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std::unorde...map::emplace(3)  C++ Standard Libary std::unorde...map::emplace(3)

NAME
     std::unordered_map::emplace - std::unordered_map::emplace

Synopsis
	template< class... Args >			      (since C++11)
	std::pair<iterator, bool> emplace( Args&&... args );

	Inserts  a  new element into the container constructed in-place with the
     given args,
	if there is no element with the key in the container.

	The constructor of the new element (i.e.  std::pair<const  Key,  T>)  is
     called with
	exactly the same arguments as supplied to emplace, forwarded via
	std::forward<Args>(args)....  The  element  may  be  constructed even if
     there already is
	an element with the key in the container, in which case the  newly  con-
     structed
	element will be destroyed immediately.

	Careful  use  of  emplace allows the new element to be constructed while
     avoiding
	unnecessary copy or move operations.

	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
	args - arguments to forward to the constructor of the element

Return value
	A pair consisting of an iterator to the inserted element (or to the ele-
     ment that
	prevented the insertion) and a bool value set to true if and only if the
     insertion
	took place.

Exceptions
	If  an	exception  is thrown for any reason, this function has no effect
     (strong
	exception safety guarantee).

Complexity
	Amortized constant on average, worst case linear in the size of the con-
     tainer.

Example
     // Run this code

      #include <iostream>
      #include <string>
      #include <utility>
      #include <unordered_map>

      int main()
      {
	  std::unordered_map<std::string, std::string> m;

	  // uses pair's move constructor
	  m.emplace(std::make_pair(std::string("a"), std::string("a")));

	  // uses pair's converting move constructor
	  m.emplace(std::make_pair("b", "abcd"));

	  // uses pair's template constructor
	  m.emplace("d", "ddd");

	  // emplace with duplicate key has no effect
	  m.emplace("d", "DDD");

	  // uses pair's piecewise constructor
	  m.emplace(std::piecewise_construct,
		    std::forward_as_tuple("c"),
		    std::forward_as_tuple(10, 'c'));
	  // an alternative is: m.try_emplace("c", 10, 'c');

	  for (const auto& p : m)
	      std::cout << p.first << " => " << p.second << '\n';
      }

Possible output:
      a => a
      b => abcd
      c => cccccccccc
      d => ddd

See also
	emplace_hint constructs elements in-place using a hint
		     (public member function)
	try_emplace  inserts in-place if the key does not exist, does nothing if
     the key
	(C++17)      exists
		     (public member function)
		     inserts elements
	insert	     or nodes
		     (since C++17)
		     (public member function)

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