编写com组件时，标准的字符串连接

STDMETHODIMP CFun::Cat(BSTR s1, BSTR s2, BSTR *pVal)
{
CComBSTR sResult( s1 );
sResult.AppendBSTR( s2 );

*pVal = sResult.Copy();
// *pVal = sResult.Detach();

return S_OK;
}