Question

Is this normal behavior; this never happened to me before. I assume it would cause an exception but why doesn’t it here? Take a look.

CWindowsApplication::MsgProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    static CWindowApplication* pApp = NULL;

    if (message == WM_NCCREATE)
    {
    //// retrieve Window instance from window creation data and associate
    //pApp = reinterpret_cast(((LPCREATESTRUCT)lParam)->lpCreateParams); 
    //::SetWindowLong(hWnd, GWL_USERDATA, reinterpret_cast(pApp));

    //pApp = reinterpret_cast(::GetWindowLong(hWnd, GWL_USERDATA));
    }

    pApp->WndProc(hWnd, message, wParam, lParam); // pApp = NULL, but it still works? I expected a exception of some sort. 
}

But, when I change the class to something else I get the exception I was expecting.
What is going on here? Never in my 10+ years as an enthusiastic programmer have I ever came across something like this.

Answer 1

As long as WndProc is not virtual, the pointer technically doesn’t need to be dereferenced at all in order to make the call. That’s not to say it won’t crash and burn when you try to use this (including calling any virtual function with an implicit this) inside WndProc, but non-virtual calls go by the type of the pointer, and don’t need to touch the vtable (or any other instance member).