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

From cppreference.com

 
C++
 
Containers library
 
std::unordered_map
Member types
Member functions
Non-member functions
(until C++20)
Deduction guides(C++17)
 
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

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