FIX: Temporary Object Memory Leak in Foundation Classes

Last reviewed: September 16, 1997
Article ID: Q98867

The information in this article applies to:

The Microsoft Foundation Classes (MFC) included with:

    - Microsoft Visual C++ for Windows, version 1.0

SYMPTOMS

Under some circumstances, an application created with Microsoft Foundation Classes version 2.0 allocates temporary objects on the application heap and does not delete them. While the memory diagnostics in the debugging version of the Foundation Classes libraries do not detect this memory leak, the error is apparent when Heap Walker is used to monitor the heap.

CAUSE

A HANDLE is attached to both a permanent object and a temporary object. When the HANDLE is detached from the permanent object, it is also detached from the temporary object. Consequently, the temporary object is never destroyed.

RESOLUTION

Do not remove the HANDLE from the temporary map when it is detached. To do so, remove one line of code from the WINHAND.CPP source file. In Visual C++, remove line 158 which reads as follows:

   m_temporaryMap.RemoveKey((MAPTYPE)h);

It is also necessary to change the arguments to some ASSERT macros to prevent detaching a temporary object from causing an assertion failure. It is not necessary to Detach a temporary object because it generally serves no useful purpose. However, because Microsoft Foundation Class Libraries version 1.0 support this practice, version 2.0 supports it as well. In WINHAND.CPP, change lines 145 and 173 from

   ASSERT(ph[0] == h);

to:

   ASSERT(ph[0] == h || ph[0] == NULL);

After making these changes, rebuild the Microsoft Foundation Class Libraries using the MAKEFILE in the library source directory (by default, C:\MSVC\MFC\SRC). The MAKEFILE supports building all variations of the Foundation Class Libraries. For more information on using the MAKEFILE, please refer to the comments at the beginning of the MAKEFILE.

STATUS

Microsoft has confirmed this to be a problem in the Microsoft Foundation Class Libraries version 2.0 for Windows. This problem was corrected in the MFC libraries version 2.5, which ships with Microsoft VC++ version 1.5.

MORE INFORMATION

The Foundation Class Libraries maintain two maps for each HANDLE used in the Microsoft Windows environment. The Libraries maintain four map pairs, for HDCs, HGDIOBJECTs, HMENUs, and HWNDs. For each handle type, the Libraries maintain two separate maps: a temporary map and a permanent map. When the Library maps a HANDLE to a C++ object, it checks the permanent map first and then checks the temporary map.

Under some circumstances, a HANDLE appears in both maps. A serious memory leak occurs when a HANDLE is Detached (or unmapped) from its permanent C++ object. This removes the HANDLE from both maps even though the temporary object is not destroyed. Because the temporary C++ object is not present in the temporary map when the CWinApp::OnIdle() function attempts to delete all temporary objects, the temporary C++ object remains in memory. For example, the following code example causes a memory leak that involves one temporary CDC object:

   CDC* pDC = CDC::FromHandle(hDC); // creates temporary object
   CDC dc;
   dc.Attach(hDC); // now in permanent map as well
   dc.Detach(hDC); // removed from temp and permanent map
   // pDC is never deleted

Even though the code sequence above is rare, it occurs quite often processing objects in the CFrameWnd class. CFrameWnd processes a WM_ERASEBKGND message in an OnEraseBkgnd handler. Windows sends WM_ERASEBKGND before it sends the WM_PAINT message. The Library creates a temporary object in the OnEraseBkgnd handler. When the application receives the WM_PAINT message, it creates a permanent CPaintDC object on the stack. This creates a situation similar to that above except that the temporary and permanent objects result from processing different messages.

Because CWinApp::OnIdle() is not called between the WM_ERASEBKGND message and WM_PAINT messages, the temporary object created processing the WM_ERASEBKGND message is not removed.

This problem does not generally affect applications created in AppWizard because CFrameWnd objects in an application that uses CView child windows do not receive WM_PAINT messages under normal circumstances.

Keywords          : kb16bitonly MfcMisc kbfasttip
Technology        : kbmfc
Version           : 1.0
Platform          : WINDOWS
Issue type        : kbbug
Solution Type     : kbfix

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Last reviewed: September 16, 1997
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