map::operator[] in C++ STL

Maps are associative containers that store elements in a mapped fashion. Each element has a key value and a mapped value. No two mapped values can have same key values.


This operator is used to reference the element present at position given inside the operator. It is similar to the at() function, the only difference is that the at() function throws an out-of-range exception when the position is not in the bounds of the size of map, while this operator causes undefined behaviour.

Syntax :

Parameters :
Key value mapped to the element to be fetched.
Returns :
Direct reference to the element at the given key value.


Input  :  map mymap;
          mymap['a'] = 1;
Output :  1

Input  :  map mymap;
          mymap["abcd"] = 7;
Output :  7

Errors and Exceptions

1. If the key is not present in the map, it shows undefined behaviour.
2. It has a no exception throw guarantee otherwise.





// CPP program to illustrate
// Implementation of [] operator
#include <map>
#include <iostream>
using namespace std;
int main()
    // map declaration
    map<int,string> mymap;
    // mapping integers to strings
    mymap[1] = "Hi";
    mymap[2] = "This";
    mymap[3] = "is";
    mymap[4] = "GeeksForGeeks";
    // using operator[] to print string 
    // mapped to integer 4
    cout << mymap[4];
    return 0;




Time Complexity: O(logn)

GeeksforGeeks has prepared a complete interview preparation course with premium videos, theory, practice problems, TA support and many more features. Please refer Placement 100 for details

My Personal Notes arrow_drop_up

Check out this Author's contributed articles.

If you like GeeksforGeeks and would like to contribute, you can also write an article using or mail your article to See your article appearing on the GeeksforGeeks main page and help other Geeks.

Please Improve this article if you find anything incorrect by clicking on the "Improve Article" button below.

Article Tags :
Practice Tags :

Be the First to upvote.

Please write to us at to report any issue with the above content.