STL unordered_set

http://www.cplusplus.com/reference/unordered_set/unordered_set/

template < class Key,                        // unordered_set::key_type/value_type
           class Hash = hash<Key>,           // unordered_set::hasher
           class Pred = equal_to<Key>,       // unordered_set::key_equal
           class Alloc = allocator<Key>      // unordered_set::allocator_type
           > class unordered_set;

stl里面的set包含了排序的数据，这些数据的值value必须是唯一的。multiset（多重集）——和集合（set）相似，然而其中的值不要求必须是唯一的（即可以有重复）

Unordered Set

Unordered sets are containers that store unique elements in no particular order, and which allow for fast retrieval of individual elements based on their value.

unordered_set是存储独一的元素，没有按照特别的顺序。

In an unordered_set, the value of an element is at the same time its key, that identifies it uniquely. Keys are immutable, therefore, the elements in an unordered_set cannot be modified once in the container - they can be inserted and removed, though.

Internally, the elements in the unordered_set are not sorted in any particular order, but organized into buckets。depending on their hash values to allow for fast access to individual elements directly by their values (with a constant average time complexity on average).

unordered_set containers are faster than set containers to access individual elements by their key, although they are generally less efficient for range iteration through a subset of their elements.

Iterators in the container are at least forward iterators.