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× C++ Standard Library C++ STL Library


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

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

Parameters

position Specify hint for the position where the element can be inserted.
args Arguments forwarded to construct the new element of the mapped type.

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 below example, 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
 101  John
 102  Marry

❮ C++ - Unordered Map