mfc中 CString转char*

char * cs2ca(CString str, char * cstr)
{
if (cstr==NULL)
{
return cstr;
}
char *ptr = cstr;
#ifdef _UNICODE
LONG len;
len = WideCharToMultiByte(CP_ACP, 0, str, -1, NULL, 0, NULL, NULL);
//ptr = new char[len + 1];
//memset(ptr, 0, len + 1);
WideCharToMultiByte(CP_ACP, 0, str, -1, ptr, len, NULL, NULL);
#else
//ptr = new char[str.GetAllocLength() + 1];
sprintf(ptr, _T("%s"), str);
#endif
return ptr;
}