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Inheritance

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R eusability --building new components by utilizing

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. This is achieve by creating new classes, reusing the

properties of existing classes.

Inheritance: Introduction

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Inheritance: Introduction

This mechanism of deriving a new classfrom existing/old class is

called inheritance.The old class is known as

base class, superclass or parent class;and the new class is

known as sub class,derived class, or childclass.

Parent

Inheritedcapability

Child

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class Rectangle{ private:

int numVertices;float *xCoord, *yCoord;

public:

void set(float *x, float *y, int nV);float area();

};

Inheritance Concept

Rectangle Triangle

Polygon

class Polygon{ private:

int numVertices;float *xCoord, *yCoord;

public:void set(float *x, float *y, int nV);

};

class Triangle{ private:

int numVertices;float *xCoord, *yCoord;

public:void set(float *x, float *y, int nV);float area();

};

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RectangleTriangle

Polygonclass Polygon{

protected:int numVertices;float *xCoord, float *yCoord;

public:void set(float *x, float *y, int nV);

};

class Triangle : public Polygon{ public:

float area();};

class Triangle{ protected:

int numVertices;float *xCoord, float *yCoord;

public:void set(float *x, float *y, int nV);float area();

};

Inheritance Concept

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Inheritance Concept

Point

Circle 3D-Point

class Point{ protected:

int x, y; public:

void set (int a, int b);};

class Circle : public Point{ private:

double r;};

class 3D-Point: public Point{ private:

int z;};

x

y

x

yr

x

yz

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A ugmenting the original class

S pecializing the original class

Inheritance Concept

RealNumber

ComplexNumber

ImaginaryNumber

R ectangleTriangle

Polygon Point

Circle

realimag

realimag

3D-Point

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W hy Inheritance ?

Inheritance is a mechanism for

building class types from existing class types

defining new class types to be a specialization

augmentationof existing types

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Define a Class Hierarchy

S yntax:

class DerivedClassName : access-level BaseClassName

where access-level specifies the type of derivation

private by default, or public

A ny class can serve as a base class Thus a derived class can also be a base class

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Class DerivationPoint

3D-Point

class Point{ protected:

int x, y; public:

void set (int a, int b);};

class 3D-Point : public Point{

private:double z;

};

class S phere : public 3D-Point{

private:double r;

};

S phere

Point is the base class of 3D-Point , while 3D-Point is the base class of S phere

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What to inherit?

In principle , every member of a base class isinherited by a derived class just with different access permission

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A ccess Control Over the MembersTwo levels of access controlover class members class definition

inheritance type

base class/ superclass/parent class

derived class/ subclass/child class

d e r i v e

f r o m

m e m b e r s g o e s t o

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

};

class Circle : public Point{

};

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The type of inheritance defines the access level for the

members of derived class that are inherited from the baseclass

A ccess Rights of Derived Classes

private protected public

private - - -

protected private protected protected public private protected public

Type of Inheritance

A c c e s s C on t r ol

f or M

e m b e r s

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class daughter : --------- mother{ private: double dPriv; public: void mFoo ( );

};

Class Derivationclass mother{

protected: int mProc; public: int mPubl; private: int mPriv;

};

class daughter : --------- mother{ private: double dPriv; public: void dFoo ( );

};

void daughter :: dFoo ( ){mPriv = 10; //error mProc = 20;

};

private/protected/publicint main() {

/*.*/}

class grandDaughter : public daughter { private: double gPriv; public: void gFoo ( );

};

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Constructor Rules for Derived ClassesThe default constructor and the destructor of the

base class are always called when a new objectof a derived class is created or destroyed.

class A { public:

A ( ){cout

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Constructor Rules for Derived ClassesYou can also specify an constructor of the

base class other than the default constructor

class A { public:

A ( ){cout

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Define its Own Members

Point

Circle

class Point{ protected:

int x, y; public:

void set(int a, int b);

};

class Circle : public Point{ private:

double r; public:

void set_r(double c);};

x

y

x

yr

class Circle{ protected:

int x, y;

private:double r;

public:void set(int a, int b);void set_r(double c);

};

T he derived class can also defineits own members, in addition to

the members inherited from thebase class

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Even more A derived class can override methods defined in its parent

class. With overriding, the method in the subclass has the identical signature to the method

in the base class. a subclass implements its own version of a base class method.

class A { protected:

int x, y; public:

void print (){cout

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class Point{ protected:

int x, y; public:

void set(int a, int b){x=a; y=b;}

void foo ();void print ();

};

class Circle : public Point{ private: double r; public:

void set (int a, int b, double c) {

Point :: set(a, b); //same name function callr = c;

}void print (); };

A ccess a Method

Circle C;C.set(10,10,100); // from class CircleC.foo (); // from base class PointC. print (); // from class Circle

Point A ;A .set(30,50); // from base class PointA . print (); // from base class Point

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Pu tting T hem T ogetherTime is the base class

E xtTime is the derived class with public inheritance

The derived class can inherit all members from the base

class, except the constructor access all public and protected

members of the base class define its private data member provide its own constructor define its public member functions override functions inherited from

the base class

E xtTime

Time

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class Time S pecification

class Time{

public :

void Set ( int h, int m, int s ) ;

void Increment ( ) ;void Write ( ) const ;Time ( int initH, int initM, int initS ) ; // constructor Time ( ) ; // default constructor

protected :

int hrs ;int mins ;int secs ;

} ;

// SPECIFIC A TION FILE ( time.h)

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Class Interface D iagram

Protected data:

hrs

mins

secs

Set

Increment

Write

Time

Time

Time class

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Derived Class Ext Time

// SPECIFIC A TION FILE ( e x ttime.h)

#include time.h

enum ZoneType { ES T, C S T, M S T, P S T, E DT, CDT, MDT, PDT } ;

class Ex

tTime : public Time// Time is the base class and use public inheritance{

public :

void Set ( int h, int m, int s, ZoneType timeZone ) ;void Write ( ) const; //overridden

E x tTime (int initH, int initM, int initS, ZoneType initZone ) ;E x tTime (); // default constructor

private :ZoneType zone ; // added data member

} ;

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Class Interface D iagram

Protected data:

hrs

mins

secs

Ext Time class

Set

Increment

Write

Time

Time

Set

Increment

Write

E x tTime

E x tTime

Private data:zone

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Implementation of Ext Time

Default Constructor

E x tTime :: E x tTime ( )

{zone = EST ;

}

The default constructor of base class, Time(), isautomatically called, when anE xtTime object is created.

E xtTime et1;

hrs = 0

mins = 0secs = 0zone = ES T

et1

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Implementation of Ext Time

A nother Constructor

E x tTime :: E x tTime (int initH, int initM, int initS, ZoneType initZone): Time (initH, initM, initS)// constructor initializer

{zone = initZone ;

}

E xtTime *et2 =new E xtTime(8,30,0, ES T);

hrs = 8mins = 30secs = 0

zone =ES

T

et2

5000

???

6000

5000

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Implementation of Ext Time

void E x tTime :: Set (int h, int m, int s, ZoneType timeZone){

Time :: Set (hours, minutes, seconds); // same name function call

zone = timeZone ;

}

void E x tTime :: Write ( ) const // function overriding{

string zone S tring[8] ={ ES T, C S T, M S T, P S T, E DT, CDT, MDT, PDT} ;

Time :: Write ( ) ;cout

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Working with Ext Time

#include e x ttime.h

int main(){

E x tTime thisTime ( 8, 35, 0, PST ) ;E x tTime thatTime ; // default constructor called

thatTime.Write( ) ; // outputs 00:00:00 ES T

thatTime.Set (16, 49, 23, C D T) ;

thatTime.Write( ) ; // outputs 16:49:23 CDTthisTime.Increment ( ) ;thisTime.Increment ( ) ;thisTime.Write ( ) ; // outputs 08:35:02 P S T

}

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T ake Home Message

Inheritance is a mechanism for defining newclass types to be a specialization or anaugmentation of existing types.

In principle, every member of a base class is

inherited by a derived class with differentaccess permissions, except for the constructors