T& operator[] ( const key_type& x );

Access element

If x matches the key of an element in the container, the function returns a reference to its mapped value.

If x does not match the key of any element in the container, the function inserts a new element with that key and returns a reference to its mapped value. Notice that this always increases the map size by one, even if no mapped value is assigned to the element (the element is constructed using its default constructor).

A call to this function is equivalent to:

(*((this->insert(make_pair(x,T()))).first)).second

Parameters

x

Key value of the element whose mapped value is accessed.
key_type is a member type defined in map containers as an alias of Key, which is the first template parameter and the type of the element keys.

Return value

Example

// accessing mapped values
#include <iostream>
#include <map>
#include <string>
using namespace std;

int main ()
{
  map<char,string> mymap;
  map<char,string>::iterator it;

  mymap['a']="an element";
  mymap['b']="another element";
  mymap['c']=mymap['b'];

  cout << "mymap['a'] is " << mymap['a'] << endl;
  cout << "mymap['b'] is " << mymap['b'] << endl;
  cout << "mymap['c'] is " << mymap['c'] << endl;
  cout << "mymap['d'] is " << mymap['d'] << endl;

  cout << "mymap now contains " << (int) mymap.size() << " elements." << endl;

  return 0;
}

Notice how the last access (to element 'd') inserts a new element in the set with that key and initialized to its default value (an empty string) even though it is accessed only to retrieve its value. Member function map::find does not produce this effect.
Output:

mymap['a'] is an element
mymap['b'] is another element
mymap['c'] is another element
mymap['d'] is
mymap now contains 4 elements.

Complexity

See also