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std::unordered_map::insert_or_assign

template <class M> pair<iterator, bool> insert_or_assign(const key_type& k, M&& obj);	(1)	(since C++17)
template <class M> pair<iterator, bool> insert_or_assign(key_type&& k, M&& obj);	(2)	(since C++17)
template <class M> iterator insert_or_assign(const_iterator hint, const key_type& k, M&& obj);	(3)	(since C++17)
template <class M> iterator insert_or_assign(const_iterator hint, key_type&& k, M&& obj);	(4)	(since C++17)

1,3) If a key equivalent to k already exists in the container, assigns std::forward<M>(obj) to the mapped_type corresponding to the key k. If the key does not exist, inserts the new value as if by insert, constructing it from value_type(k, std::forward<M>(obj))

2,4) Same as (1,3), except the mapped value is constructed from value_type(std::move(k), std::forward<M>(obj))

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(). If the insertion is successful, pointers and references to the element obtained while it is held in the node handle are invalidated, and pointers and references obtained to that element before it was extracted become valid. (since C++17).

Parameters

k	-	the key used both to look up and to insert if not found
hint	-	iterator to the position before which the new element will be inserted
args	-	arguments to forward to the constructor of the element

Return value

1,2) The bool component is true if the insertion took place and false if the assignment took place. The iterator component is pointing at the element that was inserted or updated

3,4) Iterator pointing at the element that was inserted or updated

Complexity

1,2) Same as for emplace

3,4) Same as for emplace_hint

Notes

insert_or_assign returns more information than operator[] and does not require default-constructibility of the mapped type.