std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::insert_or_assign

`template< class M > std::pair<iterator, bool> insert_or_assign( const Key& k, M&& obj );`	(1)	(since C++17)
`template< class M > std::pair<iterator, bool> insert_or_assign( Key&& k, M&& obj );`	(2)	(since C++17)
`template< class M > iterator insert_or_assign( const_iterator hint, const Key& k, M&& obj );`	(3)	(since C++17)
`template< class M > iterator insert_or_assign( const_iterator hint, Key&& 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)).

The behavior is undefined(until C++20)The program is ill-formed(since C++20) if std::is_assignable_v<mapped_type&, M&&> is false.

If after the operation the new number of elements is greater than old max_load_factor() * bucket_count() a rehashing takes place.
If rehashing occurs (due to the insertion), all iterators are invalidated. Otherwise (no rehashing), iterators are not invalidated.

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
obj	-	the value to insert or assign

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.

Feature-test macro	Value	Std	Feature
`__cpp_lib_unordered_map_try_emplace`	201411L	(C++17)	`std::unordered_map::try_emplace`, `std::unordered_map::insert_or_assign`

Example

#include <iostream>
#include <string>
#include <unordered_map>
 
void print_node(const auto& node)
{
    std::cout << '[' << node.first << "] = " << node.second << '\n';
}
 
void print_result(auto const& pair)
{
    std::cout << (pair.second ? "inserted: " : "assigned: ");
    print_node(*pair.first);
}
 
int main()
{
    std::unordered_map<std::string, std::string> myMap;
 
    print_result(myMap.insert_or_assign("a", "apple"));
    print_result(myMap.insert_or_assign("b", "banana"));
    print_result(myMap.insert_or_assign("c", "cherry"));
    print_result(myMap.insert_or_assign("c", "clementine"));
 
    for (const auto& node : myMap)
        print_node(node);
}

Possible output:

inserted: [a] = apple
inserted: [b] = banana
inserted: [c] = cherry
assigned: [c] = clementine
[c] = clementine
[a] = apple
[b] = banana

Docs

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