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Saturday, August 04, 2007

Using Static Data Members in Classes

Let us start with a question.

Suppose we want to have some information (i.e. a variable) which should be available ‘as is’ to all the objects of a particular class (ex. the number of objects of that class available at a time). Then what would you do? You can’t make it to be a regular member of the class because then, every object would have its own copy of that information which should have been common to all objects.

One way of achieving this is to declare that variable as global and access it within the class wherever needed as a member. This is illustrated in the program below; it keeps track of the number of objects present (defined) at a particular time.


  #include <iostream.h>

  int obj_count=0;

  class myclass
  {
  public:
    myclass(){obj_count++;}
    ~myclass(){obj_count--;}

    int count(){return obj_count;}
  };

  void main(void)
  {
    myclass o1;

    cout<<o1.count();
    cout<<endl;

    myclass o2;

    // here both these return
    // the same information
    cout<<o1.count();
    cout<<endl;
    cout<<o2.count();
    cout<<endl;
  }

But this is rather awkward, because how can we make a variable global which would otherwise have been a part of the class itself!

So what do we do? We use static members!

Static member of a class are those whose only one copy exist among all the objects of a class and is shared among all of them.

As I said, it is like having a global variable to store some information which is needed by all the objects of a particular class.

This is illustrated in the program below which is the modified version of the previous program to use static member variable.


  // Using static member variables
  #include <iostream.h>

  class myclass
  {
    // static variable is
    // declared
    static int obj_count;
  public:
    myclass(){obj_count++;}
    ~myclass(){obj_count--;}

    int count(){return obj_count;}
  };

  // IMPORTANT
  // define the variable
  int myclass::obj_count;

  void main(void)
  {
    myclass o1;

    cout<<o1.count();
    cout<<endl;

    myclass o2;

    // here both these return
    // the same information
    cout<<o1.count();
    cout<<endl;
    cout<<o2.count();
    cout<<endl;
  }

Points to remember:

  • Static members of a class are declared by using the ‘static’ keyword.

  • Only declaration is not enough, it must be defined somewhere outside the class.

  • All the static members are automatically initialized to 0 by the compiler.

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