# std::flat_map<Key,T,Compare,KeyContainer,MappedContainer>::insert_or_assign

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< cpp‎ | container‎ | flat map

template< class M > std::pair <iterator, bool> insert_or_assign( const key_type& k, M&& obj ); | (1) | (since C++23)
template< class M > std::pair <iterator, bool> insert_or_assign( key_type&& k, M&& obj ); | (2) | (since C++23)
template< class K, class M > std::pair <iterator, bool> insert_or_assign( K&& k, M&& obj ); | (3) | (since C++23)
template< class M > iterator insert_or_assign( const_iterator hint, const key_type& k, M&& obj ); | (4) | (since C++23)
template< class M > iterator insert_or_assign( const_iterator hint, key_type&& k, M&& obj ); | (5) | (since C++23)
template< class K, class M > iterator insert_or_assign( const_iterator hint, K&& k, M&& obj ); | (6) | (since C++23)

1,2) 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

- (1,2) try_emplace( std::forward <decltype(k)>(k), std::forward <M>(obj)),
- (4,5) try_emplace(hint, std::forward <decltype(k)>(k), std::forward <M>(obj)).

The program is ill-formed if either std::is_assignable_v <mapped_type&, M> or std::is_constructible_v <mapped_type, M> is false.
3,6) 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. Otherwise, equivalent to

- (3) try_emplace( std::forward <K>(k), std::forward <M>(obj)),
- (6) try_emplace(hint, std::forward <K>(k), std::forward <M>(obj)).

The conversion from k into `key_type` must construct an object u, for which find(k) == find(u) is true. Otherwise, the behavior is undefined.
These overloads participate in overload resolution only if :

- The qualified-id `Compare::is_transparent` is valid and denotes a type.
- std::is_constructible_v <key_type, K> is true.
- std::is_assignable_v <mapped_type&, M> is true.
- std::is_constructible_v <mapped_type, M> is true.

| Information on iterator invalidation is copied from here

### 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

1-3) 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.
4-6) Iterator pointing at the element that was inserted or updated.

### Complexity

1-3) Same as for `emplace`.
4-6) Same as for `emplace_hint`.

### Notes

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

### Example

Run this code

    #include <flat_map>
    #include <iostream>
    #include <string>

    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::flat_map<std::string, std::string> map;

        print_result(map.insert_or_assign("a", "apple"));
        print_result(map.insert_or_assign("b", "banana"));
        print_result(map.insert_or_assign("c", "cherry"));
        print_result(map.insert_or_assign("c", "clementine"));

        for (const auto& node : map)
            print_node(node);
    }

Output:

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

### See also

operator[] | access or insert specified element (public member function)
at | access specified element with bounds checking (public member function)
insert | inserts elements (public member function)
emplace | constructs element in-place (public member function)
try_emplace | inserts in-place if the key does not exist, does nothing if the key exists (public member function)

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