Function Pointer
Data type that stores a pointer to a `Function` procedure returning a value
Syntax
`
Dim variable As Function [cdecl|pascal|stdcall] [( [parameter_list] )] [ ByRef ] [As return_type] [= initializer]
`
Parameters
`parameter_list`: parameter[, parameter[, ...]]
`parameter`: `[ByRef|ByVal] identifier [As type] [= default_value]`
`identifier`: the name of the variable referenced in the subroutine
`type`: the type of variable
`default_value`: the value of the argument if none is specified in the call
`return_value`: the value returned from the function
`intializer`: address of a subroutine to set as the intial value
Description
A `Function` pointer is a procedure pointer that stores the memory location of compiled code that returns a value. If no intializer is given the default initial value is zero (0).
The memory address for the `Function` procedure can be assigned to the variable by taking the address of a subroutine with `ProcPtr` or `Operator @ (Address Of)`.
The procedure must match the same `Function` declaration as the declared `Function` pointer.
To call the subroutine assigned, use the `variable` name as if it were a normal declared `Function`.
One of the primary uses for procedure pointers is to create callback procedures. A callback procedure is a procedure created in the user program that is called by another procedure either from the user own code space or from an external library.
Example
`
Function ConcatSelf( x As String ) As String
Return x & x
End Function
Dim x As Function( x As String ) As String = ProcPtr( ConcatSelf )
Print x( "Hello" )
`
`
Function x2 (ByVal i As Integer) As Integer
Return i * 2
End Function
Function x3 (ByVal i As Integer) As Integer
Return i * 3
End Function
Function operation (ByVal i As Integer, ByVal op As Function (ByVal As Integer) As Integer) As Integer
Return op(i)
End Function
Print operation(4, @x2)
Print operation(4, @x3)
`
`
' Example of basic callback Function mechanism to implement a key pressed event:
' (the user callback Function address cannot be modified while the event thread is running)
' - An asynchronous thread tests the keyboard in a loop, and calls a user callback Function each time a key is pressed.
' - The callback Function address is passed to the thread.
' - The callback Function prints the character of the key pressed,
' but if the key pressed is <escape> it orders the thread to finish by using the function return value.
' - As the user callback address is passed to the thread as argument, it cannot be modified while the thread is running.
'' thread Sub definition
Sub threadInkey (ByVal p As Any Ptr)
If p > 0 Then '' test condition callback Function defined
Dim As Function (ByRef As String) As Integer callback = p '' convert the any ptr to a callback Function pointer
Do
Dim As String s = Inkey
If s <> "" Then '' test condition key pressed
If callback(s) Then '' test condition to finish thread
Exit Do
End If
End If
Sleep 50
Loop
End If
End Sub
'' user callback Function definition
Function printInkey (ByRef s As String) As Integer
If Asc(s) = 27 Then '' test condition key pressed = <escape>
Print
Return -1 '' order thread to finish
Else
Print s;
Return 0 '' order thread to continue
End If
End Function
'' user main code
Dim As Any Ptr p = ThreadCreate(@threadInkey, @printInkey) '' launch the thread, passing the callback Function address
ThreadWait(p) '' wait for the thread finish
`
Differences from QB

- New to FreeBASIC

See also

- `Sub`
- `ProcPtr`
- `Operator @ (Address Of)`