std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::emplace (3) - Linux Man Pages
template< class... Args > (since C++11)
std::pair<iterator,bool> emplace( Args&&... args );
Inserts a new element into the container constructed in-place with the given args if there is no element with the key in the container.
Careful use of emplace allows the new element to be constructed while avoiding unnecessary copy or move operations. The constructor of the new element (i.e. std::pair<const Key, T>) is called with exactly the same arguments as supplied to emplace, forwarded via std::forward<Args>(args).... The element may be constructed even if there already is an element with the key in the container, in which case the newly constructed element will be destroyed immediately.
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().
args - arguments to forward to the constructor of the element
Returns a pair consisting of an iterator to the inserted element, or the already-existing element if no insertion happened, and a bool denoting whether the insertion took place. True for Insertion, False for No Insertion.
If an exception is thrown by any operation, this function has no effect.
Amortized constant on average, worst case linear in the size of the container.
// Run this code
emplace_hint (public member function)
try_emplace inserts in-place if the key does not exist, does nothing if the key exists
insert (since C++17)