C++ multimap - key_comp() Function

The C++ multimap::key_comp function returns a copy of the comparison object used by the container to compare keys. By default, it is a less object, which returns the same as operator<.

Syntax

C++98
C++11

key_compare key_comp() const;

key_compare key_comp() const;

Parameters

No parameter is required.

Return Value

The comparison object associated to the container.

Time Complexity

Constant i.e, Θ(1).

Example:

In the example below, the multimap::key_comp function is used print all elements of the given multimap with key less than 'USA'.

#include <iostream>
#include <map>
using namespace std;
 
int main (){
  multimap<string, string> MMap;
  multimap<string, string>::iterator it;

  MMap.insert(pair<string, string>("USA", "New York"));
  MMap.insert(pair<string, string>("USA", "Washington"));  
  MMap.insert(pair<string, string>("CAN", "Toronto"));
  MMap.insert(pair<string, string>("CAN", "Montreal"));
  MMap.insert(pair<string, string>("IND", "Delhi"));

  //printing the content of the multimap
  cout<<"MMap contains: \n ";
  for(it = MMap.begin(); it != MMap.end(); ++it)
     cout<<it->first<<"  "<<it->second<<"\n ";

  //creating a key_comp object
  multimap<string, string>::key_compare MyComp = MMap.key_comp();

  //printing all elements of the multimap which has 
  //key than 'USA' using key_comp object
  it = MMap.begin();
  cout<<"\nElements of MMap with key less than 104:\n ";
  while(MyComp((*it).first, "USA")){
    cout<<it->first<<"  "<<it->second<<"\n ";
    it++;
  }

  return 0;
}

The output of the above code will be:

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

Elements of MMap with key less than 104:
 CAN  Toronto
 CAN  Montreal
 IND  Delhi

❮ C++ <map> Library