Information Blog: Overloading Operator II

In the previous article Overloading
[] Operator, we overloaded the [] operator in a class to access data
within the class by indexing method.

The operator [] function was defined as below:

  int myclass::operator[](int index)
  {
    // if not out of bound
    if(index<num)
      return a[index];
  }

As you can see, the above operator
function is returning values, hence it could only be used on the right
hand side of a statement. It’s a limitation!

You very well know that a statement like below is very common with respect
to arrays:

a[1]=10;

But as I said, the way we overloaded the [] operator, statement like the one
above is not possible. The good news is, it is very easy to achieve this.

For this we need to overload the [] operator like this:

  int &myclass::operator[](int index)
  {
    // if not out of bound
    if(index<num)
      return a[index];
  }

By returning a reference
to the particular element, it is possible to use the index expression on the
left hand side of the statement too.

The following program illustrates this:


  // Example Program illustrating
  // the overloading of [] operator
  // ----
  // now the index expression can be
  // used on the left side too
  #include <iostream.h>

  class myclass
  {
    // stores the number of element
    int num;
    // stores the elements
    int a[10];

  public:
    myclass(int num);

    int &operator[](int);
  };

  // takes the number of element
  // to be entered.(<=10)
  myclass::myclass(int n)
  {
    num=n;
    for(int i=0;i<num;i++)
    {
      cout<<"Enter value for element "<<i+1<<":";
      cin>>a[i];
    }
  }

  // returns a reference
  int &myclass::operator[](int index)
  {
    // if not out of bound
    if(index<num)
      return a[index];
  }

  void main()
  {
    myclass a(2);

    cout<<a[0]<<endl;
    cout<<a[1]<<endl;

    // indexing expression on the
    // left-hand side
    a[1]=21;
    cout<<a[1];
  }