CString Class Research (3)

4. CStringData Allocate and dispose.

                In prior section, we know the CStringData has two Parts, One is Header, Include 3 member variables and 1 member function, totally 12 bytes, and the other part is actual data.

                Look the following code in MFC to Allocate and Dispose CStringData and its data buffer.

CStringData* pData;

pData = (CStringData*)

                                new BYTE[sizeof(CStringData) + (nLen+1)*sizeof(TCHAR)];

pData->nAllocLength = nLen;

pData->nRefs = 1;

pData->data()[nLen] = '\0';

pData->nDataLength = nLen;

m_pchData = pData->data();

                MFC allocate memory as BYTE array, include CStringData and data buffer, and convert it to CStringData pointer, nLen is the length of string, and one more byte, store a ‘\0’.

                In debug mode, when the nLen <= 64, system allocates 64 bytes, when the nLen <= 128, system allocates 128 bytes....

delete[] (BYTE*)pData;

                To dispose buffer, because there are CStringData part (header) and data buffer part, MFC convert the CStringData* to BYTE array, then delete it.

5. CString operator =.

                There are two methods to set one CString’s value. If the source string is not a CString object, then MFC will allocate a new buffer to destination CString object, and use memcpy function to copy the content of source string to destination CString.m_pchData.

AllocBeforeWrite(nSrcLen);

memcpy(m_pchData, lpszSrcData, nSrcLen*sizeof(TCHAR));

GetData()->nDataLength = nSrcLen;

m_pchData[nSrcLen] = '\0';

In addition, if the source string is CString Object, commonly, the destination CString.m_pchData point to source CString.m_pchData. That means, the destination CString.m_pchData and the source CSting.m_pchData will point to the same address, destination CString and the source CString has the same CStringData object.

                When 2 objects share one CStringData, the nRef member variable of CStringData will be set to 2.

If (m_pchData != stringSrc.m_pchData)

{

                if ((GetData()->nRefs < 0 && GetData() != _afxDataNil) ||

                                stringSrc.GetData()->nRefs < 0)

                {

                                // actual copy necessary since one of the strings is locked

                                AssignCopy(stringSrc.GetData()->nDataLength, stringSrc.m_pchData);

                }

                else

                {

                                // can just copy references around

                                Release();

                                ASSERT(stringSrc.GetData() != _afxDataNil);

                                m_pchData = stringSrc.m_pchData;

                                InterlockedIncrement(&GetData()->nRefs);

                }

}

                Two CString share one CStringData will save system memory resource, but there is a risk, that when data in CString object’s CStringData is changed, the other CString object will be ch

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