hash_type operator()(const CopiedString& path) const { return path_hash(path.c_str()); }
hash_type operator()(const ModelKey& key) const { return hash_combine(path_hash(key.first.c_str()), path_hash(key.second.c_str())); }
hash_type operator() (const std::string& path) const { return path_hash(path.c_str()); }