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
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.
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.
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.
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."
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