Virtual function in C++ Pure Virtual Function

CE142 Runtime Polymorphism & Virtual
Function


Virtual Function
#include<iostream>
using namespace std;
class Base
{
public:
virtual void show() { cout<<" In Base n"; }
};
class Derived: public Base
{
public:
void show() { cout<<"In Derived n"; }
};
int main(void)
{
Base *bp = new Derived;
bp->show(); // RUN-TIME POLYMORPHISM
return 0;
}
In Derived


 class A{
 void f1();
 virtual void f2();
 };
 class B:public A
 {
 void f2();
 void f4(int x);
 void f1();
 }
Virtual Function
&f2() &f3() &f4()
_vtable
&f2() &f3() &f4()
_vtable
 A *p;
 A x1;
 p=&x1;
 p->f1();
 p->f2();
 p->f3();
 p->f4();
 p->f4(10);
 !!Error
//EB
//EB
//LB
//LB
//LB
p
x1
_vptr


 class A{
 void f1();
 };
 class B:public A
 {
 void f2();
 void f4(int x);
 void f1();
 }
Virtual Function
&f2() &f3() &f4()
_vtable
&f2() &f3() &f4()
_vtable
 A *p;
 B x1;
 p=&x1;
 p->f1();
 p->f2();
 p->f3();
 p->f4();
 p->f4(10);
//EB
//EB
//LB
//LB
//LB
p
x1
_vptr


Can static functions be virtual in C++?
#include<iostream>
class Test
{
public:
// Error: Virtual member functions cannot be static
virtual static void fun() { }
};


Virtual Function & Default
Arguments
#include <iostream>
class Base
{
public:
virtual void fun ( int x = 0 )
{
cout << "Base::fun(), x = " << x << endl;
}
};
class Derived : public Base
{
public:
void fun ( int x )
{
cout << "Derived::fun(), x = " << x << endl;
}
};
int main()
{
Derived d1; Base *b = &d1; b->fun();
return 0;
}
"Derived::fun(), x =0


Virtual Function & Default
Arguments
#include <iostream>
class Base
{
public:
virtual void fun ( int x = 0 )
{
cout << "Base::fun(), x = " << x << endl;
}
};
class Derived : public Base
{
public:
void fun ( int x=10 ) //Note The Changes
{
cout << "Derived::fun(), x = " << x << endl;
}
};
int main()
{
Derived d1; Base *bp = &d1; bp->fun();
return 0;
}
"Derived::fun(), x =0


Can virtual functions be private in C++?
Never.
Why?


Abstract Classes in C++
 No Implementation of function in base
class is referred as Abstract class
Example: class Shape having member
function draw().
It can only be have implementation at
derived classes. i.e. draw() in rectangle,
draw() in square.
Example: class Animal having member
function move().


 A class which does not have an implementations of
the function.
 How to create abstract class in java.
 By putting abstract keyword before class definition
 How to create abstract class in C#
 How to create abstract class in C++
 There is no abstract keyword in C++!!!!!
Abstract class


 A class in C++ is abstract if it has at least one pure
virtual function.
 What is pure virtual function?
 It is a kind of virtual function which has no
implementations!!!
Abstract class in C++


 Error: Undefined reference to vtable for Sha
 How to make any function as pure virtual
function


Abstract class in C++
 A class in C++ is abstract if it has at least one pure
virtual function.
 What is pure virtual function?
 It is a kind of virtual function which has no
implementations!!!
How to make any virtual function as pure
virtual function?
 By putting “=0” expression at function declaration.


Characteristics of Abstract
class in C++


1. Abstract class may also contain non virtual
functions.


2. Instances of the abstract class can not be created.


3. We can have pointers and references of abstract
class type.


4. If we do not override the pure virtual function in
derived class, then derived class also becomes
abstract class.


5. An abstract class can have constructors.

Virtual function in C++ Pure Virtual Function

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