C++ Primer Plus

Unordered Associative Containers (C++11)

An unordered associative container is yet another refinement of the container concept. Like an associative container, an unordered associative container associates a value with a key and uses the key to find the value. The underlying difference is that associative containers are based on tree structures, whereas unordered associative containers are based on another form of data structure called a hash table. The intent is to provide containers for which adding and deleting elements is relatively quick and for which there are efficient search algorithms. The four unordered associative containers are called unordered_set, unordered_multiset, unordered_map, and unordered_multimap. Appendix G looks a bit further ...