CMSC 202

Computer Science II for Majors

CMSC 202 UMBC

Topics

Objects and Classes Project 2 description

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Objects

What are objects ? Real-world objects

E.g. Car, Fruit A real-world object is an entity in the world

An object is an entity in an OO system OO programming approach

Programming problem is in terms of objects and communication between them

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Class

A Class is a user-defined data type whose instances are objects

Objects are variables of type class Thus, class is an abstraction of objects of

similar type E.g. automobile is a class whose objects

could be Bob’s car, Mary’s car etc.

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Class … cont

One important feature of class is Data hiding or Encapsulation

A class wraps data and functions into a single unit

Thus data is typically not accessible to outside world

Member functions provide an interface between object’s data and program

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Class … cont

Object

User

of objects

Mem

ber

Fu

nct

ion

s

Member

Variables

( Data )

• An Interface

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Class … cont

Syntaxclass class-name {

access-specifier : data and functionsaccess-specifier : data and functionsaccess-specifier : data and functions

…} ;

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Class … cont

Access – specifier private : Private to class public : Accessible to other parts of program protected : Needed in inheritance (later)

By default, all members are private

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Class … cont

E.g. of a class named Date class Date {

public : // public member functions Date (int month=1, int day=1, int year=2003); void Print ( void ) const;

private : // private data members int m_month; int m_day; int m_year;

};

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Class … cont

Defining constructor & member functions

Date :: Date (int month, int day, int year){

m_month = month;m_day = day;m_year = year;

}

void Date :: Print ( void ) const{ cout << month << “-” << day << “-” << year;}

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Class … cont

In C++, you can add new features to existing structure of object / class

Suppose we need to add a member function called NextDay NextDay increments day by 1 Where do we make changes in the class ?

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Class … cont

class Date {

public : // public member functions Date (int month=1, int day=1, int year=2003); void Print ( void ) const;

void NextDay( void );

private : // private data membersint m_month;int m_day;int m_year;

};

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Class … cont

Defining NextDay member function

void Date :: NextDay ( void )

{

.. // logic to increment date

.. // Also need to consider incrementing to next // month and next year

}

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Class … cont

Declaring an object as data member of another class A class is a data-type, a class can use an

object as a data member This is known as aggregation or composition Consider adding a Time object in our Date

class Exercise 6.10 on page 466

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Class … cont

class Time {public :

// Constructor and all member function will come // here

// Function to increment time by one secondvoid Increment ( void );

private :int m_hour; int m_minute; int m_second;

};

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Class … cont

class Date {

public :// Constructor and all member function will come // here

// Add member function Tick which calls Incrementvoid Tick ( void );

private : Time currentTime; // Declare Time object in Date// other data members

};

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Class … cont

How is the Tick() member function defined ?void Date :: Tick ( void )

{

.. // logic to increment date

// This is how we make use of Increment function of Time class

currentTime.Increment();}

But you still cannot access private members of Time class from Date class - Encapsulation

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Class … cont

m_month

m_day

m_year

m_hour

m_minute

m_second

Date Object

Time Object

User

Mem

ber

Fun

ctio

ns

Mem

ber

Fun

c

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Project 2

Objective To implement and use objects To understand aggregation To practice formatted output, makefiles

Given a quadrilateral, it can be a Parallelogram Rectangle Square Trapezoid Irregular

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Project 2 … cont

Classes A Point class

Encapsulates x and y co-ordinates of a point

A Quadrilateral class Encapsulates a quadrilateral Uses Point class for each of its four corners The corners are referred as ‘UpperRight’,

‘UpperLeft’, ‘LowerLeft’ and ‘LowerRight’

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Project 2 … cont

What you need to do ? Accept x and y co-ordinates of four corners Classify the quadrilateral as one of the five

types Compute and display its area and perimeter

Assume Co-ordinate values are integers Base is parallel to x axis Area of irregular quadrilateral is 0.0000

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Project 2 … cont

Sample run (taken from Project 2 description)linux3[3]% Proj2 This is my optional greeting Please enter x- and y-coordinates of the UpperLeft corner: 6 4 Please enter x- and y-coordinates of the LowerLeft corner: 9 1 Please enter x- and y-coordinates of the LowerRight corner: 16 1 Please enter x- and y-coordinates of the UpperRight corner: 13 4 The Quadrilateral's Corners

Upper Left: ( 6, 4) Upper Right: (13, 4) Lower Left: ( 9, 1) Lower Right: (16, 1)

This Quadrilateral is a Parallelogram Area: 21.0000 Perimeter: 22.4853

linux3[4]%