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std::unorde...t_or_assign(3)   C++ Standard Libary  std::unorde...t_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_func-
     tion()(u.first) !=
	hash_function()(k)  ||	contains(u.first) is true, the behavior is unde-
     fined. 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 de-
     notes 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  invali-
     dated. 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 in-
     serted
	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::un-
     ordered_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)

http://cppreference.com 	   2024.06.10	    std::unorde...t_or_assign(3)
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