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

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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
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
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 :
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

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