C++ unordered_map - emplace_hint() Function

The C++ unordered_map::emplace_hint function is used to insert a new element in the unordered_map with the hint of insertion position. 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.

The hint is only used to speed up the insertion process considering the actual insertion point is either specified position or close to it.

Syntax

• C++11

template <class... Args>
iterator emplace_hint (const_iterator position, Args&&... args);

Parameters

Return Value

For successfully inserted element, returns an iterator pointed to newly added element. Otherwise, returns an iterator pointed to the equivalent element.

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_hint 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;
  unordered_map<int, string>::iterator it;
    
  uMap[101] = "John";
  uMap[102] = "Marry";
  uMap[103] = "Kim";

  it = uMap.begin();
  uMap.emplace_hint(it, 104, "Jo");
  it = uMap.emplace_hint(uMap.end(), 105, "Ramesh");  

  cout<<"uMap contains: \n ";
  for(it = uMap.begin(); it != uMap.end(); ++it)
     cout<<it->first<<"  "<<it->second<<"\n ";

  return 0;
}

The output of the above code will be:

uMap contains: 
 105  Ramesh
 104  Jo
 103  Kim
 102  Marry
 101  John