Q322127: Empty Base Class/Structure Assignment Operator May Corrupt Data

Article: Q322127
Product(s): Microsoft C Compiler
Version(s): 6.0
Operating System(s): 
Keyword(s): 
Last Modified: 06-JUN-2002

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The information in this article applies to:

- Microsoft Visual C++, 32-bit Editions, version 6.0 
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SYMPTOMS
========

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.

CAUSE
=====

This problem occurs because of a bug in the Visual C++ compiler.

RESOLUTION
==========

A supported fix is now available from Microsoft, but it is only intended to
correct the problem described in this article and should be applied only to
systems experiencing this specific problem. This fix may receive additional
testing at a later time, to further ensure product quality. Therefore, if you
are not severely affected by this problem, Microsoft recommends that you wait
for the next Microsoft Visual Studio 6.0 service pack that contains this fix.

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, please go to the following
address on the World Wide Web:

  http://support.microsoft.com/default.aspx?scid=fh;EN-US;CNTACTMS

NOTE: In special cases, charges that are normally incurred for support calls may
be canceled, if a Microsoft Support Professional determines that a specific
update will resolve your problem. Normal 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



STATUS
======

Microsoft has confirmed this to be a problem in the Microsoft products that are
listed at the beginning of this article.

MORE INFORMATION
================

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 );
  }

Additional query words:

======================================================================
Keywords          :  
Technology        : kbVCsearch kbAudDeveloper kbVC600 kbVC32bitSearch
Version           : :6.0
Issue type        : kbbug
Solution Type     : kbfix

=============================================================================

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