CMSC 202

Computer Science II for Majors

CMSC 202 UMBC

Topics

Inheritance Polymorphism

CMSC 202 UMBC

Inheritance – Extending Classes

Inheritance It is a mechanism of deriving a new class

from an old class It promotes Code Reusability It implements the “Is a” relationship

between objects Thus supports the concept of hierarchical

classification

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Inheritance … cont

Vehicle

Car Truck

General Form

Special Form

Sp

ecia

liza

tion

Ge

ner

aliz

atio

n

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Inheritance … cont

Terminology General class (Vehicle) is called as base

class or super class Specialized class (Car, Truck) is called as

derived class or sub class Thus a derived class inherits all attributes

and behavior of base class and may use, extend, modify or replace the base class behavior

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Inheritance … cont

Types

Single Hierarchical

Multiple

A A

A

B B

B

C

C

D

Multilevel

A

B

C

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Inheritance … cont Defining Derived Class

class derived-class : visibility-mode base-class{

..

.. // derived class members};e.g.class Car : public Vehicle{

..};

Visibility mode specifies whether features of base class are derivedpublicly or privately

We will deal only with Public inheritance

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Inheritance … cont

Public Inheritance class ABC : public XYZ {

// members of ABC };

Public members of base class become public members of derived class

Private members of base class are not inherited

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Inheritance … cont

Inheriting Private members of Base class What if private data needs to be inherited ? Making that data public would violate data

hiding Use third access specifier : protected A protected data member in base class is accessible

within base class and any class immediately derived from it

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Inheritance … cont

Private

ProtectedPublic

Private

ProtectedPublic

Class A

Not inheritable

Class B : public A

Public Inheritance

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Inheritance … cont

Extending Classes Derived class can add / modify base class

data and member functions extending functionality of base class

Function overriding : Derives class can override an inherited method of base class by

having same signature as base class method

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Polymorphism

Polymorphism – 'One name, multiple forms' Have we seen this before ? Function overloading : A function is selected

for invoking by matching signature This information is available to compiler at

compile time – early binding or static binding

Thus object is bound to function call at compile time

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Polymorphism … cont

Consider following hierarchyclass A{

...public :

void show();};class B : public A // public inheritance{

...public :

void show();};

How would the appropriate function be selected at run time ? – Runtime Polymorphism

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Polymorphism … cont

Runtime Polymorphism C++ supports runtime polymorphism using

virtual functions Selection of appropriate function done at run

time – late binding or dynamic binding This mechanism used pointers or

references to objects

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Polymorphism … cont

Polymorphism

Compile Time Run Time

Function overloading

Operator overloading

Virtual Functions

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Polymorphism … cont

Virtual Functions When we have same function name in base

and derived classes, function in base class is declared virtual

Which function to use at run time depends on type of object pointed to by the base pointer

Thus runtime polymorphism is achieved when a virtual function is accessed through a pointer to base class

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Polymorphism … cont

Pure virtual functions A virtual function declared in base class, but

having no definition relative to base class It serves as a place holder Derived class has to provide definition for it A class containing pure virtual functions

cannot be used to declare objects of its own e.g. : virtual void show() = 0;

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Polymorphism … cont

Consider the following class hierarchy

A function GetArea() is used to display area of the particular shape

Shape

Rectangle Triangle

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Polymorphism … cont// Shape Classclass Shape { public:

virtual double GetArea() const = 0; // Pure virtual function};// Rectangle Classclass Rectangle : public Shape // Public Inheritance{ public :

Rectangle(int length=0, int breadth=0); // Constructorvirtual double GetArea() const; // Overriding Shape :: GetArea()

private :int m_length;int m_breadth;

};double Rectangle :: GetArea() const{

// code for calculating area}

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Polymorphism … cont// Triangle Classclass Triangle : public Shape // Public Inheritance{ public :

Triangle(int base=0, int height=0); // Constructorvirtual double GetArea() const;

private :int m_base;int m_height;

};

double Triangle :: GetArea() const{

// code for calculating area}

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Polymorphism … contint main(){ Rectangle rectangle1(12,10); // Create a rectangle object Triangle triangle1(2,3); // Create a triangle object Shape *ptr; // Pointer to base class

ptr = &rectangle1; // Base class pointer points to derived // class object

ptr -> GetArea(); // Will invoke function in Rectangle class

ptr = &triangle1; ptr -> GetArea(); // Will invoke function in Triangle class

return 0;

}