Re: Question on MFC internals - message maps

From:
"Doug Harrison [MVP]" <dsh@mvps.org>
Newsgroups:
microsoft.public.vc.mfc
Date:
Fri, 08 May 2009 15:07:37 -0500
Message-ID:
<8339051m4120k95bjaiokvh9ihcj1b8kk9@4ax.com>
On Fri, 8 May 2009 22:14:42 +0300, Toddintr <telephone.call.from.istanbul>
wrote:

Being one of those types that likes to understand the underlying layers of
software, I decided to look into the implementation of message maps in MFC,
e.g. the DECLARE_MESSAGE_MAP () macro and the like. However, the
implementation is incredibly convoluted. For example, the following
segment is from the .i file output:

protected:
static const AFX_MSGMAP* __stdcall GetThisMessageMap();
virtual const AFX_MSGMAP* GetMessageMap() const;

I understand the goal is (was) to avoid using virtual functions (says so in
the documentation) but nevertheless, it's not a pretty sight. I'd like to
know if there is any source that explains, in a simple manner, how the
message map mechanism works.


If you can't find it on MSDN, maybe codeproject.com or a really old back
issue of MSJ? Anyway, here's my take on it:

A class that uses DECLARE_MESSAGE_MAP has an array of (message id, pointer
to member function) pairs.

1. When a message is retrieved for a window, the HWND is looked up in a
thread-local handle map.

2. If the lookup is successful, the corresponding CWnd* is retrieved. Call
it "pWnd".

3. The virtual function pWnd->GetMessageMap() is called to retrieve a
pointer to the message map for the most derived class that used
DECLARE_MESSAGE_MAP.

4. The message map is searched for the ID. If found, the WPARAM/LPARAM
parameters are "cracked", and the associated member function is called
through pWnd. The message is thus handled.

5. If the ID isn't found, a pointer to the closest base class message map
is retrieved, and then goto (4).

If no CWnd is associated with the HWND, or the message ID lookup fails, the
message is sent on its way using standard Windows functions. Steps 1-2 are
done for posted messages. Sent messages bypass the message queue, and IIRC,
MFC overrides their window procedures and goes more directly to (3).

--
Doug Harrison
Visual C++ MVP

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