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

From cppreference.com

 
C++
 
Containers library
 
std::unordered_map
Member types
Member functions
Iterators
Capacity
Modifiers
(C++23)
unordered_map::insert_or_assign
(C++17)
(C++17)
(C++17)
(C++17)
Lookup
(C++20)
Bucket interface
Hash policy
Observers
Non-member functions
(until C++20)
(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 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 an insertion occurs and results in a rehashing of the container, all iterators are invalidated. Otherwise iterators are not affected. References are not invalidated. Rehashing occurs only if the new number of elements is greater than max_load_factor() * bucket_count().

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 Comment
__cpp_lib_unordered_map_try_emplace 201411L (C++17) std::unordered_map::try_emplace,
std::unordered_map::insert_or_assign

Example

Run this code
#include <iostream>
#include <unordered_map>
#include <string>
 
auto print_node = [](const auto &node) {
    std::cout << "[" << node.first << "] = " << node.second << '\n';
};
 
auto 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

(C++11)
 access or insert specified element
(public member function)
(C++11)
 access specified element with bounds checking
(public member function)
(C++11)
 inserts elements or nodes (since C++17)
(public member function)
(C++11)
 constructs element in-place
(public member function)
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