std::unordered_map::insert_or_assign(3)

NAME
std::unordered_map::insert_or_assign - std::unordered_map::insert_or_assign

Synopsis
   template< class M >
   std::pair<iterator, bool> insert_or_assign( const Key& k, M&& obj (1) (since C++17)
   );
   template< class M > (2) (since C++17)
   std::pair<iterator, bool> insert_or_assign( Key&& k, M&& obj );
   template< class K, class M > (3) (since C++26)
   std::pair<iterator, bool> insert_or_assign( K&& k, M&& obj );
   template< class M >
   iterator insert_or_assign( const_iterator hint, const Key& k, M&& (4) (since C++17)
   obj );
   template< class M >
   iterator insert_or_assign( const_iterator hint, Key&& k, M&& obj (5) (since C++17)
   );
   template< class K, class M > (6) (since C++26)
   iterator insert_or_assign( const_iterator hint, K&& k, M&& obj );
   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
                        Heterogeneous overloads for the __cpp_lib_associative_heterogeneous_insertion 202311L (C++26) 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)
              inserts elements
   insert or nodes
              (since C++17)
              (public member function)
   emplace constructs element in-place
              (public member function)