Unordered_map to binary file

Why I can't write unordered_map structure to a file ?

std::ostream& operator<<(std::ostream& stream, PassLibrary const& data){
    stream << data.local_login << " " //std::string
           << data.local_pass << " "  //std::string
           << data.libs;              //std::unordered_map - error
    return stream;
}

Best regards.