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From: ronb@burklabs (Ron Burk       )
Newsgroups: comp.windows.ms.programmer
Subject: Associating window handle with C++ object
Message-ID: <0J7H11w163w@burklabs>
Date: 16 Apr 91 15:36:56 GMT
Organization: Burk Labs, Redmond WA
Lines: 26

Suppose you are creating a C++ class which corresponds to a Windows 3.0
window.  Obviously, you will want to associate a Windows 3.0 window
handle with the C++ window object.  I would think the most natural way
to do this would be:

a) Allocate enough "extra" bytes to hold a pointer to "this" when you
   register your window class.

b) Using a single window function for all windows.

c) In your single window function, extract the "this" pointer and pass
   the window message on to the correct C++ window object.

Unfortunately, CreateWindow() will send messages to the window before it
returns the window handle, so the scheme just described doesn't quite
work.  In the "whello.cpp" that comes with Borland C++, they assume that
any "extra" window bytes will be initialized to zero.  When their Window
constructor calls CreateWindow(), it passes "this" in the last
parameter.  In their single window function, they first extract the
"this" pointer from the "extra" window bytes and if it is zero and the
message is WM_PAINT, they get the "this" pointer from
lParam->lpCreateParams and store it in the window "extra" bytes. 

How do other C++ classes associate the window handle with the C++ window
object? Is it safe to assume that WM_PAINT is the first message received
by any window created by CreateWindow()?