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Encapsulating WndProc message processing callback in a class

Posted on 2006-11-22
Last Modified: 2008-02-01
Currently, i'm writing a class wrapper for the basic Win32 API functions that create windows, a menu, and a few controls. I know I could use MFC, but i'm learning much more doing it this way.

I'd like to include the standard message processing callback as a method in my window class, but haven't been able to get it to compile. A cut down version of what i'm talking about is here:

// class descriptor
class WindowsGui {
    WindowsGui(HINSTANCE hInstance, int iCmdShow, const char* cClassName);
    virtual ~WindowsGui();

    WNDCLASS _WndClass;
    LRESULT CALLBACK WndProc(HWND hWnd, UINT iMessage, WPARAM wParam, LPARAM lParam);

// Constructor
WindowsGui::WindowsGui(HINSTANCE hInstance, int iCmdShow, const char* cClassName)
    _WndClass.lpfnWndProc = WndProc;

// Callback method
LRESULT CALLBACK WindowsGui::WndProc(HWND hWnd, UINT iMessage, WPARAM wParam, LPARAM lParam)
    return 0;

I've cut out all non-relevant parts. When I try to compile I get an error stating that the assignment to lpfnWndProc cannot complete as a type mismatch has occured between (LRESULT)(WindowsGui::*) and (LRESULT)(*). I've tried many different ways around this including different casts and using the 'this' operator, but I think my basic understanding of whats going on here is a little lacking.

Can anyone offer advice on what cast or fix can be applied either in the assignment of lpfnWndProc, or how I encapsulate the WndProc function into a method in my class?

Thanks for any advice
Question by:salukibob
LVL 86

Accepted Solution

jkr earned 60 total points
ID: 17997450
The problem is that you can't use a non-static member function for a callback, since they're passed an invisible 'this' parameter so that the signatures won't match. The possible solutions are:

- use a static member
- use an adapter, see e.g. http://www.codeproject.com/win32/callback_adapter.asp ("Use member functions for C-style callbacks and threads - a general solution")
LVL 39

Assisted Solution

itsmeandnobodyelse earned 65 total points
ID: 17997787
>>>> The problem is that you can't use a non-static member function for a callback  

jkr is right. Add the keyword static to the declaration of the member function

   static LRESULT CALLBACK WndProc(HWND hWnd, UINT iMessage, WPARAM wParam, LPARAM lParam);

to get it compiled.

However, as the callback function now is a static function you need to retrieve somehow the instance of class WindowsGui, the hWnd was associated to. Normally, you would do such a thing by implementing a global map where hwnd is the key and the pointer to WindowsGui is the data. You also could do that by a little 'Manager' class:

class GuiManager
     std::map<HWND, WindowsGui*> hwndMap;
     void register(HWND hwnd, WindowsGui* pWnd) { hwndMap[hwnd] = pWnd; }
     void unregister(HWND hwnd) { hwndMap.erase(hwnd); }
     WindowsGui* lookup(HWND hwnd)
          map<HWND, WindowsGui*>::iterator it = hwndMap.find(hwnd);
          if (it != hwndMap.end())
                return it->second;
    return NULL;

// the one and only GuiManager
extern  GuiManager theGuiManager;

// in windowsgui.cpp

#include "guimanager.h"

// the one and only GuiManager
GuiManager theGuiManager;      // needs to get defined in one cpp file (without extern keyword)

If doing so, you can retrieve the WindowsGui object in the callback function by

// Callback method
LRESULT CALLBACK WindowsGui::WndProc(HWND hWnd, UINT iMessage, WPARAM wParam, LPARAM lParam)
    WindowsGui* pWnd = theGuiManager.lookup(hWnd);
    if (pWnd != NULL)
           // we found the object!
    return 0;

Regards, Alex

Author Comment

ID: 17997998
Thanks for the advice. I see what you mean, and I can see now that I was missing the point that I will only want one message processing function, not one with each object created. Hence the static. Thanks also for the additional pointers.

Points split.


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