std::map<Key,T,Compare,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 as close as possible to the position just before hint. The element is constructed in-place, i.e. no copy or move operations are performed.
The constructor of the element type (value_type, that is, std::pair<const Key, T>) is called with exactly the same arguments as supplied to the function, forwarded with std::forward<Args>(args)....
No iterators or references are invalidated.
hint - iterator to the position before which the new element will be inserted
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).
Logarithmic in the size of the container in general, but amortized constant if the new element is inserted just before hint.
emplace constructs element in-place
insert (since C++17)