Skip to main content

Pure Virtual Functions

From the previous article Properties of Virtual Functions, we know that a virtual function may or may not be overridden in the derived lasses. It means, it is not necessary for a derived class to override a virtual function.

But there are times when a base class is not able to define anything meaningful for the virtual function in that case every derived class must provide its own definition of the that function. To force this type of overriding you use the following general form to declare a virtual function:

  virtual ret-type func-name(arg-list)=0;

This type of virtual function is known as Pure Virtual Function.

There are two major differences between a virtual and a pure virtual function, these are below:

  • There CAN’T be a definition of the pure virtual function in the base class.

  • There MUST be a definition of the pure virtual function in the derived class.

By making a virtual function ‘Pure’, it becomes necessary for the derived classes to override it, further since the base class can’t define a pure virtual function, we can’t have objects of that class. These types of incomplete classes (having one or more pure virtual function) are known as Abstract Classes and are used extensively.

The following program illustrates this:

  // Pure Virtual Functions
  #include <iostream.h>

  // base class
  class base
    // pure virtaul function
    // declaration
    virtual void func() = 0;
    // can't define

  // derived class
  class derived : public base
    // must define
    void func()
      cout<<"Derived1's func()\n";

  // main
  void main()
    // --CODE: base b
    // won't work because we
    // can't have objects of 
    // absract classes
    derived d1;


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

Pong Game in HTML & JavaScript (Updated)

HTML Pong Game Gameplay Pong is one of the first games that many people from the 80s or 90s had played as children. Lots of people know it as a simple arcade game but what they probably do not know is that this simple game helped establish the video game industry! In this post, we'll be making our version of a very simple but fully working Pong game in HTML, CSS and JavaScript. Basic Game Structure Games, however simple or complex they may be, follow the basic Game Loop design as shown in the chart below. Event-oriented game engines usually encapsulate the design and provide you with an event mechanism for handling various parts like the input, update, and rendering but internally the basic design is being followed. Pong Game Loop For our Game Loop, we'll be using JavaScript setInterval so that the game code remains asynchronous and separate. As you may have guessed a while (or any other loop) will freeze the page. For our input, we'll be using onmousemove event to update t