PRB: C2642 Error When Adding Message Handler

ID: Q103806

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

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*)(&amp;nMessageVariable), \ 
     /*implied 'AfxSig_lwl'*/ \ 
     (AFX_PMSG)(AFX_PMSGW)  &lt;not actual line break&gt;
     (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*)(&amp;nMessageVariable), \ 
     /*implied 'AfxSig_lwl'*/ \ 
     (AFX_PMSG)(AFX_PMSGW)  &lt;not actual line break&gt;
     (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.

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

For information about other causes of the C2642 error message, please see the following article(s) in the Microsoft Knowledge Base:

Q85521 C2642 in Message Map Caused by Incorrect Prototypes

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 kbGrpMFCATL kbArchitecture
Version : winnt:1.0,2.0,2.1,4.0
Platform : winnt
Issue type : kbprb