Non-member swap

template <typename T, typename Hash,
          typename KeyEqual, typename Allocator>
void swap( concurrent_unordered_set<T, Hash, KeyEqual, Allocator>& lhs,
           concurrent_unordered_set<T, Hash, KeyEqual, Allocator>& rhs ) noexcept(noexcept(lhs.swap(rhs)));

Equivalent to lhs.swap(rhs).