_Cache_hash_code Boolean value. True if the value of the hash function is stored along with the value. This is a time-space tradeoff. Storing it may improve lookup speed by reducing the number of times we need to call the _Equal function.
_Constant_iterators Boolean value. True if iterator and const_iterator are both constant iterator types. This is true for unordered_set and unordered_multiset, false for unordered_map and unordered_multimap.
_Unique_keys Boolean value. True if the return value of _Hashtable::count(k) is always at most one, false if it may be an arbitrary number. This is true for unordered_set and unordered_map, false for unordered_multiset and unordered_multimap.