It's exposed in the API. When you access a string field in protobuf, say foo.my_str_field(), the return type is const std::string&. We have to have a real std

It's exposed in the API. When you access a string field in protobuf, say foo.my_str_field(), the return type is const std::string&. We have to have a real std::string internally to return a reference to it.