FIX: DYNAMIC_DOWNCAST & STATIC_DOWNCAST Causes Stack Overflow (151070)

The information in this article applies to:

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

This article was previously published under Q151070

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.

Modification Type:	Major	Last Reviewed:	10/24/2003
Keywords:	kbBug kbfix kbNoUpdate kbVC420fix KB151070