CString Class Research (2)

3. CString Data Buffer

                CString use CStringData structure as a buffer to store data and other information.

struct CStringData

{

                long nRefs;             // reference count

                int nDataLength;        // length of data (including terminator)

                int nAllocLength;       // length of allocation

                // TCHAR data[nAllocLength]

                TCHAR* data()           // TCHAR* to managed data

                                { return (TCHAR*)(this+1); }

};

A CString Data Buffer (CStringData)

CStringData::data() will return to actually data address.

CString::m_pchData member variable is a pointer of actually data address.

CString::GetData() return CStringData, point to the buffer, include header and actually data.

_AFX_INLINE CStringData* CString::GetData() const

                { ASSERT(m_pchData != NULL); return ((CStringData*)m_pchData)-1; }

CString::operator LPCTSTR() return the address of m_pchData.

_AFX_INLINE CString::operator LPCTSTR() const

                { return m_pchData; }

CString::GetBuffer() function will do following  process:

create a new buffer;

copy old value to new buffer;

dispose old buffer;

return new buffer;

                But if the CString have been referenced (the data buffer is not point to _afxPchNil), and the allocate length is bigger than required, It needn’t allocate new buffer, It will return current buffer only.

For example:

                CString str1;

                char * buf;

                buf = str1.GetBuffer(10);

Before GetBuffer() function is invoked, the address of str1 is 0x0012f6f8, the address of str1.m_pchData is 0x5f4c4b14 (should be system defined “null” buffer, _afxPchNil).

                And after GetBuffer() function is invoked, the address of str1 is 0x0012f6f8, the address of str1.m_pchData is 0x004213fc, Changed!.

                So that, the LPCTSTR() operator and GetBuffer() is different, LPCTSTR() operator will not create new buffer but return the current buffer.

And other example:

                CString str1(“this buffer is big enough);

                char * buf;

                buf = str1.GetBuffer(10);

                In this example, the address of data buffer is not changed.

                The following diagram is showing the format of  system defined “null” buffer and value:

CString::ReleaseBuffer() function will update the “header” filed of CStringData.

void CString::ReleaseBuffer(int nNewLength)

{

                CopyBeforeWrite();  // just in case GetBuffer was not called

                if (nNewLength == -1)

                                nNewLength = lstrlen(m_pchData); // zero terminated

                ASSERT(nNewLength <= GetData()->nAllocLength);

&n

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