FIX: OnInitMenuPopup Deletes Temporary Objects

The information in this article applies to:

• The Microsoft Foundation Classes (MFC) included with: Microsoft Visual C++, 32-bit Edition, version 4.0

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);

REFERENCES

MFC Technical Note 3: Mapping of Windows Handles to Objects.

For more information, please see the following article in the Microsoft Knowledge Base:

   ARTICLE-ID: Q139469
   TITLE     : How to Use TrackPopupMenu() and Update UI Handlers