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Description

The C++ function std::map::operator[] if key k matches an element in the container, then method returns a reference to the element.

Declaration

Following is the declaration for std::map::operator[] function form std::map header.

C++98

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

C++11

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

Parameters

k − Key of the element whose mapped value is accessed.

Return value

Returns a reference to the element associated with key k.

Exceptions

This member doesn't throw exception.

Time complexity

Logarithmic i.e. O(lon n)

Example

The following example shows the usage of std::map::operator[] function.

#include <iostream>
#include <map>

using namespace std;

int main(void) {
   /* Initializer_list constructor */
   map<char, int> m = {
            {'a', 1},
            {'b', 2},
            {'c', 3},
            {'d', 4},
            {'e', 5},
            };

   cout << "Map contains following elements" << endl;

   cout << "m['a'] = " << m['a'] << endl;
   cout << "m['b'] = " << m['b'] << endl;
   cout << "m['c'] = " << m['c'] << endl;
   cout << "m['d'] = " << m['d'] << endl;
   cout << "m['e'] = " << m['e'] << endl;

   return 0;
}

Let us compile and run the above program, this will produce the following result −

Map contains following elements
m['a'] = 1
m['b'] = 2
m['c'] = 3
m['d'] = 4
m['e'] = 5

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