std::map<Key,T,Compare,Allocator>::emplace (3) - Linux Manuals

std::map<Key,T,Compare,Allocator>::emplace: std::map<Key,T,Compare,Allocator>::emplace


std::map<Key,T,Compare,Allocator>::emplace - std::map<Key,T,Compare,Allocator>::emplace


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.
Careful use of emplace allows the new element to be constructed while avoiding unnecessary copy or move operations. 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 constructed element will be destroyed immediately.
No iterators or references are invalidated.


args - arguments to forward to the constructor of the element

Return value

Returns a pair consisting of an iterator to the inserted element, or the already-existing element if no insertion happened, and a bool denoting whether the insertion took place. True for Insertion, False for No Insertion.


If an exception is thrown by any operation, this function has no effect.


Logarithmic in the size of the container.


// Run this code

  #include <iostream>
  #include <utility>
  #include <string>
  #include <map>

  int main()
      std::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");

      // uses pair's piecewise constructor
                std::forward_as_tuple(10, 'c'));
      // as of C++17, m.try_emplace("c", 10, 'c'); can be used

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


  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 exists
             (public member function)
             inserts elements
             or nodes
insert (since C++17)
             (public member function)