C++ map - operator[]() Function

The C++ map::operator[] function returns a reference to the mapped value of the specified key. If the specified key does not match with key of any element of the map, the function inserts a new element with that key and returns a reference to its mapped value.

Note: This function produces the same result as map::at, except map::at throws an out_of_range exception if the specified key does not match with key of any element of the map.

Syntax

• C++98
• C++11

mapped_type& operator[] (const key_type& k);

mapped_type& operator[] (const key_type& k);
mapped_type& operator[] (key_type&& k);

Parameters

Return Value

The element at the specified position in the map.

Time Complexity

Logarithmic i.e, Θ(log(n)).

Example:

In the example below, the map::operator[] function is used to get the mapped value of the specified key in MyMap.

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

  //populating MyMap
  MyMap[101] = "John";
  MyMap[102] = "Marry";
  MyMap[103] = "Kim";

  //changing the mapped value for key=103
  MyMap[103] = "Ramesh";

  //accessing mapped value of specified key
  cout<<"MyMap[101] = "<<MyMap[101]<<"\n";
  cout<<"MyMap[102] = "<<MyMap[102]<<"\n";
  cout<<"MyMap[103] = "<<MyMap[103]<<"\n";
  cout<<"MyMap[104] = "<<MyMap[104]<<"\n";
  cout<<"MyMap[105] = "<<MyMap[105]<<"\n";

  return 0;
}

The output of the above code will be:

MyMap[101] = John
MyMap[102] = Marry
MyMap[103] = Ramesh
MyMap[104] = 
MyMap[105] =