Microsoft KB Archive/141532

= FIX: OnInitMenuPopup Deletes Temporary Objects =

Article ID: 141532

Article Last Modified on 11/21/2006

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APPLIES TO


 * Microsoft Foundation Class Library 4.2, when used with:
 * Microsoft Visual C++ 4.0 Standard Edition

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This article was previously published under Q141532



SYMPTOMS
When overriding CWnd::OnInitMenuPopup, while the CMenu pointer passed in is accessed, after calling the base class OnInitMenuPopup, the program may cause an ASSERT, access violation, or other problems.



CAUSE
In MFC version 4.0 calls to AfxLockTempMaps and AfxUnlockTempMaps were added to CWnd::OnInitMenuPopup. When AfxUnlockTempMaps is called, MFC's temporary object map reference count will go to zero causing all temporary MFC objects to be deleted. When the call to OnInitMenuPopup returns, the CMenu pointer (which is a temporary) will be invalid.



RESOLUTION
Make a call to AfxLockTempMaps before the call to the base class and a call to AfxUnlockTempMaps before returning from your OnInitMenuPopup override. This will stop the internal reference count from reaching zero inside CWnd::OnInitPopupMenu and the CMenu will not be deleted.



STATUS
Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. This bug was corrected in Visual C++ 4.1.



MORE INFORMATION
MFC maintains a map from handles such as HMENUs to their C++ wrapper objects. When a call is made to, for example, GetMenu, MFC attempts to find the C++ wrapper object that corresponds to the HMENU ::GetMenu returns. If no object in the permanent map is found, MFC creates a temporary object that is normally destroyed in OnIdle processing by a call to AfxUnlockTempMaps.

Sample Code to Reproduce or Resolve Problem
/* Compile options needed: default

// Use this code to fix the problem: void CMainFrame::OnInitMenuPopup(CMenu* pPopupMenu, UINT nIndex,                                                  BOOL bSysMenu)

{   #if _MFC_VER == 0x400 AfxLockTempMaps; #endif

CMDIFrameWnd::OnInitMenuPopup(pPopupMenu, nIndex, bSysMenu);

// TODO: Add your message handler code here

#if _MFC_VER == 0x400 AfxUnlockTempMaps; #endif

}

// Use the following code to reproduce the problem in an MDI application: void CMainFrame::OnInitMenuPopup(CMenu* pPopupMenu, UINT nIndex,                                                  BOOL bSysMenu)

{

CMDIFrameWnd::OnInitMenuPopup(pPopupMenu, nIndex, bSysMenu);

// The next line will cause an ASSERT on line 1034 in a debug // build because pPopupMenu is an invalid pointer pPopupMenu->CheckMenuItem(0, MF_BYPOSITION | MF_CHECKED);

}

