Skip to main content

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.

Related Articles:

Popular posts from this blog

Fix For Toshiba Satellite "RTC Battery is Low" Error (with Pictures)

RTC Battery is Low Error on a Toshiba Satellite laptop "RTC Battery is Low..." An error message flashing while you try to boot your laptop is enough to panic many people. But worry not! "RTC Battery" stands for Real-Time Clock battery which almost all laptops and PCs have on their motherboard to power the clock and sometimes to also keep the CMOS settings from getting erased while the system is switched off.  It is not uncommon for these batteries to last for years before requiring a replacement as the clock consumes very less power. And contrary to what some people tell you - they are not rechargeable or getting charged while your computer or laptop is running. In this article, we'll learn everything about RTC batteries and how to fix the error on your Toshiba Satellite laptop. What is an RTC Battery? RTC or CMOS batteries are small coin-shaped lithium batteries with a 3-volts output. Most laptops use

The Best Way(s) to Comment out PHP/HTML Code

PHP supports various styles of comments. Please check the following example: <?php // Single line comment code (); # Single line Comment code2 (); /* Multi Line comment code(); The code inside doesn't run */ // /* This doesn NOT start a multi-line comment block /* Multi line comment block The following line still ends the multi-line comment block //*/ The " # " comment style, though, is rarely used. Do note, in the example, that anything (even a multi-block comment /* ) after a " // " or " # " is a comment, and /* */ around any single-line comment overrides it. This information will come in handy when we learn about some neat tricks next. Comment out PHP Code Blocks Check the following code <?php //* Toggle line if ( 1 ) {      // } else {      // } //*/ //* Toggle line if ( 2 ) {      // } else {      // } //*/ Now see how easy it is to toggle a part of PHP code by just removing or adding a single " / " from th

Designing a Simple Order Form Application in PHP

Ok guys, for this post we’re going to create a simple yet complete order form page. Order forms are used on many sites to take customers order online. Order forms should have the capability to take orders from visitors regarding what items they want to purchase and store the information for further processing. For this post’s example, we are going to create an order form for a Book Seller. The form will be designed to take order of five different items (books). Our order form application should be able to take order of five different items in any separate quantities tht user wants, it should also ask for shipping address and name of the customer. It should then store the information provided in a file along with the date and time order was placed. The application should also be able to take any number of orders and store them all linearly for further human processing. For this, we need a front end of a HTML form to which the user would interact a