CE00314-2 Further Programming Concepts in C++

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Charge for handling • Savings Savings • Deposit, withdrawal & interest • Investments Investment • Only interest can be withdrawn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 Cheque Savings 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(); // redefinition double Withdraw(double); // redefinition double 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: // Attributes int nXPosition; int nYposition; char strName[50]; public: // Methods Shape(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 Square int 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 class class Circle:public Shape { private: // Attributes for Circle int 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 warning Shape::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; }

Note output from constructors Running the code

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

CE00314-2 Further Programming Concepts in C++