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std::flat_map::emplace(3)      C++ Standard Libary     std::flat_map::emplace(3)

NAME
     std::flat_map::emplace - std::flat_map::emplace

Synopsis
	template< class... Args >			      (since C++23)
	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.

	Initializes an object t of type std::pair<key_type, mapped_type> with
	std::forward<Args>(args)...; if the  map  already  contains  an  element
     whose key is
	equivalent to t.first, *this is unchanged. Otherwise, equivalent to:

      auto key_it = ranges::upper_bound(c.keys, t.first, compare);
      auto value_it = c.values.begin() + std::distance(c.keys.begin(), key_it);
      c.keys.insert(key_it, std::move(t.first));
      c.values.insert(value_it, std::move(t.second));

	This overload participates in overload resolution only if
	std::is_constructible_v<std::pair<key_type,  mapped_type>,  Args...>  is
     true.

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

	 Information on iterator invalidation is copied from here

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
	If insertion occurs, linear in the size of the container, otherwise log-
     arithmic in
	the size of the container

Example
     // Run this code

      #include <iostream>
      #include <string>
      #include <utility>
      #include <flat_map>

      int main()
      {
	  std::flat_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';
      }

Output:
      a => a
      b => abcd
      c => cccccccccc
      d => ddd

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

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