+ properties_.reset(new std::unordered_map<std::string, std::string>);
+ std::unordered_map<std::string, std::string> props(properties.cbegin(), properties.cend());
+#if __cplusplus >= 201703L
+ props.merge(properties_);
+#else
+ props.insert(properties_->cbegin(), properties_->cend());
+#endif
+ properties_->swap(props);