MFC 콤마 넣기

CString CNumEdit::InsertComma(CString str)
{
CString strData = str;
 strData.Replace(_T(","), _T(""));  
 NUMBERFMT nFmt = { 0, 0, 3, _T("."), _T(","), 0 };
 TCHAR buffer[40]={0};  
 int pos=0;
 if(strData[0]==_T('-'))
 {
  pos=1;
  buffer[0]=_T('-');
 } 
 GetNumberFormat(NULL, NULL, (LPCTSTR)strData+pos, &nFmt, buffer+pos, sizeof(buffer));
 return (LPCTSTR)buffer;
}