5/26/08

What is Message Maps ?

Message Maps


MFC uses message maps to get around a fundamental problem with virtual
functions. Look at the CWnd class in the MFC help file. It contains over 200
member functions, all of which would have to be virtual if message maps were
not used. Now look at all of the classes that subclass the CWnd class. For
example, go to the contents page of the MFC help file and look at the visual
object hierarchy. 30 or so classes in MFC use CWnd as their base class. This
set includes all of the visual controls such as buttons, static labels, and lists.
Now imagine that MFC used virtual functions, and you created an application
that contained 20 controls. Each of the 200 virtual functions in CWnd would
require its own virtual function table, and each instance of a control would
therefore have a set of 200 virtual function tables associated with it. The
program would have roughly 4,000 virtual function tables floating around in
memory, and this is a problem on machines that have memory limitations.
Because the vast majority of those tables are never used, they are unneeded.
Message maps duplicate the action of a virtual function table, but do so on an
on-demand basis.

When you create an entry in a message map, you are saying
to the system, "when you see the specified message, please call the specified
function." Only those functions that actually get overridden appear in the
message map, saving memory and CPU overhead.

When you declare a message map with DECLARE_MESSAGE_MAP and
BEGIN_MESSAGE_MAP, the system routes all messages through to your
message map. If your map handles a given message, then your function gets
called and the message stops there. However, if your message map does not
contain an entry for a message, then the system sends that message to the
class specified in the second parameter of BEGIN_MESSAGE_MAP.

That class may or may not handle it and the process repeats. Eventually, if no message
map handles a given message, the message arrives at a default handler that
eats it.

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