Q114201: PRB: CString::ReleaseBuffer() Does Not Release Extra Memory

Article: Q114201
Product(s): Microsoft C Compiler
Version(s): winnt:1.0,2.0,2.1,4.0
Operating System(s): 
Keyword(s): kbnokeyword kbMFC kbVC kbVC100bug kbVC150bug kbVC200bug kbVC400bug kbVC500fix kbprb kbG
Last Modified: 07-MAY-2001

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The information in this article applies to:

- The Microsoft Foundation Classes (MFC), used with:
   - Microsoft Visual C++ for Windows, 16-bit edition, versions 1.0, 1.5, 1.51, 1.52 
   - Microsoft Visual C++, 32-bit Editions, versions 1.0, 2.0, 2.1, 4.0 
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SYMPTOMS
========

When CString::ReleaseBuffer is called and the length of the string is less than
the allocated buffer length, the extra bytes are not released. This just means
that your program might end up maintaining more memory than absolutely
necessary. This will not cause a memory leak. All of the memory will be freed
when the CString object is destroyed.

CAUSE
=====

The behavior of CString::ReleaseBuffer described above is by design.

RESOLUTION
==========

Visual C++ 2.0 and later include CString::FreeExtra to release any unused
memory.

In earlier versions, one way to make the string's buffer length equal to the
string's data length is to write a function which will free the extra memory.
The two functions in the sample code section demonstrate two different methods
of writing this function; either as a member function of a derived CString class
or as a separate function which accepts a CString parameter type. Note that if
the member function approach is taken, then the derived class will need to
override the constructors provided by the CString class in order to have access
to them.

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

By not freeing the extra bytes of memory in ReleaseBuffer, CString provides a
simple method of growing without having to allocate more memory while keeping
memory fragmentation to a minimum. For example, calling ReleaseBuffer(20) when
the string's allocated buffer length is 200 bytes will leave an extra 180 bytes
free. This way, the only time that a CString object will need to allocate more
memory is when the data length exceeds the allocated buffer length.

Sample Code
-----------

- Member function method

  class CMyString : public CString
  {
    public:
      CMyString(const char* psz = NULL) : CString(psz) {}
      void FreeExMem();
  };

  void CMyString::FreeExMem()
  {
    ASSERT(m_nDataLength<=m_nAllocLength);
    if(!IsEmpty())
    {
      char *tp = new char[m_nDataLength+1];
      memcpy(tp,m_pchData,m_nDataLength+1);
      ASSERT(m_pchData[m_nDataLength]=='\0');
      delete m_pchData;
      m_pchData = tp;
      m_nAllocLength = m_nDataLength;
    }
    ASSERT(m_pchData!=NULL);
  }

- Separate Function Method

  void FreeExMem(CString &s)
  {
    if(!s.IsEmpty())
    {
      char *p = s.GetBuffer(1);
      char *tp = new char[s.GetLength()+1];
      memcpy(tp,p,s.GetLength()+1);
      s.ReleaseBuffer();
      s.Empty();
      s = tp;
      delete tp;
    }
  }

Additional query words: 1.00 1.50 2.00 2.10 2.50 2.51 2.52 3.00 3.10 4.00

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Keywords          : kbnokeyword kbMFC kbVC kbVC100bug kbVC150bug kbVC200bug kbVC400bug kbVC500fix kbprb kbGrpDSMFCATL 
Technology        : kbAudDeveloper kbMFC
Version           : winnt:1.0,2.0,2.1,4.0
Issue type        : kbprb

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