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
`
Procedure pointers store procedure addresses, which are retrieved using `Operator @` (Address of) or the `Procptr Operator`:
`
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
`
Calling a procedure pointer
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)
`
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)
`
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:
`
'' .. 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)
`
Pointers to procedure pointers
Because the syntax of a procedure pointer does not allow declaration of a pointer to procedure pointer, 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):
`
Sub Halve (ByRef i As Integer)
i /= 2
End Sub
Sub Triple (ByRef i As Integer)
i *= 3
End Sub
Type operation As Sub (ByRef 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 (0 <> *op)
' call the procedure that is pointed to, note the extra parenthesis
(*op)(i)
op += 1
Wend
Print "Value of 'i' after all operations performed: " & i
`
See also

- `Sub`
- `Function`
- `Pointer`
- `Operator * (Address Of)`
- `Procptr Operator`