Microsoft KB Archive/114201

= PRB: CString::ReleaseBuffer Does Not Release Extra Memory =

Article ID: 114201

Article Last Modified on 11/21/2006

-

APPLIES TO

 Microsoft Foundation Class Library 4.2, when used with:  Microsoft Visual C++ 1.0 Professional Edition

 Microsoft Visual C++ 1.5 Professional Edition

 Microsoft Visual C++ 1.51

 Microsoft Visual C++ 1.52 Professional Edition</li></ul>

 Microsoft Visual C++ 1.0 Professional Edition</li></ul>

 Microsoft Visual C++ 2.0 Professional Edition</li></ul>

 Microsoft Visual C++ 2.1</li></ul>

 Microsoft Visual C++ 4.0 Standard Edition</li></ul> </li></ul>

-

<div class="notice_section">

This article was previously published under Q114201

<div class="symptoms_section">

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.

<div class="cause_section">

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

<div class="resolution_section">

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.

<div class="moreinformation_section">

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); }                       </li>  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; } }                       </li></ul>

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

Keywords: kbbug kbvc500fix kbprb KB114201

-

[mailto:TECHNET@MICROSOFT.COM Send feedback to Microsoft]

© Microsoft Corporation. All rights reserved.