High Quality Content by WIKIPEDIA articles! In the C++ programming language, unordered associative containers are a group of class templates in the C++ Standard Library that implement variations of hash tables. Being templates, they can be used to store arbitrary elements, such as integers or custom classes. The following containers are defined in the current revision of the C++ standard: unordered_set, unordered_map, unordered_multiset, unordered_multimap. Each of these containers differ only on constraints placed on their elements.