Pointers to Procedures
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:
' declares a pointer to sub procedure that takes no arguments
Dim pointerToProcedure As Sub
Dim pointerToProcedure As Sub
Procedure pointers store procedure addresses, which are retrieved using Operator @ (Address of) or the ProcPtr operator:
Calling a procedure pointer'' 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
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
The interesting thing about procedure pointers is that they can be called just like a procedure:
'' .. Add and pFunc as before ..
#include once "pfunc.bi"
Print "3 + 4 = " & pFunc(3, 4)
#include once "pfunc.bi"
Print "3 + 4 = " & pFunc(3, 4)
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.
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:
'' .. 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)
#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)
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:
Pointers to procedure pointers'' .. 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)
#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)
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):
Pointers to member proceduresFunction 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
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
Before fbc 1.10.0, method pointers are not implemented yet, but it is possible to work-around that by using a static wrapper:
Since fbc 1.10.0, method pointers are implemented via updating the 'Procptr' operator:
Typing rule for procedure pointer declaration/''
' 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
' 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
/''
' 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
' 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
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:
Assigning to a function pointer a function with a contravariant parameter and a covariant result, both by reference:
See also:
'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
' - 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 "'Dim As A Ptr pab = pf(@b0)':"
Dim As A Ptr pab = pf(@b0)
Print "'Delete CPtr(B Ptr, pab)':"
Delete CPtr(B Ptr, pab)
Print "'End Scope':"
End Scope
Sleep
'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
' - 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 "'Dim Byref As A rab = pf(b0)':"
Dim ByRef As A rab = pf(b0)
Print "'Delete @rab':"
Delete @rab
Print "'End Scope':"
End Scope
Sleep
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