perltie - how to hide an object class in a simple variable


SYNOPSIS

        tie VARIABLE, CLASSNAME, LIST

        $object = tied VARIABLE

        untie VARIABLE


DESCRIPTION

       Prior to release 5.0 of Perl, a programmer could use
       dbmopen() to connect an on-disk database in the standard
       Unix dbm(3x) format magically to a %HASH in their program.
       However, their Perl was either built with one particular
       dbm library or another, but not both, and you couldn't
       extend this mechanism to other packages or types of vari­
       ables.

       Now you can.

       The tie() function binds a variable to a class (package)
       that will provide the implementation for access methods
       for that variable.  Once this magic has been performed,
       accessing a tied variable automatically triggers method
       calls in the proper class.  The complexity of the class is
       hidden behind magic methods calls.  The method names are
       in ALL CAPS, which is a convention that Perl uses to indi­
       cate that they're called implicitly rather than explic­
       itly--just like the BEGIN() and END() functions.

       In the tie() call, `VARIABLE' is the name of the variable
       to be enchanted.  `CLASSNAME' is the name of a class
       implementing objects of the correct type.  Any additional
       arguments in the `LIST' are passed to the appropriate con­
       structor method for that class--meaning TIESCALAR(),
       TIEARRAY(), TIEHASH(), or TIEHANDLE().  (Typically these
       are arguments such as might be passed to the dbminit()
       function of C.) The object returned by the "new" method is
       also returned by the tie() function, which would be useful
       if you wanted to access other methods in `CLASSNAME'. (You
       don't actually have to return a reference to a right
       "type" (e.g., HASH or `CLASSNAME') so long as it's a prop­
       erly blessed object.)  You can also retrieve a reference
       to the underlying object using the tied() function.

       Unlike dbmopen(), the tie() function will not `use' or
       `require' a module for you--you need to do that explicitly
       yourself.


       A class implementing a tied scalar should define the fol­
       lowing methods: TIESCALAR, FETCH, STORE, and possibly
       DESTROY.

       Let's look at each in turn, using as an example a tie
       class for scalars that allows the user to do something
       like:

           tie $his_speed, 'Nice', getppid();
           tie $my_speed,  'Nice', $$;

       And now whenever either of those variables is accessed,
       its current system priority is retrieved and returned.  If
       those variables are set, then the process's priority is
       changed!

       We'll use Jarkko Hietaniemi <jhi@iki.fi>'s BSD::Resource
       class (not included) to access the PRIO_PROCESS, PRIO_MIN,
       and PRIO_MAX constants from your system, as well as the
       getpriority() and setpriority() system calls.  Here's the
       preamble of the class.

           package Nice;
           use Carp;
           use BSD::Resource;
           use strict;
           $Nice::DEBUG = 0 unless defined $Nice::DEBUG;

       TIESCALAR classname, LIST
           This is the constructor for the class.  That means it
           is expected to return a blessed reference to a new
           scalar (probably anonymous) that it's creating.  For
           example:

               sub TIESCALAR {
                   my $class = shift;
                   my $pid = shift || $$; # 0 means me

                   if ($pid !~ /^\d+$/) {
                       carp "Nice::Tie::Scalar got non-numeric pid $pid" if $^W;
                       return undef;
                   }

                   unless (kill 0, $pid) { # EPERM or ERSCH, no doubt
                       carp "Nice::Tie::Scalar got bad pid $pid: $!" if $^W;
                       return undef;
                   }

                   return bless \$pid, $class;
               }

           than raising an exception if its constructor should
           fail.  While this is how dbmopen() works, other
           classes may well not wish to be so forgiving.  It
           checks the global variable `$^W' to see whether to
           emit a bit of noise anyway.

       FETCH this
           This method will be triggered every time the tied
           variable is accessed (read).  It takes no arguments
           beyond its self reference, which is the object repre­
           senting the scalar we're dealing with.  Because in
           this case we're using just a SCALAR ref for the tied
           scalar object, a simple $$self allows the method to
           get at the real value stored there.  In our example
           below, that real value is the process ID to which
           we've tied our variable.

               sub FETCH {
                   my $self = shift;
                   confess "wrong type" unless ref $self;
                   croak "usage error" if @_;
                   my $nicety;
                   local($!) = 0;
                   $nicety = getpriority(PRIO_PROCESS, $$self);
                   if ($!) { croak "getpriority failed: $!" }
                   return $nicety;
               }

           This time we've decided to blow up (raise an excep­
           tion) if the renice fails--there's no place for us to
           return an error otherwise, and it's probably the right
           thing to do.

       STORE this, value
           This method will be triggered every time the tied
           variable is set (assigned).  Beyond its self refer­
           ence, it also expects one (and only one) argument--the
           new value the user is trying to assign.

               sub STORE {
                   my $self = shift;
                   confess "wrong type" unless ref $self;
                   my $new_nicety = shift;
                   croak "usage error" if @_;

                   if ($new_nicety < PRIO_MIN) {
                       carp sprintf
                         "WARNING: priority %d less than minimum system priority %d",
                             $new_nicety, PRIO_MIN if $^W;
                       $new_nicety = PRIO_MIN;
                   }

                       carp sprintf
                         "WARNING: priority %d greater than maximum system priority %d",
                             $new_nicety, PRIO_MAX if $^W;
                       $new_nicety = PRIO_MAX;
                   }

                   unless (defined setpriority(PRIO_PROCESS, $$self, $new_nicety)) {
                       confess "setpriority failed: $!";
                   }
                   return $new_nicety;
               }

       DESTROY this
           This method will be triggered when the tied variable
           needs to be destructed.  As with other object classes,
           such a method is seldom necessary, because Perl deal­
           locates its moribund object's memory for you automati­
           cally--this isn't C++, you know.  We'll use a DESTROY
           method here for debugging purposes only.

               sub DESTROY {
                   my $self = shift;
                   confess "wrong type" unless ref $self;
                   carp "[ Nice::DESTROY pid $$self ]" if $Nice::DEBUG;
               }

       That's about all there is to it.  Actually, it's more than
       all there is to it, because we've done a few nice things
       here for the sake of completeness, robustness, and general
       aesthetics.  Simpler TIESCALAR classes are certainly pos­
       sible.

       Tying Arrays

       A class implementing a tied ordinary array should define
       the following methods: TIEARRAY, FETCH, STORE, FETCHSIZE,
       STORESIZE and perhaps DESTROY.

       FETCHSIZE and STORESIZE are used to provide `$#array' and
       equivalent `scalar(@array)' access.

       The methods POP, PUSH, SHIFT, UNSHIFT, SPLICE, DELETE, and
       EXISTS are required if the perl operator with the corre­
       sponding (but lowercase) name is to operate on the tied
       array. The Tie::Array class can be used as a base class to
       implement the first five of these in terms of the basic
       methods above.  The default implementations of DELETE and
       EXISTS in Tie::Array simply `croak'.

       In addition EXTEND will be called when perl would have
       pre-extended allocation in a real array.

       below needs upgrading to illustrate this. (The documenta­
       tion in Tie::Array is more complete.)

       For this discussion, we'll implement an array whose
       indices are fixed at its creation.  If you try to access
       anything beyond those bounds, you'll take an exception.
       For example:

           require Bounded_Array;
           tie @ary, 'Bounded_Array', 2;
           $| = 1;
           for $i (0 .. 10) {
               print "setting index $i: ";
               $ary[$i] = 10 * $i;
               $ary[$i] = 10 * $i;
               print "value of elt $i now $ary[$i]\n";
           }

       The preamble code for the class is as follows:

           package Bounded_Array;
           use Carp;
           use strict;

       TIEARRAY classname, LIST
           This is the constructor for the class.  That means it
           is expected to return a blessed reference through
           which the new array (probably an anonymous ARRAY ref)
           will be accessed.

           In our example, just to show you that you don't really
           have to return an ARRAY reference, we'll choose a HASH
           reference to represent our object.  A HASH works out
           well as a generic record type: the `{BOUND}' field
           will store the maximum bound allowed, and the
           `{ARRAY}' field will hold the true ARRAY ref.  If
           someone outside the class tries to dereference the
           object returned (doubtless thinking it an ARRAY ref),
           they'll blow up.  This just goes to show you that you
           should respect an object's privacy.

               sub TIEARRAY {
                   my $class = shift;
                   my $bound = shift;
                   confess "usage: tie(\@ary, 'Bounded_Array', max_subscript)"
                       if @_ || $bound =~ /\D/;
                   return bless {
                       BOUND => $bound,
                       ARRAY => [],
                   }, $class;
               }

           This method will be triggered every time an individual
           element the tied array is accessed (read).  It takes
           one argument beyond its self reference: the index
           whose value we're trying to fetch.

               sub FETCH {
                 my($self,$idx) = @_;
                 if ($idx > $self->{BOUND}) {
                   confess "Array OOB: $idx > $self->{BOUND}";
                 }
                 return $self->{ARRAY}[$idx];
               }

           As you may have noticed, the name of the FETCH method
           (et al.) is the same for all accesses, even though the
           constructors differ in names (TIESCALAR vs TIEARRAY).
           While in theory you could have the same class servic­
           ing several tied types, in practice this becomes cum­
           bersome, and it's easiest to keep them at simply one
           tie type per class.

       STORE this, index, value
           This method will be triggered every time an element in
           the tied array is set (written).  It takes two argu­
           ments beyond its self reference: the index at which
           we're trying to store something and the value we're
           trying to put there.  For example:

               sub STORE {
                 my($self, $idx, $value) = @_;
                 print "[STORE $value at $idx]\n" if _debug;
                 if ($idx > $self->{BOUND} ) {
                   confess "Array OOB: $idx > $self->{BOUND}";
                 }
                 return $self->{ARRAY}[$idx] = $value;
               }

       DESTROY this
           This method will be triggered when the tied variable
           needs to be destructed.  As with the scalar tie class,
           this is almost never needed in a language that does
           its own garbage collection, so this time we'll just
           leave it out.

       The code we presented at the top of the tied array class
       accesses many elements of the array, far more than we've
       set the bounds to.  Therefore, it will blow up once they
       try to access beyond the 2nd element of @ary, as the fol­
       lowing output demonstrates:

           setting index 1: value of elt 1 now 10
           setting index 2: value of elt 2 now 20
           setting index 3: Array OOB: 3 > 2 at Bounded_Array.pm line 39
                   Bounded_Array::FETCH called at testba line 12

       Tying Hashes

       As the first Perl data type to be tied (see dbmopen()),
       hashes have the most complete and useful tie() implementa­
       tion.  A class implementing a tied hash should define the
       following methods: TIEHASH is the constructor.  FETCH and
       STORE access the key and value pairs.  EXISTS reports
       whether a key is present in the hash, and DELETE deletes
       one.  CLEAR empties the hash by deleting all the key and
       value pairs.  FIRSTKEY and NEXTKEY implement the keys()
       and each() functions to iterate over all the keys.  And
       DESTROY is called when the tied variable is garbage col­
       lected.

       If this seems like a lot, then feel free to inherit from
       merely the standard Tie::Hash module for most of your
       methods, redefining only the interesting ones.  See the
       Tie::Hash manpage for details.

       Remember that Perl distinguishes between a key not exist­
       ing in the hash, and the key existing in the hash but hav­
       ing a corresponding value of `undef'.  The two possibili­
       ties can be tested with the `exists()' and `defined()'
       functions.

       Here's an example of a somewhat interesting tied hash
       class:  it gives you a hash representing a particular
       user's dot files.  You index into the hash with the name
       of the file (minus the dot) and you get back that dot
       file's contents.  For example:

           use DotFiles;
           tie %dot, 'DotFiles';
           if ( $dot{profile} =~ /MANPATH/ ||
                $dot{login}   =~ /MANPATH/ ||
                $dot{cshrc}   =~ /MANPATH/    )
           {
               print "you seem to set your MANPATH\n";
           }

       Or here's another sample of using our tied class:

           tie %him, 'DotFiles', 'daemon';
           foreach $f ( keys %him ) {
               printf "daemon dot file %s is size %d\n",
                   $f, length $him{$f};
           }

       for the object containing several important fields, of
       which only the `{LIST}' field will be what the user thinks
       of as the real hash.

       USER whose dot files this object represents

       HOME where those dot files live

       CLOBBER
            whether we should try to change or remove those dot
            files

       LIST the hash of dot file names and content mappings

       Here's the start of Dotfiles.pm:

           package DotFiles;
           use Carp;
           sub whowasi { (caller(1))[3] . '()' }
           my $DEBUG = 0;
           sub debug { $DEBUG = @_ ? shift : 1 }

       For our example, we want to be able to emit debugging info
       to help in tracing during development.  We keep also one
       convenience function around internally to help print out
       warnings; whowasi() returns the function name that calls
       it.

       Here are the methods for the DotFiles tied hash.

       TIEHASH classname, LIST
           This is the constructor for the class.  That means it
           is expected to return a blessed reference through
           which the new object (probably but not necessarily an
           anonymous hash) will be accessed.

           Here's the constructor:

               sub TIEHASH {
                   my $self = shift;
                   my $user = shift || $>;
                   my $dotdir = shift || '';
                   croak "usage: @{[&whowasi]} [USER [DOTDIR]]" if @_;
                   $user = getpwuid($user) if $user =~ /^\d+$/;
                   my $dir = (getpwnam($user))[7]
                           || croak "@{[&whowasi]}: no user $user";
                   $dir .= "/$dotdir" if $dotdir;

                       USER    => $user,
                       HOME    => $dir,
                       LIST    => {},
                       CLOBBER => 0,
                   };

                   opendir(DIR, $dir)
                           || croak "@{[&whowasi]}: can't opendir $dir: $!";
                   foreach $dot ( grep /^\./ && -f "$dir/$_", readdir(DIR)) {
                       $dot =~ s/^\.//;
                       $node->{LIST}{$dot} = undef;
                   }
                   closedir DIR;
                   return bless $node, $self;
               }

           It's probably worth mentioning that if you're going to
           filetest the return values out of a readdir, you'd
           better prepend the directory in question.  Otherwise,
           because we didn't chdir() there, it would have been
           testing the wrong file.

       FETCH this, key
           This method will be triggered every time an element in
           the tied hash is accessed (read).  It takes one argu­
           ment beyond its self reference: the key whose value
           we're trying to fetch.

           Here's the fetch for our DotFiles example.

               sub FETCH {
                   carp &whowasi if $DEBUG;
                   my $self = shift;
                   my $dot = shift;
                   my $dir = $self->{HOME};
                   my $file = "$dir/.$dot";

                   unless (exists $self->{LIST}->{$dot} || -f $file) {
                       carp "@{[&whowasi]}: no $dot file" if $DEBUG;
                       return undef;
                   }

                   if (defined $self->{LIST}->{$dot}) {
                       return $self->{LIST}->{$dot};
                   } else {
                       return $self->{LIST}->{$dot} = `cat $dir/.$dot`;
                   }
               }

           It was easy to write by having it call the Unix cat(1)
           command, but it would probably be more portable to
           open the file manually (and somewhat more efficient).
           Of course, because dot files are a Unixy concept,


       STORE this, key, value
           This method will be triggered every time an element in
           the tied hash is set (written).  It takes two argu­
           ments beyond its self reference: the index at which
           we're trying to store something, and the value we're
           trying to put there.

           Here in our DotFiles example, we'll be careful not to
           let them try to overwrite the file unless they've
           called the clobber() method on the original object
           reference returned by tie().

               sub STORE {
                   carp &whowasi if $DEBUG;
                   my $self = shift;
                   my $dot = shift;
                   my $value = shift;
                   my $file = $self->{HOME} . "/.$dot";
                   my $user = $self->{USER};

                   croak "@{[&whowasi]}: $file not clobberable"
                       unless $self->{CLOBBER};

                   open(F, "> $file") || croak "can't open $file: $!";
                   print F $value;
                   close(F);
               }

           If they wanted to clobber something, they might say:

               $ob = tie %daemon_dots, 'daemon';
               $ob->clobber(1);
               $daemon_dots{signature} = "A true daemon\n";

           Another way to lay hands on a reference to the under­
           lying object is to use the tied() function, so they
           might alternately have set clobber using:

               tie %daemon_dots, 'daemon';
               tied(%daemon_dots)->clobber(1);

           The clobber method is simply:

               sub clobber {
                   my $self = shift;
                   $self->{CLOBBER} = @_ ? shift : 1;
               }

       DELETE this, key
           This method is triggered when we remove an element
           from the hash, typically by using the delete()

           they really want to clobber files.

               sub DELETE   {
                   carp &whowasi if $DEBUG;

                   my $self = shift;
                   my $dot = shift;
                   my $file = $self->{HOME} . "/.$dot";
                   croak "@{[&whowasi]}: won't remove file $file"
                       unless $self->{CLOBBER};
                   delete $self->{LIST}->{$dot};
                   my $success = unlink($file);
                   carp "@{[&whowasi]}: can't unlink $file: $!" unless $success;
                   $success;
               }

           The value returned by DELETE becomes the return value
           of the call to delete().  If you want to emulate the
           normal behavior of delete(), you should return what­
           ever FETCH would have returned for this key.  In this
           example, we have chosen instead to return a value
           which tells the caller whether the file was success­
           fully deleted.

       CLEAR this
           This method is triggered when the whole hash is to be
           cleared, usually by assigning the empty list to it.

           In our example, that would remove all the user's dot
           files!  It's such a dangerous thing that they'll have
           to set CLOBBER to something higher than 1 to make it
           happen.

               sub CLEAR    {
                   carp &whowasi if $DEBUG;
                   my $self = shift;
                   croak "@{[&whowasi]}: won't remove all dot files for $self->{USER}"
                       unless $self->{CLOBBER} > 1;
                   my $dot;
                   foreach $dot ( keys %{$self->{LIST}}) {
                       $self->DELETE($dot);
                   }
               }

       EXISTS this, key
           This method is triggered when the user uses the
           exists() function on a particular hash.  In our exam­
           ple, we'll look at the `{LIST}' hash element for this:

                   carp &whowasi if $DEBUG;
                   my $self = shift;
                   my $dot = shift;
                   return exists $self->{LIST}->{$dot};
               }

       FIRSTKEY this
           This method will be triggered when the user is going
           to iterate through the hash, such as via a keys() or
           each() call.

               sub FIRSTKEY {
                   carp &whowasi if $DEBUG;
                   my $self = shift;
                   my $a = keys %{$self->{LIST}};          # reset each() iterator
                   each %{$self->{LIST}}
               }

       NEXTKEY this, lastkey
           This method gets triggered during a keys() or each()
           iteration.  It has a second argument which is the last
           key that had been accessed.  This is useful if you're
           carrying about ordering or calling the iterator from
           more than one sequence, or not really storing things
           in a hash anywhere.

           For our example, we're using a real hash so we'll do
           just the simple thing, but we'll have to go through
           the LIST field indirectly.

               sub NEXTKEY  {
                   carp &whowasi if $DEBUG;
                   my $self = shift;
                   return each %{ $self->{LIST} }
               }

       DESTROY this
           This method is triggered when a tied hash is about to
           go out of scope.  You don't really need it unless
           you're trying to add debugging or have auxiliary state
           to clean up.  Here's a very simple function:

               sub DESTROY  {
                   carp &whowasi if $DEBUG;
               }

       Note that functions such as keys() and values() may return
       huge lists when used on large objects, like DBM files.
       You may prefer to use the each() function to iterate over


           # print out history file offsets
           use NDBM_File;
           tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
           while (($key,$val) = each %HIST) {
               print $key, ' = ', unpack('L',$val), "\n";
           }
           untie(%HIST);

       Tying FileHandles

       This is partially implemented now.

       A class implementing a tied filehandle should define the
       following methods: TIEHANDLE, at least one of PRINT,
       PRINTF, WRITE, READLINE, GETC, READ, and possibly CLOSE
       and DESTROY.  The class can also provide: BINMODE, OPEN,
       EOF, FILENO, SEEK, TELL - if the corresponding perl opera­
       tors are used on the handle.

       It is especially useful when perl is embedded in some
       other program, where output to STDOUT and STDERR may have
       to be redirected in some special way. See nvi and the
       Apache module for examples.

       In our example we're going to create a shouting handle.

           package Shout;

       TIEHANDLE classname, LIST
           This is the constructor for the class.  That means it
           is expected to return a blessed reference of some
           sort. The reference can be used to hold some internal
           information.

               sub TIEHANDLE { print "<shout>\n"; my $i; bless \$i, shift }

       WRITE this, LIST
           This method will be called when the handle is written
           to via the `syswrite' function.

               sub WRITE {
                   $r = shift;
                   my($buf,$len,$offset) = @_;
                   print "WRITE called, \$buf=$buf, \$len=$len, \$offset=$offset";
               }

       PRINT this, LIST
           This method will be triggered every time the tied

           Beyond its self reference it also expects the list
           that was passed to the print function.

               sub PRINT { $r = shift; $$r++; print join($,,map(uc($_),@_)),$\ }

       PRINTF this, LIST
           This method will be triggered every time the tied han­
           dle is printed to with the `printf()' function.
           Beyond its self reference it also expects the format
           and list that was passed to the printf function.

               sub PRINTF {
                   shift;
                   my $fmt = shift;
                   print sprintf($fmt, @_)."\n";
               }

       READ this, LIST
           This method will be called when the handle is read
           from via the `read' or `sysread' functions.

               sub READ {
                   my $self = shift;
                   my $$bufref = \$_[0];
                   my(undef,$len,$offset) = @_;
                   print "READ called, \$buf=$bufref, \$len=$len, \$offset=$offset";
                   # add to $$bufref, set $len to number of characters read
                   $len;
               }

       READLINE this
           This method will be called when the handle is read
           from via <HANDLE>.  The method should return undef
           when there is no more data.

               sub READLINE { $r = shift; "READLINE called $$r times\n"; }

       GETC this
           This method will be called when the `getc' function is
           called.

               sub GETC { print "Don't GETC, Get Perl"; return "a"; }

       CLOSE this
           This method will be called when the handle is closed
           via the `close' function.

               sub CLOSE { print "CLOSE called.\n" }

           As with the other types of ties, this method will be
           called when the tied handle is about to be destroyed.
           This is useful for debugging and possibly cleaning up.

               sub DESTROY { print "</shout>\n" }

       Here's how to use our little example:

           tie(*FOO,'Shout');
           print FOO "hello\n";
           $a = 4; $b = 6;
           print FOO $a, " plus ", $b, " equals ", $a + $b, "\n";
           print <FOO>;

       The `untie' Gotcha

       If you intend making use of the object returned from
       either tie() or tied(), and if the tie's target class
       defines a destructor, there is a subtle gotcha you must
       guard against.

       As setup, consider this (admittedly rather contrived)
       example of a tie; all it does is use a file to keep a log
       of the values assigned to a scalar.

           package Remember;

           use strict;
           use warnings;
           use IO::File;

           sub TIESCALAR {
               my $class = shift;
               my $filename = shift;
               my $handle = new IO::File "> $filename"
                                or die "Cannot open $filename: $!\n";

               print $handle "The Start\n";
               bless {FH => $handle, Value => 0}, $class;
           }

           sub FETCH {
               my $self = shift;
               return $self->{Value};
           }

               my $self = shift;
               my $value = shift;
               my $handle = $self->{FH};
               print $handle "$value\n";
               $self->{Value} = $value;
           }

           sub DESTROY {
               my $self = shift;
               my $handle = $self->{FH};
               print $handle "The End\n";
               close $handle;
           }

           1;

       Here is an example that makes use of this tie:

           use strict;
           use Remember;

           my $fred;
           tie $fred, 'Remember', 'myfile.txt';
           $fred = 1;
           $fred = 4;
           $fred = 5;
           untie $fred;
           system "cat myfile.txt";

       This is the output when it is executed:

           The Start
           1
           4
           5
           The End

       So far so good.  Those of you who have been paying atten­
       tion will have spotted that the tied object hasn't been
       used so far.  So lets add an extra method to the Remember
       class to allow comments to be included in the file -- say,
       something like this:

           sub comment {
               my $self = shift;
               my $text = shift;
               my $handle = $self->{FH};
               print $handle $text, "\n";
           }

       And here is the previous example modified to use the `com­
       ment' method (which requires the tied object):

           use Remember;

           my ($fred, $x);
           $x = tie $fred, 'Remember', 'myfile.txt';
           $fred = 1;
           $fred = 4;
           comment $x "changing...";
           $fred = 5;
           untie $fred;
           system "cat myfile.txt";

       When this code is executed there is no output.  Here's
       why:

       When a variable is tied, it is associated with the object
       which is the return value of the TIESCALAR, TIEARRAY, or
       TIEHASH function.  This object normally has only one ref­
       erence, namely, the implicit reference from the tied vari­
       able.  When untie() is called, that reference is
       destroyed.  Then, as in the first example above, the
       object's destructor (DESTROY) is called, which is normal
       for objects that have no more valid references; and thus
       the file is closed.

       In the second example, however, we have stored another
       reference to the tied object in $x.  That means that when
       untie() gets called there will still be a valid reference
       to the object in existence, so the destructor is not
       called at that time, and thus the file is not closed.  The
       reason there is no output is because the file buffers have
       not been flushed to disk.

       Now that you know what the problem is, what can you do to
       avoid it?  Well, the good old `-w' flag will spot any
       instances where you call untie() and there are still valid
       references to the tied object.  If the second script above
       this near the top `use warnings 'untie'' or was run with
       the `-w' flag, Perl prints this warning message:

           untie attempted while 1 inner references still exist

       To get the script to work properly and silence the warning
       make sure there are no valid references to the tied object
       before untie() is called:

           undef $x;
           untie $fred;


SEE ALSO

       See the DB_File manpage or the Config manpage for some
       interesting tie() implementations.

       Tied arrays are incomplete.  They are also distinctly
       lacking something for the `$#ARRAY' access (which is hard,
       as it's an lvalue), as well as the other obvious array
       functions, like push(), pop(), shift(), unshift(), and
       splice().

       You cannot easily tie a multilevel data structure (such as
       a hash of hashes) to a dbm file.  The first problem is
       that all but GDBM and Berkeley DB have size limitations,
       but beyond that, you also have problems with how refer­
       ences are to be represented on disk.  One experimental
       module that does attempt to address this need partially is
       the MLDBM module.  Check your nearest CPAN site as
       described in the perlmodlib manpage for source code to
       MLDBM.


AUTHOR

       Tom Christiansen

       TIEHANDLE by Sven Verdoolaege <skimo@dns.ufsia.ac.be> and
       Doug MacEachern <dougm@osf.org>


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