void SetAt( int nIndex, TCHAR ch );
Zero-based index of the character in the CString object. The nIndex parameter must be greater than or equal to 0 and less than the value returned by GetLength. The Debug version of the Microsoft Foundation Class Library will validate the bounds of nIndex; the Release version will not.
The character to insert.
You can think of a CString object as an array of characters. The SetAt member function overwrites a single character specified by an index number. SetAt will not enlarge the string if the index exceeds the bounds of the existing string.