CE00314-2 Further Programming Concepts in C++ Lecture 5 Inheritance & Polymorphism

CE00314-2Further Programming Concepts in C++

Lecture 5

Inheritance & Polymorphism

Introduction

Basic inheritance Polymorphism Constructors and destructors Extending inheritance

Bank accounts

“Normal” account Account Already considered Owner details, balance, methods etc.

Transfer Cheque Technique to transfer money to other account Charge for handling

Savings Savings Deposit, withdrawal & interest

Investments Investment Only interest can be withdrawn

Accounts relationships

Account

ChequeSavings

Investment

Declarations - abridged

Within header file

class Account{protected:

double dBalance;Public:

Account(double dBal = 0){

dBalance = dBal;}

double GetBalance(void){

return dBalance;}

};

Default values - revisited

Use can be esoteric Especially when removing implementation from

class definition

See DefaultValues.cpp & DefaultValues.h

Protected

New keyword Allows derived classes / objects to access.

“Family” Not available to “outsiders”

Private members Only within Or by friends

Savings Account

class Savings:public Account{protected:double dRate;

public:Savings(double = 0.0, double = 0.5);double Compound(void);double Withdraw(double);

};

Savings Account

See Savings.cpp & Savings.h Acc2 is Savings

Inherited from base Account Attributes Methods

Adds new as required for speciality Attributes Interest Rate – dRate Methods Own constructor(s)

Own method(s) – Compound()

More Inheritance - Investment

class Investment:public Savings{protected:

double dFundsAvailable;

public:Investment(double = 0.0, double = 1.5, int = -99);

double Compound(); // redefinitiondouble Withdraw(double); // redefinitiondouble GetAvailable();

};

Investment Account

Inherited from Savings NOT directly from Account

See Investment.cpp & Investment.h Note

Investment Withdraw method Complete redefinition

Investment Partial, adds to Savings

Polymorphism

Meaning The ability to appear in many forms

Programming Objects react differently to same “instruction”

Depending upon data types or class

Practically Ability to redefine methods for derived classes

Polymorphism

Shape

Circle Square Triangle

Polymorphism

Shape – base type Anchor position, “name” field, colour etc.

Circle – inherits from Shape Radius, method to draw circle.

Square – inherits from Shape Height, width, methods to draw square

Triangle – inherits from Shape Height, base width, methods to draw triagle

Polymorphism

Let the object look after itself Carry what it needs to know Remove functionality to the object Do not carry items relating to others

Shape base class

class Shape{private:

// Attributesint nXPosition;int nYposition;char strName[50];

public:// MethodsShape(int nX, int nY, char *ptrName){

nXPosition = nX;nYposition = nY;strcpy(strName, ptrName);

}

void Draw(void){

cout << "I am a " << strName << ", I am at " << nXPosition <<" and " << nYposition << endl;

}};

Square class

class Square:public Shape{private:

// Attributes for Squareint nLength;int nWidth;

public:Square(int nX, int nY, int nL, int nW, char *ptrName):Shape(nX,nY,ptrName){

nLength = nL;nWidth = nW;cout << "Square object created" << endl;

}

void Draw(void){

int nArea;nArea = nLength * nWidth;Shape::Draw();cout << "My area is " << nArea << " square units" << endl;

}

};

Circle classclass Circle:public Shape{private:

// Attributes for Circleint nRadius;

public:Circle(int nX, int nY, int nR, char *ptrName):Shape(nX, nY, ptrName){

nRadius = nR;cout << "Circle object created" << endl;

}

~Circle(void){

cout << "Circle object being deleted (Circle destructor)" << endl;}void Draw(void){

float nArea;nArea = (float)(PI * nRadius * nRadius); // Note casting to avoid warningShape::Draw();cout << "My area is " << nArea << " square units" << endl;

}};

Main// ShapesMain.cpp#include <iostream>#include <string.h>#define PI 3.142

using namespace std;

#include "ShapesClasses.h"

int main(void){

Shape MyBase(10, 20, "Base Shape");Square MySquare(20, 50, 162, 75, "Square");Circle MyCircle(70, 26, 24, "Circle");

cout << endl << "-------------------------------------------" << endl;MyBase.Draw();cout << endl << "-------------------------------------------" << endl;MySquare.Draw();cout << endl << "-------------------------------------------" << endl;MyCircle.Draw();cout << endl << "-------------------------------------------" << endl;

return 0;}

Running the code

Note output fromconstructors

Constructor Order

Starts with least specialised Works sequentially towards most specialised.

Shape MyBase(10, 20, "Base Shape");

Square MySquare(20, 50, 162, 75, "Square");

Circle MyCircle(70, 26, 24, "Circle");

Destructors~Shape(void)

{cout << strName << " being deleted (Shape destructor)" << endl;

}---------------------------------------------------~Square(void)

{cout << "Square object being deleted (Square destructor)" << endl;

}---------------------------------------------------~Circle(void)

{cout << "Circle object being deleted (Circle destructor)" << endl;

}

Destructor Order

Starts with most specialised Works sequentially towards least specialised. Opposite of constructors

Destructor call

Review As before – when “delete” used

Not included within Main – see slide 19 Only difference within class

Addition of Destructors When application terminates

All associated objects deleted Destructors call automatically

Example code

See ShapesMain.cpp & ShapesClasses.h Within ShapeTypes.doc

Tutorial Work

Develop Cheque using inheritance As per diagram within slide 4 Able to transfer money from one account to

another

Extend Shapes to include Triangle Extend Shapes with Triangle

Different types of triangle See next slide

Triangles

Shape

Circle Square Triangle

Isosceles Equilateral Scalene

Summary

Basic inheritance Polymorphism Constructors and inheritance Destructors and inheritance Extending the inheritance