inline const value_type at const key_type key const auto it map_ templ

        inline const value_type& at(const key_type& key) const {
                auto it = map_.template get<forward>().find(key);
                if(it == map_.template get<forward>().cend()) {
                        throw std::out_of_range("object_manager::at() const");
                }
                return *(it->second);
        }
        inline value_type& at(const key_type& key) {
                auto it = map_.template get<forward>().find(key);
                if(it == map_.template get<forward>().end()) {
                        throw std::out_of_range("object_manager::at()");
                }
                return *(it->second);
        }