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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
  {
  public:
    // pure virtaul function
    // declaration
    virtual void func() = 0;
    // can't define
  };

  // derived class
  class derived : public base
  {
  public:
    // 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;

    d1.func();
  }

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