C++ unordered_set - emplace_hint() Function

The C++ unordered_set::emplace_hint function is used to insert a new unique element in the unordered_set with the hint of insertion position. The insertion of new element happens only when it is not already present in the unordered_set. If insertion happens, it increases the size of the unordered_set 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_set::emplace_hint function is used to insert new element in the unordered_set called uSet.

#include <iostream>
#include <unordered_set>
using namespace std;
 
int main (){
  unordered_set<int> uSet{10, 20, 30, 40, 50}; 
  unordered_set<int>::iterator it;

  it = uSet.begin();
  uSet.emplace_hint(it, 60);
  it = uSet.emplace_hint(uSet.end(), 70);

  cout<<"uSet contains: ";
  for(it = uSet.begin(); it != uSet.end(); ++it)
    cout<<*it<<" ";

  return 0;
}

The output of the above code will be:

uSet contains: 70 60 10 20 30 40 50