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

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