Question

I am attempting to learn about creating windows in c++, I have looked at an article about creating a wrapper class but I don’t really understand it. So far I know that you can’t have a class method WndProc (I dont know why) but honestly, that is all. Can somebody give an explanation, also explaining the reinterpret_cast? Here is the article.

LRESULT CALLBACK Window::MsgRouter(HWND hwnd, UINT message,
                                   WPARAM wparam, LPARAM lparam)
{
  Window *wnd = 0;

  if(message == WM_NCCREATE)
  {
    // retrieve Window instance from window creation data and associate
    wnd = reinterpret_cast<Window *>((LPCREATESTRUCT)lparam)->lpCreateParams;
    ::SetWindowLong(hwnd, GWL_USERDATA, reinterpret_cast<long>(wnd));

    // save window handle
    wnd->SetHWND(hwnd);
  }
  else
    // retrieve associated Window instance
    wnd = reinterpret_cast<Window *>(::GetWindowLong(hwnd, GWL_USERDATA));

  // call the windows message handler
  wnd->WndProc(message, wparam, lparam);
}

Thanks in advance, ell.

Answer 1

The MsgRouter() procedure acts as a proxy between the Windows message handling system to the Window instance associated with a HWND. It routes Windows messages to C++ objects.

A pointer to the Window instance is passed to the MsgRouter() procedure via the last parameter of the CreateWindow() function. When you first create a HWND via CreateWindow() (or CreateWindowEx()), some messages are sent – one of them being WM_NCCREATE. When the procedure receives a WM_NCCREATE message, the LPARAM parameter contains a pointer to a CREATESTRUCT which contains the arguments that was passed into the CreateWindow() function. The procedure retrieves the Window instance pointer from the CREATESTRUCT and saves it in the HWND by setting it as an attribute of the HWND (GWL_USERDATA via SetWindowLong()).

Now that the pointer has been saved, the window procedure can from now on retrieve a pointer to the original Window instance from a HWND via GetWindowLong() when it receives a message. Finally, the window procedure calls WndProc() on the retrieved Window pointer, passing in the exact message and parameters, so the Window instance can handle the message.