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 M>
   iterator insert_or_assign( const_iterator hint, const Key& k, M&& (3) (since C++17)
   obj );
   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: __cpp_lib_unordered_map_try_emplace

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