Another Way to Define A Class: Inheritance

Inheritance Concept

Rectangle Triangle

Polygon

class Polygon{

private: int width, length;public: void set(int w, int

l);}

class Rectangle{private: int width, length;public: void set(int w, int

l); int area();

}class Triangle{

private: int width, length;public: void set(int w, int

l); int area();

}

Inheritance Concept

Rectangle Triangle

Polygonclass Polygon{

protected: int width, length;public: void set(int w, int l);

}

class Rectangle : public Polygon

{public: int area();

}

class Rectangle{protected: int width, length;public: void set(int w, int l); int area();

}

Inheritance Concept

Rectangle Triangle

Polygonclass Polygon{

protected: int width, length;public: void set(int w, int l);

}

class Triangle : public Polygon

{public: int area();

}

class Triangle{protected: int width, length;public: void set(int w, int l); int area();

}

Why Inheritance? Reusability--building new components by

utilising existing components- is yet another important aspect of OO paradigm.

It is always good/“productive” if we are able to reuse something that is already exists rather than creating the same all over again.

Save time & money

Cond.This mechanism of

deriving a new class from existing/old class is called “inheritance”.

The old class is known as “base” class, “super” class or “parent” class”; and the new class is known as “sub” class, “derived” class, or “child” class.

Parent

Child

Inherited Capability

Inheritance Concepts

Inheritance creates a hierarchy of related class es (types) which share code and interface.

This is also called a “is a” relationship:

A dog is an animal

A teacher is a person

A car is a vehicle

Inheritance Examples

Base Class Derived Classes

Student CommuterStudentResidentStudent

Shape CircleTriangleRectangle

Loan CarLoanHomeImprovementLoanMortgageLoan

Shape class hierarchy

TwoDimensionalShape

Shape

ThreeDimensionalShape

Circle Square Triangle Sphere Cube Tetrahedron

What to inherit?

In principle, every member of a base class (data + member function) is inherited by a derived class just with different access permission

However, there are exceptions forbase class's constructor and destructor base class's friends

Since all these functions are class-specific

What a derived class can add?

New data membersNew member functions (also overwrite existing

ones)

New constructors and destructorNew friends

GenerializationIn UML, inheritance is called generalization

Parent class is more general form of the child class

Or

Child is more specific version of the parent

Defining Derived Classes

Defined by specifying the relationship with the base class in addition to its own details.

Class derived-class : visibility mode base-class

{………….…………. //members of derived class};

Accessing Base Class MembersMember of base class used by derived class

object called as accessibility.

Protected: is accessible by the member functions within its class and any class immediately derived from it.

Inheritance & AccessibilityAccess Specifier

Accessible from own class

From derived class

Form objects outside class

public yes yes yes

protected Yes yes no

private yes no no

Overriding member functions

To override a base-class member function In derived class, supply new version of that function

Same function name, different definitionThe scope-resolution operator may be used to

access the base class version from the derived class

Access Control Over the Members

Two levels of access control over class membersclass definitionVisibility mode

base c lass / supe rc lass /pa ren t c lass

deriv ed c lass / subc lass /ch ild c lass

deriv

e fro

m

mem

bers

goe

s to class Point{

protected: int x, y;public: void set(int a, int b);

}

class Circle : public Point{… …

}

Visibility modePrivate (default)PublicExamples:

This is an example of Single inheritance.

Class A: public X{//members of A};

Class A : private X{ //members of A};

Public Inheritance

class A : public B{ // Class A now inherits the members of Class B

// with no change in the “access specifier” for} // the inherited members

public base class (B)

public members

protected members

private members

derived class (A)

public

protected

not inherited

Private Inheritanceclass A : private B{ // Class A now inherits the members of Class B

// with public and protected members} // “promoted” to private

private base class (B)

public members

protected members

private members

derived class (A)

private

private

not inherited

Protected visibilityWhen a protected member is inherited in

public mode, it becomes protected in derived class. It is ready for further inheritance

A Protected member, inherited in private mode, it becomes private in the derived class. But not further inherited.

Protected Inheritance

class A : protected B{ // Class A now inherits the members of Class B

// with public members “promoted” to protected} // but no other changes to the inherited members

protected base class (B)

public members

protected members

private members

derived class (A)

protected

protected

not inherited

Visibility of inherited members

private protected public

private Not inher. Not inher. Not inher.

protected private Protected protected

public private protected public

Derived Class Visibility

#include <iostream.h>

class B

{ int a;

public:

int b;

void get_ab();

int get_a();

void show_a();

};

class D: public B

{ int c;

public:

void mul();

void display();

};

void B::get_ab()

{ a=5; b=10; }

int B::get_a()

{ return a;

}

void B::show_a()

{ cout<<"a="<<a<<endl; }

void D::mul()

{ c=b*get_a();

}

void D:: display()

{

cout <<"a="<<get_a()<<endl;

cout<<"b="<<b<<endl;

cout<<"c="<<c<<endl;

}

void main()

{ D d;

d.get_ab();

d. mul();

d.show_a();

d.display();

d.b =20;

d.mul();

d.display(); }

Output

a=5

a=5

b=10

c=50

a=5

b=20

c=100

Forms of Inheritance:

The different forms of inheritance are:

Single inheritance (only one super class)Multiple inheritance (several super classes)Hierarchical inheritance (one super class,

many sub classes)Multi-Level inheritance (derived from a

derived class)Hybrid inheritance (more than two types)

A

B

(a) Single Inheritance

C

(b) Multiple Inheritance

A B

(c) Hierarchical Inheritance

B

A

C D

A

C

B

(e) Hybrid Inheritance

B

D

c

A

(f) Multipath Inheritance

B

A

D

c

(d) Multilevel Inheritance

Multilevel Inheritance

A

C

B

Class A{…………..};Class B: public A{……….………};Class C : public B{……..……..};

Class B-----intermediate class

Chain ABC is known as inheritance path

Process can be extended to any number of levels.

Multiple Inheritance

A class can inherit the attributes of two or more classes.

Class D: public A, public B{------………}

A C

D

B

Combines the features of several existing classes to define a new class.

Child inheriting the smartness from his mother & intelligence from father..

Ambiguity Resolution in inheritanceWhen a function with the same name

appears in more than one base class.We can solve this problem by defining a

named instance within the derived class, using the scope resolution operator with the function…

Ambiguity in single inheritance applications

Class A{Public:Void display(){Cout<<“A”;}};

Class B:public A{Public:Void display(){Cout<<“B”;}};

Void main(){B b;b.display();b.A::display();}

Hierarchical Inheritance

A

C DB

Class A{………..};Class B:public A{…..…..};Class C:public A{…….……};

Hybrid Inheritance There could be situations where we need to apply

two or more types of inheritance to design a program. For example

test

Result

Sports

Student

MultilevelMultiple

Virtual Base ClassesThe duplication of inherited members due to

multiple paths can be avoided by making the common base class as virtual base class.

Class A{….};Class B1:virtual public A{…};

Class B2:public virtual A{…};Class C:public B1,public B2{….};

Abstract ClassesAn abstract class is one that is not used to

create objects.It is designed only to act as a base class.

Constructors in Derived class

No parameterized constructor in base class: no need to define constructor in derived class.

If contains then there is a need to define ctor in derived class.

Usually we create objects of derived classes, it make sense to pass arguments to the base class constructor.

If both classes are having constructors, then the base constructor is executed first and then the constructor in the derived class is executed.

General SyntaxDerived-ctor (Arg1, n(argn)arg2,…argn, argd):base1(arg1),base2(arg2),….basen(argn){

//body of derived constructor}

D(int a1, int a2, int d1):B(a1,a2){d=d1;}

Using Constructors and Destructors in Derived Classes

Derived-class constructor Calls the constructor for its base class first to

initialize its base-class membersIf the derived-class constructor is omitted, its

default constructor calls the base-class’ default constructor

Destructors are called in the reverse order of constructor calls.Derived-class destructor is called before its base-

class destructor

Example 1:default in base no need of ctor in derivedClass A class B{ {int a; public:public: void display()A(){a=0;} {A::display();}void display() };{cout<<a;} void main()}; {B b1;

b1.display();}

Output is: 0

Example 2:if default ctor in derived Class A class B{ {int b;int a; public:public: B(){b=5;}

void display()A(){a=0;} {A::display();void display() cout<<b;}};{cout<<a;} void main()}; {B b1;

b1.display();}

Output is:0 5

Example 3Class A class B{ {int b;int a; public:public: B(){b=5;}A(){a=0;} void display()A(int m) {A::display();{a=m;} cout<<b;}};void display() {cout<<a;} void main()}; {B b1;

b1.display();}

Output:0 5

Example 4

Class A class B{ {int b;int a; public:public: B(){b=5;}A(){a=0;} B(int n){b=n;}A(int m) void display(){A::display();{a=m;} cout<<b;}};void display() {cout<<a;} void main()}; {B b1(10);

b1.display();}

Output is:0 10

Example 5Class A class B{ {int b;int a; public:public: B(){b=5;}A(){a=0;} B(int m,int n):A(m),b(n){}A(int m) void display(){A::display();{a=m;} cout<<b;}};void display() {cout<<a;} void main()}; {B b1(10,20);

b1.display();}

Multiple Inheritance with constructorsBase classes are constructed in the order in

which they appear in the declaration of the derived class.

ExampleClass D:public A, protected B, private C

Order of execution is:ABCD

Multilevel Inheritance with constructorsIn multilevel inheritance, the constructors

will be executed in the order of inheritance.

A

C

B

Order is

ABC

Execution of base class constructorsConstructors for virtual base classes are

invoked before any non-virtual base classes.

Class A:public virtual B, public C, public DClass A:public B, virtual public C, public DClass A:virtual public B, public C, virtual

public D

class A void show() void main()

{ { {

int a; cout<<b; C c1(4,5,6);

public: }}; c1.A::show();

A(int x) class C:public A, public Bc1.B::show();

{a=x;} { c1.show();

void show() int c; }

{cout<<a;} public:

}; C(int m, int n, int z):B(n),A(m),C(z)

class B {cout<<“objects initialized”;}

{int b; void show()

public: {cout<<c;}

B(int y) };

{b=y;

}

Case Study: Point, Circle, Cylinder

Define class PointDerive Circle

Derive Cylinder