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Q151070: FIX: DYNAMIC_DOWNCAST & STATIC_DOWNCAST Causes Stack Overflow

Article: Q151070
Product(s): Microsoft C Compiler
Version(s): winnt:4.1
Operating System(s): 
Keyword(s): kbMFC kbVC400bug kbVC410bug kbVC420fix kbGrpDSMFCATL kbNoUpdate
Last Modified: 30-JUL-2001

-------------------------------------------------------------------------------
The information in this article applies to:

- The Microsoft Foundation Classes (MFC), used with:
   - Microsoft Visual C++, 32-bit Editions, version 4.1 
-------------------------------------------------------------------------------

SYMPTOMS
========

When the DYNAMIC_DOWNCAST and STATIC_DOWNCAST macros are called, and a pointer
to a "const" object is passed, you see an error message similar to the following
in debug mode:

  Unhandled exception in downcast.exe (MFC40D.DLL); Stack Overflow.

In release builds, the application hangs.

CAUSE
=====

The DYNAMIC_DOWNCAST and STATIC_DOWNCAST macros invoke the AfxDynamicDownCast
and AfxStaticDownCast functions respectively. A code defect in Visual C++
version 4.1 causes these calls to be recursive if the object pointer passed as
the second parameter to either macro points to a "const" object. The end result
is a stack overflow.

RESOLUTION
==========

To work around this problem, remove the "const" attribute for the object pointer
in the call to the appropriate DOWNCAST macro. You can do this by using the
"const_cast" operator as follows:

     //CMyDlg is derived from CDialog
     const CMyDlg dlg;

     //pdlg is a pointer to a const object
     const CMyDlg* pdlg = &dlg;

     DYNAMIC_DOWNCAST(CDialog, const_cast<CMyDlg*>(pdlg));

     // or

     STATIC_DOWNCAST(CDialog, const_cast<CMyDlg*>(pdlg));

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++ 32-bit Edition
version 4.2.

Additional query words: MfcMisc

======================================================================
Keywords          : kbMFC kbVC400bug kbVC410bug kbVC420fix kbGrpDSMFCATL kbNoUpdate 
Technology        : kbAudDeveloper kbMFC
Version           : winnt:4.1
Issue type        : kbbug
Solution Type     : kbfix

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