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;
  }