Empty Base Class or Structure Assignment Operator May Corrupt Data

This article was previously published under Q322127
This article has been archived. It is offered "as is" and will no longer be updated.
When you use a compiler-generated assignment operator for an empty base class or structure, the code that the compiler generates may corrupt data or write beyond the end of the class or the structure.
This problem occurs because of a bug in the Visual C++ compiler.
A supported hotfix is now available from Microsoft, but it is only intended to correct the problem that is described in this article. Only apply it to systems that are experiencing this specific problem. This hotfix may receive additional testing. Therefore, if you are not severely affected by this problem, we recommend that you wait for the next Microsoft Visual Studio 6.0 service pack that contains this hotfix.

To resolve this problem immediately, contact Microsoft Product Support Services to obtain the fix. For a complete list of Microsoft Product Support Services phone numbers and information about support costs, visit the following Microsoft Web site:NOTE: In special cases, charges that are ordinarily incurred for support calls may be canceled if a Microsoft Support Professional determines that a specific update will resolve your problem. The typical support costs will apply to additional support questions and issues that do not qualify for the specific update in question.

The English version of this fix has the file attributes (or later) that are listed in the following table. The dates and times for these files are listed in coordinated universal time (UTC). When you view the file information, it is converted to local time. To find the difference between UTC and local time, use the Time Zone tab in the Date and Time tool in Control Panel.
   Date         Time   Version         Size       File name   --------------------------------------------------------------   14-May-2002  10:40  12.0.9586.0     1,224,755  C1xx.dll				

Microsoft has confirmed that this is a problem in the Microsoft products that are listed at the beginning of this article.
You can use the following sample code to demonstrate the problem. Note that the values of 'a.m_n' and 'b.m_n' differ.
#include <stdio.h>class CEmpty{public:	CEmpty() throw()	{	}	CEmpty( const CEmpty& ) throw()	{	}	~CEmpty() throw()	{	}	CEmpty get() const throw()	{		return( *this );	}};class CDerived :	public CEmpty  // Empty base class {public:	int m_n;	CDerived& operator=( const CDerived& src ) throw()	{		m_n = src.m_n;                // Overwrites low byte of m_n.		CEmpty::operator=( src.get() ); 		return( *this );	}	};int main( int, char** ){	CDerived a;	a.m_n = 0x01020304;	CDerived b;	b = a;	printf( "0x%08x\n", b.m_n );	return( 0 );}				

Article ID: 322127 - Last Review: 01/07/2015 15:27:43 - Revision: 1.3

Microsoft Visual C++ 6.0 Service Pack 5

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