Lecture 19 (Inheritance)

8/7/2019 Lecture 19 (Inheritance)

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INHERITANCELECTURE-19

Qasim M. Rajpoot

NUST School of Electrical Engineering

and Computer Science

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Object Oriented

Programming using C++


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WHAT TO STUDY

Inheritance Examples

Inheritance Syntax

Protected access specifierRedefining member functions

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INHERITANCE

New classes created from existing classesy Absorb attributes and behaviors

Derived classy Class that inherits data members and member

functions from a previously defined base class

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IINHERITANCENHERITANCE

Inheritance is a mechanism for

Building class types from other class types

Defining new class types to be a .

y Specialization

y Augmentation

of existing types

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INHERITANCE EXAMPLES

Base class Derived classes

Student GraduateStudent

UndergraduateStudent

Shape Circle

Triangle

Rectangle

Employee FacultyMember

StaffMember

Account CheckingAccount

SavingsAccount

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THE STUDENT CLASS HIERARCHY

student

print()

graduate_student

print()

inherits (isa)

student_id,

year, name

dept,

thesis

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A natural way to reuse code

y Programming by extension rather than

reinvention

y Object-oriented paradigm is well-suited forthis style of programming

Terminology

y Base class (superclass)

y Derived class (subclass)

INHERITANCE

Bicycle

Mountain

Bikes

Racing

Bikes

Tandem

Bikes

is-a relationships

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CAN YOU BUILD INHERITANCE HIERARCHY

OUT OF THIS VENN-DIAGRAM

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INHERITANCE SYNTAX

The simplest example of inheritance requires twoclasses: a base class and a derived class.

y The base class OR super-class does not needany special syntax.

y The derived class OR subclass on the otherhand, must indicate that its derived from thebase class.

This is done by placing a colon after the nameof the derived class, followed by a keywordsuch as public and then the base class name.

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SYNTAX FOR INHERITANCE

class derivedClass : public baseClass {

private :

// Declarations of additional members, if needed.

public:


protected:


}

The derived class inherits from the base class: all public members,

all protected members (will see later)

The additional member functions defined can have the same name

as those of the base class (when some base members are

to be redefined)

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class Teacher{ // Base classprivate:

string name;int age, numOfStudents;

public:void setName (const string & new_name){

name = new_name;}

};

class Principal : public Teacher{ // Derived classstring school_name;

int numOfTeachers;public:

void setSchool(const string & s_name){school_name = s_name;

}};

INHERITANCE SYNTAX

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INHERITANCE SYNTAX

int main()

{

Teacher t1;

Principal p1;

p1.setName(" Principal 1");

t1.setName(" Teacher 1");

p1.setSchool(" Elementary School");

return 0;

} The p1 object can also access, in addition to its own

member function setSchool(), the member functionfrom Parent (Base), which is setName().

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PROTECTED ACCESS

We have seen two access modes in C++classes:public and privatey Public members are directly accessible by

users of the classy Private members are NOT directly accessible

by users of the class, not even by inheritors

There is a 3rd access mode:protectedy Protected members are directly accessible by

derived classes but not by other users

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EXAMPLE OF INHERITED CLASSES

class Shape {

protected:

int width, height;

public:

void setDims (int a, int b){

width=a; height=b;}

};

class Rectangle: public Shape {

public:

int area ( ) {

return (width * height);

}

};

class Triangle: public Shape {

public:

int area ( ) {

return (width * height/2);

}};

class Square: public Rectangle {

public:void setDims (int a){

width=a; height=a;}

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ACCESS CONTROL FOR PUBLIC INHERITANCE

An object of a derived class inherits all the

member data and functions of the base class.

y Private members

Not visible in the derived class. The derived class may access them only through the public

interface of the base class.

y Protected Data members

Visible in the derived class.

The derived class may access protected members directly.

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REDEFINING MEMBERS

Some member functions of the base class may

not be suitable for the derived class. These

members should be redefined in the derived

class.

For example, assume that the Teacher class

has a print function that prints properties of

teachers on the screen. But this function is notsufficient for the class Principal, because

principals have more properties to be printed.

So the print function must be redefined. 16


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REDEFINING MEMBERS

class Teacher{ // Base class

private:string name;

int age, numOfStudents;

public:

void setName (const string & new_name){

name = new_name;

}void print() const;

};

voidTeacher::print() const

{

cout


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REDEFINING MEMBERSclass Principal : public Teacher{ // Derived class

string school_name;

int numOfTeachers;

public:

void setSchool(const string & s_name){

school_name = s_name;

}void print() const;

};

void Principal::print() const

{// We can write this only if base class has protected data members

//cout


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REDEFINING MEMBER FUNCTION OR

OVERRIDING

print() function of the Principal class overrides (hides)the print() function of the Teacher class. Now thePrincipal class has two print() functions. The membersof the base class can be accessed by using the scoperesolution operator (::).

// Print method of Principal class

void Principal::print() const

{

//invoking the print function of the teacher class

Teacher::print();

cout


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ACCESS CONTROL PROTECTED MEMBERS

A class can have public orprivate members

Once inheritance enters the picture, other accesspossibilities arise for derived classes. In addition toprivate and public members a base class can alsohave protected members

Derived class inherits all members of base classwhether public, private, or protected. Memberfunctions of a derived class can access publicand protected members of the base class, butnot private members.

Objects of a derived class in main can accessonly public members of the base class. 20


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ACCESS CONTROL PUBLIC INHERITANCE

Access Specifier Accessible from Accessible from Accessible from

In Base Class Base Class Derived Class Outside Objects

public yes yes yes

protected yes yes no

private no no no`

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WHAT IS NEXT?

Inheritance continued

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Lecture 19 (Inheritance)