Issue When Using [] Operator To Access Elements Of A map

January 29, 2009

I had the habit of using [] operator to access elements of a map. For example, if we have

map<int, int> myMap, to get the element corresponding to 10, we can use

myMap[10]. This is perfectly ok as long as we have an element corresponding to 10. But, what if we don’t have one corresponding to 10?

There lies the problem. In this situation, myMap is already added with a default elemnent corresponding to 10 ([] operator provides no facility to validate if an element exists) and in the above situation, it is 0. As a result, a new (unwanted) element is added to the map (we can see that the size of map has been incremented by 1).

So a better alternative is to use the find() function associated with map. It does not add an element and if the element is not found, it will return map::end().

The example given below illustrates this.


using namespace std;

int main()
   map myMap;
   cout << "Size of myMap = " << myMap.size() << '\n';   // will be 0    int content = myMap[0];    // Now the size of myMap will be 1 as we used the [] operator    // and the value of content = myMap[0] = 0    cout << "Size of myMap = " << myMap.size() << '\n';       // Instead, if we use find, the size will not change.    // It will look for the element only.    map::const_iterator it = myMap.find(1);    // Here the element has not been found. So it = myMap.end()    cout << "Size of myMap = " << myMap.size() << '\n';   // will be 1 only     return 0;    } [/sourcecode] So the best practice is to avoid [] operator for accessing individual elements of a map.