std::string result(len + 1, '\0');
    int retLen = ::WideCharToMultiByte(CP_UTF8, 0, str.c_str(), str.size(), &result[0], len + 1, NULL, NULL);
    if (!(retLen && retLen == len && SUCCEEDED(::GetLastError())))
        return std::string();
    result.resize(retLen);
    return result;