This set of notes is adapted from that provided by “Computer Science – A Structured Programming Approach Using C++”, B.A. Forouzan & R.F. Gilberg, Thomson Learning, 2000.

InheritancInheritancee

Chapter 12

COMP 103 - Inheritance 2

Extending Class from other classes

We have seen basic class functions Data hiding (private, protected, public) Friend Functions Overloading (functions, operators)

One of the most powerful elements of ADT is the ability to derive classes from other classes This provides the ability to create new classes while

retaining the basic characteristics of the original

This concept is called: Inheritance

COMP 103 - Inheritance 3

Figure 12-1

Case Study: Simple Polygons

COMP 103 - Inheritance 4

Polygon Base Class

Figure 12-2

COMP 103 - Inheritance 5

Figure 12-3, Part Iclass Polygon { protected: double area; double perimeter; public: Polygon() {}; ~Polygon() {}; void printArea(); void printPeri();}; // class Polygon

Class Definition of Polygon

COMP 103 - Inheritance 6

Private, Protected and Public

Private data and functions Can only be accessed within the class

Protected data and functions Can be accessed within the class as well as

the derived class from this class Public data and functions

Can be accessed by “everyone”.

COMP 103 - Inheritance 7

Polygon : Function definition

void Polygon :: printArea (){ cout << "The area of your polygon is "

<< area << endl; } // Polygons printArea

void Polygon :: printPeri (){ cout << "The perimeter of your polygon is "

<< perimeter << endl;

} // Polygons printPeri

COMP 103 - Inheritance 8

Figure 12-3, Part III

class Rectangle : public Polygon { private: double sideA, sideB; void calcArea(); void calcPeri(); public: Rectangle(double a, double b);};

Rectangle Object & Polygon

COMP 103 - Inheritance 9

Derived Class Syntax

class derived_class_name : inheritance_type base_class_name

Example:class Rectangle : public Polygonsclass Rectangle : protected Polygonsclass Rectangle : private Polygons

Inheritance rules:• All data members of the base object are inherited• All function members of the base object are

inherited, except:a. Constructorsb. Destructorsc. friend functionsd. Assignment operators

COMP 103 - Inheritance 10

Inheritance of data and functions

class Rectangle : public Polygon{ private: double sideA, sideB; void calcArea(); void calcPeri(); protected: double area; double perimeter; public: Rectangle(double a, double b); void printArea(); void printPeri();};

InheritedData andFunctions

COMP 103 - Inheritance 11

Rectangle: Constructor and functions

Rectangle::Rectangle(double A, double B) { sideA = A; sideB = B; calcPeri( ); // update perimeter field calcArea( ); // update area field}

void Rectangle::calcPeri() { perimeter = 2*(sideA + sideB);}

void Rectangle::calcArea() { area = sideA * sideB;}

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Main( ) : using Rectangle type

void main() { Rectangle rect(3,4); rect.printArea( ); rect.printPeri( );}

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Figure 12-3, Part II

Polygon: Triangle Object

class Triangle : public Polygon { private: double sideA, sideB, sideC; void calcArea(); void calcPeri(); public: Triangle(double a, double b,

double c );};

COMP 103 - Inheritance 14

Inheritance of data and functions

class Triangle : public Polygon { private: double sideA, sideB, sideC; void calcArea(); void calcPeri(); protected: double area; double perimeter; public: Triangle(double a, double b, double c); void printArea(); void printPeri();};

COMP 103 - Inheritance 15

Triangle: Constructor and Functions

Triangle::Triangle(double A,double B,double C){ sideA = A; sideB = B; sideC = C; calcPeri(); // update perimeter field calcArea(); // update area field}

void Triangle::calcPeri(){ perimeter = sideA + sideB + sideC;}

void Triangle::calcArea(){ // Compute the area of a triangle based on // the given side lengths}

COMP 103 - Inheritance 16

Main( ) : using Triangle type

void main() { Triangle tri(3, 4, 5); tri.printArea(); tri.printPeri();}

void main() { Rectangle rect(3, 4); rect.printArea(); rect.printPeri(); }

compared with

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Base and Derived Class Access

Inheritance Type Base Access Type Derived Access Type

privateprivate inaccessible

protected private

public private

protectedprivate inaccessible

protected protected

public protected

publicprivate inaccessible

protected protected

public public

Note: This is confusing for C++ programmers

COMP 103 - Inheritance 18

Another Example

class EmpInfo{private:

int ID;protected:

int Salary;public:

char Name[200];};

Base Classclass EmpInfo : private SecurityEmp{ . . .}ID (private) inaccessibleSalary (protected) privateName (public) private

class EmpInfo : protected ImportantEmp { . . .};ID (private) inaccessibleSalary (protected) protectedName (public) protected

class EmpInfo : public RegularEmp{ . . .};ID (private) inaccessibleSalary (protected) protectedName (public) public

Derived Classes

COMP 103 - Inheritance 19

Inheritance types (p.601)

For all types, Private member data/functions in the base class are inaccessible in the derived class

Private (default type) Protected and public data/functions in the base

class become private in the derived class Protected

Protected and public data/functions in the base class become protected in the derived class

Public Protected and public types are preserved!

COMP 103 - Inheritance 20

Constructors and Derived Types

Since Constructors are not inherited, derived types must declare their own constructors, e.g. Polygon case.

When a derived class is constructed, it must first execute the constructor for the base class

Sometimes we need to pass data to the base class’ constructor (initialize constructor). How?

with base-member-initialization list, e.g. Employee case.

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Example: Employee Class (p.609-611) class Employee { protected: int id; public: Employee(int idIn);};

class SalaryEmp:public Employee { protected: int salary; public: SalaryEmp(int idIn, int sal);};

class HourlyEmp:public Employee { protected: float payrate, hours; public: HourlyEmp(int idIn, float rate);};

Figure 12-6(p.608)

Employee

id : integer

SalaryEmp

salary : integer

HourlyEmp

payRate : real

hours : real

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Constructors

Employee::Employee(int idIn) { id = idIn;}

SalaryEmp::SalaryEmp(int idIn, int sal) : Employee(idIn) { salary = sal;}

HourlyEmp::HourlyEmp(int idIn, float rate) : Employee(idIn) { payRate = rate; hours = 0.0;}

pass idIn to Employee(idIn) constructor

pass idIn to Employee(idIn) constructor

COMP 103 - Inheritance 23

Base-Class-Initialization

SalaryEmp::SalaryEmp(int idIn, int sal): Employee(idIn)

{

salary = sal;

}

Explicit “base-class-initialization” - Employee(idIn)

Inherited-class constructor SalaryEmp::SalaryEmp(int idIn, int sal) calls the explicit-value base-class constructor

Employee(idIn)

COMP 103 - Inheritance 24

Summary

Polygon Case Study Access types

Private, Protected, Public (Confusing part of C++)

Constructor initialization syntax Employee example