Q172398: FIX: Debug Assertion When Assigning to STL String

Article: Q172398
Product(s): Microsoft C Compiler
Version(s): winnt:5.0
Operating System(s): 
Keyword(s): kbVC500bug kbVC600fix
Last Modified: 05-MAY-2001

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The information in this article applies to:

- The Standard C++ Library, used with:
   - Microsoft Visual C++, 32-bit Enterprise Edition, version 5.0 
   - Microsoft Visual C++, 32-bit Professional Edition, version 5.0 
-------------------------------------------------------------------------------

SYMPTOMS
========

When you assign a shorter string to an existing string that originally contained
a longer string, the assignment corrupts the heap.

When running a debug build, you may see an assertion similar to the following:

  

  Debug Error!
   Program <your program name>
   DAMAGE: after Normal block (#NNN) at 0xNNNNNNNN

CAUSE
=====

This problem is due to a bug in the Standard C++ Library basic_string class
implementation. When assigning a shorter string to an existing string that
originally contained a longer string, the heap is corrupted. The assignment can
be done either through operator=() or assign().

RESOLUTION
==========

To work around the problem, call the string::erase member function before
assigning the new value to the existing string.

STATUS
======

Microsoft has confirmed this to be a bug in the Microsoft products listed at the
beginning of this article. This problem was corrected in Visual C++ version 6.0
for Windows.

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

Steps to Reproduce Behavior
---------------------------

     //Compile options needed: /GX
     #include <crtdbg.h>
     #include <string>

     int main()
     {
         std::string  str, str2;
         str = "abcdefghijklmnopqrstuvwxyzabcdefghij" ;
         str2 = str;

         //Workaround, uncomment the following line
         //str.erase() ;

         str = "zyxw" ;
         _CrtCheckMemory() ;

         return 0;

     }

======================================================================
Keywords          : kbVC500bug kbVC600fix 
Technology        : kbVCsearch kbAudDeveloper kbVCLibrary
Version           : winnt:5.0
Issue type        : kbbug
Solution Type     : kbfix

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