std::unordered_set<Key,Hash,KeyEqual,Allocator>::emplace_hint (3) - Linux Man Pages
template <class... Args> (since C++11)
iterator emplace_hint( const_iterator hint, Args&&... args );
Inserts a new element to the container, using hint as a suggestion where the element should go. The element is constructed in-place, i.e. no copy or move operations are performed.
The constructor of the element is called with exactly the same arguments as supplied to the function, forwarded with std::forward<Args>(args)....
If rehashing occurs due to the insertion, all iterators are invalidated. Otherwise iterators are not affected. References are not invalidated. Rehashing occurs only if the new number of elements is greater than max_load_factor()*bucket_count().
hint - iterator, used as a suggestion as to where to insert the new element
args - arguments to forward to the constructor of the element
Returns an iterator to the newly inserted element.
If the insertion failed because the element already exists, returns an iterator to the already existing element with the equivalent key.
If an exception is thrown by any operation, this function has no effect (strong exception guarantee).
Amortized constant on average, worst case linear in the size of the container.
emplace (public member function)
insert (since C++17)