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);
}