What is a Pure Virtual Function Write a C plus plus program using Pure Virtual Function?

Answer 1

A pure-virtual method is similar to a virtual method. A virtual method is a method that is expected to be overridden in a derived class while a pure-virtual method must be overridden in a derived class. The base class may provide a default implementation for a pure-virtual method, but it is not a requirement, whereas it must provide an implementation for a virtual method. In addition,

any base class that has a pure-virtual method becomes abstract; you cannot instantiate abstract classes -- you are expectedto derive from them.

#include

class Abstract

{

public:

// Pure-virtual method; note the '=0' to denote pure-virtual.

virtual void Show()=0;

};

class Derived : public Abstract

{

public:

virtual void Show()

{

// Implementation is required here!

std:: cout << "Derived" << std::end;

}

};

void main()

{

// Abstract a; // Compiler error!

Derived d;

d.Show(); // Call pure-virtual method.

return( 0 );

}

Answer 2

The following program demonstrates pure virtual functions.

#include<iostream>

#include<list>

class shape

{

public:

// pure-virtual

virtual void draw() = 0 { std::cout<<"Drawing..."; }

};

class circle: public shape

{

public:

void draw(){ shape::draw(); std::cout<<"circle"<<std::endl; }

};

class square: public shape

{

public:

void draw(){ shape::draw(); std::cout<<"square"<<std::endl; }

};

void process_shapes(std::list<shape*>& shapes)

{

for(std::list<shape*>::iterator i=shapes.begin(); i!= shapes.end(); ++i )

{

shape& sh=*(*i);

sh.draw();

}

}

int main()

{

circle c;

square s;

std::list<shape*> shapes;

shapes.push_back( &c );

shapes.push_back( &s );

process_shapes( shapes );

}