hashset in c 2b 2b

1template < class Key,                        // unordered_set::key_type/value_type
2           class Hash = hash<Key>,           // unordered_set::hasher
3           class Pred = equal_to<Key>,       // unordered_set::key_equal
4           class Alloc = allocator<Key>      // unordered_set::allocator_type
5           > class unordered_set;