C++ unordered_map - emplace() Function

The C++ unordered_map::emplace function is used to insert a new unique element in the unordered_map. The insertion of new element happens only when the key is not already present in the unordered_map. If insertion happens, it increases the size of the unordered_map by one.

Syntax

• C++11

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

Parameters

Return Value

For successfully inserted element, returns a pair of an iterator pointed to newly added element and a value of true. Otherwise, returns a pair of an iterator pointed to equivalent element and a value of false.

Time Complexity

Average case: Constant i.e, Θ(1).
Worst case: Linear i.e, Θ(n).

Example:

In the example below, the unordered_map::emplace function is used to insert new element in the unordered_map called uMap.

#include <iostream>
#include <unordered_map>
using namespace std;
 
int main (){
  unordered_map<int, string> uMap;
  
  uMap[101] = "John";
  uMap[102] = "Marry";
  uMap[103] = "Kim";

  //insert a new element with unique key in the uMap
  auto NewInsert = uMap.emplace(104, "Jo");
  
  if(!NewInsert.second) 
     cout<<"104 key is already present in uMap.\n";
  else
     cout<<"[104, Jo] is added in uMap.\n";

  //insert a new element with already present key in the uMap
  NewInsert = uMap.emplace(102, "Ramesh");

  if(!NewInsert.second) 
     cout<<"102 key is already present in uMap.\n";
  else
     cout<<"[102, Ramesh] is added in uMap.\n";

  return 0;
}

The output of the above code will be:

[104, Jo] is added in uMap.
102 key is already present in uMap.