C++ Unordered_map Library - emplace() Function


The C++ function std::unordered_map::emplace() extends container by inserting new element.

insertion takes place only and only if key is not present already.


Following is the declaration for std::unordered_map::emplace() function form std::unordered_map header.


template <class... Args>
pair<iterator, bool> emplace ( Args&&... args );


args − Arguments to forward to the constructor of the element.

Return value

Returns a pair consisting of bool to indicate whether insertion is happened or not and returns an iterator to the newly inserted element.

Time complexity

Constant i.e. O(1) in average case.

Linear i.e. O(n) in worst case.


The following example shows the usage of std::unordered_map::emplace() function.

#include <iostream>
#include <unordered_map>

using namespace std;

int main(void) {
   unordered_map<char, int> um;

   um.emplace('a', 1);
   um.emplace('b', 2);
   um.emplace('c', 3);
   um.emplace('d', 4);
   um.emplace('e', 5);

   cout << "Unordered map contains following elements" << endl;

   for (auto it = um.begin(); it != um.end(); ++it)
      cout << it->first << " = " << it->second << endl;

   return 0;

Let us compile and run the above program, this will produce the following result −

Unordered map contains following elements
e = 5
d = 4
c = 3
b = 2
a = 1