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Function Pointers in VB


This code makes it possible to use function pointers (no, not OBJPtr) basic funciton pointers, in VB.
It uses two API calls. Basically, it uses a window callback as the entry point for your new function, redirecting the window call to the function supplied.
A little out of the norm, but a handy thing to have when callbacks to non-objects is needed.
If you are attempting to call objects back, I recommend either the undocumented ObjPtr() or CallByName (when you know the name of the object function to call)

Original Author: unknown

Inputs

No inputs

Assumptions

Nothing, absolutely nothing.

Returns

No Returns, just reap the benefits.

Side Effects

None that I know of, been using this for two years.

API Declarations

Declare Function CallWindowProc& Lib "user32" Alias "CallWindowProcA" (ByVal
lpPrevWndFunc&, ByVal hWnd&, ByVal Msg&, ByVal wParam&, ByVal lParam&)
Declare Sub CopyMemory Lib "kernel32" Alias "RtlMoveMemory" (lpvDest As Any,
lpvSource As Any, ByVal cbCopy As Long)

Code

Option Explicit
Private Function StripStringFromPointer$(ByVal lpString&, ByVal nStrLen&)
  Dim Info$
  Info = String$(nStrLen, vbNullChar)
  CopyMemory ByVal StrPtr(Info), ByVal lpString, nStrLen * 2
  StripStringFromPointer = Info
End Function
Private Function GetAddress(Addr&)
  GetAddress = Addr
End Function
Private Function MyFunction&(ByVal lpString&, ByVal nStrLen&, ByVal param3&,
ByVal param4&)
  Debug.Print StripStringFromPointer(lpString, nStrLen)
End Function
Public Sub Main()
  Dim FunctAddr&, Info$
  Info = "Holy Smoke"
  FunctAddr = GetAddress(AddressOf MyFunction)
  CallWindowProc FunctAddr, StrPtr(Info), CLng(Len(Info)), 0&, 0&
  End
End Sub

About this post

Posted: 2002-06-01
By: ArchiveBot
Viewed: 81 times

Categories

Visual Basic 6

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