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;
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.