Q103806: PRB: C2642 Error When Adding Message Handler

Article: Q103806
Product(s): Microsoft C Compiler
Version(s): winnt:1.0,2.0,2.1,4.0
Operating System(s): 
Keyword(s): kbMFC kbVC100 kbVC150 kbVC200 kbVC400 kbGrpDSMFCATL kbArchitecture
Last Modified: 29-AUG-2001

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The information in this article applies to:

- The Microsoft Foundation Classes (MFC), used with:
   - Microsoft C/C++ for MS-DOS, version 7.0 
   - Microsoft Visual C++ for Windows, 16-bit edition, versions 1.0, 1.5, 1.51, 1.52 
   - Microsoft Visual C++, 32-bit Editions, versions 1.0, 2.0, 2.1, 4.0 
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SYMPTOMS
========

The following error message can occur after adding a message handler for a class
when using the Microsoft Foundation Classes (MFC):

  error C2642: cast to pointer to member must be from related pointer to member

CAUSE
=====

This compiler error message occurs if a window message handler macro has been
added to the message map of a class that is not derived from CWnd. For example,
if you declare a class as

     class CMyApp: public CWinApp
     {
        ...
        DECLARE_MESSAGE_MAP()
        afx_msg LRESULT OnMyRegisteredMessage( WPARAM, LPARAM );
        ...
     };

with a message map such as the following

     BEGIN_MESSAGE_MAP(CMyApp,CWinApp)
        ON_REGISTERED_MESSAGE( MyMSGID, OnMyRegisteredMessage )
     END_MESSAGE_MAP()

the C2642 error will occur. Examine the macro ON_REGISTERED_MESSAGE() in
AFXMSG_.H; you can see why the error occurs. Here is the definition of the
macro:

     // For registered Windows messages
     #define ON_REGISTERED_MESSAGE(nMessageVariable, memberFxn) \ 
       { 0xC000, 0, (UINT)(UINT NEAR*)(&nMessageVariable), \ 
       /*implied 'AfxSig_lwl'*/ \ 
       (AFX_PMSG)(AFX_PMSGW)  <not actual line break>
       (LRESULT (AFX_MSG_CALL CWnd::*)(WPARAM, LPARAM))memberFxn },

In MFC 4.0 (included with Visual C++ 4.0), this macro is defined as:

     // for Registered Windows messages
     #define ON_REGISTERED_MESSAGE(nMessageVariable, memberFxn) \ 
       { 0xC000, 0, 0, 0, (UINT)(UINT*)(&nMessageVariable), \ 
       /*implied 'AfxSig_lwl'*/ \ 
       (AFX_PMSG)(AFX_PMSGW)  <not actual line break>
       (LRESULT (AFX_MSG_CALL CWnd::*)(WPARAM, LPARAM))memberFxn },

Notice that the function pointer is cast to a pointer to a function in the CWnd
scope. In the example above, CMyApp is derived only from CWinApp. As it does not
have CWnd as a base class, CMyApp will not receive window messages. It only
receives command messages that are sent through the command routing process.

RESOLUTION
==========

The following are two possible workarounds to resolve the problem:

- Trap the message in a CWnd object. If necessary, then send a WM_COMMAND
  message or notification to the CCmdTarget.

-or-

- Trap the message in a CWnd object and then call a member function of the
  intended object directly.

MORE INFORMATION
================

Message maps contain six different categories of messages:

- Control notification messages

- Update command UI messages

- Command messages

- VBX and OCX event notifications

- Normal window messages

- Registered window messages

The first four categories are notifications that are routed through MFC's
command-routing methods. You can handle these messages in any CCmdTarget-
derived class.

The last two categories are standard window messages and can be handled only in a
CWnd-derived class. These messages are not routed via the framework's command
routing mechanism.

REFERENCES
==========

Information about command routing can be found in Chapter 3 of the "Class
Library Reference," MFC Technical Note 21, and Chapter 6 of the "Class Library
User's Guide."

Additional query words: 7.00 1.00 1.50 1.51 1.52 2.00 2.10 2.50 2.51 2.52 3.00 3.10 4.00

======================================================================
Keywords          : kbMFC kbVC100 kbVC150 kbVC200 kbVC400 kbGrpDSMFCATL kbArchitecture 
Technology        : kbAudDeveloper kbMFC
Version           : winnt:1.0,2.0,2.1,4.0
Issue type        : kbprb

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