Pointers to Procedures

• Source: https://www.freebasic.net/wiki/wikka.php?wakka=ProPgProcedurePointers
• Last revised: 2024-02-15

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Pointers that point to procedures

Just as pointers can be made to point to an Integer or Single type, pointers can also point to procedures, that is, they can store the address of a procedure.

Declaration

To declare a pointer to procedure, use the Sub or Function keywords, followed by any parameters and return value type:

start GeSHi

' declares a pointer to sub procedure that takes no arguments
Dim pointerToProcedure As Sub

end GeSHi

Procedure pointers store procedure addresses, which are retrieved using Operator @ (Address of) or the ProcPtr operator:

start GeSHi

'' pfunc.bi

Function Add (a As Integer, b As Integer) As Integer
    Return a + b
End Function

Dim pFunc As Function (As Integer, As Integer) As Integer = @Add

end GeSHi

Calling a procedure pointer

The interesting thing about procedure pointers is that they can be called just like a procedure:

start GeSHi

'' .. Add and pFunc as before ..
#include once "pfunc.bi"

Print "3 + 4 = " & pFunc(3, 4)

end GeSHi

For a calling example of subroutine pointer, see the Operator @ (Address of) page.

Note: When calling a procedure through a procedure pointer, parentheses surrounding the argument list (even empty) are mandatory to resolve ambiguity with a simple access to the pointer value.

Passing procedure pointers to procedures

Passing procedure pointers to other procedures is similar as well:

start GeSHi

'' .. Add and pFunc as before ..
#include once "pfunc.bi"

Function DoOperation (a As Integer, b As Integer, operation As Function (As Integer, As Integer) As Integer) As Integer
    Return operation(a, b)
End Function

Print "3 + 4 = " & DoOperation(3, 4, @Add)

end GeSHi

Because procedure pointer declarations can be lengthy, it often helps to create a type alias for the procedure pointer, in an effort to make clearer code:

start GeSHi

'' .. Add and pFunc as before ..
#include once "pfunc.bi"

Type operation As Function (As Integer, As Integer) As Integer

Function DoOperation (a As Integer, b As Integer, op As operation) As Integer
    Return op(a, b)
End Function

Print "3 + 4 = " & DoOperation(3, 4, @Add)

end GeSHi

Pointers to procedure pointers

Because the syntax of a procedure pointer does not allow declaration of a pointer to procedure pointer when the procedure is a function (because ptr applies on return type and not on procedure), a type alias is used. Notice how it is necessary to surround a dereferenced pointer to procedure pointer by parenthesis when calling the procedure. This is because the function-call operator '()' has higher precedence than Operator * (Value of):

start GeSHi

Function Halve (ByVal i As Integer) As Integer
    Return i / 2
End Function

Function Triple (ByVal i As Integer) As Integer
    Return i * 3
End Function

Type operation As Function (ByVal As Integer) As Integer

' an array of procedure pointers, NULL indicates the
' end of the array
Dim operations(20) As operation = _
{ @Halve, @Triple, 0 }

Dim i As Integer = 280

' apply all of the operations to a variable by iterating through the array
' with a pointer to procedure pointer
Dim op As operation Ptr = @operations(0)
While (*op <> 0)
    ' call the procedure that is pointed to, note the extra parenthesis
    i = (*op)(i)
    op += 1
Wend

Print "Value of 'i' after all operations performed: " & i

end GeSHi

Pointers to member procedures

Before fbc 1.10.0, method pointers are not implemented yet, but it is possible to work-around that by using a static wrapper:

start GeSHi

/''
 ' This example shows, before fbc 1.10.0, how you can simulate getting a class method pointer,
 ' until support is properly implemented in the compiler.
 '
 ' When this is supported (since fbc 1.10.0), you will only need to remove the static wrapper
 ' function presented here, to maintain compatibility.
 '/

Type T
    Declare Function test(ByVal number As Integer) As Integer
    Declare Static Function test(ByRef This As T, ByVal number As Integer) As Integer
    Dim As Integer i = 420
End Type

Function T.test(ByVal number As Integer) As Integer
    Return i + number
End Function

Function T.test(ByRef This As T, ByVal number As Integer) As Integer
    Return This.test(number)
End Function

Dim p As Function(ByRef As T, ByVal As Integer) As Integer
p = @T.test

Dim As T obj

Print p(obj, 69) '' prints 489

end GeSHi

Since fbc 1.10.0, method pointers are implemented via updating the 'Procptr' operator:

start GeSHi

/''
 ' This example shows, since fbc 1.10.0, how to directly obtain a class method pointer,
 '/

Type T
    Declare Function test(ByVal number As Integer) As Integer
    Dim As Integer i = 420
End Type

Function T.test(ByVal number As Integer) As Integer
    Return i + number
End Function

Dim p As Function(ByRef As T, ByVal As Integer) As Integer
p = ProcPtr(T.test)  '' or 'p = @T.test'

Dim As T obj

Print p(obj, 69) '' prints 489

end GeSHi

Typing rule for procedure pointer declaration

The procedure pointer declaration allows to assign to the pointer:

• not only a procedure with the same parameters types, and if any, the same result type,
• but also a procedure with contravariant parameters (by reference or by pointer) or/and a covariant result (by reference or by pointer).

Assigning to a function pointer a fonction with a contravariant parameter and a covariant result, both by pointer:

start GeSHi

'Example of assigning to a function pointer a function with:
'   - a contravariant parameter by pointer,
'   - and a covariant result by pointer.

Type A
    Dim As Integer I
    Declare Constructor ()
    Declare Destructor ()
End Type
Constructor A ()
    Print "    A instance constructed", @This
End Constructor
Destructor A ()
    Print "    A instance destroyed", @This
End Destructor

Type B Extends A
    Dim As Integer J
    Declare Constructor ()
    Declare Constructor (ByRef a0 As A)
    Declare Destructor ()
End Type
Constructor B ()
    Print "    B instance constructed", @This
End Constructor
Constructor B (ByRef a0 As A)
    Cast(A, This) = a0
    Print "    B instance constructed", @This
End Constructor
Destructor B ()
    Print "    B instance destroyed", @This
End Destructor

Function f (ByVal pa0 As A Ptr) As B Ptr
    Return New B(*pa0)
End Function

Scope
    Dim As Function (ByVal As B Ptr) As A Ptr pf = @f
    Print "'Scope : Dim As B b0':"
    Dim As B b0
    Print
    Print "'Dim As A Ptr pab = pf(@b0)':"
    Dim As A Ptr pab = pf(@b0)
    Print
    Print "'Delete CPtr(B Ptr, pab)':"
    Delete CPtr(B Ptr, pab)
    Print
    Print "'End Scope':"
End Scope

Sleep

end GeSHi

Assigning to a function pointer a function with a contravariant parameter and a covariant result, both by reference:

start GeSHi

'Example of assigning to a function pointer a function with:
'   - a contravariant parameter by reference,
'   - and a covariant result by reference.

Type A Extends Object
    Dim As Integer I
    Declare Constructor ()
    Declare Virtual Destructor ()
End Type
Constructor A ()
    Print "    A instance constructed", @This
End Constructor
Destructor A ()
    Print "    A instance destroyed", @This
End Destructor

Type B Extends A
    Dim As Integer J
    Declare Constructor ()
    Declare Constructor (ByRef a0 As A)
    Declare Virtual Destructor ()
End Type
Constructor B ()
    Print "    B instance constructed", @This
End Constructor
Constructor B (ByRef a0 As A)
    Cast(A, This) = a0
    Print "    B instance constructed", @This
End Constructor
Destructor B ()
    Print "    B instance destroyed", @This
End Destructor

Function f (ByRef a0 As A) ByRef As B
    Return *New B(a0)
End Function

Scope
    Dim As Function (ByRef As B) ByRef As A pf = @f
    Print "'Scope : Dim As B b0':"
    Dim As B b0
    Print
    Print "'Dim Byref As A rab = pf(b0)':"
    Dim ByRef As A rab = pf(b0)
    Print
    Print "'Delete @rab':"
    Delete @rab
    Print
    Print "'End Scope':"
End Scope

Sleep

end GeSHi

See also

• Sub
• Function
• Pointer
• Operator @ (Address of)
• ProcPtr operator

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