Question

I am writing a Windows Api wrapper and I have run into a problem. I am abstracting the Window Procedure (WndProc) and wrote a static MsgHandler which would call the WndProc of my WinHandler class. Here is the code:

LRESULT CALLBACK WinHandle::MsgHandler(HWND hwnd,UINT msg,WPARAM wParam,LPARAM lParam)
{
    WinHandle* wnd = 0;

    if (msg == WM_NCCREATE)
    {
        ::SetWindowLong(hwnd,GWL_USERDATA,
                        long((LPCREATESTRUCT(lParam))->lpCreateParams));
    }

    wnd = (WinHandle*) (::GetWindowLong(hwnd,GWL_USERDATA));

    if (wnd)
        return wnd->WndProc(msg,wParam,lParam);
    else
        return ::DefWindowProc(hwnd,msg,wParam,lParam);
}

Since this functions is static , there is no this pointer and it works perfectly with WNDCLASSEX, but for some reason it never calls wnd->WndProc() and always returns the default window procedure. Need Help. What is wrong? Does anyone know a better approach?

Answer 1

Call SetWindowLong when you receive WM_CREATE, not WM_NCCREATE. I don’t think lpCreateParams is valid in WM_NCCREATE. That is:

if (msg == WM_CREATE)
{
    ::SetWindowLong(hwnd,GWL_USERDATA,
                    long((LPCREATESTRUCT(lParam))->lpCreateParams));
}