std::unordered_map insert_or_assign() method

od C++17

// (1) Non const version only
template <class M>
std::pair<iterator, bool> insert_or_assign( const Key& k, M&& obj );

// (2) Non const version only
template <class M>
std::pair<iterator, bool> insert_or_assign( Key&& k, M&& obj );

// (3) Non const version only
template <class M>
iterator insert_or_assign( const_iterator hint, const Key& k, M&& obj );

// (4) Non const version only
template <class M>
iterator insert_or_assign( const_iterator hint, Key&& k, M&& obj );

• (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)).

od C++20

The behavior is undefined if std::is_assignable_v<mapped_type&, M&&> is false.

do C++20

The program is ill-formed if std::is_assignable_v<mapped_type&, M&&> is false.

Invalidation

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

Exceptions

(none)

Notes

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

Feature testing macro: __cpp_lib_unordered_map_try_emplace.

Example

Main.cpp

#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