations and program crashes.



               10.6          Inheritance


               As noted in the previous section, it is possible to create descendants of helper classes.  Since
               only the last helper class in the current scope can be used, it is necessary to descend a helper
               class from another one if methods of both helpers must be used.  More on this in a subsequent
               section.

               A descendent of a class helper can extend a different class than its parent.  The following is
               a valid class helper for TMyObject:

               TObjectHelper  =  class  helper  for  TObject
                   procedure  SomeMethod;
               end;


               TMyObject  =  class(TObject)
               end;


               TMyObjectHelper  =  class  helper(TObjectHelper)  for  TMyObject
                   procedure  SomeOtherMethod;
               end;

               The TMyObjectHelper extends TObjectHelper, but does not extend the TObject class, it
               only extends the TMyObject class.

               Since records know no inheritance, it is obvious that descendants of record helpers can only
               extend the same record.

Remark:         For maximum delphi compatibility, it is impossible to create descendants of record helpers
               in Delphi mode.



               10.7          Usage


               Once a helper class is defined, its methods can be used whenever the helper class is in scope.
               This means that if it is defined in a separate unit, then this unit should be in the uses clause



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wherever the methods of the helper class are used.

Consider the following unit:


{$mode  objfpc}
{$h+}
unit  oha;


interface


Type
   TObjectHelper  =  class  helper  for  TObject
      function  AsString(const  aFormat:  String):  String;
   end;


implementation


uses  sysutils;


function  TObjectHelper.AsString(const  aFormat:  String):  String;


begin
   Result  :=  Format(aFormat,  [ToString]);
end;


end.


Then the following will compile:


Program  Example113;


uses  oha;


{  Program  to  demonstrate  the  class  helper  scope.  }


Var
   o  :  TObject;


begin
   O:=TObject.Create;
   Writeln(O.AsString('O  as  a  string  :  %s'));
end.


But, if a second unit (ohb) is created:


{$mode  objfpc}
{$h+}
unit  ohb;


interface


Type
   TAObjectHelper  =  class  helper  for  TObject
      function  MemoryLocation:  String;
   end;



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 *___



implementation


uses  sysutils;


function  TAObjectHelper.MemoryLocation:  String;


begin
   Result  :=  format('%p',[pointer(Self)]);
end;


end.


And is added after the first unit in the uses clause:


Program  Example113;


uses  oha,ohb;


{  Program  to  demonstrate  the  class  helper  scope.  }


Var
   o  :  TObject;


begin
   O:=TObject.Create;
   Writeln(O.AsString('O  as  a  string  :  %s'));
   Writeln(O.MemoryLocation);
end.


Then the compiler will complain that it does not know the method 'AsString'.  This is because
the compiler stops looking for class helpers as soon as the first class helper is encountered.
Since the ohb unit comes last in the uses clause, the compiler will only use TAObjectHelper
as the class helper.

The solution is to re-implement unit ohb:


{$mode  objfpc}
{$h+}
unit  ohc;


interface


uses  oha;


Type
   TAObjectHelper  =  class  helper(TObjectHelper)  for  TObject
      function  MemoryLocation:  String;
   end;


implementation


uses  sysutils;


function  TAObjectHelper.MemoryLocation:  String;



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 *___



begin
   Result  :=  format('%p',[pointer(Self)]);
end;


end.


And after replacing unit ohb with ohc,  the example program will compile and function as
expected.

Note that it is not enough to include a unit with a class helper once in a project; The unit
must be included whenever the class helper is needed.



                                                                 140




Chapter   11



Ob jective-Pascal   Classes



11.1          Introduction


The preferred programming language to access Mac OS X system frameworks is Objective-C.
In order to fully realize the potential offered by system interfaces written in that language,
a  variant  of  Object  Pascal  exists  in  the  Free  Pascal  compiler  that  tries  to  offer  the  same
functionality as Objective-C. This variant is called Objective-Pascal.

The compiler has mode switches to enable the use of these Objective-C-related constructs.
There are 2 kinds of Objective-C language features, discerned by a version number: Objective-
C 1.0 and Objective-C 2.0.

The Objective-C 1.0 language features can be enabled by adding a modeswitch to the source
file:


{$modeswitch  objectivec1}


or by using the -Mobjectivec1 command line switch of the compiler.

The Objective-C 2.0 language features can be enabled using a similar modewitch:


{$modeswitch  objectivec2}


or the command-line option -Mobjectivec2.

The Objective-C 2.0 language features are a superset of the Objective-C 1.0 language fea-
tures,  and  therefor  the  latter  switch  automatically  implies  the  former.   Programs  using
Objective-C 2.0 language features will only work on Mac OS X 10.5 and later.

The fact that objective-C features are enabled using mode switches rather than actual syntax
modes, means they can be used in combination with every general syntax mode (fpc, objfpc,
tp, delphi, macpas).  Note that a {$Mode  } directive switch will reset the mode switches, so
the {$modeswitch  } statement should be located after it.



11.2          Ob jective-Pascal  class  declarations


Objective-C or -Pascal classes are declared much as Object Pascal classes are declared, but
they use the objcclass keyword:

        |______________________________________________________________________________________________________________|
        Objective C Class types



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 *___



     - - ___ objcclass __  __|______________________________________________________|___-
                             |_ external __  __|____________________________________|__|
                                               |_ name __    string constant __  _|
     -  ______|____________________________________________________________|_____________________________________________-oe
              |_heritage__|______________________________________|_ end __ _|
                          |___  ______ component list __    ________|
                              6||_ class visibility specifier __ _||


     - - _ heritage __  ( ____|____________________________________________|____|__________________________________|__ ) ______-oe
                              |_ objective-Cclass type identifier __    _|      |_ implemented protocols __      _|


     - - ___ implemented protocols __      __  _ protocol identifier __ ______________________________________________-oe
                                             6||____________ , ___________|__|


     - - ___ component list __    __|__________________________|____|____________________________|___-
                                    |_ visibility specifier ___|    |___  _ field definition _____|
                                                                        6||______________________|_|
     -  ______|__________________________________________________|______________________________________________________-oe
              |___|___ _ class variable declaration part __    ___|__|
                  6|  |_________ method definition __    _______|_|
                  |   ||________                     __   ________|||
                  ||_____________property_definition______________|_________|


     - - ___ class variable declaration part __     class __  variable declaration part __    _______________________-oe


     - - ___ field definition __  identifier list __: __ type __ ; ____|______________|_________________________________-oe
                                                                       |_ static; __ _|


     - - ___ method definition __    __ ___ _______________ __ function header __  ______ ; ___-
                                       |   ||_       __ |_|||_                  __    |_||
                                       ||______class___________procedure_header_________|_____________|

     -  ______|________________________________________________|_________________________________________________________-oe
              |___|______________________________________|_ ; ___|

                  |_____________|override___  _____________|
                                           message __     string constant __  _|

     - - ___ class visibility specifier __ __|______________________________|____________________________________________-oe
                                             |___ _______________ private __  _|
                                             |   ||_        __ _||             |
                                             |       strict                    |
                                             |_______ protected __   _______|
                                             |_________|public_____________|_________________|


       |_______________________________________________________________________________________________________________|


As can be seen, the syntax is roughly equivalent to Object Pascal syntax, with some exten-
sions.

In order to use Objective-C classes, an external modifier exists:  this indicates to the compiler
that the class is implemented in an external object file or library, and that the definition is
meant  for  import  purposes.  The  following  is  an  example  of  an  external  Objective-C  class
definition:


NSView  =  objcclass  external(NSResponder)
private
   _subview    :  id;
public
   function  initWithFrame(rect  :  NSRect):  id;



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        message  'initWithFrame:';
   procedure  addSubview(aview:  NSView);
        message  'addSubview:';
   procedure  setAutoresizingMask(mask:  NSUInteger);
        message  'setAutoresizingMask:';
   procedure  setAutoresizesSubviews(flag:  LongBool);
        message  'setAutoresizesSubviews:';
   procedure  drawRect(dirtyRect:  NSRect);
        message  'drawRect:';
end;


As can be seen, the class definition is not so different from an Object Pascal class definition;
Only  the  message  directive  is  more  prominently  present:  each  Objective-C  or  Objective-
Pascal  method  must  have  a  message  name  associated  with  it.   In  the  above  example,  no
external  name  was  specified  for  the  class  definition,  meaning  that  the  Pascal  identifier  is
used as the name for the Objective-C class.  However, since Objective-C is not so strict in
its naming conventions, sometimes an alias must be created for an Objective-C class name
that doesn't obey the Pascal identifier rules.

The following example defines an Objective-C class which is implemented in Pascal:


MyView  =  objcclass(NSView)
public
   data  :  Integer;
   procedure  customMessage(dirtyRect:  NSRect);
      message  'customMessage';
   procedure  drawRect(dirtyRect:  NSRect);  override;
end;


The absence of the external keyword tells the compiler that the methods must be imple-
mented later in the source file:  it will be treated much like a regular object pascal class.  Note
the presence of the override directive:  in Objective-C, all methods are virtual.  In Object
Pascal, overriding a virtual method must be done through the override directive.  This has
been extended to Objective-C classes:  it allows the compiler to verify the correctness of the
definition.

Unless the class is implementing the method of a protocol (more about this in a subsequent
section), one of message or override is expected:  all methods are virtual, and either a new
method  is  started  (or  re-introduced),  or  an  existing  is  overridden.   Only  in  the  case  of  a
method that is part of a protocol, the method can be defined without message or override.

Note  that  the  Objective-C  class  declaration  may  or  may  not  specify  a  parent  class.   In
Object Pascal, omitting a parent class will automatically make the new class a descendant
of  TObject.  In  Objective-C,  this  is  not  the  case:  the  new  class  will  be  a  new  root  class.
However,  Objective-C  does  have  a  class  which  fullfills  the  function  of  generic  root  class:
NSObject, which can be considered the equivalent of TObject in Object Pascal.  It has other
root  classes,  but  in  general,  Objective-Pascal  classes  should  descend  from  NSObject.  If  a
new  root  class  is  constructed  anyway,  it  must  implement  the  NSObjectProtocol  -  just  as
the NSObject class itself does.

Finally, objective-Pascal classes can have properties, but these properties are only usable in
Pascal code:  the compiler currently does not export the properties in a way that makes them
usable from Objective-C.



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11.3          Formal  declaration


Object Pascal has the concept of Forward declarations.  Objective-C takes this concept a bit
further:  it allows to declare a class which is defined in another unit.  This has been dubbed
'Formal declaration' in Objective-Pascal.  Looking at the syntax diagram, the following is a
valid declaration:


MyExternalClass  =  objcclass  external;


This is a formal declaration.  It tells the compiler that MyExternalClass is an Objective-C
class type, but that there is no declaration of the class members.  The type can be used in
the remainder of the unit, but its use is restricted to storage allocation (in a field or method
parameter definition) and assignment (much like a pointer).

As soon as the class definition is encountered, the compiler can enforce type compatibility.

The following unit uses a formal declaration:


unit  ContainerClass;


{$mode  objfpc}
{$modeswitch  objectivec1}


interface


type
    MyItemClass  =  objcclass  external;


    MyContainerClass  =  objcclass
       private
        item:  MyItemClass;
       public
        function  getItem:  MyItemClass;  message  'getItem';
    end;


implementation


function  MyContainerClass.getItem:  MyItemClass;
begin
    result:=item;  //  Assignment  is  OK.
end;


end.


A second unit can contain the actual class declaration:


unit  ItemClass;


{$mode  objfpc}
{$modeswitch  objectivec1}


interface


type
    MyItemClass  =  objcclass(NSObject)



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   private
      content  :  longint;
   public
      function  initWithContent(c:  longint):  MyItemClass;
           message  'initWithContent:';
        function  getContent:  longint;
           message  'getContent';
   end;


implementation


function  MyItemClass.initWithContent(c:  longint):
    MyItemClass;
begin
   content:=c;
   result:=self;
end;


function  MyItemClass.getContent:  longint;
begin
   result:=content;
end;


end.


If both units are used in a program, the compiler knows what the class is and can verify the
correctness of some assignments:


Program  test;


{$mode  objfpc}
{$modeswitch  objectivec1}


uses
   ItemClass,  ContainerClass;


var
   c:  MyContainerClass;
   l:  longint;
begin
   c:=MyContainerClass.alloc.init;
   l:=c.getItem.getContent;
end.



11.4          Allocating  and  de-allocating  Instances


The syntax diagram of Objective-C classes shows that the notion of constructor and destruc-
tor is not supported in Objective-C. New instances are created in a 2-step process:


    1.  Call the 'alloc' method (send an 'alloc' message):  This is a class method of NSObject,
        and returns a pointer to memory for the new instance.  The use of alloc is a convention
        in Objective-C.



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    2.  Send  an  'initXXX'  message.  By  convention,  all  classes  have  one  or  more  'InitXXX'
        methods  that  initializes  all  fields  in  the  instance.  This  method  will  return  the  final
        instance pointer, which may be Nil.


The following code demonstrates this:


var
    obj:  NSObject;
begin
    //  First  allocate  the  memory.
    obj:=NSObject.alloc;
    //  Next,  initialise.
    obj:=obj.init;
    //  Always  check  the  result  !!
    if  (Obj=Nil)  then
       //  Some  error;


By convention, the initXXX method will return Nil if initialization of some fields failed, so
it is imperative that the result of the function is tested.

Similarly,  no  privileged  destructor  exists;  By  convention,  the  dealloc  method  fullfills  the
cleanup of the instances.  This method can be overridden to perform any cleanup necessary.
Like  Destroy,  it  should  never  be  called  directly,  instead,  the  release  method  should  be
called  instead:  All  instances  in  Objective-C  are  reference  counted,  and  release  will  only
call dealloc if the reference count reaches zero.



11.5          Protocol  definitions


In Objective-C, protocols play the role that interfaces play in Object Pascal, but there are
some differences:


     o  Protocol methods can be marked optional, i.e.  the class implementing the protocol can
        decide not to implement these methods.

     o  Protocols can inherit from multiple other protocols.


Objective-C classes can indicate which protocols they implement in the class definition, as
could be seen in the syntax diagram for Objective-C classes.

The  following  diagram  shows  how  to  declare  a  protocol.  It  starts  with  the  objcprotocol
keyword:

        |______________________________________________________________________________________________________________|
        Protocol type


      --  ___ objcprotocol __    __|______________________________________________________|____|____________|___-
                                   |_ external __  __|____________________________________|__| |_heritage_|
                                                     |_ name __    string constant __  _|
      -  ______|_ protocol method list __    _|__ end __ ________________________________________________________________-oe

      --  ___ heritage __  ( ____  _ protocol type identifier __  ___ ) ________________________________________________-oe
                                 6||_______________ , ______________|__|


      --  ___ protocol method list __    __|___ _____________________|_ method definition _______|__|_________________-oe
                                           6|  |_ required __  _  |                                 |
                                           |   |||_          __ |_||                                |
                                           ||_______optional___________________________________|    |



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 *___



       |_______________________________________________________________________________________________________________|


As in the case of objective-Pascal classes, the external specifier tells the compiler that the
declaration  is  an  import  of  a  protocol  defined  elsewhere.   For  methods,  almost  the  same
rules apply as for methods in the Objective-Pascal class declarations.  The exception is that
message specifiers must be present.

The required and optional specifiers before a series of method declarations are optional.
If none is specified, required is assumed.  The following is a definition of a protocol:


type
   MyProtocol  =  objccprotocol
      //  default  is  required
      procedure  aRequiredMethod;
         message  'aRequiredMethod';
   optional
      procedure  anOptionalMethodWithPara(para:  longint);
         message  'anOptionalMethodWithPara:';
      procedure  anotherOptionalMethod;
         message  'anotherOptionalMethod';
   required
      function  aSecondRequiredMethod:  longint;
         message  'aSecondRequiredMethod';
   end;


   MyClassImplementingProtocol  =  objcclass(NSObject,MyProtocol)
      procedure  aRequiredMethod;
      procedure  anOptionalMethodWithPara(para:  longint);
      function  aSecondRequiredMethod:  longint;
   end;


Note  that  in  the  class  declaration,  the  message  specifier  was  omitted.  The  compiler  (and
runtime) can deduce it from the protocol definition.



11.6          Categories


Similar to class helpers in Object Pascal, Objective-C has Categories.  Categories allow to
extend classes without actually creating a descendant of these classes.  However, Objective-C
categories provide more functionality than a class helper:


    1.  In Object Pascal, only 1 helper class can be in scope (the last one).  In Objective-C,
        multiple categories can be in scope at the same time for a particular class.

    2.  In Object Pascal,  a helper method cannot change an existing method present in the
        original class (but it can hide a method).  In Objective-C, a category can also replace
        existing methods in another class rather than only add new ones.  Since all methods
        are virtual in Objective-C, this also means that this method changes for all classes that
        inherit from the class in which the method was replaced (unless they override it).

    3.  Object Pascal helpers cannot be used to add interfaces to existing classes.  By contrast,
        an Objective-C category can also implement protocols.


The definition of an objective-C class closely resembles a protocol definition, and is started
with the objccategory keyword:



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 *___



       |_______________________________________________________________________________________________________________|
       Category type



     - - ___ objccategory __    __|______________________________________________________|____|____________|___-
                                  |_ external __  __|____________________________________|__| |_heritage_|
                                                    |_ name __    string constant __  _|
     -  ______|_ category method list __    _|__ end __ ________________________________________________________________-oe


     - - ___ heritage __  ( __ Objective Class type identifier __     __  _ protocol type identifier __  ___ ) ________-oe
                                                                        6||_______________ , ______________|__|


     - - ___ category method list __    __  ___|_ method definition __    __ ________________________|_______________________-oe
                                          6||                               ||_ reintroduce __    _||   ||
                                          |______________________________________________________|


       |_______________________________________________________________________________________________________________|


Note again the possibility of an alias for externally defined categories:  objective-C 2.0 allows
an empty category name.  Note that the reintroduce modifier must be used if an existing
method is being replaced rather than that a new method is being added.

When replacing a method, calling 'inherited' will not call the original method of the class,
but instead will call the parent class' implementation of the method.

The following is an example of a category definition:


MyProtocol  =  objcprotocol
   procedure  protocolmethod;  message  'protocolmethod';
end;


MyCategory  =  objccategory(NSObject,MyProtocol)
   function  hash:  cuint;  reintroduce;
   procedure  protocolmethod;  //  from  MyProtocol.
   class  procedure  newmethod;  message  'newmethod';
end;


Note  that  this  declaration  replaces  the  Hash  method  of  every  class  that  descends  from
NSObject (unless it specifically overrides it).



11.7          Name  scope  and  Identifiers


In Object Pascal, each identifier must be unique in it's namespace:  the unit.  In Objective-C,
this need not be the case and each type identifier must be unique among its kind:  classes,
protocols, categories, fields or methods.  This is shown in the definitions of the basic protocol
and class of Objective-C: Both protocol and class are called NSObject.

When  importing  Objective-C  classes  and  protocols,  the  Objective-Pascal  names  of  these
types  must  conform  to  the  Object  Pascal  rules,  and  therefor  must  have  distinct  names.
Likewise, names that are valid identifiers in Objective-C may be reserved words in Object
Pascal.  They also must be renamed when imported.

To make this possible, the External and 'message' modifiers allow to specify a name:  this
is the name of the type or method as it exists in Objective-C:


NSObjectProtocol  =  objcprotocol  external  name  'NSObject'
    function  _class:  pobjc_class;  message  name  'class';



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 *___



end;


NSObject  =  objcclass  external  (NSObjectProtocol)
   function  _class:  pobjc_class;
   class  function  classClass:  pobjc_class;  message  'class';
end;



11.8          Selectors


A Selector in Objective-C can be seen as an equivalent to a procedural type in Object Pascal.

In difference with the procedural type, Objective-C has only 1 selector type:  SEL. It is defined
in the objc unit - which is automatically included in the uses clause of any unit compiled
with the objectivec1 modeswitch.

To assign a value to a variable of type SEL, the objcselector method must be used:


{$modeswitch  objectivec1}
var
    a:  SEL;
begin
    a:=objcselector('initiWithWidth:andHeight:');
    a:=objcselector('myMethod');
end.


The objc unit contains methods to manipulate and use the selector.



11.9          The  id  type


The  id  type  is  special  in  Objective-C/Pascal.  It  is  much  like  the  pointer  type  in  Object
Pascal,  except  that  it  is  a  real  class.  It  is  assignment-compatible  with  instances  of  every
objcclass and objcprotocol type, in two directions:


    1.  variables of any  objcclass/objcprotocol type can be assigned to a variable of the
        type id.

    2.  variables of type id can be assigned to variables of any particular objcclass/objcprotocol
        type.


No explicit typecast is required for either of these assignments.

Additionally, any Objective-C method declared in an objcclass or objccategory that is in
scope can be called when using an id-typed variable.

If,  at  run  time,  the  actual  objcclass  instance  stored  in  the  id-typed  variable  does  not
respond to the sent message, the program will terminate with a run time error:  much like
the dispatch mechanism for variants under MS-Windows.

When there are multiple methods with the same Pascal identifier, the compiler will use the
standard  overload  resolution  logic  to  pick  the  most  appropriate  method.   In  this  process,
it  will  behave  as  if  all  objcclass/objccategory  methods  in  scope  have  been  declared  as
global procedures/functions with the overload specifier.  Likewise,  the compiler will print
an error if it cannot determine which overloaded method to call.

In such cases, a list of all methods that could be used to implement the call will be printed
as a hint.



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To resolve the error, an explicit type cast must be used to tell the compiler which objcclass
type contains the needed method.



11.10            Enumeration  in  Ob jective-C  classes


Fast enumeration in Objective-C is a construct which allows to enumerate the elements in a
Cocoa container class in a generic way.  It is implemented using a for-in loop in Objective-C.

This  has  been  translated  to  Objective-Pascal  using  the  existing  for-in  loop  mechanism.
Therefor,  the  feature  behaves  identically  in  both  languages.    Note  that  it  requires  the
Objective-C 2.0 mode switch to be activated.

The following is an example of the use of for-in:


{$mode  delphi}
{$modeswitch  objectivec2}


uses
    CocoaAll;


var
    arr:  NSMutableArray;
    element:  NSString;
    pool:  NSAutoreleasePool;
    i:  longint;
begin
    pool:=NSAutoreleasePool.alloc.init;
    arr:=NSMutableArray.arrayWithObjects(
       NSSTR('One'),
       NSSTR('Two'),
       NSSTR('Three'),
       NSSTR('Four'),
       NSSTR('Five'),
       NSSTR('Six'),
       NSSTR('Seven'),
       nil);


    i:=0;
    for  element  in  arr  do
       begin
          inc(i);
          if  i=2  then
             continue;
          if  i=5  then
             break;
          if  i  in  [2,5..10]  then
             halt(1);
          NSLog(NSSTR('element:  %@'),element);
       end;
    pool.release;
end.



                                                                 150




               Chapter   12



               Expressions



               Expressions  occur  in  assignments  or  in  tests.   Expressions  produce  a  value  of  a  certain
               type.  Expressions are built with two components:  operators and their operands.  Usually an
               operator is binary, i.e.  it requires 2 operands.  Binary operators occur always between the
               operands (as in X/Y). Sometimes an operator is unary, i.e.  it requires only one argument.  A
               unary operator occurs always before the operand, as in -X.

               When using multiple operands in an expression, the precedence rules of table (12.1  ) are used.



                                                      Table 12.1:  Precedence of operators


                     __Operator_________________________________________________Precedence_____________Category____________________*
 *____
                       Not,  @                                                  Highest (first)        Unary operators
                       *  /  div  mod  and  shl  shr  as  <<  >>                Second                 Multiplying operators
                       +  -  or  xor                                            Third                  Adding operators
                     __=__<>__<__>__<=__>=__in__is______________________________Lowest_(Last)__________relational_operators________*
 *____



               When determining the precedence, the compiler uses the following rules:


                   1.  In operations with unequal precedences the operands belong to the operator with the
                       highest precedence.  For example, in 5*3+7, the multiplication is higher in precedence
                       than the addition, so it is executed first.  The result would be 22.

                   2.  If parentheses are used in an expression, their contents is evaluated first.  Thus, 5*(3+7)
                       would result in 50.


Remark:         The order in which expressions of the same precedence are evaluated is not guaranteed to
               be left-to-right.  In general, no assumptions on which expression is evaluated first should be
               made in such a case.  The compiler will decide which expression to evaluate first based on
               optimization rules.  Thus, in the following expression:


                   a  :=  g(3)  +  f(2);


               f(2)  may  be  executed  before  g(3).  This  behaviour  is  distinctly  different  from  Delphi  or
               Turbo Pascal.

               If one expression must be executed before the other, it is necessary to split up the statement
               using temporary results:



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               ________________________________________________________________________________________CHAPTER_12.___EXPRESSIONS___*
 *__________________



                  e1  :=  g(3);
                  a    :=  e1  +  f(2);


Remark:         The exponentiation operator (**) is available for overloading, but is not defined on any of
               the standard Pascal types (floats and/or integers).



               12.1          Expression  syntax


               An expression applies relational operators to simple expressions.  Simple expressions are a
               series of terms (what a term is, is explained below), joined by adding operators.


                       |___________________________________________________________________________________________________________*
 *___|
                       Expressions



                     --  ___ expression __   simple expression __   __|______________________________________|_____________________*
 *______-oe
                                                                      |___|__  *  __|_ simple expression __   _|

                                                                          |_  <=  _|
                                                                          |__  >  __|
                                                                          |_  >=  _|
                                                                          |__  =  __|
                                                                          |_  <>  _|
                                                                          |_|_in  _|
                                                                                is  _|

                     --  ___ simple expression __   __  ___ term __ _______________________________________________________________*
 *_____-oe
                                                      6||___|__ + __ __||__|

                                                            |___ - _____|
                                                            |__|or_____|
                                                                    xor __ _|

                       |___________________________________________________________________________________________________________*
 *___|


               The following are valid expressions:


               GraphResult<>grError
               (DoItToday=Yes)  and  (DoItTomorrow=No);
               Day  in  Weekend


               And here are some simple expressions:


               A  +  B
               -Pi
               ToBe  or  NotToBe


               Terms consist of factors, connected by multiplication operators.


                       |___________________________________________________________________________________________________________*
 *___|
                       Terms



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 *___



     - - ___ term __ __  ___ factor ___________________________________________________________________________________-oe
                       6||___|____  *  ___|_|__|

                             |____  /  ____|
                             |__ div __ __|
                             |_ mod __   _|
                             |__ and __  __|
                             |___ shl _____|
                             |__|shr______|
                                         as __ ___|

       |_______________________________________________________________________________________________________________|


Here are some valid terms:


2  *  Pi
A  Div  B
(DoItToday=Yes)  and  (DoItTomorrow=No);


Factors are all other constructions:


       |_______________________________________________________________________________________________________________|
       Factors



     - - ___ factor __ __|_ ( __ expression __   ) ___|__________________________________________________________________-oe

                         |__ variable reference __   __|
                         |_____ function call __  _____|
                         ||_ unsigned constant __     _||
                         |_____ not __  factor __ _____|
                         |_____ sign __  factor _______|
                         |___ set constructor __   ___|
                         |____|value_typecast___   ____|
                                               address factor __   ____|

     - - ___ unsigned constant __     __|__ unsigned number __     __|___________________________________________________-oe

                                        |___ character string __   ___|
                                        |_|constant_identifier___   _|
                                                                Nil __ __________|

       |_______________________________________________________________________________________________________________|



12.2          Function  calls


Function calls are part of expressions (although, using extended syntax, they can be state-
ments too).  They are constructed as follows:


        |______________________________________________________________________________________________________________|
        Function calls



      --  ___ function call __  __|________ function identifier __  ________|____|________________________________|_____________-oe

                                  |_______ method designator __     _______|     |_ actual parameter list __    _|
                                  |_|qualified_method_designator __      _|
                                                       variable reference __   ________|

      --  ___ actual parameter list __     ( ____|______________________|__ ) ____________________________________________-oe
                                                 |___  _ expression ______|
                                                     6||_______ , _______|_|



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 *___



       |_______________________________________________________________________________________________________________|


The  variable reference          must be a procedural type variable reference.  A method designator
can only be used inside the method of an object.  A qualified method designator can be used
outside object methods too.  The function that will get called is the function with a declared
parameter list that matches the actual parameter list.  This means that


    1.  The number of actual parameters must equal the number of declared parameters (unless
        default parameter values are used).

    2.  The types of the parameters must be compatible.  For variable reference parameters,
        the parameter types must be exactly the same.


If  no  matching  function  is  found,  then  the  compiler  will  generate  an  error.   Which  error
depends - among other things - on whether the function is overloaded or not:  i.e.  multiple
functions with the same name, but different parameter lists.

There are cases when the compiler will not execute the function call in an expression.  This
is the case when assigning a value to a procedural type variable, as in the following example
in Delphi or Turbo Pascal mode:


Type
    FuncType  =  Function:  Integer;
Var  A  :  Integer;
Function  AddOne  :  Integer;
begin
    A  :=  A+1;
    AddOne  :=  A;
end;
Var  F  :  FuncType;
       N  :  Integer;
begin
    A  :=  0;
    F  :=  AddOne;  {  Assign  AddOne  to  F,  Don't  call  AddOne}
    N  :=  AddOne;  {  N  :=  1  !!}
end.


In  the  above  listing,  the  assignment  to  F  will  not  cause  the  function  AddOne  to  be  called.
The assignment to N, however, will call AddOne.

Sometimes, the call is desired, for instance in recursion in that case, the call must be forced.
This can be done by adding the parenthesis to the function name:


function  rd  :  char;


var
    c  :  char;


begin
    read(c);
    if  (c='\')  then
       c:=rd();
    rd:=c;
end;


var  ch  :  char;



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 *___



begin
    ch:=rd;
    writeln(ch);
end.


The above will read a character and print it.  If the input is a backslash, a second character
is read.

A problem with this syntax is the following construction:


If  F  =  AddOne  Then
   DoSomethingHorrible;


Should the compiler compare the addresses of F and AddOne, or should it call both functions,
and compare the result?  In fpc and objfpc mode this is solved by considering a procedural
variable as equivalent to a pointer.  Thus the compiler will give a type mismatch error, since
AddOne is considered a call to a function with integer result, and F is a pointer.

How then, should one check whether F points to the function AddOne?  To do this, one should
use the address operator @:


If  F  =  @AddOne  Then
   WriteLn  ('Functions  are  equal');


The left hand side of the boolean expression is an address.  The right hand side also, and so
the compiler compares 2 addresses.  How to compare the values that both functions return ?
By adding an empty parameter list:


   If  F()=Addone  then
      WriteLn  ('Functions  return  same  values  ');


Remark that this last behaviour is not compatible with Delphi syntax.  Switching on Delphi
mode will allow you to use Delphi syntax.



12.3          Set  constructors


When a set-type constant must be entered in an expression, a set constructor must be given.
In  essence  this  is  the  same  thing  as  when  a  type  is  defined,  only  there  is  no  identifier  to
identify the set with.  A set constructor is a comma separated list of expressions, enclosed in
square brackets.


        |______________________________________________________________________________________________________________|
        Set constructors



      --  ___ set constructor __    [ ____|______________________|__ ] ____________________________________________________-oe
                                          |___  _ set group ______|
                                              6||_______ , ______|__|


      --  ___ set group __   expression __  __|________________________|__________________________________________________-oe
                                              |_ .. __ expression __ _|


        |______________________________________________________________________________________________________________|



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 *__________________



               All set groups and set elements must be of the same ordinal type.  The empty set is denoted
               by [], and it can be assigned to any type of set.  A set group with a range [A..Z] makes all
               values in the range a set element.  The following are valid set constructors:


               [today,tomorrow]
               [Monday..Friday,Sunday]
               [  2,  3*2,  6*2,  9*2  ]
               ['A'..'Z','a'..'z','0'..'9']


Remark:         If the first range specifier has a bigger ordinal value than the second, the resulting set will
               be empty, e.g., ['Z'..'A'] denotes an empty set.  One should be careful when denoting a
               range.



               12.4          Value  typecasts


               Sometimes it is necessary to change the type of an expression, or a part of the expression,
               to be able to be assignment compatible.  This is done through a value typecast.  The syntax
               diagram for a value typecast is as follows:


                       |___________________________________________________________________________________________________________*
 *___|
                       Typecasts


                     --  ___ value typecast __    type identifier __  ( __ expression __  ) _______________________________________*
 *_-oe


                       |___________________________________________________________________________________________________________*
 *___|


               Value typecasts cannot be used on the left side of assignments, as variable typecasts.  Here
               are some valid typecasts:


               Byte('A')
               Char(48)
               boolean(1)
               longint(@Buffer)


               In general, the type size of the expression and the size of the type cast must be the same.
               However, for ordinal types (byte, char, word, boolean, enumerates) this is not so, they can
               be used interchangeably.  That is, the following will work, although the sizes do not match.


               Integer('A');
               Char(4875);
               boolean(100);
               Word(@Buffer);


               This is compatible with Delphi or Turbo Pascal behaviour.



               12.5          Variable  typecasts


               A variable can be considered a single factor in an expression.  It can therefore be typecast
               as well.  A variable can be typecast to any type, provided the type has the same size as the
               original variable.

               It is a bad idea to typecast integer types to real types and vice versa.  It's better to rely on
               type assignment compatibility and using some of the standard type changing functions.



                                                                                156

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 *___



Note that variable typecasts can occur on either side of an assignment, i.e.  the following are
both valid typecasts:


Var
   C  :  Char;
   B  :  Byte;


begin
   B:=Byte(C);
   Char(B):=C;
end;


Pointer variables can be typecasted to procedural types, but not to method pointers.

A typecast is an expression of the given type, which means the typecast can be followed by
a qualifier:


Type
   TWordRec  =  Packed  Record
      L,H  :  Byte;
   end;


Var
   P  :  Pointer;
   W  :  Word;
   S  :  String;


begin
   TWordRec(W).L:=$FF;
   TWordRec(W).H:=0;
   S:=TObject(P).ClassName;



12.6          Unaligned  typecasts


A special typecast is the Unaligned typecast of a variable or expression.  This is not a real
typecast, but is rather a hint for the compiler that the expression may be misaligned (i.e.  not
on an aligned memory address).  Some processors do not allow direct access to misaligned
data structures, and therefor must access the data byte per byte.

Typecasting an expression with the unaligned keyword signals the compiler that it should
access the data byte per byte.

Note that the compiler assumes that access to all fields/elements of packed data structures
is unaligned.

Example:


program  me;


Var
    A  :  packed  Array[1..20]  of  Byte;
    I  :  LongInt;


begin
    For  I:=1  to  20  do



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      A[I]:=I;
   I:=PInteger(Unaligned(@A[13]))^;
end.



12.7          The  @  operator


The address operator @ returns the address of a variable, procedure or function.  It is used
as follows:


        |______________________________________________________________________________________________________________|
        Address factor


      --  ___ addressfactor __    @ ____|_______ variable reference __   _______|_________________________________________-oe

                                        |______ procedure identifier __   ______|
                                        |_______|function_identifier __  _______|
                                                           qualified method identifier __    _|

        |______________________________________________________________________________________________________________|


The @ operator returns a typed pointer if the $T switch is on.  If the $T switch is off then
the address operator returns an untyped pointer,  which is assignment compatible with all
pointer types.  The type of the pointer is ^T, where T is the type of the variable reference.
For example, the following will compile


Program  tcast;
{$T-}  {  @  returns  untyped  pointer  }


Type  art  =  Array[1..100]  of  byte;
Var  Buffer  :  longint;
       PLargeBuffer  :  ^art;


begin
  PLargeBuffer  :=  @Buffer;
end.


Changing the {$T-} to {$T+} will prevent the compiler from compiling this.  It will give a
type mismatch error.

By default, the address operator returns an untyped pointer:  applying the address operator
to a function, method, or procedure identifier will give a pointer to the entry point of that
function.  The result is an untyped pointer.

This means that the following will work:


Procedure  MyProc;


begin
end;


Var
    P  :  PChar;


begin
    P:=@MyProc;
end;



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By default, the address operator must be used if a value must be assigned to a procedural
type variable.  This behaviour can be avoided by using the -Mtp or -MDelphi switches, which
result in a more compatible Delphi or Turbo Pascal syntax.



12.8          Operators


Operators  can  be  classified  according  to  the  type  of  expression  they  operate  on.  We  will
discuss them type by type.



12.8.1         Arithmetic  operators

Arithmetic operators occur in arithmetic operations, i.e.  in expressions that contain integers
or reals.  There are 2 kinds of operators :  Binary and unary arithmetic operators.  Binary
operators are listed in table (12.2  ), unary operators are listed in table (12.3  ).



                                    Table 12.2:  Binary arithmetic operators


                                          __Operator_________Operation______________
                                            +               Addition
                                            -               Subtraction
                                            *               Multiplication
                                            /               Division
                                            Div             Integer division
                                          __Mod_____________Remainder_______________



With the exception of  Div and Mod, which accept only integer expressions as operands, all
operators accept real and integer expressions as operands.

For  binary  operators,  the  result  type  will  be  integer  if  both  operands  are  integer  type  ex-
pressions.  If one of the operands is a real type expression, then the result is real.

As an exception, division (/) results always in real values.



                                    Table 12.3:  Unary arithmetic operators


                                            _Operator_________Operation_____________
                                              +               Sign identity
                                            __-_______________Sign_inversion________



For unary operators, the result type is always equal to the expression type.  The division (/)
and Mod operator will cause run-time errors if the second argument is zero.

The sign of the result of a Mod operator is the same as the sign of the left side operand of
the Mod operator.  In fact, the Mod operator is equivalent to the following operation :


    I  mod  J  =  I  -  (I  div  J)  *  J


But it executes faster than the right hand side expression.



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               12.8.2         Logical  operators

               Logical operators act on the individual bits of ordinal expressions.  Logical operators require
               operands that are of an integer type, and produce an integer type result.  The possible logical
               operators are listed in table (12.4  ).



                                                          Table 12.4:  Logical operators


                                         __Operator_________Operation______________________________________________
                                           not             Bitwise negation (unary)
                                           and             Bitwise and
                                           or              Bitwise or
                                           xor             Bitwise xor
                                           shl             Bitwise shift to the left
                                         __shr_____________Bitwise_shift_to_the_right______________________________
                                           <<              Bitwise shift to the left (same as shl)
                                         __>>______________Bitwise_shift_to_the_right_(same_as_shr)________________



               The following are valid logical expressions:


               A  shr  1    {  same  as  A  div  2,  but  faster}
               Not  1       {  equals  -2  }
               Not  0       {  equals  -1  }
               Not  -1     {  equals  0    }
               B  shl  2    {  same  as  B  *  4  for  integers  }
               1  or  2     {  equals  3  }
               3  xor  1    {  equals  2  }



               12.8.3         Boolean  operators

               Boolean operators can be considered as logical operations on a type with 1 bit size.  Therefore
               the shl and shr operations have little sense.  Boolean operators can only have boolean type
               operands, and the resulting type is always boolean.  The possible operators are listed in table
               (12.5  )



                                                         Table 12.5:  Boolean operators


                                                    __Operator________Operation__________________________
                                                      not             logical negation (unary)
                                                      and             logical and
                                                      or              logical or
                                                    __xor_____________logical_xor________________________



Remark:          By  default,  boolean  expressions  are  evaluated  with  short-circuit  evaluation.   This  means
               that from the moment the result of the complete expression is known, evaluation is stopped
               and the result is returned.  For instance, in the following expression:


                 B  :=  True  or  MaybeTrue;


               The compiler will never look at the value of MaybeTrue, since it is obvious that the expression



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will always be True.  As a result of this strategy, if  MaybeTrue is a function, it will not get
called !  (This can have surprising effects when used in conjunction with properties)



12.8.4         String  operators

There  is  only  one  string  operator:  +.  Its  action  is  to  concatenate  the  contents  of  the  two
strings (or characters) it acts on.  One cannot use + to concatenate null-terminated (PChar)
strings.  The following are valid string operations:


    'This  is  '  +  'VERY  '  +  'easy  !'
    Dirname+'\'


The following is not:


Var
    Dirname  :  PChar;
...
    Dirname  :=  Dirname+'\';


Because Dirname is a null-terminated string.

Note that if all strings in a string expressions are short strings, the resulting string is also a
short string.  Thus, a truncation may occur:  there is no automatic upscaling to ansistring.

If all strings in a string expression are ansistrings, then the result is an ansistring.

If the expression contains a mix of ansistrings and shortstrings, the result is an ansistring.

The value of the {$H} switch can be used to control the type of constant strings; by default,
they are short strings (and thus limited to 255 characters).



12.8.5         Set  operators

The following operations on sets can be performed with operators:  union, difference, sym-
metric difference,  inclusion and intersection.  Elements can be added or removed from the
set with the Include or Exclude operators.  The operators needed for this are listed in table
(12.6  ).



                                              Table 12.6:  Set operators


                             __Operator________Action____________________________________________
                               +               Union
                               -               Difference
                               *               Intersection
                               ><              Symmetric difference
                               <=              Contains
                               include         include an element in the set
                               exclude         exclude an element from the set
                             __in______________check_wether_an_element_is_in_a_set_______________



The set type of the operands must be the same, or an error will be generated by the compiler.

The following program gives some valid examples of set operations:



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Type
   Day  =  (mon,tue,wed,thu,fri,sat,sun);
   Days  =  set  of  Day;


Procedure  PrintDays(W  :  Days);
Const
   DayNames  :  array  [Day]  of  String[3]
                  =  ('mon','tue','wed','thu',
                       'fri','sat','sun');
Var
   D  :  Day;
   S  :  String;
begin
   S:='';
   For  D:=Mon  to  Sun  do
      if  D  in  W  then
         begin
         If  (S<>'')  then  S:=S+',';
         S:=S+DayNames[D];
         end;
   Writeln('[',S,']');
end;


Var
   W  :  Days;


begin
    W:=[mon,tue]+[wed,thu,fri];  //  equals  [mon,tue,wed,thu,fri]
    PrintDays(W);
    W:=[mon,tue,wed]-[wed];        //  equals  [mon,tue]
    PrintDays(W);
    W:=[mon,tue,wed]-[wed,thu];        //  also  equals  [mon,tue]
    PrintDays(W);
    W:=[mon,tue,wed]*[wed,thu,fri];  //  equals  [wed]
    PrintDays(W);
    W:=[mon,tue,wed]><[wed,thu,fri];  //  equals  [mon,tue,thu,fri]
    PrintDays(W);
end.


As can be seen, the union is equivalent to a binary OR, while the intersection is equivalent
to a binary AND, and the symmetric difference equals a XOR operation.

The Include and Exclude operations are equivalent to a union or a difference with a set of
1 element.  Thus,


   Include(W,wed);


is equivalent to


   W:=W+[wed];


and


   Exclude(W,wed);



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is equivalent to


   W:=W-[wed];


The In operation results in a True if the left operand (an element) is included of the right
operand (a set), the result will be False otherwise.



12.8.6         Relational  operators

The relational operators are listed in table (12.7  )



                                         Table 12.7:  Relational operators


                                      __Operator________Action__________________________
                                        =               Equal
                                        <>              Not equal
                                        <               Stricty less than
                                        >               Strictly greater than
                                        <=              Less than or equal
                                        >=              Greater than or equal
                                      __in______________Element_of______________________



Normally, left and right operands must be of the same type.  There are some notable excep-
tions, where the compiler can handle mixed expressions:


    1.  Integer and real types can be mixed in relational expressions.

    2.  If the operator is overloaded, and an overloaded version exists whose arguments types
        match the types in the expression.

    3.  Short-, Ansi- and widestring types can be mixed.


Comparing strings is done on the basis of their character code representation.

When comparing pointers, the addresses to which they point are compared.  This also is true
for PChar type pointers.  To compare the strings the PChar point to, the StrComp function
from  the  strings  unit  must  be  used.  The  in  returns  True  if  the  left  operand  (which  must
have  the  same  ordinal  type  as  the  set  type,  and  which  must  be  in  the  range  0..255)  is  an
element of the set which is the right operand, otherwise it returns False.



12.8.7         Class  operators

Class  operators  are  slightly  different  from  the  operators  above  in  the  sense  that  they  can
only be used in class expressions which return a class.  There are only 2 class operators, as
can be seen in table (12.8  ).



                                            Table 12.8:  Class operators


                                       __Operator________Action_________________________
                                         is              Checks class type
                                         as              Conditional typecast



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An  expression  containing  the  is  operator  results  in  a  boolean  type.  The  is  operator  can
only  be  used  with  a  class  reference  or  a  class  instance.   The  usage  of  this  operator  is  as
follows:


 Object  is  Class


This expression is completely equivalent to


 Object.InheritsFrom(Class)


If  Object is Nil, False will be returned.

The following are examples:


Var
   A  :  TObject;
   B  :  TClass;


begin
   if  A  is  TComponent  then  ;
   If  A  is  B  then;
end;


The  as  operator  performs  a  conditional  typecast.  It  results  in  an  expression  that  has  the
type of the class:


   Object  as  Class


This is equivalent to the following statements:


   If  Object=Nil  then
      Result:=Nil
   else  if  Object  is  Class  then
      Result:=Class(Object)
   else
      Raise  Exception.Create(SErrInvalidTypeCast);


Note that if the object is nil, the as operator does not generate an exception.

The following are some examples of the use of the as operator:


Var
   C  :  TComponent;
   O  :  TObject;


begin
   (C  as  TEdit).Text:='Some  text';
   C:=O  as  TComponent;
end;


The as and is operators also work on COM interfaces.  They can be used to check whether
an interface also implements another interface as in the following example:


{$mode  objfpc}



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 *___



uses
   SysUtils;


type
   IMyInterface1  =  interface
      ['{DD70E7BB-51E8-45C3-8CE8-5F5188E19255}']
      procedure  Bar;
   end;


   IMyInterface2  =  interface
      ['{7E5B86C4-4BC5-40E6-A0DF-D27DBF77BCA0}']
      procedure  Foo;
   end;


   TMyObject  =  class(TInterfacedObject,  IMyInterface1,  IMyInterface2)
      procedure  Bar;
      procedure  Foo;
   end;


procedure  TMyObject.Bar;
begin


end;


procedure  TMyObject.Foo;
begin


end;


var
   i:  IMyInterface1;
begin
   i  :=  TMyObject.Create;
   i.Bar;
   Writeln(BoolToStr(i  is  IMyInterface2,  True));  //  prints  true
   Writeln(BoolToStr(i  is  IDispatch,  True));  //  prints  false
   (i  as  IMyInterface2).Foo;
end.


Additionally, the is operator can be used to check if a class implements an interface, and
the varas operator can be used to typecast an interface back to the class:


{$mode  objfpc}
var
   i:  IMyInterface;
begin
   i  :=  TMyObject.Create;
   Writeln(BoolToStr(i  is  TMyObject,True));  //  prints  true
   Writeln(BoolToStr(i  is  TObject,True));  //  prints  true
   Writeln(BoolToStr(i  is  TAggregatedObject,True));  //  prints  false
   (i  as  TMyObject).Foo;
end.


Although the interfaces must be COM interfaces, the typecast back to a class will only work



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if the interface comes from an Object Pascal class.  It will not work on interfaces obtained
from the system by COM.



                                                                 166




Chapter   13



Statements



The  heart  of  each  algorithm  are  the  actions  it  takes.   These  actions  are  contained  in  the
statements  of  a  program  or  unit.   Each  statement  can  be  labeled  and  jumped  to  (within
certain limits) with Goto statements.  This can be seen in the following syntax diagram:


        |______________________________________________________________________________________________________________|
        Statements


      --  ___ statement __   __|________________|____|________________________________|_______________________________________-oe
                               |_ label __ : ___|    |____ simple statement __    ____|

                                                     |_|structured_statement___     _|
                                                                              asm statement __    _____|

        |______________________________________________________________________________________________________________|


A label can be an identifier or an integer digit.



13.1          Simple  statements


A  simple  statement  cannot  be  decomposed  in  separate  statements.  There  are  basically  4
kinds of simple statements:


        |______________________________________________________________________________________________________________|
        Simple statements


      --  ___ simple statement __    __|_ assignment statement __      _|_______________________________________________-oe

                                       |__ procedure statement __     __|
                                       |______|goto_statement___    ______|
                                                               raise statement __   _____|

        |______________________________________________________________________________________________________________|


Of  these  statements,  the  raise  statement  will  be  explained  in  the  chapter  on  Exceptions
(chapter 17 , page 230  )



13.1.1         Assignments

Assignments give a value to a variable, replacing any previous value the variable might have
had:



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                      |____________________________________________________________________________________________________________*
 *___|
                      Assignments



                    - - ___ assignment statement __      __|__ variable reference __  ___|___|__ := __ __|__ expression __  _______*
 *_________-oe
                                                           |_ function identifier __  _|     |_ += __   _|

                                                                                             |__ -= __ __|
                                                                                             |__|*=____ __|
                                                                                                        /= __  __|

                      |____________________________________________________________________________________________________________*
 *___|


               In addition to the standard Pascal assignment operator (:=), which simply replaces the value
               of the variable with the value resulting from the expression on the right of the := operator,
               Free Pascal supports some C-style constructions.  All available constructs are listed in table
               (13.1  ).



                                             Table 13.1:  Allowed C constructs in Free Pascal

                                  __Assignment__________________________________________________________________Result_____
                                    a += b                         Adds b to a, and stores the result in a.
                                    a -= b               Subtracts b from a, and stores the result in a.
                                    a *= b               Multiplies a with b, and stores the result in a.
                                  __a_/=_b______________Divides_a_through_b,_and_stores_the_result_in_a.___________________



               For these constructs to work, the -Sc command-line switch must be specified.

Remark:        These constructions are just for typing convenience, they don't generate different code.  Here
               are some examples of valid assignment statements:


               X  :=  X+Y;
               X+=Y;          {  Same  as  X  :=  X+Y,  needs  -Sc  command  line  switch}
               X/=2;          {  Same  as  X  :=  X/2,  needs  -Sc  command  line  switch}
               Done  :=  False;
               Weather  :=  Good;
               MyPi  :=  4*  Tan(1);


               Keeping in mind that the dereferencing of a typed pointer results in a variable of the type
               the pointer points to, the following are also valid assignments:


               Var
                  L  :  ^Longint;
                  P  :  PPChar;


               begin
                  L^:=3;
                  P^^:='A';


               Note the double dereferencing in the second assignment.



               13.1.2         Procedure  statements

               Procedure statements are calls to subroutines.  There are different possibilities for procedure
               calls:



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                    o A normal procedure call.

                    o An object method call (fully qualified or not).

                    o Or even a call to a procedural type variable.


               All types are present in the following diagram:


                      |____________________________________________________________________________________________________________*
 *___|
                      Procedure statements


                    - - ___ procedure statement __     __|______ procedure identifier __   ______|____|____________________________*
 *____|_____-oe

                                                         |_______ method identifier __   _______|     |_ actual parameter list __  *
 *  _|
                                                         |_|qualified_method_identifier __    _|
                                                                              variable reference __   _______|

                      |____________________________________________________________________________________________________________*
 *___|


               The  Free  Pascal  compiler  will  look  for  a  procedure  with  the  same  name  as  given  in  the
               procedure statement, and with a declared parameter list that matches the actual parameter
               list.  The following are valid procedure statements:


               Usage;
               WriteLn('Pascal  is  an  easy  language  !');
               Doit();


Remark:        When looking for a function that matches the parameter list of the call, the parameter types
               should be assignment-compatible for value and const parameters, and should match exactly
               for parameters that are passed by reference.



               13.1.3         Goto  statements

               Free Pascal supports the goto jump statement.  Its prototype syntax is


                       |___________________________________________________________________________________________________________*
 *___|
                       Goto statement


                     --  ___ goto statement __     goto __  label __ ______________________________________________________________*
 *__-oe


                       |___________________________________________________________________________________________________________*
 *___|


               When using goto statements, the following must be kept in mind:


                   1.  The jump label must be defined in the same block as the Goto statement.

                   2.  Jumping  from  outside  a  loop  to  the  inside  of  a  loop  or  vice  versa  can  have  strange
                       effects.

                   3.  To be able to use the Goto statement, the -Sg compiler switch must be used, or {$GOTO
                       ON} must be used.


Remark:         In iso or macpas mode, or with the modeswitch "nonlocalgoto", the compiler will also allow
               non-local gotos.

               Goto  statements  are  considered  bad  practice  and  should  be  avoided  as  much  as  possible.
               It  is  always  possible  to  replace  a  goto  statement  by  a  construction  that  doesn't  need  a
               goto, although this construction may not be as clear as a goto statement.  For instance, the
               following is an allowed goto statement:



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 *___



label
   jumpto;
...
Jumpto  :
   Statement;
...
Goto  jumpto;
...



13.2          Structured  statements


Structured statements can be broken into smaller simple statements, which should be exe-
cuted repeatedly, conditionally or sequentially:

        |______________________________________________________________________________________________________________|
        Structured statements


      --  ___ structured statement __     __|__ compound statement __      ___|_________________________________________-oe

                                            |_ conditional statement __     _|
                                            |__ repetitive statement __    __|
                                            |_____|with_statement___    _____|
                                                                   exception statement __     __|

        |______________________________________________________________________________________________________________|


Conditional statements come in 2 flavours :

        |______________________________________________________________________________________________________________|
        Conditional statements


      --  ___ conditional statement __     __|_ case statement __   __|________________________________________________-oe
                                             |___ if statement __  ___|

        |______________________________________________________________________________________________________________|


Repetitive statements come in 3 flavours:

        |______________________________________________________________________________________________________________|
        Repetitive statements


      --  ___ repetitive statement __    __|____ for statament __   ____|__________________________________________________-oe

                                           |_|repeat_statement___    _|
                                                                while statement __    __|

        |______________________________________________________________________________________________________________|


The following sections deal with each of these statements.



13.2.1         Compound  statements

Compound  statements  are  a  group  of  statements,  separated  by  semicolons,  that  are  sur-
rounded  by  the  keywords  Begin  and  End.   The  last  statement  -  before  the  End  keyword
-  doesn't  need  to  be  followed  by  a  semicolon,  although  it  is  allowed.  A  compound  state-
ment is a way of grouping statements together, executing the statements sequentially.  They
are treated as one statement in cases where Pascal syntax expects 1 statement, such as in
if...then...else statements.



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                      |____________________________________________________________________________________________________________*
 *___|
                      Compound statements



                    - - ___ compound statement __        begin __  __  _ statement __ ___end __ ___________________________________*
 *_-oe
                                                                     6||_______ ; _______|_|

                      |____________________________________________________________________________________________________________*
 *___|



               13.2.2         The  Case  statement

               Free Pascal supports the case statement.  Its syntax diagram is


                       |___________________________________________________________________________________________________________*
 *___|
                       Case statement



                     --  ___ case statement __     case __  expression __   of  ____  _case_____  ____________________ __________ e*
 *nd __ _______-oe
                                                                                    6||__ ; |___|||_ else part __ |_| ||_ ; ___||


                     --  ___ case __ __  _ constant __  __ __________________________ : __ statement __  __________________________*
 *________-oe
                                       6||                ||_ .. __ constant __ _||||
                                       |__________________ , ____________________|


                     --  ___ else part __ __|_____ else __ _____|__ statementlist __  _____________________________________________*
 *______-oe
                                            |_ otherwise __   _|


                       |___________________________________________________________________________________________________________*
 *___|


               The constants appearing in the various case parts must be known at compile-time, and can
               be of the following types :  enumeration types, Ordinal types (except boolean), and chars or
               string types.  The case expression must be also of this type, or a compiler error will occur.
               All case constants must have the same type.

               The compiler will evaluate the case expression.  If one of the case constants' value matches
               the value of the expression, the statement that follows this constant is executed.  After that,
               the program continues after the final end.

               If none of the case constants match the expression value, the statement list after the else
               or otherwise keyword is executed.  This can be an empty statement list.  If no else part is
               present,  and  no  case  constant  matches  the  expression  value,  program  flow  continues  after
               the final end.

               The case statements can be compound statements (i.e.  a Begin..End block).

Remark:          Contrary  to  Turbo  Pascal,  duplicate  case  labels  are  not  allowed  in  Free  Pascal,  so  the
               following code will generate an error when compiling:


               Var  i  :  integer;
               ...
               Case  i  of
                 3  :  DoSomething;
                 1..5  :  DoSomethingElse;
               end;


               The compiler will generate a Duplicate  case  label error when compiling this, because the
               3 also appears (implicitly) in the range 1..5.  This is similar to Delphi syntax.

               The following are valid case statements:



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 *___



Case  C  of
 'a'  :  WriteLn  ('A  pressed');
 'b'  :  WriteLn  ('B  pressed');
 'c'  :  WriteLn  ('C  pressed');
else
   WriteLn  ('unknown  letter  pressed  :  ',C);
end;


Or


Case  C  of
 'a','e','i','o','u'  :  WriteLn  ('vowel  pressed');
 'y'  :  WriteLn  ('This  one  depends  on  the  language');
else
   WriteLn  ('Consonant  pressed');
end;


Case  Number  of
 1..10     :  WriteLn  ('Small  number');
 11..100  :  WriteLn  ('Normal,  medium  number');
else
 WriteLn  ('HUGE  number');
end;


Free Pascal allows the use of strings as case labels, and in that case the case variable must also
be a string.  When using string types, the case variable and the various labels are compared
in a case-sensitive way.


Case  lowercase(OS)  of
 'windows',
 'dos'     :  WriteLn  ('Microsoft  playtform);
 'macos',
 'darwin'  :  Writeln('Apple  platform');
 'linux',
 'freebsd',
 'netbsd'  :  Writeln('Community  platform');
else
   WriteLn  ('Other  platform');
end;


The case with strings is equivalent to a series of if  then  else statements, no optimizations
are performed.

However, ranges are allowed, and are the equivalent of an


if  (value>=beginrange)  and  (value<=endrange)  then
   begin
   end;



13.2.3         The  If..then..else  statement

The If  ..    then  ..    else..  prototype syntax is

        |______________________________________________________________________________________________________________|
        If then statements



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 *___



     - - ___ if statement __   if  __expression __  then __   statement __  __|__________________________|______________-oe
                                                                              |_ else __  statement __  _|

       |_______________________________________________________________________________________________________________|


The expression between the if and then keywords must have a Boolean result type.  If the
expression evaluates to True then the statement following the then keyword is executed.

If  the  expression  evaluates  to  False,  then  the  statement  following  the  else  keyword  is
executed, if it is present.

Some points to note:


     o Be aware of the fact that the boolean expression by default will be short-cut evaluated,
       meaning that the evaluation will be stopped at the point where the outcome is known
       with certainty.

     o Also, before the else keyword, no semicolon (;) is allowed, but all statements can be
       compound statements.

     o In nested If..    then  ..    else constructs,  some ambiguity may araise as to which
       else  statement  pairs  with  which  if  statement.   The  rule  is  that  the  else  keyword
       matches the first if keyword (searching backwards) not already matched by an else
       keyword.


For example:


If  exp1  Then
   If  exp2  then
      Stat1
else
   stat2;


Despite its appearance, the statement is syntactically equivalent to


If  exp1  Then
    begin
    If  exp2  then
         Stat1
    else
         stat2
    end;


and not to


{  NOT  EQUIVALENT  }
If  exp1  Then
    begin
    If  exp2  then
         Stat1
    end
else
    stat2;


If it is this latter construct which is needed, the begin and end keywords must be present.
When in doubt, it is better to add them.

The following is a valid statement:



                                                                 173

               _________________________________________________________________________________________CHAPTER_13.___STATEMENTS___*
 *__________________



               If  Today  in  [Monday..Friday]  then
                  WriteLn  ('Must  work  harder')
               else
                  WriteLn  ('Take  a  day  off.');



               13.2.4         The  For..to/downto..do  statement

               Free  Pascal  supports  the  For  loop  construction.  A  for  loop  is  used  in  case  one  wants  to
               calculate something a fixed number of times.  The prototype syntax is as follows:


                       |___________________________________________________________________________________________________________*
 *___|
                       For statement



                     --  ___ for statement __    for __ control variable __   := __  initial value ____|_____ to __ _____|_-
                                                                                                       |_ downto __   _|
                     -  ______ final value __  do __  statement __  _______________________________________________________________*
 *__-oe


                     --  ___ control variable __   variable identifier __ _________________________________________________________*
 *_-oe


                     --  ___ initial value __  expression __ ______________________________________________________________________*
 *_-oe


                     --  ___ final value __  expression __  _______________________________________________________________________*
 *__-oe


                       |___________________________________________________________________________________________________________*
 *___|


               Here, Statement can be a compound statement.  When the For statement is encountered,
               the control variable is initialized with the initial value, and is compared with the final value.
               What happens next depends on whether to or downto is used:


                   1.  In the case To is used, if the initial value is larger than the final value then Statement
                       will never be executed.

                   2.  In the case DownTo is used, if the initial value is less than the final value then Statement
                       will never be executed.


               After this check, the statement after Do is executed.  After the execution of the statement,
               the control variable is increased or decreased with 1, depending on whether To or Downto is
               used.  The control variable must be an ordinal type, no other types can be used as counters
               in a loop.

Remark:         Free Pascal always calculates the upper bound before initializing the counter variable with
               the initial value.

Remark:         It is not allowed to change (i.e.  assign a value to) the value of a loop variable inside the
               loop.

               The following are valid loops:


               For  Day  :=  Monday  to  Friday  do  Work;
               For  I  :=  100  downto  1  do
                   WriteLn  ('Counting  down  :  ',i);
               For  I  :=  1  to  7*dwarfs  do  KissDwarf(i);


               The following will generate an error:



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 *___



For  I:=0  to  100  do
   begin
   DoSomething;
   I:=I*2;
   end;


because the loop variable I cannot be assigned to inside the loop.

If the statement is a compound statement,  then the Break and Continue system routines
can be used to jump to the end or just after the end of the For statement.  Note that Break
and Continue are not reserved words and therefor can be overloaded.



13.2.5         The  For..in..do  statement

As of version 2.4.2, Free Pascal supports the For..in loop construction.  A for..in loop is
used in case one wants to calculate something a fixed number of times with an enumerable
loop variable.  The prototype syntax is as follows:


        |______________________________________________________________________________________________________________|
        For statement



      --  ___ for in statement __    for __ control variable __   in __ enumerable __    do __  statement __   _______-oe


      --  ___ control variable __   variable identifier __ __________________________________________________________-oe


      --  ___ enumerable __   __|_ enumerated type __    __|__________________________________________________________-oe
                                |______ expression __  ______|


        |______________________________________________________________________________________________________________|


Here,  Statement  can  be  a  compound  statement.   The  enumerable  must  be  an  expression
that consists of a fixed number of elements:  the loop variable will be made equal to each of
the elements in turn and the statement following the do keyword will be executed.

The enumerable expression can be one of 5 cases:


    1.  An enumeration type identifier.  The loop will then be over all elements of the enumer-
        ation type.  The control variable must be of the enumeration type.

    2.  A  set  value.  The  loop  will  then  be  over  all  elements  in  the  set,  the  control  variable
        must be of the base type of the set.

    3.  An  array  value.   The  loop  will  be  over  all  elements  in  the  array,  and  the  control
        variable  must  have  the  same  type  as  an  element  in  the  array.   As  a  special  case,  a
        string is regarded as an array of characters.

    4.  An  enumeratable  class  instance.   This  is  an  instance  of  a  class  that  supports  the
        IEnumerator  and  IEnumerable  interfaces.   In  this  case,  the  control  variable's  type
        must equal the type of the IEnumerator.GetCurrent return value.

    5.  Any  type  for  which  an  enumerator  operator  is  defined.   The  enumerator  operator
        must  return  a  class  that  implements  the  IEnumerator  interface.   The  type  of  the
        control variable's type must equal the type of the enumerator class GetCurrent return
        value type.


The simplest case of the for..in loop is using an enumerated type:



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 *___



Type
   TWeekDay  =  (monday,  tuesday,  wednesday,  thursday,
                       friday,saturday,sunday);


Var
   d  :  TWeekday;


begin
   for  d  in  TWeekday  do
      writeln(d);
end.


This will print all week days to the screen.

The above for..in construct is equivalent to the following for..to construct:


Type
   TWeekDay  =  (monday,  tuesday,  wednesday,  thursday,
                       friday,saturday,sunday);


Var
   d  :  TWeekday;


begin
   for  d:=Low(TWeekday)  to  High(TWeekday)  do
      writeln(d);
end.


A second case of for..in loop is when the enumerable expression is a set, and then the loop
will be executed once for each element in the set:


Type
   TWeekDay  =  (monday,  tuesday,  wednesday,  thursday,
                       friday,saturday,sunday);


Var
   Week  :  set  of  TWeekDay
           =  [monday,  tuesday,  wednesday,  thursday,  friday];
   d  :  TWeekday;


begin
   for  d  in  Week  do
      writeln(d);
end.


This will print the names of the week days to the screen.  Note that the variable d is of the
same type as the base type of the set.

The above for..in construct is equivalent to the following for..to construct:


Type
   TWeekDay  =  (monday,  tuesday,  wednesday,  thursday,
                       friday,saturday,sunday);


Var



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 *___



   Week  :  set  of  TWeekDay
           =  [monday,  tuesday,  wednesday,  thursday,  friday];


   d  :  TWeekday;


begin
   for  d:=Low(TWeekday)  to  High(TWeekday)  do
      if  d  in  Week  then
         writeln(d);
end.


The third possibility for a for..in loop is when the enumerable expression is an array:


var
   a  :  Array[1..7]  of  string
      =  ('monday','tuesday','wednesday','thursday',
           'friday','saturday','sunday');


Var
   S  :  String;


begin
   For  s  in  a  do
      Writeln(s);
end.


This will also print all days in the week, and is equivalent to


var
   a  :  Array[1..7]  of  string
      =  ('monday','tuesday','wednesday','thursday',
           'friday','saturday','sunday');


Var
   i  :  integer;


begin
   for  i:=Low(a)  to  high(a)  do
      Writeln(a[i]);
end.


A string type is equivalent to an array  of  char,  and therefor a string can be used in a
for..in loop.  The following will print all letters in the alphabet, each letter on a line:


Var
   c  :  char;


begin
 for  c  in  'abcdefghijklmnopqrstuvwxyz'  do
    writeln(c);
end.


The  fourth  possibility  for  a  for..in  loop  is  using  classes.   A  class  can  implement  the
IEnumerable interface, which is defined as follows:



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 *___



IEnumerable  =  interface(IInterface)
   function  GetEnumerator:  IEnumerator;
end;


The  actual  return  type  of  the  GetEnumerator  must  not  necessarily  be  an  IEnumerator
interface, instead, it can be a class which implements the methods of  IEnumerator:


IEnumerator  =  interface(IInterface)
   function  GetCurrent:  TObject;
   function  MoveNext:  Boolean;
   procedure  Reset;
   property  Current:  TObject  read  GetCurrent;
end;


The Current property and the MoveNext method must be present in the class returned by the
GetEnumerator method.  The actual type of the Current property need not be a TObject.
When encountering a for..in loop with a class instance as the 'in' operand, the compiler
will check each of the following conditions:


     o Whether the class in the enumerable expression implements a method GetEnumerator

     o Whether the result of  GetEnumerator is a class with the following method:


       Function  MoveNext  :  Boolean


     o Whether the result of GetEnumerator is a class with the following read-only property:


       Property  Current  :  AType;


       The type of the property must match the type of the control variable of the for..in
       loop.


Neither the IEnumerator nor the IEnumerable interfaces must actually be declared by the
enumerable  class:  the  compiler  will  detect  whether  these  interfaces  are  present  using  the
above  checks.   The  interfaces  are  only  defined  for  Delphi  compatibility  and  are  not  used
internally.  (it would also be impossible to enforce their correctness).

The Classes unit contains a number of classes that are enumerable:


TFPList       Enumerates all pointers in the list.

TList     Enumerates all pointers in the list.

TCollection         Enumerates all items in the collection.

TStringList        Enumerates all strings in the list.

TComponent            Enumerates all child components owned by the component.


Thus, the following code will also print all days in the week:


{$mode  objfpc}
uses  classes;


Var
   Days  :  TStrings;
   D  :  String;



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 *___



begin
   Days:=TStringList.Create;
   try
      Days.Add('Monday');
      Days.Add('Tuesday');
      Days.Add('Wednesday');
      Days.Add('Thursday');
      Days.Add('Friday');
      Days.Add('Saturday');
      Days.Add('Sunday');
      For  D  in  Days  do
         Writeln(D);
   Finally
      Days.Free;
   end;
end.


Note that the compiler enforces type safety:  declaring D as an integer will result in a compiler
error:


testsl.pp(20,9)  Error:  Incompatible  types:  got  "AnsiString"  expected  "LongInt"


The above code is equivalent to the following:


{$mode  objfpc}
uses  classes;


Var
   Days  :  TStrings;
   D  :  String;
   E  :  TStringsEnumerator;


begin
   Days:=TStringList.Create;
   try
      Days.Add('Monday');
      Days.Add('Tuesday');
      Days.Add('Wednesday');
      Days.Add('Thursday');
      Days.Add('Friday');
      Days.Add('Saturday');
      Days.Add('Sunday');
      E:=Days.getEnumerator;
      try
         While  E.MoveNext  do
             begin
             D:=E.Current;
             Writeln(D);
             end;
      Finally
         E.Free;
      end;
   Finally



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 *___



      Days.Free;
   end;
end.


Both programs will output the same result.

The fifth and last possibility to use a for..in loop can be used to enumerate almost any
type,  using  the  enumerator  operator.  The  enumerator  operator  must  return  a  class  that
has the same signature as the IEnumerator approach above.  The following code will define
an enumerator for the Integer type:


Type



TEvenEnumerator  =  Class
   FCurrent  :  Integer;
   FMax  :  Integer;
   Function  MoveNext  :  Boolean;
   Property  Current  :  Integer  Read  FCurrent;
end;


Function  TEvenEnumerator.MoveNext  :  Boolean;


begin
   FCurrent:=FCurrent+2;
   Result:=FCurrent<=FMax;
end;


operator  enumerator(i  :  integer)  :  TEvenEnumerator;


begin
   Result:=TEvenEnumerator.Create;
   Result.FMax:=i;
end;


var
   I  :  Integer;
   m  :  Integer  =  4;


begin
   For  I  in  M  do
      Writeln(i);
end.


The loop will print all nonzero even numbers smaller or equal to the enumerable.  (2 and 4
in the case of the example).

Care must be taken when defining enumerator operators:  the compiler will find and use the
first available enumerator operator for the enumerable expression.  For classes this also means
that the GetEnumerator method is not even considered.  The following code will define an
enumerator operator which extracts the object from a stringlist:


{$mode  objfpc}
uses  classes;



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 *___



Type
   TDayObject  =  Class
      DayOfWeek  :  Integer;
      Constructor  Create(ADayOfWeek  :  Integer);
   end;


   TObjectEnumerator  =  Class
      FList  :  TStrings;
      FIndex  :  Integer;
      Function  GetCurrent  :  TDayObject;
      Function  MoveNext:  boolean;
      Property  Current  :  TDayObject  Read  GetCurrent;
   end;


Constructor  TDayObject.Create(ADayOfWeek  :  Integer);


begin
   DayOfWeek:=ADayOfWeek;
end;


Function  TObjectEnumerator.GetCurrent  :  TDayObject;
begin
   Result:=FList.Objects[Findex]  as  TDayObject;
end;


Function  TObjectEnumerator.MoveNext:  boolean;


begin
   Inc(FIndex);
   Result:=(FIndex<FList.Count);
end;


operator  enumerator  (s  :  TStrings)  :  TObjectEnumerator;


begin
   Result:=TObjectEnumerator.Create;
   Result.Flist:=S;
   Result.FIndex:=-1;
end;


Var
   Days  :  TStrings;
   D  :  String;
   O  :  TdayObject;


begin
   Days:=TStringList.Create;
   try
      Days.AddObject('Monday',TDayObject.Create(1));
      Days.AddObject('Tuesday',TDayObject.Create(2));
      Days.AddObject('Wednesday',TDayObject.Create(3));
      Days.AddObject('Thursday',TDayObject.Create(4));
      Days.AddObject('Friday',TDayObject.Create(5));
      Days.AddObject('Saturday',TDayObject.Create(6));



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 *__________________



                     Days.AddObject('Sunday',TDayObject.Create(7));
                     For  O  in  Days  do
                        Writeln(O.DayOfWeek);
                  Finally
                     Days.Free;
                  end;
               end.


               The above code will print the day of the week for each day in the week.

               If a class is not enumerable,  the compiler will report an error when it is encountered in a
               for...in loop.

Remark:        Like the for..to loop, it is not allowed to change (i.e.  assign a value to) the value of a loop
               control variable inside the loop.



               13.2.6         The  Repeat..until  statement

               The repeat statement is used to execute a statement until a certain condition is reached.
               The statement will be executed at least once.  The prototype syntax of the Repeat..until
               statement is


                       |___________________________________________________________________________________________________________*
 *___|
                       Repeat statement



                     --  ___ repeat statement __     repeat __  __  _ statement __ ___until __  expression __ _____________________*
 *__-oe
                                                                  6||_______ ; _______|_|

                       |___________________________________________________________________________________________________________*
 *___|


               This will execute the statements between repeat and until up to the moment when Expression
               evaluates to True.  Since the expression is evaluated after the execution of the statements,
               they are executed at least once.

               Be  aware  of  the  fact  that  the  boolean  expression  Expression  will  be  short-cut  evaluated
               by default, meaning that the evaluation will be stopped at the point where the outcome is
               known with certainty.

               The following are valid repeat statements


               repeat
                   WriteLn  ('I  =',i);
                   I  :=  I+2;
               until  I>100;


               repeat
                 X  :=  X/2
               until  x<10e-3;


               Note that the last statement before the until keyword does not need a terminating semi-
               colon, but it is allowed.

               The Break and Continue system routines can be used to jump to the end or just after the
               end of the repeat  ..    until statement.  Note that Break and Continue are not reserved
               words and therefor can be overloaded.



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 *___



13.2.7         The  While..do  statement

A while statement is used to execute a statement as long as a certain condition holds.  In
difference with the repeat loop, this may imply that the statement is never executed.

The prototype syntax of the While..do statement is


        |______________________________________________________________________________________________________________|
        While statements



      --  ___ while statement __     while __  expression __   do __ statement __   _________________________________-oe


        |______________________________________________________________________________________________________________|


This will execute Statement as long as Expression evaluates toTrue.  Since Expression is
evaluated before the execution of Statement, it is possible that Statement isn't executed at
all.  Statement can be a compound statement.

Be  aware  of  the  fact  that  the  boolean  expression  Expression  will  be  short-cut  evaluated
by default, meaning that the evaluation will be stopped at the point where the outcome is
known with certainty.

The following are valid while statements:


I  :=  I+2;
while  i<=100  do
    begin
    WriteLn  ('I  =',i);
    I  :=  I+2;
    end;
X  :=  X/2;
while  x>=10e-3  do
    X  :=  X/2;


They correspond to the example loops for the repeat statements.

If the statement is a compound statement, then the Break and Continue reserved words can
be used to jump to the end or just after the end of the While statement.  Note that Break
and Continue are not reserved words and therefor can be overloaded.



13.2.8         The  With  statement

The  with  statement  serves  to  access  the  elements  of  a  record  or  object  or  class,  without
having to specify the element's name each time.  The syntax for a with statement is


        |______________________________________________________________________________________________________________|
        With statement



      --  ___ with statement __    __  _ variable reference __ ___do __  statement __  _______________________________-oe
                                     6||___________ , ____________|_|

        |______________________________________________________________________________________________________________|


The  variable  reference  must  be  a  variable  of  a  record,  object  or  class  type.   In  the  with
statement,  any  variable  reference,  or  method  reference  is  checked  to  see  if  it  is  a  field  or
method of the record or object or class.  If so, then that field is accessed, or that method is
called.  Given the declaration:



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 *___



Type
   Passenger  =  Record
      Name  :  String[30];
      Flight  :  String[10];
   end;


Var
   TheCustomer  :  Passenger;


The following statements are completely equivalent:


TheCustomer.Name  :=  'Michael';
TheCustomer.Flight  :=  'PS901';


and


With  TheCustomer  do
   begin
   Name  :=  'Michael';
   Flight  :=  'PS901';
   end;


The statement


With  A,B,C,D  do  Statement;


is equivalent to


With  A  do
 With  B  do
   With  C  do
    With  D  do  Statement;


This also is a clear example of the fact that the variables are tried last to first, i.e., when the
compiler encounters a variable reference,  it will first check if it is a field or method of the
last variable.  If not, then it will check the last-but-one, and so on.  The following example
shows this;


Program  testw;
Type  AR  =  record
         X,Y  :  Longint;
        end;
        PAR  =  ^Ar;


Var  S,T  :  Ar;
begin
   S.X  :=  1;S.Y  :=  1;
   T.X  :=  2;T.Y  :=  2;
   With  S,T  do
      WriteLn  (X,'  ',Y);
end.


The output of this program is


2  2



                                                                 184

               _________________________________________________________________________________________CHAPTER_13.___STATEMENTS___*
 *__________________



               Showing thus that the X,Y in the WriteLn statement match the T record variable.

Remark:         When using a With statement with a pointer, or a class, it is not permitted to change the
               pointer  or  the  class  in  the  With  block.  With  the  definitions  of  the  previous  example,  the
               following illustrates what it is about:


               Var  p  :  PAR;


               begin
                  With  P^  do
                   begin
                   //  Do  some  operations
                   P:=OtherP;
                   X:=0.0;    //  Wrong  X  will  be  used  !!
                   end;


               The reason the pointer cannot be changed is that the address is stored by the compiler in a
               temporary register.  Changing the pointer won't change the temporary address.  The same
               is true for classes.



               13.2.9         Exception  Statements

               Free Pascal supports exceptions.  Exceptions provide a convenient way to program error and
               error-recovery mechanisms, and are closely related to classes.  Exception support is explained
               in chapter 17 , page 230



               13.3          Assembler  statements


               An assembler statement allows to insert assembler code right in the Pascal code.


                       |___________________________________________________________________________________________________________*
 *___|
                       Assembler statements



                     --  ___ asm statement __      asm __  assembler code __     end __ __|__________________|_____________________*
 *_____-oe
                                                                                          |_ registerlist ___|


                     --  ___ registerlist __ [ ____  _ stringconstant __ ___] _____________________________________________________*
 *___-oe
                                                   6||__________ , _________|__|


                       |___________________________________________________________________________________________________________*
 *___|


               More  information  about  assembler  blocks  can  be  found  in  the  Programmer's Guide           .  The
               register list is used to indicate the registers that are modified by an assembler statement in
               the assembler block.  The compiler stores certain results in the registers.  If the registers are
               modified in an assembler statement, the compiler should, sometimes, be told about it.  The
               registers are denoted with their Intel names for the I386 processor, i.e., 'EAX', 'ESI' etc...
               As an example, consider the following assembler code:


               asm
                   Movl  $1,%ebx
                   Movl  $0,%eax
                   addl  %eax,%ebx
               end  ['EAX','EBX'];



                                                                                185

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 *___



This will tell the compiler that it should save and restore the contents of the EAX and EBX
registers when it encounters this asm statement.

Free Pascal supports various styles of assembler syntax.  By default, AT&T syntax is assumed
for the 80386 and compatibles platform.  The default assembler style can be changed with
the {$asmmode  xxx} switch in the code, or the -R command-line option.  More about this
can be found in the Programmer's Guide           .



                                                                 186




               Chapter   14



               Using   functions   and   procedures



               Free Pascal supports the use of functions and procedures.  It supports


                    o  Function overloading, i.e.  functions with the same name but different parameter lists.

                    o  Const parameters.

                    o  Open arrays (i.e.  arrays without bounds).

                    o  Variable number of arguments as in C.

                    o  Return-like construct as in C, through the Exit keyword.


Remark:          In  many  of  the  subsequent  paragraphs  the  words  procedure  and  function  will  be  used
               interchangeably.  The statements made are valid for both, except when indicated otherwise.



               14.1          Procedure  declaration


               A  procedure  declaration  defines  an  identifier  and  associates  it  with  a  block  of  code.  The
               procedure can then be called with a procedure statement.


                       |___________________________________________________________________________________________________________*
 *___|
                       Procedure declaration


                     --  ___ procedure declaration __      procedure header __     ; __subroutine block __     ; __________________*
 *_-oe

                     --  ___ procedure header __     procedure __    __|____________ identifier ______________|_-
                                                                       |_ qualified method identifier __    _|
                     -  ______ formal parameter list __    __|__________________|__ hintdirectives __  ____________________________*
 *________-oe
                                                             |_ modifiers __  _|


                     --  ___ subroutine block __    __|_________ block __ _________|_______________________________________________*
 *_______-oe

                                                      |_ external directive __  _|
                                                      |______|asm_block____  ______|
                                                                            forward __  _______|

                       |___________________________________________________________________________________________________________*
 *___|


               See  section  14.4  ,  page  189    for  the  list  of  parameters.      A  procedure  declaration  that  is
               followed by a block implements the action of the procedure in that block.  The following is
               a valid procedure :



                                                                            187

_________________________________________________CHAPTER_14.___USING_FUNCTIONS_AND_PROCEDURES______________________________________*
 *___



Procedure  DoSomething  (Para  :  String);
begin
   Writeln  ('Got  parameter  :  ',Para);
   Writeln  ('Parameter  in  upper  case  :  ',Upper(Para));
end;


Note that it is possible that a procedure calls itself.



14.2          Function  declaration


A function declaration defines an identifier and associates it with a block of code.  The block
of code will return a result.  The function can then be called inside an expression, or with a
procedure statement, if extended syntax is on.


        |______________________________________________________________________________________________________________|
        Function declaration



      --  ___ function declaration __     function header __    ; __subroutine block __     ; _______________________-oe


      --  ___ function header __     function __  __|____________ identifier ______________|_-
                                                    |_ qualified method identifier __    _|
      -  ______ formal parameter list __     : __result type __  __|__________________|__ hintdirectives __  ________________-oe
                                                                   |_ modifiers __  _|


      --  ___ subroutine block __    __|_________ block __ _________|______________________________________________________-oe

                                       |_ external directive __  _|
                                       |______|asm_block____  ______|
                                                             forward __  _______|

        |______________________________________________________________________________________________________________|


The result type of a function can be any previously declared type.  contrary to Turbo Pascal,
where only simple types could be returned.



14.3          Function  results


The result of a function can be set by setting the result variable:  this can be the function
identifier or, (only in ObjFPC or Delphi mode) the special Result identifier:


Function  MyFunction  :  Integer;


begin
    MyFunction:=12;  //  Return  12
end;


In Delphi or ObjFPC mode, the above can also be coded as:


Function  MyFunction  :  Integer;


begin
    Result:=12;
end;



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As an extension to Delphi syntax, the ObjFPC mode also supports a special extension of the
Exit procedure:


Function  MyFunction  :  Integer;


begin
   Exit(12);
end;


The  Exit  call  sets  the  result  of  the  function  and  jumps  to  the  final  End  of  the  function
declaration block.  It can be seen as the equivalent of the C return instruction.



14.4          Parameter  lists


When  arguments  must  be  passed  to  a  function  or  procedure,  these  parameters  must  be
declared in the formal parameter list of that function or procedure.  The parameter list is a
declaration of identifiers that can be referred to only in that procedure or function's block.


        |______________________________________________________________________________________________________________|
        Parameters


      --  ___ formal parameter list __     ( ____  _ parameter declaration __   ___) __________________________________-oe
                                                 6||______________ ; ______________|_|


      --  ___ parameter declaration __     __|____ value parameter __    ____|_____________________________________________-oe

                                             |__ variable parameter __    __|
                                             |_____ out parameter __    _____|
                                             |_|constant_parameter___     _|
                                                                    out parameter __    _____|

        |______________________________________________________________________________________________________________|


Constant parameters, out parameters and variable parameters can also be untyped param-
eters if they have no type identifier.

As of version 1.1, Free Pascal supports default values for both constant parameters and value
parameters, but only for simple types.  The compiler must be in OBJFPC or DELPHI mode to
accept default values.



14.4.1         Value  parameters

Value parameters are declared as follows:


        |______________________________________________________________________________________________________________|
        Value parameters


      --   value parameter __    __ _______ identifier list __ : ___ _____________________ parameter type __    ____________-oe
                                   |                                ||_       __      ___||                            |
                                   ||_                                  array     of                                   |
                                       identifier __ : __ type identifier __  = __  default parameter value __     _|

        |______________________________________________________________________________________________________________|


When parameters are declared as value parameters, the procedure gets a copy of the param-
eters that the calling statement passes.  Any modifications to these parameters are purely
local to the procedure's block, and do not propagate back to the calling block.



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A block that wishes to call a procedure with value parameters must pass assignment com-
patible parameters to the procedure.  This means that the types should not match exactly,
but can be converted to the actual parameter types.  This conversion code is inserted by the
compiler itself.

Care must be taken when using value parameters:  value parameters makes heavy use of the
stack, especially when using large parameters.  The total size of all parameters in the formal
parameter  list  should  be  below  32K  for  portability's  sake  (the  Intel  version  limits  this  to
64K).

Open  arrays  can  be  passed  as  value  parameters.   See  section  14.4.5  ,  page  193    for  more
information on using open arrays.

For a parameter of a simple type (i.e.  not a structured type), a default value can be specified.
This can be an untyped constant.  If the function call omits the parameter, the default value
will be passed on to the function.  For dynamic arrays or other types that can be considered
as equivalent to a pointer, the only possible default value is Nil.

The following example will print 20 on the screen:


program  testp;


Const
   MyConst  =  20;


Procedure  MyRealFunc(I  :  Integer  =  MyConst);


begin
   Writeln('Function  received  :  ',I);
end;


begin
   MyRealFunc;
end.



14.4.2         Variable  parameters

Variable parameters are declared as follows:


        |______________________________________________________________________________________________________________|
        Variable parameters


      --  ___ variable parameter __     var __  identifier list ____|________________________________________________|_____-oe
                                                                    |_ : ____|____________________|_ type identifier __ _|
                                                                             |_ array __  of  ___|

        |______________________________________________________________________________________________________________|


When  parameters  are  declared  as  variable  parameters,  the  procedure  or  function  accesses
immediately the variable that the calling block passed in its parameter list.  The procedure
gets a pointer to the variable that was passed, and uses this pointer to access the variable's
value.  From this, it follows that any changes made to the parameter, will propagate back to
the calling block.  This mechanism can be used to pass values back in procedures.  Because
of  this,  the  calling  block  must  pass  a  parameter  of  exactly  the  same  type  as  the  declared
parameter's type.  If it does not, the compiler will generate an error.

Variable  and  constant  parameters  can  be  untyped.  In  that  case  the  variable  has  no  type,
and hence is incompatible with all other types.  However, the address operator can be used



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on it, or it can be passed to a function that has also an untyped parameter.  If an untyped
parameter is used in an assignment, or a value must be assigned to it, a typecast must be
used.

File type variables must always be passed as variable parameters.

Open  arrays  can  be  passed  as  variable  parameters.  See  section  14.4.5  ,  page  193    for  more
information on using open arrays.

Note that default values are not supported for variable parameters.  This would make little
sense since it defeats the purpose of being able to pass a value back to the caller.



14.4.3         Out  parameters

Out parameters (output parameters) are declared as follows:


        |______________________________________________________________________________________________________________|
        Out parameters



      --  ___ out parameter __     out __  identifier list ____|________________________________________________|_________-oe
                                                               |_ : ____|____________________|_ type identifier __ _|
                                                                        |_ array __  of  ___|

        |______________________________________________________________________________________________________________|


The purpose of an out parameter is to pass values back to the calling routine:  the variable
is passed by reference.  The initial value of the parameter on function entry is discarded, and
should not be used.

If a variable must be used to pass a value to a function and retrieve data from the function,
then a variable parameter must be used.  If only a value must be retrieved, a out parameter
can be used.

Needless to say, default values are not supported for out parameters.

The  difference  of  out  parameters  and  parameters  by  reference  is  very  small  (however,  see
below  for  managed  types):   the  former  gives  the  compiler  more  information  about  what
happens to the arguments when passed to the procedure:  it knows that the variable does
not have to be initialized prior to the call.  The following example illustrates this:


Procedure  DoA(Var  A  :  Integer);


begin
    A:=2;
    Writeln('A  is  ',A);
end;


Procedure  DoB(Out  B  :  Integer);


begin
    B:=2;
    Writeln('B  is  ',B);
end;


Var
    C,D  :  Integer;


begin



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                  DoA(C);
                  DoB(D);
               end.


               Both  procedures  DoA  and  DoB  do  practically  the  same.   But  DoB's  declaration  gives  more
               information to the compiler, allowing it to detect that D does not have to initialized before
               DoB is called.  Since the parameter A in DoA can receive a value as well as return one,  the
               compiler notices that C was not initialized prior to the call to DoA:


               home:  >fpc  -S2  -vwhn  testo.pp
               testo.pp(19,8)  Hint:  Variable  "C"  does  not  seem  to  be  initialized


               This shows that it is better to use out parameters when the parameter is used only to return
               a value.

Remark:         Out parameters are only supported in Delphi and ObjFPC mode.  For the other modes, out
               is a valid identifier.

Remark:         For managed types (reference counted types), using Out parameters incurs some overhead:
               the compiler must be sure that the value is correctly initialized (i.e.  has a reference count of
               zero (0)).  This initialization is normally done by the caller.



               14.4.4         Constant  parameters

               In addition to variable parameters and value parameters Free Pascal also supports Constant
               parameters.  A constant parameter can be specified as follows:


                       |___________________________________________________________________________________________________________*
 *___|
                       Constant parameters


                     --   constant parameter __      const __ __|______ identifier list ____|______________________________________*
 *__________|_______|_-
                                                                |                           |_ : ____ _____________________ type id*
 *entifier __ _|    |
                                                                |                                    ||_       __      ___||       *
 *                  |
                                                                ||_                                      array     of              *
 *                  |
                     -  ____________________________________________identifier____:____type_identifier_____=_____default_parameter_*
 *value___-oe  _|


                       |___________________________________________________________________________________________________________*
 *___|


               Specifying  a  parameter  as  Constant  is  giving  the  compiler  a  hint  that  the  contents  of  the
               parameter will not be changed by the called routine.  This allows the compiler to perform
               optimizations  which  it  could  not  do  otherwise,  and  also  to  perform  certain  checks  on  the
               code inside the routine:  namely, it can forbid assignments to the parameter.  Furthermore a
               const parameter cannot be passed on to another function that requires a variable parameter:
               the compiler can check this as well.  The main use for this is reducing the stack size, hence
               improving performance, and still retaining the semantics of passing by value...

Remark:         Contrary to Delphi, no assumptions should be made about how const parameters are passed
               to the underlying routine.  In particular, the assumption that parameters with large size are
               passed by reference is not correct.  For this the constref parameter type should be used,
               which is available as of version 2.5.1 of the compiler.

               An  exception  is  the  stdcall  calling  convention:  for  compatibility  with  COM  standards,
               large const parameters are passed by reference.

Remark:         Note that specifying const is a contract between the programmer and the compiler.  It is
               the programmer who tells the compiler that the contents of the const parameter will not be
               changed when the routine is executed, it is not the compiler who tells the programmer that
               the parameter will not be changed.



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This is particularly important and visible when using refcounted types.  For such types, the
(invisible)  incrementing  and  decrementing  of  any  reference  count  is  omitted  when  const
is  used.   Doing  so  often  allows  the  compiler  to  omit  invisible  try/finally  frames  for  these
routines.

As a side effect, the following code will produce not the expected output:


Var
   S  :  String  =  'Something';


Procedure  DoIt(Const  T  :  String);


begin
   S:='Something  else';
   Writeln(T);
end;


begin
   DoIt(S);
end.


Will write


Something  else


This behaviour is by design.

Constant parameters can also be untyped.  See section 14.4.2  , page 190   for more information
about untyped parameters.

As for value parameters, constant parameters can get default values.

Open arrays can be passed as constant parameters.  See section 14.4.5  ,  page 193   for more
information on using open arrays.



14.4.5         Open  array  parameters

Free Pascal supports the passing of open arrays,  i.e.  a procedure can be declared with an
array  of  unspecified  length  as  a  parameter,  as  in  Delphi.   Open  array  parameters  can  be
accessed in the procedure or function as an array that is declared with starting index 0, and
last element index High(parameter).  For example, the parameter


Row  :  Array  of  Integer;


would be equivalent to


Row  :  Array[0..N-1]  of  Integer;


Where N would be the actual size of the array that is passed to the function.  N-1 can be
calculated as High(Row).

Specifically, if an empty array is passed, then High(Parameter) returns -1, while low(Parameter)
returns 0.

Open parameters can be passed by value, by reference or as a constant parameter.  In the
latter cases the procedure receives a pointer to the actual array.  In the former case, it receives
a copy of the array.  In a function or procedure, open arrays can only be passed to functions
which are also declared with open arrays as parameters, not to functions or procedures which
accept arrays of fixed length.  The following is an example of a function using an open array:



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Function  Average  (Row  :  Array  of  integer)  :  Real;
Var  I  :  longint;
      Temp  :  Real;
begin
   Temp  :=  Row[0];
   For  I  :=  1  to  High(Row)  do
      Temp  :=  Temp  +  Row[i];
   Average  :=  Temp  /  (High(Row)+1);
end;


As of FPC 2.2, it is also possible to pass partial arrays to a function that accepts an open
array.  This can be done by specifying the range of the array which should be passed to the
open array.

Given the declaration


Var
   A  :  Array[1..100];


the following call will compute and print the average of the 100 numbers:


   Writeln('Average  of  100  numbers:  ',Average(A));


But the following will compute and print the average of the first and second half:


   Writeln('Average  of  first  50  numbers:  ',Average(A[1..50]));
   Writeln('Average  of  last    50  numbers:  ',Average(A[51..100]));



14.4.6         Array  of  const

In Object Pascal or Delphi mode, Free Pascal supports the Array  of  Const construction to
pass parameters to a subroutine.

This is a special case of the Open  array construction, where it is allowed to pass any expres-
sion in an array to a function or procedure.  The expression must have a simple result type:
structures  cannot  be  passed  as  an  argument.   This  means  that  all  ordinal,  float  or  string
types can be passed, as well as pointers, classes and interfaces.

The elements of the array  of  const are converted to a special variant record:


Type
    PVarRec  =  ^TVarRec;
    TVarRec  =  record
        case  VType  :  Ptrint  of
            vtInteger       :  (VInteger:  Longint);
            vtBoolean       :  (VBoolean:  Boolean);
            vtChar            :  (VChar:  Char);
            vtWideChar     :  (VWideChar:  WideChar);
            vtExtended     :  (VExtended:  PExtended);
            vtString        :  (VString:  PShortString);
            vtPointer       :  (VPointer:  Pointer);
            vtPChar          :  (VPChar:  PChar);
            vtObject        :  (VObject:  TObject);
            vtClass          :  (VClass:  TClass);
            vtPWideChar    :  (VPWideChar:  PWideChar);



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           vtAnsiString  :  (VAnsiString:  Pointer);
           vtCurrency     :  (VCurrency:  PCurrency);
           vtVariant       :  (VVariant:  PVariant);
           vtInterface    :  (VInterface:  Pointer);
           vtWideString  :  (VWideString:  Pointer);
           vtInt64          :  (VInt64:  PInt64);
           vtQWord          :  (VQWord:  PQWord);
    end;


Therefor, inside the procedure body, the array  of  const argument is equivalent to an open
array of  TVarRec:


Procedure  Testit  (Args:  Array  of  const);


Var  I  :  longint;


begin
   If  High(Args)<0  then
      begin
      Writeln  ('No  aguments');
      exit;
      end;
   Writeln  ('Got  ',High(Args)+1,'  arguments  :');
   For  i:=0  to  High(Args)  do
      begin
      write  ('Argument  ',i,'  has  type  ');
      case  Args[i].vtype  of
         vtinteger       :
             Writeln  ('Integer,  Value  :',args[i].vinteger);
         vtboolean       :
             Writeln  ('Boolean,  Value  :',args[i].vboolean);
         vtchar            :
             Writeln  ('Char,  value  :  ',args[i].vchar);
         vtextended     :
             Writeln  ('Extended,  value  :  ',args[i].VExtended^);
         vtString        :
             Writeln  ('ShortString,  value  :',args[i].VString^);
         vtPointer       :
             Writeln  ('Pointer,  value  :  ',Longint(Args[i].VPointer));
         vtPChar          :
             Writeln  ('PChar,  value  :  ',Args[i].VPChar);
         vtObject        :
             Writeln  ('Object,  name  :  ',Args[i].VObject.Classname);
         vtClass          :
             Writeln  ('Class  reference,  name  :',Args[i].VClass.Classname);
         vtAnsiString  :
             Writeln  ('AnsiString,  value  :',AnsiString(Args[I].VAnsiString);
      else
             Writeln  ('(Unknown)  :  ',args[i].vtype);
      end;
      end;
end;


In code, it is possible to pass an arbitrary array of elements to this procedure:



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                  S:='Ansistring  1';
                  T:='AnsiString  2';
                  Testit  ([]);
                  Testit  ([1,2]);
                  Testit  (['A','B']);
                  Testit  ([TRUE,FALSE,TRUE]);
                  Testit  (['String','Another  string']);
                  Testit  ([S,T])    ;
                  Testit  ([P1,P2]);
                  Testit  ([@testit,Nil]);
                  Testit  ([ObjA,ObjB]);
                  Testit  ([1.234,1.234]);
                  TestIt  ([AClass]);


               If the procedure is declared with the cdecl modifier, then the compiler will pass the array
               as a C compiler would pass it.  This, in effect, emulates the C construct of a variable number
               of arguments, as the following example will show:


               program  testaocc;
               {$mode  objfpc}


               Const
                  P  :  PChar  =  'example';
                  Fmt  :  PChar  =
                            'This  %s  uses  printf  to  print  numbers  (%d)  and  strings.'#10;


               //  Declaration  of  standard  C  function  printf:
               procedure  printf  (fm  :  pchar;  args  :  array  of  const);cdecl;  external  'c';


               begin
                printf(Fmt,[P,123]);
               end.


               Remark that this is not true for Delphi, so code relying on this feature will not be portable.

Remark:         Note that there is no support for QWord arguments in array  of  const.  This is for Delphi
               compatibility, and the compiler will ignore any resulting range checks when in mode Delphi.



               14.5          Managed  types  and  reference  counts


               Some  types  (Unicodestring,  Ansistring,  interfaces,  dynamic  arrays)  are  treated  somewhat
               specially  by  the  compiler:  the  data  has  a  reference  count  which  is  increased  or  decreased
               depending on how many reference to the data exists.

               The qualifiers for parameters in function or procedure calls influence influence what happens
               to the reference count of the managed types:


                    o  nothing (pass by value):  the reference count of the parameter is increased on entry and
                       decreased on exit.

                    o  out:  the  reference  count  of  the  value  that  is  passed  in  is  decreased  by  1,  and  the
                       variable  that's  passed  into  the  procedure  is  initialized  to  "empty"  (usually  Nil,  but
                       that is an implementation detail which should not be relied on).



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     o var  nothing  happens  to  the  reference  count.   A  reference  to  the  original  variable  is
       passed in, and changing it or reading it has exactly the same effect as changing/reading
       the original variable.

     o const this case is slightly tricky.  Nothing happens to the reference count because you
       can pass non-lvalues here.  In particular, you can pass a class implementing an interface
       rather than the interface itself which can cause the class to be freed unexpectedly.


The following example demonstrates the dangers:


{$mode  objfpc}


Type
   ITest  =  Interface
      Procedure  DoTest(ACount  :  Integer);
   end;


   TTest  =  Class(TInterfacedObject,ITest)
      Procedure  DoTest(ACount  :  Integer);
      Destructor  destroy;  override;
   end;


Destructor  TTest.Destroy;


begin
   Writeln('Destroy  called');
end;


Procedure  TTest.DoTest(ACount  :  Integer);


begin
   Writeln('Test  ',ACount,'  :  ref  count:  ',RefCount);
end;


procedure  DoIt1(x:  ITest;  ACount  :  Integer);


begin
   //  Reference  count  is  increased
   x.DoTest(ACount);
   //  And  decreased
end;


procedure  DoIt2(const  x:  ITest;  ACount  :  Integer);


begin
   //  No  change  to  reference  count.
   x.DoTest(ACount);
end;


Procedure  Test1;


var
   y:  ITest;
begin



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   y  :=  TTest.Create;
   //  Ref.  count  is  1  at  this  point.
   y.DoTest(1);
   //  Calling  DoIT  will  increase  reference  count  and  decrease  on  exit.
   DoIt1(y,2);
   //  Reference  count  is  still  one.
   y.DoTest(3);
end;


Procedure  Test2;


var
   Y  :  TTest;
begin
   Y  :=  TTest.Create;  //  no  count  on  the  object  yet
   //  Ref.  count  is  0  at  this  point.
   y.DoTest(3);
   //  Ref  count  will  remain  zero.
   DoIt2(y,4);
   Y.DoTest(5);
   Y.Free;
end;


Procedure  Test3;


var
   Y  :  TTest;
begin
   Y  :=  TTest.Create;  //  no  count  on  the  object  yet
   //  Ref.  count  is  0  at  this  point.
   y.DoTest(6);
   //  Ref  count  will  remain  zero.
   DoIt1(y,7);
   y.DoTest(8);
end;



begin
   Test1;
   Test2;
   Test3;
end.


The output of this example is:


Test  1  :  ref  count:  1
Test  2  :  ref  count:  2
Test  3  :  ref  count:  1
Destroy  called
Test  3  :  ref  count:  0
Test  4  :  ref  count:  0
Test  5  :  ref  count:  0
Destroy  called
Test  6  :  ref  count:  0



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Test  7  :  ref  count:  1
Destroy  called
Test  8  :  ref  count:  0


As can be seen, in test3, the reference count is decreased from 1 to 0 at the end of the DoIt
call, causing the instance to be freed before the call returns.

The following small program demonstrates the reference counts used in strings:



14.6          Function  overloading


Function overloading simply means that the same function is defined more than once, but
each time with a different formal parameter list.  The parameter lists must differ at least in
one of its elements type.  When the compiler encounters a function call, it will look at the
function parameters to decide which one of the defined functions it should call.  This can be
useful when the same function must be defined for different types.  For example, in the RTL,
the Dec procedure could be defined as:


...
Dec(Var  I  :  Longint;decrement  :  Longint);
Dec(Var  I  :  Longint);
Dec(Var  I  :  Byte;decrement  :  Longint);
Dec(Var  I  :  Byte);
...


When the compiler encounters a call to the Dec function, it will first search which function
it should use.  It therefore checks the parameters in a function call, and looks if there is a
function definition which matches the specified parameter list.  If the compiler finds such a
function, a call is inserted to that function.  If no such function is found, a compiler error is
generated.

Functions  that  have  a  cdecl  modifier  cannot  be  overloaded.   (Technically,  because  this
modifier prevents the mangling of the function name by the compiler).

Prior to version 1.9 of the compiler, the overloaded functions needed to be in the same unit.
Now the compiler will continue searching in other units if it doesn't find a matching version
of an overloaded function in one unit, and if the overload keyword is present.

If the overload keyword is not present, then all overloaded versions must reside in the same
unit,  and  if  it  concerns  methods  part  of  a  class,  they  must  be  in  the  same  class,  i.e.  the
compiler will not look for overloaded methods in parent classes if the overload keyword was
not specified.



14.7          Forward  declared  functions


A  function  can  be  declared  without  having  it  followed  by  its  implementation,  by  having
it followed by the forward procedure.  The effective implementation of that function must
follow later in the module.  The function can be used after a forward declaration as if it had
been implemented already.  The following is an example of a forward declaration.


Program  testforward;
Procedure  First  (n  :  longint);  forward;



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Procedure  Second;
begin
   WriteLn  ('In  second.  Calling  first...');
   First  (1);
end;
Procedure  First  (n  :  longint);
begin
   WriteLn  ('First  received  :  ',n);
end;
begin
   Second;
end.


A function can be forward declared only once.  Likewise, in units, it is not allowed to have
a forward declared function of a function that has been declared in the interface part.  The
interface declaration counts as a forward declaration.  The following unit will give an error
when compiled:


Unit  testforward;
interface
Procedure  First  (n  :  longint);
Procedure  Second;
implementation
Procedure  First  (n  :  longint);  forward;
Procedure  Second;
begin
   WriteLn  ('In  second.  Calling  first...');
   First  (1);
end;
Procedure  First  (n  :  longint);
begin
   WriteLn  ('First  received  :  ',n);
end;
end.


Reversely,  functions  declared  in  the  interface  section  cannot  be  declared  forward  in  the
implementation section.  Logically, since they already have been declared.



14.8          External  functions


The external modifier can be used to declare a function that resides in an external object
file.  It allows to use the function in some code, and at linking time, the object file containing
the implementation of the function or procedure must be linked in.


        |______________________________________________________________________________________________________________|
        External directive



      --  ___ external directive __   external __  __|______________________________________________________________|_____-oe
                                                     |_ string constant __   __|____________________________________|__|

                                                                               |__|name___    string constant __  __|
                                                                                         index __   integer constant __   _|

        |______________________________________________________________________________________________________________|



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               It replaces, in effect, the function or procedure code block.  As an example:


               program  CmodDemo;
               {$Linklib  c}
               Const  P  :  PChar  =  'This  is  fun  !';
               Function  strlen  (P  :  PChar)  :  Longint;  cdecl;  external;
               begin
                  WriteLn  ('Length  of  (',p,')  :  ',strlen(p))
               end.


Remark:         The parameters in the declaration of the external function should match exactly the ones
               in the declaration in the object file.

               If the external modifier is followed by a string constant:


               external  'lname';


               Then this tells the compiler that the function resides in library 'lname'.  The compiler will
               then automatically link this library to the program.

               The name that the function has in the library can also be specified:


               external  'lname'  name  'Fname';


               This tells the compiler that the function resides in library 'lname', but with name 'Fname'.
               The compiler will then automatically link this library to the program, and use the correct
               name for the function.  Under Windows and os/2, the following form can also be used:


               external  'lname'  Index  Ind;


               This tells the compiler that the function resides in library 'lname', but with index Ind.  The
               compiler will then automatically link this library to the program, and use the correct index
               for the function.

               Finally, the external directive can be used to specify the external name of the function :


               external  name  'Fname';
               {$L  myfunc.o}


               This tells the compiler that the function has the name 'Fname'.  The correct library or object
               file (in this case myfunc.o) must still be linked, ensuring that the function 'Fname' is indeed
               included in the linking stage.



               14.9          Assembler  functions


               Functions and procedures can be completely implemented in assembly language.  To indicate
               this, use the assembler keyword:

                       |___________________________________________________________________________________________________________*
 *___|
                       Assembler functions


                     --  ___ asm block __    assembler __    ; __ declaration part __   asm statement __    _______________________*
 *_-oe

                       |___________________________________________________________________________________________________________*
 *___|


               Contrary to Delphi, the assembler keyword must be present to indicate an assembler function.
               For more information about assembler functions, see the chapter on using assembler in the
               Programmer's Guide           .



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               14.10            Modifiers


               A function or procedure declaration can contain modifiers.  Here we list the various possibil-
               ities:


                       |___________________________________________________________________________________________________________*
 *___|
                       Modifiers



                     --  ___ modifiers __  __|_;__ _ public __ __ ________________________________________|________________________*
 *__________-oe
                                             6|   |              ||_ name __    string constant __  _|| | |
                                             |    |___________________        __  ___________________   | |
                                             |    |                    export                        |    |
                                             |     ________ alias __  : __string constant __  ________    |
                                             |    |__________________           __  __________________|   |
                                             |    |                   interrupt                       |   |
                                             |     __________________ iochecks __   __________________    |
                                             |    ||________________                __  ______________|_| |
                                             ||______________________call_modifiers_______________________|_____|



                     --  ___ call modifiers __  __|_______ cdecl __ _______|_______________________________________________________*
 *______-oe

                                                  |_______ inline __ _______|
                                                  |_______ local __ _______|
                                                  |_ nostackframe __     _|
                                                  |_____ overload __   _____|
                                                  |______ pascal __  ______|
                                                  |_____ register __  _____|
                                                  |_____ safecall __ _____|
                                                  |__ saveregisters __   __|
                                                  |_____ softfloat __  _____|
                                                  |______|stdcall__________|
                                                                      varargs __  ______|

                       |___________________________________________________________________________________________________________*
 *___|


               Free  Pascal  doesn't  support  all  Turbo  Pascal  modifiers  (although  it  parses  them  for  com-
               patibility),  but  does  support  a  number  of  additional  modifiers.  They  are  used  mainly  for
               assembler and reference to C object files.



               14.10.1          alias

               The  alias  modifier  allows  the  programmer  to  specify  a  different  name  for  a  procedure
               or  function.  This  is  mostly  useful  for  referring  to  this  procedure  from  assembly  language
               constructs or from another object file.  As an example, consider the following program:


               Program  Aliases;


               Procedure  Printit;alias  :  'DOIT';
               begin
                   WriteLn  ('In  Printit  (alias  :  "DOIT")');
               end;
               begin
                   asm
                   call  DOIT
                   end;
               end.


Remark:         The specified alias is inserted straight into the assembly code, thus it is case sensitive.



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               The alias modifier does not make the symbol public to other modules, unless the routine
               is also declared in the interface part of a unit, or the public modifier is used to force it as
               public.  Consider the following:



               unit  testalias;


               interface


               procedure  testroutine;


               implementation


               procedure  testroutine;alias:'ARoutine';
               begin
                  WriteLn('Hello  world');
               end;


               end.


               This will make the routine testroutine available publicly to external object files under the
               label name ARoutine.

Remark:         The alias directive is considered deprecated.  Please use the public  name directive.  See
               section 14.10.12   , page 207  .



               14.10.2          cdecl

               The cdecl modifier can be used to declare a function that uses a C type calling convention.
               This must be used when accessing functions residing in an object file generated by standard
               C compilers, but must also be used for Pascal functions that are to be used as callbacks for
               C libraries.

               The cdecl modifier allows to use C function in the code.  For external C functions, the object
               file containing the C implementation of the function or procedure must be linked in.  As an
               example:


               program  CmodDemo;
               {$LINKLIB  c}
               Const  P  :  PChar  =  'This  is  fun  !';
               Function  StrLen(P:  PChar):  Longint;cdecl;  external  name  'strlen';
               begin
                   WriteLn  ('Length  of  (',p,')  :  ',StrLen(p));
               end.


               When compiling this, and linking to the C-library, the strlen function can be called through-
               out the program.  The external directive tells the compiler that the function resides in an
               external object file or library with the 'strlen' name (see 14.8  ).

Remark:         The parameters in our declaration of the C function should match exactly the ones in the
               declaration in C.

               For functions that are not external, but which are declared using cdecl, no external linking is
               needed.  These functions have some restrictions, for instance the array  of  const construct
               can not be used (due the way this uses the stack).  On the other hand, the cdecl modifier
               allows these functions to be used as callbacks for routines written in C, as the latter expect
               the 'cdecl' calling convention.



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               14.10.3          export

               The export modifier is used to export names when creating a shared library or an executable
               program.  This means that the symbol will be publicly available, and can be imported from
               other  programs.   For  more  information  on  this  modifier,  consult  the  section  on  "Making
               libraries" in the Programmer's Guide           .



               14.10.4          inline

               Procedures that are declared inline are copied to the places where they are called.  This
               has the effect that there is no actual procedure call, the code of the procedure is just copied
               to  where  the  procedure  is  needed,  this  results  in  faster  execution  speed  if  the  function  or
               procedure is used a lot.  It is obvious that inlining large functions does not make sense.

               By  default,  inline  procedures  are  not  allowed.   Inline  code  must  be  enabled  using  the
               command-line switch -Si or {$inline  on} directive.

Remark:


                   1.  inline  is  only  a  hint  for  the  compiler.   This  does  not  automatically  mean  that  all
                       calls are inlined; sometimes the compiler may decide that a function simply cannot be
                       inlined, or that a particular call to the function cannot be inlined.  If so, the compiler
                       will emit a warning.

                   2.  In old versions of Free Pascal, inline code was not exported from a unit.  This meant
                       that when calling an inline procedure from another unit, a normal procedure call will
                       be performed.  Only inside units, Inline procedures are really inlined.  As of version
                       2.0.2, inline works accross units.

                   3.  Recursive inline functions are not allowed.  i.e.  an inline function that calls itself is not
                       allowed.



               14.10.5          interrupt

               The  interrupt  keyword  is  used  to  declare  a  routine  which  will  be  used  as  an  interrupt
               handler.  On entry to this routine, all the registers will be saved and on exit, all registers will
               be restored and an interrupt or trap return will be executed (instead of the normal return
               from subroutine instruction).

               On platforms where a return from interrupt does not exist, the normal exit code of routines
               will be done instead.  For more information on the generated code, consult the Programmer's Guide           .



               14.10.6          iocheck

               The  iocheck  keyword  is  used  to  declare  a  routine  which  causes  generation  of  I/O  result
               checking code within a {$IOCHECKS  ON} block whenever it is called.

               The result is that if a call to this procedure is generated, the compiler will insert I/O checking
               code if the call is within a {$IOCHECKS  ON} block.

               This modifier is intended for RTL internal routines, not for use in application code.



               14.10.7          local

               The  local  modifier  allows  the  compiler  to  optimize  the  function:  a  local  function  cannot
               be in the interface section of a unit:  it is always in the implementation section of the unit.
               From this it follows that the function cannot be exported from a library.



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On Linux, the local directive results in some optimizations.  On Windows, it has no effect.
It was introduced for Kylix compatibility.



14.10.8          noreturn

The  noreturn  modifier  can  be  used  to  tell  the  compiler  the  procedure  does  not  return.
This information can used by the compiler to avoid emitting warnings about uninitialized
variables or results not being set.

In the following example, the compiler will not emit a warning that the result may not be
set in function f:


procedure  do_halt;noreturn;
begin
    halt(1);
end;


function  f(i  :  integer)  :  integer  ;


begin
    if  (i<0)    then
       do_halt
    else
       result:=i;
end;



14.10.9          nostackframe

The nostackframe modifier can be used to tell the compiler it should not generate a stack
frame for this procedure or function.  By default, a stack frame is always generated for each
procedure or function.

One  should  be  extremely  careful  when  using  this  modifier:  most  procedures  or  functions
need a stack frame.  Particularly for debugging they are needed.



14.10.10           overload

The  overload  modifier  tells  the  compiler  that  this  function  is  overloaded.   It  is  mainly
for Delphi compatibility, as in Free Pascal, all functions and procedures can be overloaded
without this modifier.

There  is  only  one  case  where  the  overload  modifier  is  mandatory:  if  a  function  must  be
overloaded that resides in another unit.  Both functions must be declared with the overload
modifier:  the overload modifier tells the compiler that it should continue looking for over-
loaded versions in other units.

The following example illustrates this.  Take the first unit:


unit  ua;


interface


procedure  DoIt(A  :  String);  overload;


implementation



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procedure  DoIt(A  :  String);


begin
   Writeln('ua.DoIt  received  ',A)
end;


end.


And a second unit, which contains an overloaded version:


unit  ub;


interface


procedure  DoIt(A  :  Integer);  overload;


implementation


procedure  DoIt(A  :  integer);


begin
   Writeln('ub.DoIt  received  ',A)
end;


end.


And the following program, which uses both units:


program  uab;


uses  ua,ub;


begin
   DoIt('Some  string');
end.


When the compiler starts looking for the declaration of DoIt, it will find one in the ub unit.
Without the overload directive, the compiler would give an argument mismatch error:


home:  >fpc  uab.pp
uab.pp(6,21)  Error:  Incompatible  type  for  arg  no.  1:
Got  "Constant  String",  expected  "SmallInt"


With the overload directive in place at both locations, the compiler knows it must continue
searching for an overloaded version with matching parameter list.  Note that both declarations
must have the overload modifier specified; it is not enough to have the modifier in unit ub.
This  is  to  prevent  unwanted  overloading:  the  programmer  who  implemented  the  ua  unit
must mark the procedure as fit for overloading.



14.10.11           pascal

The  pascal  modifier  can  be  used  to  declare  a  function  that  uses  the  classic  Pascal  type
calling  convention  (passing  parameters  from  left  to  right).   For  more  information  on  the
Pascal calling convention, consult the Programmer's Guide           .



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14.10.12           public

The Public keyword is used to declare a function globally in a unit.  This is useful if the
function should not be accessible from the unit file (i.e.  another unit/program using the unit
doesn't see the function), but must be accessible from the object file.  As an example:


Unit  someunit;
interface
Function  First  :  Real;
Implementation
Function  First  :  Real;
begin
    First  :=  0;
end;
Function  Second  :  Real;  [Public];
begin
    Second  :=  1;
end;
end.


If another program or unit uses this unit, it will not be able to use the function Second, since it
isn't declared in the interface part.  However, it will be possible to access the function Second
at the assembly-language level, by using its mangled name (see the Programmer's Guide           ).

The public modifier can also be followed by a name directive to specify the assembler name,
as follows:


Unit  someunit;
interface
Function  First  :  Real;
Implementation
Function  First  :  Real;
begin
    First  :=  0;
end;
Function  Second  :  Real;  Public  name  'second';
begin
    Second  :=  1;
end;
end.


The  assembler  symbol  as  specified  by  the  'public  name'  directive  will  be  'second',  in  all
lowercase letters.



14.10.13           register

The register keyword is used for compatibility with Delphi.  In version 1.0.x of the compiler,
this directive has no effect on the generated code.  As of the 1.9.X versions, this directive is
supported.  The first three arguments are passed in registers EAX,ECX and EDX.



14.10.14           safecall

The safecall modifier ressembles closely the stdcall modifier.  It sends parameters from
right to left on the stack.  Additionally, the called procedure saves and restores all registers.



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More information about this modifier can be found in the Programmer's Guide           , in the section
on the calling mechanism and the chapter on linking.



14.10.15           saveregisters

The saveregisters modifier tells the compiler that all CPU registers should be saved prior
to calling this routine.  Which CPU registers are saved, depends entirely on the CPU.



14.10.16           softfloat

The softfloat modifier makes sense only on the ARM architecture.



14.10.17           stdcall

The stdcall modifier pushes the parameters from right to left on the stack, it also aligns
all the parameters to a default alignment.

More information about this modifier can be found in the Programmer's Guide           , in the section
on the calling mechanism and the chapter on linking.



14.10.18           varargs

This modifier can only be used together with the cdecl modifier, for external C procedures.
It indicates that the procedure accepts a variable number of arguments after the last declared
variable.  These arguments are passed on without any type checking.  It is equivalent to using
the array  of  const construction for cdecl procedures, without having to declare the array
of  const.  The square brackets around the variable arguments do not need to be used when
this form of declaration is used.

The following declarations are 2 ways of referring to the same function in the C library:


Function  PrintF1(fmt  :  pchar);  cdecl;  varargs;
                                                   external  'c'  name  'printf';
Function  PrintF2(fmt  :  pchar;  Args  :  Array  of  const);  cdecl;
                                                   external  'c'  name  'printf';


But they must be called differently:


PrintF1('%d  %d\n',1,1);
PrintF2('%d  %d\n',[1,1]);



14.11            Unsupported  Turbo  Pascal  modifiers


The modifiers that exist in Turbo Pascal, but aren't supported by Free Pascal, are listed in
table (14.1  ).



                                       Table 14.1:  Unsupported modifiers


                                _Modifier______________________Why_not_supported_?______________
                                  Near           Free Pascal is a 32-bit compiler.
                                  Far            Free Pascal is a 32-bit compiler.



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The  compiler  will  give  a  warning  when  it  encounters  these  modifiers,  but  will  otherwise
completely ignore them.



                                                                 209




Chapter   15



Operator   overloading



15.1          Introduction


Free Pascal supports operator overloading.  This means that it is possible to define the action
of some operators on self-defined types, and thus allow the use of these types in mathematical
expressions.

Defining the action of an operator is much like the definition of a function or procedure, only
there are some restrictions on the possible definitions, as will be shown in the subsequent.

Operator  overloading  is,  in  essence,  a  powerful  notational  tool;  but  it  is  also  not  more
than that, since the same results can be obtained with regular function calls.  When using
operator overloading, it is important to keep in mind that some implicit rules may produce
some unexpected results.  This will be indicated.



15.2          Operator  declarations


To define the action of an operator is much like defining a function:


        |______________________________________________________________________________________________________________|
        Operator definitions



      --  ___ operator definition __    operator __   __|__ assignment operator definition __      __|___-

                                                        |___|arithmetic_operator definition __     ___|
                                                                         comparision operator definition __      _|
      -  ______|_ result identifier ____|_ : __ result type __  ; __subroutine block __    ______________________________-oe
               |________________________|


      --  ___ assignment operator definition __      __|____ := __ ____|__ ( __ value parameter __     ) ________________-oe
                                                       |_ explicit __ _|


      --  ___ arithmetic operator definition __     __|___ + __ ___|__ ( __ parameter list __   ) _________________________-oe

                                                      |___ - _____|
                                                      |___ * _____|
                                                      |___ / _____|
                                                      |__|**___ __|
                                                              >< __  _|



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 *__________________



                    - - ___ comparision operator definition __      __||_  =  __||_ ( __ parameter list __   ) ____________________*
 *_____-oe

                                                                      ||_  <   _||
                                                                      ||_  <=  _||
                                                                      ||_  >   _||
                                                                      ||_  >=  _||
                                                                      ||_|_<>  _||
                                                                            in ___|

                      |____________________________________________________________________________________________________________*
 *___|


               The parameter list for a comparison operator or an arithmetic operator must always contain
               2  parameters,  with  the  exception  of  the  unary  minus,  where  only  1  parameters  is  needed.
               The result type of the comparison operator must be Boolean.

Remark:         When  compiling  in  Delphi  mode  or  Objfpc  mode,  the  result  identifier  may  be  dropped.
               The result can then be accessed through the standard Result symbol.

               If  the  result  identifier  is  dropped  and  the  compiler  is  not  in  one  of  these  modes,  a  syntax
               error will occur.

               The statement block contains the necessary statements to determine the result of the oper-
               ation.  It can contain arbitrary large pieces of code; it is executed whenever the operation is
               encountered in some expression.  The result of the statement block must always be defined;
               error conditions are not checked by the compiler, and the code must take care of all possible
               cases, throwing a run-time error if some error condition is encountered.

               In the following, the three types of operator definitions will be examined.  As an example,
               throughout this chapter the following type will be used to define overloaded operators on :


               type
                  complex  =  record
                     re  :  real;
                     im  :  real;
                  end;


               This type will be used in all examples.

               The sources of the Run-Time Library contain 2 units that heavily use operator overloading:


               ucomplex        This unit contains a complete calculus for complex numbers.

               matrix      This unit contains a complete calculus for matrices.



               15.3          Assignment  operators


               The assignment operator defines the action of a assignent of one type of variable to another.
               The result type must match the type of the variable at the left of the assignment statement,
               the single parameter to the assignment operator must have the same type as the expression
               at the right of the assignment operator.

               This system can be used to declare a new type, and define an assignment for that type.  For
               instance, to be able to assign a newly defined type 'Complex'


               Var
                   C,Z  :  Complex;  //  New  type  complex


               begin
                   Z:=C;    //  assignments  between  complex  types.
               end;



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               The following assignment operator would have to be defined:


               Operator  :=  (C  :  Complex)  z  :  complex;


               To be able to assign a real type to a complex type as follows:


               var
                  R  :  real;
                  C  :  complex;


               begin
                  C:=R;
               end;


               the following assignment operator must be defined:


               Operator  :=  (r  :  real)  z  :  complex;


               As can be seen from this statement, it defines the action of the operator := with at the right
               a real expression, and at the left a complex expression.

               An example implementation of this could be as follows:


               operator  :=  (r  :  real)  z  :  complex;


               begin
                  z.re:=r;
                  z.im:=0.0;
               end;


               As can be seen in the example, the result identifier (z in this case) is used to store the result
               of the assignment.  When compiling in Delphi mode or ObjFPC mode, the use of the special
               identifier Result is also allowed,  and can be substituted for the z,  so the above would be
               equivalent to


               operator  :=  (r  :  real)  z  :  complex;


               begin
                  Result.re:=r;
                  Result.im:=0.0;
               end;


               The assignment operator is also used to convert types from one type to another.  The compiler
               will consider all overloaded assignment operators till it finds one that matches the types of
               the left hand and right hand expressions.  If no such operator is found,  a 'type mismatch'
               error is given.

Remark:        The assignment operator is not commutative; the compiler will never reverse the role of the
               two arguments.  In other words, given the above definition of the assignment operator, the
               following is not possible:


               var
                  R  :  real;
                  C  :  complex;


               begin
                  R:=C;
               end;



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               If the reverse assignment should be possible then the assignment operator must be defined
               for that as well.  (This is not so for reals and complex numbers.)

Remark:         The assignment operator is also used in implicit type conversions.  This can have unwanted
               effects.  Consider the following definitions:


               operator  :=  (r  :  real)  z  :  complex;
               function  exp(c  :  complex)  :  complex;


               Then the following assignment will give a type mismatch:


               Var
                  r1,r2  :  real;


               begin
                  r1:=exp(r2);
               end;


               The mismatch occurs because the compiler will encounter the definition of the exp function
               with the complex argument.  It implicitly converts r2 to a complex, so it can use the above
               exp function.  The result of this function is a complex, which cannot be assigned to r1, so
               the compiler will give a 'type mismatch' error.  The compiler will not look further for another
               exp which has the correct arguments.

               It is possible to avoid this particular problem by specifying


                  r1:=system.exp(r2);


               When  doing  an  explicit  typecast,  the  compiler  will  attempt  an  implicit  conversion  if  an
               assignment operator is present.  That means that


               Var
                  R1  :  T1;
                  R2  :  T2;


               begin
                  R2:=T2(R1);


               Will be handled by an operator


               Operator  :=  (aRight:  T1)  Res:  T2;
               \begin{verbatim}
               However,  an  \var{Explicit}  operator  can  be  defined,  and  then  it  will  be
               used  instead  when  the  compiler  encounters  a  typecast.


               The  reverse  is  not  true:  In  a  regular  assignment,  the  compiler  will  not
               consider  explicit  assignment  operators.


               Given  the  following  definitions:
               \begin{verbatim}
               uses
                  sysutils;


               type
                  TTest1  =  record



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      f:  LongInt;
   end;
   TTest2  =  record
      f:  String;
   end;
   TTest3  =  record
      f:  Boolean;
   end;


It is possible to create assignment operators:


operator  :=  (aRight:  TTest1)  Res:  TTest2;
begin
   Writeln('Implicit  TTest1  =>  TTest2');
   Res.f  :=  IntToStr(aRight.f);
end;


operator  :=  (aRight:  TTest1)  Res:  TTest3;
begin
   Writeln('Implicit  TTest1  =>  TTest3');
   Res.f  :=  aRight.f  <>  0;
end;


But one can also define typecasting operators:


operator  Explicit(aRight:  TTest2)  Res:  TTest1;
begin
   Writeln('Explicit  TTest2  =>  TTest1');
   Res.f  :=  StrToIntDef(aRight.f,  0);
end;


operator  Explicit(aRight:  TTest1)  Res:  TTest3;
begin
   Writeln('Explicit  TTest1  =>  TTest3');
   Res.f  :=  aRight.f  <>  0;
end;


Thus, the following code


var
   t1:  TTest1;
   t2:  TTest2;
   t3:  TTest3;
begin
   t1.f  :=  42;
   //  Implicit
   t2  :=  t1;
   //  theoretically  explicit,  but  implicit  op  will  be  used,
   //  because  no  explicit  operator  is  defined
   t2  :=  TTest2(t1);
   //  the  following  would  not  compile,
   //  no  assignment  operator  defined  (explicit  one  won't  be  used  here)
   //t1  :=  t2;
   //  Explicit



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   t1  :=  TTest1(t2);
   //  first  explicit  (TTest2  =>  TTest1)  then  implicit  (TTest1  =>  TTest3)
   t3  :=  TTest1(t2);
   //  Implicit
   t3  :=  t1;
   //  explicit
   t3  :=  TTest3(t1);
end.


will produce the following output:


Implicit  TTest1  =>  TTest2
Implicit  TTest1  =>  TTest2
Explicit  TTest2  =>  TTest1
Explicit  TTest2  =>  TTest1
Implicit  TTest1  =>  TTest3
Implicit  TTest1  =>  TTest3
Explicit  TTest1  =>  TTest3



15.4          Arithmetic  operators


Arithmetic operators define the action of a binary operator.  Possible operations are:


multiplication          To multiply two types, the * multiplication operator must be overloaded.

division      To divide two types, the / division operator must be overloaded.

addition       To add two types, the + addition operator must be overloaded.

substraction         To subtract two types, the - subtraction operator must be overloaded.

exponentiation           To exponentiate two types, the ** exponentiation operator must be over-
        loaded.

Unary minus           is used to take the negative of the argument following it.

Symmetric Difference                To take the symmetric difference of 2 structures, the >< operator
        must be overloaded.


The definition of an arithmetic operator takes two parameters, except for unary minus, which
needs only 1 parameter.  The first parameter must be of the type that occurs at the left of
the operator, the second parameter must be of the type that is at the right of the arithmetic
operator.  The result type must match the type that results after the arithmetic operation.

To compile an expression as


var
    R  :  real;
    C,Z  :  complex;


begin
    C:=R*Z;
end;


One needs a definition of the multiplication operator as:



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Operator  *  (r  :  real;  z1  :  complex)  z  :  complex;


begin
   z.re  :=  z1.re  *  r;
   z.im  :=  z1.im  *  r;
end;


As can be seen, the first operator is a real, and the second is a complex.  The result type is
complex.

Multiplication and addition of reals and complexes are commutative operations.  The com-
piler,  however,  has  no  notion  of  this  fact  so  even  if  a  multiplication  between  a  real  and  a
complex is defined, the compiler will not use that definition when it encounters a complex
and a real (in that order).  It is necessary to define both operations.

So, given the above definition of the multiplication, the compiler will not accept the following
statement:


var
   R  :  real;
   C,Z  :  complex;


begin
   C:=Z*R;
end;


Since the types of  Z and R don't match the types in the operator definition.

The reason for this behaviour is that it is possible that a multiplication is not always com-
mutative.  E.g.  the  multiplication  of  a  (n,m)  with  a  (m,n)  matrix  will  result  in  a  (n,n)
matrix, while the multiplication of a (m,n) with a (n,m) matrix is a (m,m) matrix, which
needn't be the same in all cases.



15.5          Comparison  operator


The comparison operator can be overloaded to compare two different types or to compare
two equal types that are not basic types.  The result type of a comparison operator is always
a boolean.

The comparison operators that can be overloaded are:


equal to       (=) To determine if two variables are equal.

unequal to         (<>) To determine if two variables are different.

less than       (<) To determine if one variable is less than another.

greater than         (>) To determine if one variable is greater than another.

greater than or equal to                (>=) To determine if one variable is greater than or equal to
        another.

less than or equal to             (<=) To determine if one variable is greater than or equal to an-
        other.


If  there  is  no  separate  operator  for  unequal  to  (<>),  then,  to  evaluate  a  statement  that
contains the unequal to operator, the compiler uses the equal to operator (=), and negates



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the result.  The opposite is not true:  if no "equal to" but an "unequal to" operator exists,
the compiler will not use it to evaluate an expression containing the equal (=) operator.

As an example, the following operator allows to compare two complex numbers:


operator  =  (z1,  z2  :  complex)  b  :  boolean;


the above definition allows comparisons of the following form:


Var
   C1,C2  :  Complex;


begin
   If  C1=C2  then
      Writeln('C1  and  C2  are  equal');
end;


The comparison operator definition needs 2 parameters, with the types that the operator is
meant to compare.  Here also, the compiler doesn't apply commutativity:  if the two types
are different, then it is necessary to define 2 comparison operators.

In the case of complex numbers,  it is,  for instance necessary to define 2 comparisons:  one
with the complex type first, and one with the real type first.

Given the definitions


operator  =  (z1  :  complex;r  :  real)  b  :  boolean;
operator  =  (r  :  real;  z1  :  complex)  b  :  boolean;


the following two comparisons are possible:


Var
   R,S  :  Real;
   C  :  Complex;


begin
   If  (C=R)  or  (S=C)  then
    Writeln  ('Ok');
end;


Note that the order of the real and complex type in the two comparisons is reversed.



15.6          In  operator


As of version 2.6 of Free pascal, the In operator can be overloaded as well.  The first argument
of the in operator must be the operand on the left of the in keyword.  The following overloads
the in operator for records:



{$mode  objfpc}{$H+}


type
    TMyRec  =  record  A:  Integer  end;


operator  in  (const  A:  Integer;  const  B:  TMyRec):  boolean;



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begin
   Result  :=  A  =  B.A;
end;


var
   R:  TMyRec;
begin
   R.A  :=  10;
   Writeln(1  in  R);  //  false
   Writeln(10  in  R);  //  true
end.


The in operator can also be overloaded for other types than ordinal types, as in the following
example:


{$mode  objfpc}{$H+}


type
   TMyRec  =  record  A:  Integer  end;


operator  in  (const  A:  TMyRec;  const  B:  TMyRec):  boolean;
begin
   Result  :=  A.A  =  B.A;
end;


var
   S,R:  TMyRec;
begin
   R.A  :=  10;
   S.A:=1;
   Writeln(S  in  R);  //  false
   Writeln(R  in  R);  //  true
end.



                                                                 218




Chapter   16



Programs,   units,   blocks



A Pascal program can consist of modules called units.  A unit can be used to group pieces
of code together, or to give someone code without giving the sources.  Both programs and
units consist of code blocks, which are mixtures of statements, procedures, and variable or
type declarations.



16.1          Programs


A Pascal program consists of the program header, followed possibly by a 'uses' clause, and
a block.


        |______________________________________________________________________________________________________________|
        Programs



      --  ___ program __    program header __     ; ____|____________________|__ block __  . ______________________________-oe
                                                        |_ uses clause __  _|


      --  ___ program header __      program __    identifier ____|________________________________________|_____________-oe
                                                                  |_ ( __ program parameters __       ) ___|

      --  ___ program parameters __       identifier list ___________________________________________________________-oe


      --  ___ uses clause __   uses __ __  _ identifier ____ ______________________________ ; _____________________________-oe
                                         6||                ||_ in __ string literal __|_|||
                                         |____________________ , ______________________|


        |______________________________________________________________________________________________________________|


The program header is provided for backwards compatibility, and is ignored by the compiler.

The uses clause serves to identify all units that are needed by the program.  All identifiers
which are declared in the interface section of the units in the uses clause are added to the
known identifiers of the program.  The system unit doesn't have to be in this list, since it is
always loaded by the compiler.

The  order  in  which  the  units  appear  is  significant,  it  determines  in  which  order  they  are
initialized.  Units are initialized in the same order as they appear in the uses clause.  Identifiers
are  searched  in  the  opposite  order,  i.e.  when  the  compiler  searches  for  an  identifier,  then
it  looks  first  in  the  last  unit  in  the  uses  clause,  then  the  last  but  one,  and  so  on.  This  is
important in case two units declare different types with the same identifier.



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The compiler will look for compiled versions or source versions of all units in the uses clause
in the unit search path.  If the unit filename was explicitly mentioned using the in keyword,
the source is taken from the filename specified:


program  programb;


uses  unita  in  '..\unita.pp';


unita is searched in the parent directory of the programb source file.

When the compiler looks for unit files, it adds the extension .ppu to the name of the unit.
On  linux  and  in  operating  systems  where  filenames  are  case  sensitive  when  looking  for  a
unit, the following mechanism is used:


    1.  The unit is first looked for in the original case.

    2.  The unit is looked for in all-lowercase letters.

    3.  The unit is looked for in all-uppercase letters.


Additionally, If a unit name is longer than 8 characters, the compiler will first look for a unit
name  with  this  length,  and  then  it  will  truncate  the  name  to  8  characters  and  look  for  it
again.  For compatibility reasons, this is also true on platforms that support long file names.

Note that the above search is performed in each directory in the search path.

The  program  block  contains  the  statements  that  will  be  executed  when  the  program  is
started.  Note  that  these  statements  need  not  necessarily  be  the  first  statements  that  are
executed:  the initialization code of the units may also contain statements that are executed
prior to the program code.

The structure of a program block is discussed below.



16.2          Units


A unit contains a set of declarations, procedures and functions that can be used by a program
or another unit.  The syntax for a unit is as follows:

        |______________________________________________________________________________________________________________|
        Units


      --  ___ unit __ unit header __   interface part __   implementation part __    -_
      -  ______|______________________________________________________|__ end __   . ______________________________________-oe
               |_ initialization part __ __ ____________________________|
               |                           ||_                   __  _|||
               ||____________                  finalization part        |
                              begin __  __|_ statement __  __|_____________|
                                          6|_______ ; _________|


                                          _________ . ___________|
      --  ___ unit header __   unit __ __|?|_ unit identifier ____|___|________________________|__ ; ___________________________-oe
                                                                      |_ hint directives __  _|

      --  ___ interface part __   interface __  __|____________________|____ ___|____________________________________|__________-oe
                                                  |_ uses clause __  _|     6|| |_ constant declaration part __     _|  ||
                                                                            |    ____ type declaration part __    ____  |
                                                                            |   |__                           __    __| |
                                                                            |   |   variable declaration part         | |
                                                                            |    __ property declaration part __     __ |
                                                                            |   ||___                        __     ___|||
                                                                            ||________procedure_headers_part____________|_________|



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     - - ___ procedure headers part __     __|_ procedure header __    _|__ ; ____|__________________________|_____________-oe
                                             |__ function header __   __|         |_ call modifiers __   ; ___|


     - - ___ implementation part __      implementation __      __|____________________|__ declaration part __   _________-oe
                                                                  |_ uses clause __  _|


     - - ___ initialization part __  initialization __  __  _ statement __ ____________________________________________-oe
                                                          6||_______ ; _______|_|


     - - ___ finalization part __   finalization __ __  _ statement __ _______________________________________________-oe
                                                      6||_______ ; _______|_|


       |_______________________________________________________________________________________________________________|


As can be seen from the syntax diagram, a unit always consists of a interface and an imple-
mentation part.  Optionally, there is an initialization block and a finalization block, contain-
ing code that will be executed when the program is started, and when the program stops,
respectively.

Both the interface part or implementation part can be empty, but the keywords Interface
and implementation must be specified.  The following is a completely valid unit;


unit  a;


interface


implementation


end.


The interface part declares all identifiers that must be exported from the unit.  This can be
constant, type or variable identifiers, and also procedure or function identifier declarations.
The interface part cannot contain code that is executed:  only declarations are allowed.  The
following is a valid interface part:


unit  a;


interface


uses  b;


Function  MyFunction  :  SomeBType;


Implementation


The type SomeBType is defined in unit b.

All functions and methods that are declared in the interface part must be implemented in the
implementation part of the unit, except for declarations of external functions or procedures.
If a declared method or function is not implemented in the implementation part, the compiler
will give an error, for example the following:


unit  unita;


interface


Function  MyFunction  :  Integer;



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implementation


end.


Will result in the following error:


unita.pp(5,10)  Error:  Forward  declaration  not  solved  "MyFunction:SmallInt;"


The implementation part is primarily intended for the implementation of the functions and
procedures declared in the interface part.  However,  it can also contain declarations of it's
own:  the declarations inside the implementation part are not accessible outside the unit.

The initialization and finalization part of a unit are optional.

The initialization block is used to initialize certain variables or execute code that is necessary
for the correct functioning of the unit.  The initialization parts of the units are executed in
the order that the compiler loaded the units when compiling a program.  They are executed
before the first statement of the program is executed.

The  finalization  part  of  the  units  are  executed  in  the  reverse  order  of  the  initialization
execution.  They are used for instance to clean up any resources allocated in the initialization
part  of  the  unit,  or  during  the  lifetime  of  the  program.   The  finalization  part  is  always
executed in the case of a normal program termination:  whether it is because the final end is
reached in the program code or because a Halt instruction was executed somewhere.

In case the program stops during the execution of the initialization blocks of one of the units,
only the units that were already initialized will be finalized.    Note that in difference with
Delphi,  in  Free  Pascal  a  finalization  block  can  be  present  without  an  Initialization
block.  That means the following will compile in Free Pascal, but not in Delphi.


Finalization
   CleanupUnit;
end.


An  initialization  section  by  itself  (i.e.   without  finalization)  may  simply  be  replaced  by  a
statement block.  That is, the following:


Initialization
   InitializeUnit;
end.


is completely equivalent to


Begin
   InitializeUnit;
end.



16.3          Namespaces:   Dotted  Units


As can be seen in the syntax diagram for a unit, a unit name can contain dots.  This means
that the units can be organized in namespaces.

So, the following is a correct unit declaration:


unit  a.b;



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interface


Function  C  :  integer;


implementation


Function  C  :  integer;


begin
   Result:=1;
end;


end.


The unit can be used as follows:


program  d;


uses  a.b;


begin
   Writeln(c);
end.


When resolving symbols, unit scopes always take precedence over symbols inside units.

Given the following units:


unit  myunit;


interface


var
   test:  record
      a:  longint;
   end;


implementation


initialization
   test.a:=2;
end.


and


unit  myunit.test;


interface


var
   a:  longint;


implementation



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initialization
   a:=1;
end.


The following program will resolve myunit.test.a to the variable a in unit myunit.test:


uses
    myunit,  myunit.test;


begin
   Writeln('myunit.test.a  :  ',myunit.test.a);
end.


So it will print:


myunit.test.a  :  1


Reversing the order of the units will not change this:


uses
    myunit.test,  myunit;


begin
   Writeln('myunit.test.a  :  ',myunit.test.a);
end.


will also print


myunit.test.a  :  1


Similarly, the following program will resolve myunit.test.a to the variable a in unit myunit.test:


uses
    myunit.test,  myunit;


begin
   Writeln('a  :  ',a);
end.


it will print:


a  :  1


Similarly, the following program will resolve test.a to the variable test.a in unit myunit:


uses
    myunit.test,  myunit;


begin
   Writeln('test.a  :  ',test.a);
end.


will print


test.a  :  2



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16.4          Unit  dependencies


When a program uses a unit (say unitA) and this units uses a second unit, say unitB, then the
program depends indirectly also on unitB. This means that the compiler must have access
to unitB when trying to compile the program.  If the unit is not present at compile time, an
error occurs.

Note that the identifiers from a unit on which a program depends indirectly, are not accessible
to the program.  To have access to the identifiers of a unit, the unit must be in the uses clause
of the program or unit where the identifiers are needed.

Units can be mutually dependent, that is, they can reference each other in their uses clauses.
This is allowed, on the condition that at least one of the references is in the implementation
section of the unit.  This also holds for indirect mutually dependent units.

If  it  is  possible  to  start  from  one  interface  uses  clause  of  a  unit,  and  to  return  there  via
uses clauses of interfaces only, then there is circular unit dependence, and the compiler will
generate an error.  For example, the following is not allowed:


Unit  UnitA;
interface
Uses  UnitB;
implementation
end.


Unit  UnitB
interface
Uses  UnitA;
implementation
end.


But this is allowed :


Unit  UnitA;
interface
Uses  UnitB;
implementation
end.
Unit  UnitB
implementation
Uses  UnitA;
end.


Because UnitB uses UnitA only in its implentation section.

In general, it is a bad idea to have unit interdependencies, even if it is only in implementation
sections.



16.5          Blocks


Units and programs are made of blocks.  A block is made of declarations of labels, constants,
types,  variables  and  functions  or  procedures.   Blocks  can  be  nested  in  certain  ways,  i.e.,
a procedure or function declaration can have blocks in themselves.  A block looks like the
following:



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       |_______________________________________________________________________________________________________________|
       Blocks



     - - ___ block __  declaration part __   statement part __    __________________________________________________-oe


     - - ___ declaration part __   __|___ ____________________________________________________|____________________________-oe
                                     6|| |__________ label declaration part __   __________ | ||
                                     |   |________ constant declaration part __     _______|_ |
                                     |   |_____                                 __     _____ ||
                                     |   |      resourcestring declaration part             | |
                                     |    __________ type declaration part __    __________   |
                                     |   |________                           __    ________|  |
                                     |   |         variable declaration part               |  |
                                     |    _____ threadvariable declaration part __      _____ |
                                     |   ||_                                     __        _|||
                                     ||______procedure/function_declaration_part______________|_____________|



     - - ___ label declaration part __    label __ __  _ label _____ ; _________________________________________________-oe
                                                     6||____ , ___|__|


     - - ___ constant declaration part __      const __ __  ___ _____ constant declaration __    ________________________-oe
                                                          6||  ||_ typed constant declaration __      |_|||
                                                          |__________________________________________|


     - - __ resourcestring declaration part __      resourcestring __    __  _ string constant declaration __   ______-oe
                                                                           6||_____________________________________|_|


     - - ___ type declaration part __     type __ __  _ type declaration __ __________________________________________-oe
                                                    6||________________________|_|


     - - ___ variable declaration part __     var ____  _ variable declaration __ ___________________________________-oe
                                                      6||____________________________|_|


     - - ___ threadvariable declaration part __       threadvar __  __  _ variable declaration __ ___________________-oe
                                                                      6||____________________________|_|


     - - ___ procedure/function declaration part __        __|___ __ procedure declaration __     ____|___________________-oe
                                                             6|  |___ function declaration __    ___| |
                                                             |   |_                         __    _ | |
                                                             |   |  constructor declaration        |  |
                                                             |    __ destructor declaration __    __  |
                                                             |   ||_________________________________|_||
                                                             ||______________________________________||


     - - ___ statement part __    compound statement __       ______________________________________________________-oe


       |_______________________________________________________________________________________________________________|


Labels that can be used to identify statements in a block are declared in the label declaration
part of that block.  Each label can only identify one statement.

Constants that are to be used only in one block should be declared in that block's constant
declaration part.

Variables that are to be used only in one block should be declared in that block's variable
declaration part.

Types that are to be used only in one block should be declared in that block's type declaration
part.

Lastly,  functions  and  procedures  that  will  be  used  in  that  block  can  be  declared  in  the
procedure/function declaration part.



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These 4 declaration parts can be intermixed, there is no required order other than that you
cannot use (or refer to) identifiers that have not yet been declared.

After  the  different  declaration  parts  comes  the  statement  part.  This  contains  any  actions
that the block should execute.  All identifiers declared before the statement part can be used
in that statement part.



16.6          Scope


Identifiers are valid from the point of their declaration until the end of the block in which the
declaration occurred.  The range where the identifier is known is the scope of the identifier.
The exact scope of an identifier depends on the way it was defined.



16.6.1         Block  scope

The scope of a variable declared in the declaration part of a block, is valid from the point
of declaration until the end of the block.  If a block contains a second block,  in which the
identifier  is  redeclared,  then  inside  this  block,  the  second  declaration  will  be  valid.  Upon
leaving the inner block, the first declaration is valid again.  Consider the following example:


Program  Demo;
Var  X  :  Real;
{  X  is  real  variable  }
Procedure  NewDeclaration
Var  X  :  Integer;    {  Redeclare  X  as  integer}
begin
  //  X  :=  1.234;  {would  give  an  error  when  trying  to  compile}
  X  :=  10;  {  Correct  assignment}
end;
{  From  here  on,  X  is  Real  again}
begin
  X  :=  2.468;
end.


In this example, inside the procedure, X denotes an integer variable.  It has its own storage
space, independent of the variable X outside the procedure.



16.6.2         Record  scope

The field identifiers inside a record definition are valid in the following places:


    1.  To the end of the record definition.

    2.  Field designators of a variable of the given record type.

    3.  Identifiers inside a With statement that operates on a variable of the given record type.



16.6.3         Class  scope

A component identifier (one of the items in the class' component list) is valid in the following
places:


    1.  From the point of declaration to the end of the class definition.



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    2.  In all descendent types of this class, unless it is in the private part of the class decla-
        ration.

    3.  In all method declaration blocks of this class and descendent classes.

    4.  In a With statement that operators on a variable of the given class's definition.


Note that method designators are also considered identifiers.



16.6.4         Unit  scope

All identifiers in the interface part of a unit are valid from the point of declaration, until the
end of the unit.  Furthermore, the identifiers are known in programs or units that have the
unit in their uses clause.

Identifiers  from  indirectly  dependent  units  are  not  available.   Identifiers  declared  in  the
implementation part of a unit are valid from the point of declaration to the end of the unit.

The system unit is automatically used in all units and programs.  Its identifiers are therefore
always known, in each Pascal program, library or unit.

The rules of unit scope imply that an identifier of a unit can be redefined.  To have access
to an identifier of another unit that was redeclared in the current unit, precede it with that
other units name, as in the following example:


unit  unitA;
interface
Type
    MyType  =  Real;
implementation
end.
Program  prog;
Uses  UnitA;


{  Redeclaration  of  MyType}
Type  MyType  =  Integer;
Var  A  :  Mytype;          {  Will  be  Integer  }
       B  :  UnitA.MyType  {  Will  be  real  }
begin
end.


This is especially useful when redeclaring the system unit's identifiers.



16.7          Libraries


Free Pascal supports making of dynamic libraries (DLLs under Win32 and os/2) trough the
use of the Library keyword.

A Library is just like a unit or a program:


        |______________________________________________________________________________________________________________|
        Libraries



      --  ___ library __ library header __   ; ____|____________________|__ block __  . ___________________________________-oe
                                                   |_ uses clause __  _|



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 *___



     - - ___ library header __   library __  identifier _____________________________________________________________-oe


       |_______________________________________________________________________________________________________________|


By default, functions and procedures that are declared and implemented in library are not
available to a programmer that wishes to use this library.

In order to make functions or procedures available from the library, they must be exported
in an exports clause:


       |_______________________________________________________________________________________________________________|
       Exports clause



     - - ___ exports clause __    exports __   exports list __ ; ___________________________________________________-oe


     - - ___ exports list __ __  _ exports entry __ __________________________________________________________________-oe
                               6||_________ , _________|_|


     - -  exports entry __    identifier ____|____________________________________|____|____________________________________|_-
     -  _____________________________________|__index______integer_constant_______|____|__name_______string_constant___-o_|e



       |_______________________________________________________________________________________________________________|


Under Win32, an index clause can be added to an exports entry.  An index entry must be a
positive number larger or equal than 1, and less than MaxInt.

Optionally, an exports entry can have a name specifier.  If present, the name specifier gives
the exact name (case sensitive) by which the function will be exported from the library.

If neither of these constructs is present, the functions or procedures are exported with the
exact names as specified in the exports clause.



                                                                 229




               Chapter   17



               Exceptions



               Exceptions provide a convenient way to program error and error-recovery mechanisms, and
               are closely related to classes.  Exception support is based on 3 constructs:


               Raise       statements.  To raise an exception.  This is usually done to signal an error condition.
                       It is however also usable to abort execution and immediately return to a well-known
                       point in the executable.

               Try ...  Except          blocks.  These block serve to catch exceptions raised within the scope of
                       the block, and to provide exception-recovery code.

               Try ...  Finally         blocks.  These block serve to force code to be executed irrespective of an
                       exception occurrence or not.  They generally serve to clean up memory or close files in
                       case  an  exception  occurs.  The  compiler  generates  many  implicit  Try  ...    Finally
                       blocks around procedure, to force memory consistency.



               17.1          The  raise  statement


               The raise statement is as follows:


                       |___________________________________________________________________________________________________________*
 *___|
                       Raise statement



                     --  ___ raise statement __    raise __ __|__________________________________________________________|_________*
 *______-oe
                                                              |_ exception instance __    __|____________________________|__|
                                                                                            |_ exception address __    _|


                     --  ___ exception address __     at __ exception address __    __|________________________________|___________*
 *_____-oe
                                                                                      |_ , __ address expression __   _|

                       |___________________________________________________________________________________________________________*
 *___|


               This statement will raise an exception.  If it is specified, the exception instance must be an
               initialized instance of any class, which is the raise type.  The exception address and frame
               are optional.  If they are not specified, the compiler will provide the address by itself.  If the
               exception  instance  is  omitted,  then  the  current  exception  is  re-raised.  This  construct  can
               only be used in an exception handling block (see further).

Remark:          Control  never  returns  after  an  exception  block.   The  control  is  transferred  to  the  first
               try...finally or try...except statement that is encountered when unwinding the stack.



                                                                            230

               _________________________________________________________________________________________CHAPTER_17.___EXCEPTIONS___*
 *__________________



               If no such statement is found,  the Free Pascal Run-Time Library will generate a run-time
               error  217  (see  also  section  17.5  ,  page  234  ).  The  exception  address  will  be  printed  by  the
               default exception handling routines.

               As an example:  The following division checks whether the denominator is zero,  and if so,
               raises an exception of type EDivException


               Type  EDivException  =  Class(Exception);
               Function  DoDiv  (X,Y  :  Longint)  :  Integer;
               begin
                  If  Y=0  then
                     Raise  EDivException.Create  ('Division  by  Zero  would  occur');
                  Result  :=  X  Div  Y;
               end;


               The class Exception is defined in the Sysutils unit of the rtl.  (section 17.5  , page 234  )

Remark:         Although the Exception class is used as the base class for exceptions throughout the code,
               this is just an unwritten agreement: the class can be of any type, and need not be a descendent
               of the Exception class.

               Of course, most code depends on the unwritten agreement that an exception class descends
               from Exception.

               The following code shows how to omit an error reporting routine from the stack shown in
               the exception handler:


               {$mode  objfpc}
               uses  sysutils;


               procedure  error(Const  msg  :  string);


               begin
                  raise  exception.create(Msg)  at
                     get_caller_addr(get_frame),
                     get_caller_frame(get_frame);
               end;


               procedure  test2;


               begin
                  error('Error');
               end;


               begin
                  test2;
               end.


               The program, when run, will show a backtrace as follows:


               An  unhandled  exception  occurred  at  $00000000004002D3  :
               Exception  :  Error
                  $00000000004002D3  line  15  of  testme.pp
                  $00000000004002E6  line  19  of  testme.pp


               Line 15 is in procedure Test2, not in Error, which actually raised the exception.



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_________________________________________________________________________________________CHAPTER_17.___EXCEPTIONS__________________*
 *___



17.2          The  try...except  statement


A try...except exception handling block is of the following form :


        |______________________________________________________________________________________________________________|
        Try..except statement



      --  ___ try statement __    try __ statement list __   except __   exceptionhandlers __     end __ ____________-oe


      --  ___ statement list __  __  _ statement __ ___________________________________________________________________-oe
                                   6||_______ ; _______|_|


      --  ___ exceptionhandlers __    __|________________________________________________________________|_______________-oe
                                        |___  _ exception handler __  ____    __________________________________|
                                        |   6||____________   ___________|__|||_      __                 __  _|||
                                        ||__________________;___                 else     statement list        |
                                                                 statement list __  ______________________|

      --   exception handler __     on ____|______________________|__ class type identifier __   do __  statement __  ____-oe
                                           |_ identifier __ : ___|

        |______________________________________________________________________________________________________________|


If no exception is raised during the execution of the statement  list,  then all statements
in the list will be executed sequentially, and the except block will be skipped, transferring
program flow to the statement after the final end.

If an exception occurs during the execution of the statement  list, the program flow will
be transferred to the except block.  Statements in the statement list between the place where
the exception was raised and the exception block are ignored.

In  the  exception  handling  block,  the  type  of  the  exception  is  checked,  and  if  there  is  an
exception  handler  where  the  class  type  matches  the  exception  object  type,  or  is  a  parent
type of the exception object type, then the statement following the corresponding Do will be
executed.  The first matching type is used.  After the Do block was executed,  the program
continues after the End statement.

The  identifier  in  an  exception  handling  statement  is  optional,  and  declares  an  exception
object.  It can be used to manipulate the exception object in the exception handling code.
The scope of this declaration is the statement block following the Do keyword.

If none of the On handlers matches the exception object type, then the statement list after
else is executed.  If no such list is found, then the exception is automatically re-raised.  This
process allows to nest try...except blocks.

If, on the other hand, the exception was caught, then the exception object is destroyed at
the  end  of  the  exception  handling  block,  before  program  flow  continues.  The  exception  is
destroyed through a call to the object's Destroy destructor.

As an example, given the previous declaration of the DoDiv function, consider the following


Try
    Z  :=  DoDiv  (X,Y);
Except
    On  EDivException  do  Z  :=  0;
end;


If  Y happens to be zero, then the DoDiv function code will raise an exception.  When this
happens, program flow is transferred to the except statement, where the Exception handler
will set the value of  Z to zero.  If no exception is raised, then program flow continues past



                                                                 232

_________________________________________________________________________________________CHAPTER_17.___EXCEPTIONS__________________*
 *___



the last end statement.  To allow error recovery, the Try  ...    Finally block is supported.
A  Try...Finally  block  ensures  that  the  statements  following  the  Finally  keyword  are
guaranteed to be executed, even if an exception occurs.



17.3          The  try...finally  statement


A Try..Finally statement has the following form:


        |______________________________________________________________________________________________________________|
        Try...finally statement



      --  ___ trystatement __    try __ statement list __   finally __  finally statements __   end __ ______________-oe


      --  ___ finally statements __    statementlist __  ____________________________________________________________-oe


        |______________________________________________________________________________________________________________|


If  no  exception  occurs  inside  the  statement  List,  then  the  program  runs  as  if  the  Try,
Finally  and  End  keywords  were  not  present,  unless  an  exit  command  is  given:  an  exit
command first executes all statements in the finally blocks before actually exiting.

If, however, an exception occurs, the program flow is immediately transferred from the point
where the exception was raised to the first statement of the Finally  statements.

All  statements  after  the  finally  keyword  will  be  executed,  and  then  the  exception  will  be
automatically re-raised.  Any statements between the place where the exception was raised
and the first statement of the Finally  Statements are skipped.

As an example consider the following routine:


Procedure  Doit  (Name  :  string);
Var  F  :  Text;
begin
    Assign  (F,Name);
    Rewrite  (name);
    Try
       ...  File  handling  ...
    Finally
       Close(F);
    end;
end;


If  during  the  execution  of  the  file  handling  an  exception  occurs,  then  program  flow  will
continue at the close(F) statement, skipping any file operations that might follow between
the place where the exception was raised, and the Close statement.  If no exception occurred,
all file operations will be executed, and the file will be closed at the end.

Note  that  an  Exit  statement  enclosed  by  a  try  ..    finally  block,  will  still  execute  the
finally block.  Reusing the previous example:


Procedure  Doit  (Name  :  string);
Var
    F  :  Text;
    B  :  Boolean;
begin



                                                                 233

_________________________________________________________________________________________CHAPTER_17.___EXCEPTIONS__________________*
 *___



   B:=False;
   Assign  (F,Name);
   Rewrite  (name);
   Try
      //  ...  File  handling  ...
      if  B  then
         exit;  //  Stop  processing  prematurely
      //  More  file  handling
   Finally
      Close(F);
   end;
end;


The file will still be closed, even if the processing ends prematurely using the Exit statement.



17.4          Exception  handling  nesting


It is possible to nest Try...Except blocks with Try...Finally blocks.  Program flow will
be done according to a lifo (last in, first out) principle:  The code of the last encountered
Try...Except or Try...Finally block will be executed first.  If the exception is not caught,
or it was a finally statement, program flow will be transferred to the last-but-one block, ad
infinitum.

If an exception occurs, and there is no exception handler present which handles this exception,
then a run-time error 217 will be generated.  When using the SysUtils unit, a default handler is
installed which will show the exception object message, and the address where the exception
occurred, after which the program will exit with a Halt instruction.



17.5          Exception  classes


The sysutils unit contains a great deal of exception handling.  It defines the base exception
class, Exception


Exception  =  class(TObject)
private
    fmessage  :  string;
    fhelpcontext  :  longint;
public
    constructor  create(const  msg  :  string);
    constructor  createres(indent  :  longint);
    property  helpcontext  :  longint  read  fhelpcontext  write  fhelpcontext;
    property  message  :  string  read  fmessage  write  fmessage;
end;
ExceptClass  =  Class  of  Exception;


And uses this declaration to define quite a number of exceptions, for instance:


{  mathematical  exceptions  }
EIntError  =  class(Exception);
EDivByZero  =  class(EIntError);
ERangeError  =  class(EIntError);
EIntOverflow  =  class(EIntError);
EMathError  =  class(Exception);



                                                                 234

_________________________________________________________________________________________CHAPTER_17.___EXCEPTIONS__________________*
 *___



The  SysUtils  unit  also  installs  an  exception  handler.  If  an  exception  is  unhandled  by  any
exception handling block, this handler is called by the Run-Time library.  Basically, it prints
the exception address, and it prints the message of the Exception object, and exits with an
exit code of 217.  If the exception object is not a descendent object of the Exception object,
then the class name is printed instead of the exception message.

It is recommended to use the Exception object or a descendant class for all raise statements,
since then the message field of the exception object can be used.



                                                                 235




Chapter   18



Using   assembler



Free  Pascal  supports  the  use  of  assembler  in  code,  but  not  inline  assembler  macros.   To
have more information on the processor specific assembler syntax and its limitations, see the
Programmer's Guide           .



18.1          Assembler  statements


The following is an example of assembler inclusion in Pascal code.


  ...
  Statements;
  ...
  Asm
     the  asm  code  here
     ...
  end;
  ...
  Statements;


The assembler instructions between the Asm and end keywords will be inserted in the assem-
bler generated by the compiler.  Conditionals can be used in assembler code,  the compiler
will recognise them, and treat them as any other conditionals.



18.2          Assembler  procedures  and  functions


Assembler procedures and functions are declared using the Assembler directive.  This permits
the code generator to make a number of code generation optimizations.

The code generator does not generate any stack frame (entry and exit code for the routine)
if it contains no local variables and no parameters.  In the case of functions, ordinal values
must be returned in the accumulator.  In the case of floating point values, these depend on
the target processor and emulation options.



                                                             236




Index



Abstract, 75                                                    Dispatch, 92
Address, 158                                                    DispatchStr, 92
Alias, 202                                                      Double, 28
Ansistring, 31 , 35
Array, 39 , 193  , 194                                          else, 171  , 172
      Constructor, 44                                           except, 232  , 234
      Dynamic, 40                                               Exception, 230
      Of const, 194                                             Exceptions, 230
      Static, 39                                                      Catching, 230  , 232
array, 56                                                             Classes, 234
Array constructor, 44                                                 Handling, 233  , 234
Asm, 185                                                              Raising, 230
Assembler, 185  , 201  , 236                                    export, 204
                                                                Expression, 182  , 183
block, 225                                                      Expressions, 151
Boolean, 25                                                     Extended, 28
                                                                Extended records, 126
Case, 171                                                       External, 200
cdecl, 203                                                      external, 61 , 201
Char, 29
Class, 79 , 87                                                  Fields, 45 , 69
Class helpers, 131                                              File, 49
Classes, 79                                                     finally, 233  , 234
CodePage, 31 , 33                                               For, 174  , 175
      Conversion, 33                                                  downto, 174
COM, 55 , 108                                                         in, 175
Comments, 12                                                          to, 174
Comp, 28                                                        Forward, 51 , 199
Const, 22                                                       Function, 188
      String, 22                                                Functions, 187
Constants, 20                                                         Assembler, 201  , 236
      Ordinary, 20                                                    External, 200
      String, 18 , 21 , 35                                            Forward, 199
      Typed, 21                                                       Modifiers, 202
Constructor, 71 , 84 , 155                                            Overloaded, 199
CORBA, 55 , 108
Currency, 28                                                    Generics, 110
                                                                      Overloads, 117
Delphi                                                                Type Restrictions, 114
      Compatibility, 116
      Syntax, 116                                               Hint directives, 16
      Type overloads, 117
Destructor, 71 , 85                                             Identifiers, 15
Directives                                                      If, 172
      Hint, 16                                                  index,I96n,h201erited, 87



                                                             237

___________________________________________________________________________________________________________________________INDEX___*
 *___



inherited, 75 , 99                                              Objective-Pascal, 141
inline, 204                                                     Objective-Pascal Classes, 141
interface, 103                                                  Objects, 68
Interfaces, 55 , 57 , 103                                       Operators, 20 , 36 , 51 , 151  , 158  , 159
      COM, 108                                                        Arithmetic, 159  , 215
      CORBA, 108                                                      Assignment, 211
      Implementations, 105                                            Binary, 215
interrupt, 204                                                        Boolean, 160
iocheck, 204                                                          Comparison, 216
                                                                      Logical, 160
Labels, 18                                                            Relational, 163
Libraries, 228                                                        Set, 161
library, 228                                                          String, 161
local, 204                                                            Unary, 159

                                                                operators, 210
Message, 92                                                     otherwise, 171
message, 92                                                     overload, 205
Methods, 72 , 86                                                overloading
      Abstract, 75                                                    operators, 210
      Class, 87                                                 Override, 87
      Message, 92                                               override, 75
      Static, 73 , 90
      Virtual, 74 , 75 , 86                                     Packed, 46 , 69 , 85
Modifiers, 14 , 202  , 208                                      Parameters, 189
      Alias, 202                                                      Constant, 189  , 192
      cdecl, 203                                                      Open Array, 193  , 194
      export, 204                                                     Out, 191
      inline, 204                                                     Untypes, 189
      interrupt, 204                                                  Value, 189
      iocheck, 204                                                    Var, 96 , 189  , 190
      local, 204                                                pascal, 206
      noreturn, 205                                             PChar, 32 , 35
      nostackframe, 205                                         Pointer, 49
      overload, 205                                             Private, 78 , 81 , 95
      pascal, 206                                                     strict, 81
      public, 207                                               private, 69
      register, 207                                             Procedural, 52
      safecall, 207                                             Procedure, 52 , 187
      saveregisters, 208                                        Procedures, 187
      softfloat, 208                                            program, 219
      stdcall, 208                                              Properties, 64 , 94
      varargs, 208                                                    Array, 97

                                                                      Indexed, 96
name, 201                                                       Property, 88 , 94
noreturn, 205                                                   Protected, 78 , 81
nostackframe, 205                                               Public, 78 , 81
Numbers, 17                                                     public, 69 , 207
      Binary, 17                                                Published, 81 , 95
      Decimal, 17                                               PUnicodeChar, 35
      Hexadecimal, 17
      Octal, 17                                                 Raise, 230
      Real, 17                                                  Rawbytestring, 34

                                                                Read, 95
object, 68                                                      Real, 28



                                                                 238

___________________________________________________________________________________________________________________________INDEX___*
 *___



Record, 45                                                            WideString, 35
      Constant, 63                                              Symbols, 11
Record helpers, 131                                             Syntax diagrams, 9
register, 207
reintroduce, 87                                                 Text, 49
Repeat, 182                                                     then, 172
Reserved words, 13                                              Thread Variables, 64
      Delphi, 14                                                Threadvar, 64
      Free Pascal, 14                                           Tokens, 11
      Modifiers, 14                                                   Comments, 12
      Turbo Pascal, 13                                                Identifiers, 15
Resourcestring, 22                                                    Numbers, 17
                                                                      Reserved words, 13
safecall, 207                                                         Strings, 18
saveregisters, 208                                                    Symbols, 11
Scope, 32 , 42 , 62 , 68 , 78 , 80 , 227                        try, 233  , 234
      block, 227                                                Type, 23
      Class, 227                                                      Compatibility, 25 , 43 , 59 , 118  , 156  , 158  ,
      record, 227                                                           190
      unit, 228                                                       Generics, 110
Self, 72 , 88 , 93                                                    Helpers, 131
Set, 48                                                               Restrictions, 114
Shortstring, 30                                                       Type overloads, 117
Single, 28                                                      Typecast, 31 , 32 , 35 , 156  , 157
softfloat, 208                                                        Unaligned, 157
Statements, 167                                                       Value, 156
      Assembler, 185  , 236                                           Variable, 156
      Assignment, 167                                           Types, 23
      Case, 171                                                       Ansistring, 31
      Compound, 170                                                   Array, 39 , 40
      Exception, 185                                                  Base, 23
      For, 174  , 175                                                 Boolean, 25
      Goto, 169                                                       Char, 29
      if, 172                                                         Class, 79
      Loop, 174  , 175  , 182  , 183                                  Class helpers, 131
      Procedure, 168                                                  Constructor, 44
      Repeat, 182                                                     Enumeration, 26
      Simple, 167                                                     Extended record, 126
      Structured, 170                                                 File, 49
      While, 183                                                      Forward declaration, 51
      With, 183                                                       Integer, 24
Static class methods, 90                                              Object, 68
stdcall, 208                                                          Ordinal, 24
String, 18                                                            PChar, 32 , 36
      Ansistring, 31                                                  Pointer, 36 , 49
      CodePage, 31                                                    Procedural, 52
      Codepage, 33                                                    Rawbytestring, 34
      Constant, 35                                                    Real, 28
      Multi-byte, 35                                                  Record, 45
      RawByteString, 34                                               Record helpers, 131
      ShortString, 30                                                 Reference counted, 31 , 33 -35 , 40 , 42 , 108  ,
      Single-byte string, 29                                                196
      UnicodeString, 35                                               Set, 48
      UTF8String, 34                                                  ShortString, 30



                                                                 239

___________________________________________________________________________________________________________________________INDEX___*
 *___



      String, 29
      Structured, 37
      Subrange, 28
      Type helpers, 131
      UnicodeString, 34 , 35
      UTF8String, 34
      Variant, 55
      Widestring, 35


Unicodestring, 35
unit, 220  , 228
uses, 219
UTF8String, 34


Var, 60
varargs, 208
Variable, 60
Variables, 60
      Initialized, 21 , 62 , 63
Variant, 55
Virtual, 71 , 72 , 74 , 86 , 92
Visibility, 68 , 78 , 103
      Private, 68 , 81
      Protected, 81
      Public, 68 , 81
      Published, 81
      Strict Private, 81
      Strict Protected, 81


While, 183
WideChar, 29
Widestring, 35
With, 183
Write, 95



                                                                 240
