string w2s( const wstring &src, int codepage=- 1  )
{
    if( src.empty() )
    {
        return "";
    }
    const size_t len_origin = src.length();
    if( len_origin ==  0  )
    {
        return "";
    }
    if( codepage == - 1  )
    {
        codepage = CP_ACP;  // default to ANSI code page
    }
    const int expected_size = WideCharToMultiByte( codepage,  0 , src.c_str(), (int)len_origin, NULL,  0 , NULL, NULL ); 
    if( expected_size <=  0  )
    {
        return "";
    }
    char *buf = new char[expected_size +  1 ];
    if( buf )
    {
        memset( buf,  0 , expected_size +  1  );
        ::WideCharToMultiByte( codepage,  0 , src.c_str(), (int)len_origin, buf, expected_size, NULL, NULL );
        buf[expected_size] = '\0';
        string ret(buf);
        delete [] buf;
        return ret;
    }
    return "";
}