stdunorderedmap

std::unordered_map is a standard library container that implements a hash-table based associative container. It stores key-value pairs with unique keys and provides fast lookup, insertion, and removal by key. The value_type is pair<const Key, T>, and the key type must be hashable by std::hash<Key> and comparable by the configured key_equal predicate (default std::equal_to<Key>). The container uses a hash function to assign keys to buckets and stores elements in those buckets.

Key features include that it is unordered: iteration yields elements in an unspecified order, not in insertion order. It offers typical operations such as insert, emplace, try_emplace, operator[], at, find, count, erase, clear, and swap. It also exposes metadata and control functions like bucket_count, load_factor, max_load_factor, rehash, and reserve. The container allows customization through allocators, and users can influence performance by preallocating buckets or adjusting the maximum load factor.

Performance characteristics: average-case constant time for lookup, insertion, and deletion, with worst-case linear time in degenerate scenarios caused by many collisions. The container automatically rehashes as needed when the load factor grows beyond the specified limit; rehashing can be triggered manually via rehash or reserve to allocate enough buckets in advance.

Notes: std::unordered_map is defined in the <unordered_map> header and resides in the std namespace. It requires hash specialization for the key type (via std::hash) and an equality predicate (by default std::equal_to). It is a fundamental building block for fast associative mappings in C++ programs.