Subclassing a window with a functor (Win32)

Question

Quick sanity check: Is it possible to subclass a window using a functor? I'm running into a situation where I want to have some data available in the win proc, but GWLP_USERDATA is already being used. A functor seems like a good alternative, but I'm having trouble getting it to work.

Here's the basics:

class MyWinProc { // Win Proc Functor
public:
 MyWinProc(ExternalClass* obj, HWND window) :
                obj(obj), window(window) {
                oldWinProc = SubclassWindow(window, this); // Apply Subclass
            }

 virtual ~MyWinProc() {
                SubclassWindow(window, oldWinProc); // Remove Subclass
            }

 LRESULT CALLBACK operator()(HWND, UINT, WPARAM, LPARAM) {
                switch( uMsg ) {
  case WM_MOUSEMOVE: {
   obj->onMouseMove(/*etc*/);
   break;
  }
                }
                return CallWindowProc(oldWinProc, hWnd, uMsg, wParam, lParam);
            }

private:
 ExternalClass* obj;
 HWND  window;
 WNDPROC oldWinProc;
};

Seems all well and good, but when I hit DispatchMessage() in me message pump, I "Access Violation Writing Location 0x00000000", obviously not a good sign. Remove the call to the above code and life is happy again. :( So is this even possible, or am I going about it entirely the wrong way?

Answer 1

A CALLBACK function must be a static member function or an otherwise straight C-style function. The Windows API doesn't really know anything about C++ objects.

Something along the lines of this should work:

class MyWinProc { 
public:
        MyWinProc(ExternalClass* obj, HWND window) :
                obj(obj), window(window) {
                pContext = this;

                oldWinProc = SubclassWindow(window, &MyWinProc::wndproc); // Apply Subclass
            }

        virtual ~MyWinProc() {
                SubclassWindow(window, oldWinProc); // Remove Subclass
            }


private:
        static MyWinProc* pContext;

        static
        LRESULT CALLBACK wndproc( HWND, UINT, WPARAM, LPARAM) {
            MyWndProc& me = *pContext;

            // do your WndProc work...
        }

        ExternalClass* obj;
        HWND  window;
        WNDPROC oldWinProc;
};

Answer 2

GWLP_USERDATA is already being used

I don't know what your SubclassWindow function is, but CWnd::SubclassWindow says, "The window must not already be attached to an MFC object when this function is called".

I'm running into a situation where I want to have some data available in the win proc

A usual (non-MFC) way to implement that is to have a global/static dictionary, whose key/index is the HWND value of the subclassed windows, and whose data is the data that you want to associate with that window: that data is often the this pointer of a C++ class of yours.

You subclass the window procedure with a static callback function of yours: your static callback function then, when it's invoked, uses the HWND which it's passed to look up the data in the static dictionary.

Answer 3

GWLP_USERDATA is not the only way to store data associated with a window, you can also use SetProp().

And at least on x86, you can do ATL style thunking (A small piece of asm code that puts your class pointer in ecx and then jumps to your wndproc) You can find some links about that in a answer I posted here

Answer 4

The problem with using a functor is the calling convention: Windows is expecting the address to be the address of a static function, and will use/invoke that address as such; whereas the 'this' which you're passing is not the address of a static function.

Windows is going to use the address like this (pseudo-coded assembly):

; push the necessary parameters
push [hWnd]
push etc...
; invoke the specified address (of the static function)
call [callback]

To invoke a functor, the Windows code would need to be like this

; push the necessary parameters
push [hWnd]
push etc...
; invoke the specified address (of the functor object)
; ... first, put the 'this' pointer as a hidden parameter into the ecx register
mov ecx,[callback]
; ... next, invoke the address (where is it?) of the class' functor method
call MyWinProc::operator()

... or instead of the last two statements, the following statements if the operator is virtual ...

; ... first, put the 'this' pointer as a hidden parameter into the ecx register
mov ecx,[callback]
; ... next, invoke the address of the operator via an (which?) entry
;     in the class' vtable
call [ecx+8]

Neither of these is possible because the O/S isn't aware of the calling conventions for non-static C++ methods, especially including:

• The way in which the implicit 'this' parameter is passed
• The address of the class' non-virtual methods
• The vtable entries of the class' virtual methods