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Pure Virtual Function

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Pure Virtual Function`

Usually, a function declared inside the base class and redefined in the derived class.

Not defined any object of class and function in the base class have been defined “empty”.

Such functions are called “do-nothing” functions.

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Such functions are defined as:

Virtual void display()=0;

Such functions are called Pure virtual functions.
A class containing pure virtual function can not be used to declare any object of its own.
Such classes are called Abstract base classes.
The main objective of base class is to create base class ptr required for achieving run-time polymorphism.

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#include<iostream>

using namespace std;

class Base

{

int x;

public:

virtual void fun() = 0;

int getX() { return x; }

};

class Derived: public Base

{

int y;

public:

void fun()

{

cout << "fun() called"; }

};

int main(void)

{

Derived d;

d.fun();

return 0;

}

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class B

{

public:

virtual void s() = 0;

};

class D:public B

{

public:

void s()

{

cout << "Virtual Function in Derived class\n";

}

};

int main()

{

B *b;

D dobj;

b = &dobj;

b->s();

}

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class A
{
Int a;
public:
A()
{
a=1;�}
virtual void show()
{
cout<<a;
}
};
class B:public A
{
int b;
public:
B()
{
b=2;
}
virtual void show()
{
cout<<b;
}
};
int main()
{
A *ptr;
B ob;
ptr= &ob;
ptr->show();
return 0;
}

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Virtual Constructor/Virtual Destructor

The virtual mechanism works only when we have a base class pointer to a derived class object.
Destructors in the Base class can be Virtual.
Destructors of the Base class must be made virtual for proper destruction of the object when the program exits.
NOTE: Constructors are never Virtual, only Destructors can be Virtual.

New & delete operators/keyword
initialize/destroying objects

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class b
{
public:
b()
{ cout<<"Constructing base \n";
}
virtual ~b()
{ cout<<"Destructing base \n";
}
};
class d: public b
{
public:
d()
{ cout<<"Constructing derived \n";
}
~d()
{ cout<<"Destructing derived \n";
}
};
int main()
{
b *ptr=new d();
delete ptr;
return 0;
}

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Constructing base
Constructing derived
Destructing derived
Destructing base

(Without virtual)

Constructing base
Constructing derived
Destructing base

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class b
{
public:
b()
{ cout<<"Constructing base \n";
}
~b()
{
cout<<"Destructing base \n";
}
};
class d: public b
{
public:
d()
{
cout<<"Constructing derived \n";
}
~d()
{
cout<<"Destructing derived \n";
}
};
int main()
{
b *ptr1= new b();
b *ptr2=new d();
delete ptr1;
delete ptr2;
return 0;
}

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Constructing base
Constructing base
Constructing derived

Destructing base
Destructing derived
Destructing base
(Without virtual)
Constructing base
Constructing base
Constructing derived
Destructing base
Destructing base