Inheritance and Polymorphism

Contents
2

 What is inheritance?
 Derived and base class
 Inheritance types:
 Single level
 Multi level
 Hierarchical
 Multiple
 Hybrid

 Virtual Base Class

By: Kanika Sharma

 Cont…
3

 Abstract Classes
 Pointer to object
 This Pointer
 Pointer to derived Class
 Virtual Function
 Pure Virtual function
 Early vs Late binding

By: Kanika Sharma

 Inheritance
4

 Inheritance allows us to define a class in terms of

another class, which makes it easier to create and
maintain an application.
 This also provides an opportunity to reuse the code
functionality and fast implementation time.
 When creating a class, instead of writing completely
new data members and member functions, the
programmer can designate that the new class should
inherit the members of an existing class.

By: Kanika Sharma

 Cont…
5

 This existing class is called the base class, and the

new class is referred to as the derived class.
 Syntax:
class derived-class: access-specifier base-class
 Where access-specifier is one of public,
protected, or private, and base-class is the name
of a previously defined class.
 If the access-specifier is not used, then it is private by
default.

By: Kanika Sharma

 Access Control and Inheritance
6

 A derived class can access all the non-private

members of its base class.
 Thus base-class members that should not be
accessible to the member functions of derived classes
should be declared private in the base class.
 A derived class inherits all base class methods with
the following exceptions:
 Constructors, destructors and copy constructors of the base
class.
 Overloaded operators of the base class.
 The friend functions of the base class.

By: Kanika Sharma

 7

By: Kanika Sharma

 8

By: Kanika Sharma

 Types of Inheritance
9

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 Single level inheritance
10

 In this type of inheritance one

derived class inherits from only
one base class.

By: Kanika Sharma

 Program on single level inheritance
11
class B : public A
class A {
{ public:
void display2()
{
cout<<"Derived Class";
public: }
void display()
};
{ int main()
cout<<"Base {
B b;
Class"; b.display();
} b.display2();
return 0;
}; }

By: Kanika Sharma

 Multilevel inheritance
12

 In this type of
inheritance the derived
class inherits from a
class, which in turn
inherits from some
other class. The Super
class for one, is sub
class for the other.

By: Kanika Sharma

 Program on multilevel inheritance
class B : public A 13
{
#include <iostream> public: int main()
int c; {
using namespace std; void sum() C c;
class A { c.getdata();
{ cout<<endl;
c=a+b; c.sum();
public: cout<<endl;
int a,b; } c.msg();
}; cout<<endl;
void getdata()
{ class C : public B return 0;
{ b"<<endl;
cout<<"Enter a and }
cin>>a>>b; public:
} void msg()
}; {
cout<<"Sum is"<<c;

}
By: Kanika Sharma
};
 Multiple Inheritance
14

 In this type of
inheritance a single
derived class may
inherit from two or
more than two base
classes.

By: Kanika Sharma

 Ambiguity in multiple inheritance
15

 In multiple inheritance, there may be possibility that

a class may inherit member functions with same
name from two or more base classes and the derived
class may not have functions with same name as
those of its base classes.
 If the object of the derived class need to access one of
the same named member function of the base classes
then it result in ambiguity as it is not clear to the
compiler which base’s class member function should
be invoked. The ambiguity simply means the state
when the compiler confused.

By: Kanika Sharma

 Solution of ambiguity in multiple
inheritance
16

 The ambiguity can be resolved by using the scope

resolution operator to specify the class in which the
member function lies.

By: Kanika Sharma

 Hierarchical inheritance
17

 In this type of
inheritance, multiple
derived classes inherits
from a single base class.

By: Kanika Sharma

 Hybrid Inheritance
18

 "Hybrid
Inheritance" is a
method where one or
more types of inheritance
are combined together
and used.

By: Kanika Sharma

 Diamond Problem
19

 An ambiguity can arise when several

paths exist to a class from the same base
class. This means that a child class could
have duplicate sets of members
inherited from a single base class.
 The "diamond problem" is an
ambiguity that arises when two classes B
and C inherit from A, and class D
inherits from both B and C.
 C++ solves this issue by introducing a
virtual base class. When a class is made
virtual, necessary care is taken so that
the duplication is avoided regardless of
the number of paths that exist to the
child class.

By: Kanika Sharma

 Virtual Base Class
20

 When two or more objects are derived from a

common base class, we can prevent multiple copies
of the base class being present in an object derived
from those objects by declaring the base class as
virtual when it is being inherited.
 Such a base class is known as virtual base class. This
can be achieved by preceding the base class’ name
with the word virtual.

By: Kanika Sharma

 Program on virtual base class
21
class D: public B, public C
#include<iostream> {
using namespace std; public:
class A int sum;
{ };
public: int main()
int i; {
}; D ob;
ob.i = 10;//unambiguous since only one
class B : virtual public A copy of i is inherited.
{ ob.j = 20;
public: ob.k = 30;
int j; ob.sum = ob.i + ob.j + ob.k;
}; cout<<"Value of i is : "<< ob.i<<"\n";
class C: virtual public A cout<<"Value of j is :"<< ob.j<<"\n";
{ cout << "Value of k is :"<< ob.k<<"\n";
public: cout << "Sum is : "<< ob.sum <<"\n";
int k; return 0;
};By: Kanika Sharma }
 Abstract Class
22

 In C++ an abstract class is one which defines an

interface, but does not necessarily provide
implementations for all its member functions.
 An abstract class is meant to be used as the base
class from which other classes are derived.
 The derived class is expected to provide
implementations for the member functions that are
not implemented in the base class.
 A derived class that implements all the missing
functionality is called a concrete class .

By: Kanika Sharma

 Cont…
23

 Abstract Class is a class which contains atleast one

Pure Virtual function in it.
 Abstract classes are used to provide an Interface for
its sub classes.
 Classes inheriting an Abstract Class must provide
definition to the pure virtual function, otherwise they
will also become abstract class.

By: Kanika Sharma

 Why we cant create object of abstract class
24

 When we create a pure virtual function in Abstract

class, we reserve a slot for a function in the VTABLE,
but doesn't put any address in that slot. Hence the
VTABLE will be incomplete.
 As the VTABLE for Abstract class is incomplete,
hence the compiler will not let the creation of object
for such class and will display an errror message
whenever you try to do so.

By: Kanika Sharma

 Program on abstract class
25

#include <iostream> int main()

{
using namespace std; Base *b;
class Base //Abstract base class Derived d;
{ b = &d;
public: b->show();
virtual void show() = 0; //Pure Virtual Function }
};

class Derived:public Base

{
public:
void show()
{
cout << "Implementation of Virtual Function in
Derived class"; }
};
By: Kanika Sharma
 Characteristics of abstract classes
26

 Abstract class cannot be instantiated, but pointers

and references of Abstract class type can be created.
 Abstract class can have normal functions and
variables along with a pure virtual function.
 Abstract classes are mainly used for Up casting, so
that its derived classes can use its interface.
 Classes inheriting an Abstract Class must implement
all pure virtual functions, or else they will become
Abstract too.

By: Kanika Sharma

 Virtual Functions
27

 Virtual Function is a function in base class, which is

override in the derived class, and which tells the compiler
to perform Late Binding on this function.
 Virtual Keyword is used to make a member function of
the base class Virtual.
 Virtual functions allow us to create a list of base class
pointers and call methods of any of the derived classes
without even knowing kind of derived class object
 A virtual member function for which no implementation
is given is called a pure virtual function . If a C++ class
contains a pure virtual function, it is an abstract class.

By: Kanika Sharma

 Points to be remember
28

 Only the Base class Method's declaration needs

the Virtual Keyword, not the definition.
 If a function is declared as virtual in the base class,
it will be virtual in all its derived classes.
 The address of the virtual Function is placed in
the VTABLE and the compiler
uses VPTR(vpointer) to point to the Virtual
Function.

By: Kanika Sharma

 Cont…
29

 To accomplish late binding, Compiler creates VTABLEs, for each class

with virtual function. The address of virtual functions is inserted into
these tables. Whenever an object of such class is created the compiler
secretly inserts a pointer called vpointer, pointing to VTABLE for that
object. Hence when function is called, compiler is able to resolve the
call by binding the correct function using the vpointer.

By: Kanika Sharma

 Without Virtual functions
30

#include<iostream> class B:public A

using namespace std; {
Class A public:
{ void show()
public: {
void show() cout << "Derived Class";
{ }
cout << "Base class"; };
}
}; int main()
{
A * a; //Base class pointer
B b; //Derived class object
a= &b;
a->show(); //Early Binding Ocuurs
}

By: Kanika Sharma

 With virtual function
31
#include<iostream>
using namespace std; int main()
{
class A A *a;
{ B b;
public: a = &b;
virtual void show() a -> show();
{ }
cout << "Base class\n";
}
};

class B: public A
{
private:
virtual void show()
{
cout << "Derived class\n";
} Kanika Sharma
By:
};
 Pure Virtual Functions
32

 Pure virtual Functions are virtual functions with no

definition. They start with virtual keyword and ends
with= 0.
 Pure Virtual functions can be given a small definition
in the Abstract class, which you want all the derived
classes to have. Still you cannot create object of
Abstract class.
 Also, the Pure Virtual function must be defined
outside the class definition. If you will define it
inside the class definition, complier will give an
error. Inline pure virtual definition is Illegal.

By: Kanika Sharma

 Program on pure virtual function
33
class Derived:public Base
#include <iostream>
{
public:
using namespace std;
void show()
{ cout << "Implementation of Vir
class Base //Abstract base class
Function in Derived class"; }
{
};
public:
virtual void show() = 0; //Pure Virtual Function
int main()
};
{
Base *b;
void Base :: show() //Pure Virtual definition
Derived d;
{
b = &d;
cout << "Pure Virtual definition\n";
b->show();
}
}

By: Kanika Sharma

 Comparison between virtual and pure virtual
functions
34

Virtual functions Pure Virtual functions

 Virtual function have a  Pure virtual function

function body. have no function body.
 Overloaded can be done  Overloading is must in
by the virtual funciton. pure virtual funciton.
(Optional) (Must)
 It is define as : virtual int  It is define as : virtual
myfunction(); int myfunction() = 0;

By: Kanika Sharma

 Pointer to object
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 The pointers pointing to objects are referred to as

Object Pointers.
 Just like other pointers, the object pointers are
declared by placing in front of a object pointer's
name. It takes the following general form :
 class-name ∗ object-pointer
 When accessing members of a class using an object
pointer, the arrow operator (->) is used instead of
dot operator.

By: Kanika Sharma

 This pointer
36

 As soon as you define a class,

the member functions are
created and placed in the
memory space only once.
 That is, only one copy of
member functions is
maintained that is shared by
all the objects of the class.
 Only space for data members
is allocated separately for each
object

By: Kanika Sharma

 This pointer
37

 When a member function is called, it is automatically

passed an implicit (in-built) argument that is a
pointer to the object that invoked the function. This
pointer is called this.

By: Kanika Sharma

 Consider a situation
38

 The this pointer can be thought of analogous to the

ATM card.
 For instance, in a bank there are many accounts.
 The account holders can withdraw amount or view
their bank-statements through Automatic-Teller-
Machines.
 Now, these ATMs can withdraw from any account
in the bank, but which account are they supposed
to work upon ?
 This is resolved by the ATM card, which gives the
identification of user and his accounts, from where
the amount is withdrawn.
 Similarly, the this pointer is the ATM cards for
objects, which identifies the currently-calling
object.
 The this pointer stores the address of currently-
calling object.

By: Kanika Sharma

 Early binding vs late binding
39

Early binding Late binding

 Compiler knows at  Compiler doesn't know

compile time which until runtime which
function to invoke. function to invoke.
 one way to get late
 Direct function calls
binding is to use
can be resolved using a function pointers or
process known as early the other way is the use
binding. of virtual functions in
inheritance

By: Kanika Sharma