C++ unordered_multimap - emplace_hint() Function

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

`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

Returns an iterator pointed to newly added element.

Time Complexity

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

Example:

In the example below, the unordered_multimap::emplace_hint function is used to insert a new element in the unordered_multimap called uMMap.

#include <iostream>
#include <unordered_map>
using namespace std;
 
int main (){
  unordered_multimap<string, string> uMMap;
  unordered_multimap<string, string>::iterator it;

  uMMap.insert(pair<string, string>("USA", "New York"));
  uMMap.insert(pair<string, string>("USA", "Washington"));  
  uMMap.insert(pair<string, string>("CAN", "Toronto"));

  it = uMMap.begin();
  uMMap.emplace_hint(it, "CAN", "Montreal");
  it = uMMap.emplace_hint(uMMap.end(), "IND", "Delhi"); 

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

  return 0;
}

The output of the above code will be:

uMMap contains: 
 IND  Delhi
 CAN  Toronto
 CAN  Montreal
 USA  Washington
 USA  New York

❮ C++ <unordered_map> Library