Data type that stores a pointer to a FUNCTION procedure returning a value

dim variable as Function [CDecl|Pascal|StdCall] [( [parameter_list] )] [ ByRef ] [As return_type] [= initializer]

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

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 Function pointers is to create callback procedures:
- A callback Function is a Function that is passed through an argument (a Function pointer) to another procedure which is expected to call back (execute) the "argument" at a convenient time.
- If the callback Function is completely executed before the invocation returns to the caller code, then the callback process is said to be "synchronous".
- If the invocation immediately returns to the caller code, and the callback Function and the caller's next code are running in parallel, then the callback process is said to be "asynchronous".

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 (asynchronous) 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
        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, 1
    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>
      Return -1                                                '' order thread to finish
      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:
See also:
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