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.
 
map::operator[]

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 inserts a new element with that key and returns a reference to its mapped value, increasing the container size by one, the element is constructed using its default constructor if no value is provided.
Syntax : 
 

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

Examples: 
 

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

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

// CPP program to illustrate
// Implementation of [] operator
#include <iostream>
#include <map>
#include <string>
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] is " << mymap[4] << '\n';
    // using operator[] to access the key is not in the map
    // it displays the default value in the constructor
    cout << "mymap[5] is " << mymap[5] << '\n';
    // the map size increases to 5
    cout << "mymap now contains " << mymap.size()
         << " elements.\n";

    return 0;
}

mymap[4] is GeeksForGeeks
mymap[5] is 
mymap now contains 5 elements.

Time Complexity: O(logn)