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

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< cpp‎ | container‎ | unordered map

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 K, class M > std::pair <iterator, bool> insert_or_assign( K&& k, M&& obj ); | (3) | (since C++26)
template< class M > iterator insert_or_assign( const_iterator hint, const Key& k, M&& obj ); | (4) | (since C++17)
template< class M > iterator insert_or_assign( const_iterator hint, Key&& k, M&& obj ); | (5) | (since C++17)
template< class K, class M > iterator insert_or_assign( const_iterator hint, K&& k, M&& obj ); | (6) | (since C++26)

1,4) 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)).
2,5) Same as (1,4), except the mapped value is constructed from value_type(std::move(k), std::forward <M>(obj)).
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. If the key does not exist, constructs an object `u` of `value_type` with std::forward <K>(k), std::forward <M>(obj)), then inserts `u` into *this. If hash_function()(u.first) != hash_function()(k) || contains(u.first) is true, the behavior is undefined. The `value_type` must be EmplaceConstructible into `unordered_map` from std::forward <K>(k), std::forward <M>(obj). This overload participates in overload resolution only if Hash::is_transparent and KeyEqual::is_transparent are valid and each denotes a type. This assumes that such `Hash` is callable with both `K` and `Key` type, and that the `KeyEqual` is transparent, which, together, allows calling this function without constructing an instance of `Key`.

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

1-3) 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.

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`
`__cpp_lib_associative_heterogeneous_insertion` | `202311L` | (C++26) | Heterogeneous overloads for the remaining member functions in ordered and unordered associative containers. Overloads (3) and (6).

### Example

Run this code

    #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

### See also

operator[] | access or insert specified element (public member function)
at | access specified element with bounds checking (public member function)
insert | inserts elements or nodes(since C++17) (public member function)
emplace | constructs element in-place (public member function)

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