title: What's an alternative to GWL_USERDATA for storing an object pointer?
body: <p>In the Windows applications I work on, we have a custom framework that sits directly above Win32 (don't ask).  When we create a window, our normal practice is to put <code>this</code> in the window's user data area via <code>SetWindowLong(hwnd, GWL_USERDATA, this)</code>, which allows us to have an MFC-like callback or a tightly integrated <code>WndProc</code>, depending.  The problem is that this will not work on 64-bit Windows, since LONG is only 32-bits wide.  What's a better solution to this problem that works on both 32- and 64-bit systems?</p>