Skip to main content

Properties of Pure Virtual Functions

In the article Properties of Virtual Functions, we discussed about two properties of virtual functions in detail. In this article we’ll be discussing about some properties of Pure Virtual Functions.

Property #1: We know that a base class can’t define a pure virtual function and at the same time its derived class must define it. But what if the derived class is used as base for deriving yet another class; is this possible?

Yes, it is, as is obvious from the following program:


  #include <iostream.h>

  // base class
  class base
  {
  public:
    // pure virtual function
    // declaration
    virtual void func() = 0;
  };

  // derived class
  class derived1 : public base
  {
  public:
    // must define
    void func()
    {
      cout<<"Derived1's func()\n";
    }
  };

  // derived from class derived1
  class derived2 : public derived1
  {
  public:
    void func()
    {
      cout<<"Derived2's func()\n";
    }
  };

  // main
  void main()
  {
    derived1 d1;
    derived2 d2;

    d1.func();
    d2.func();
  }

Hence strictly speaking, a pure virtual function cannot be declared and defined in the same class.

Property #2: While it’s necessary for a derived class to override a pure virtual function but if we derive yet another class from that derived class then it’s not necessary for the newly derived class to override that function.

So that means, if we have a hierarchy of derived classes then a pure virtual function must be overridden at least once.

It is clear from the following program:


  #include <iostream.h>

  // base class
  class base
  {
  public:
    // pure virtual function
    // declaration
    virtual void func() = 0;
  };

  // derived class
  class derived1 : public base
  {
  public:
    // must define
    void func()
    {
      cout<<"Derived1's func()\n";
    }
  };

  // derived from class derived1
  class derived2 : public derived1
  {
  public:
    // may not override the
    // pure virtual function
  };

  // main
  void main()
  {
    derived1 d1;
    derived2 d2;

    d1.func();

    // derived1's function will
    // be called
    d2.func();
  }

Property #3: We know that we can’t declare objects of Abstract Classes but since C++ offers very powerful Polymorphism capability, we can have their pointer (as well as references).

It is clear from the following program:


  #include <iostream.h>

  // base class
  class base
  {
  public:
    // pure virtual function
    // declaration
    virtual void func() = 0;
  };

  // derived class
  class derived1 : public base
  {
  public:
    // must define
    void func()
    {
      cout<<"Derived1's func()\n";
    }
  };

  // derived from class derived1
  class derived2 : public derived1
  {
  public:
    void func()
    {
      cout<<"Derived2's func()\n";
    }
  };

  // main
  void main()
  {
    // although objects of abstract classes
    // cannot be declared but we can have
    // their pointers
    base *bptr;

    derived1 d1;
    derived2 d2;

    // point to derived1 's object
    bptr=&d1;

    // derived1's func will be called
    bptr->func();

    // point to derived2's object
    bptr=&d2;

    // derived2's func will be called
    // same interface is used
    // but different function
    // is called
    bptr->func();
  }

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

Introduction to Operator Overloading in C++

a1 = a2 + a3; The above operation is valid, as you know if a1, a2 and a3 are instances of in-built Data Types . But what if those are, say objects of a Class ; is the operation valid? Yes, it is, if you overload the ‘+’ Operator in the class, to which a1, a2 and a3 belong. Operator overloading is used to give special meaning to the commonly used operators (such as +, -, * etc.) with respect to a class. By overloading operators, we can control or define how an operator should operate on data with respect to a class. Operators are overloaded in C++ by creating operator functions either as a member or a s a Friend Function of a class. Since creating member operator functions are easier, we’ll be using that method in this article. As I said operator functions are declared using the following general form: ret-type operator#(arg-list); and then defining it as a normal member function. Here, ret-type is commonly the name of the class itself as the ope