meets all of the requirements of a container, of an unordered associative container, and of an allocator-aware container (Table 76
It provides the operations described in the preceding requirements table for unique keys; that is, an unordered_set
supports the a_uniq
operations in that table, not the a_eq
For an unordered_set<Key>
the key type
and the value type are both Key.
types are both constant iterator types.
It is unspecified whether they are the same type.