CS300: Object Oriented Analysis and Design

Classes and Objects

(Using C++)

Case study-Vending Machine

Vending Machine that allows users to buy snack items. In addition, a user can find out the caloric content of her choice.

Object Oriented Anslysis and Design (CS 212)

Specification

A Vending machine holds a number of snack items and displays the list of snack items and their prices through an user interface with a display screen and buttons for making selections. In addition, the vending machine has a receptacle for money and an item dispenser.

​

A user can make a selection and query for the number of calories of a snack item. The calories are displayed on pressing a button. A user can place the money in the receptacle and select an item.

​

Object Oriented Anslysis and Design (CS 212)

Step 1

A Vending machine holds a number of snack items and displays the list of snack items and their prices through an user interface with a display screen and buttons for making selections. In addition, the vending machine has a receptacle for money and an item dispenser.

​

A user can make a selection and query for the number of calories of a snack item. The calories are displayed on pressing a button. A user can place the money in the receptacle and select an item.

Object Oriented Anslysis and Design (CS 212)

Step 1

A Vending machine holds a number of snack items and displays the list of snack items and their prices through an user interface with a display screen and buttons for making selections. In addition, the vending machine has a receptacle for money and an item dispenser.

​

A user can make a selection and query for the number of calories of a snack item. The calories are displayed on pressing a button. A user can place the money in the receptacle and select an item.

Object Oriented Anslysis and Design (CS 212)

Example-Vending Machine

• Most of the nouns are objects/classes.
• Some nouns are attributes of these classes.
• The verbs are actions that can be attached to these objects.
• In order to focus on the problem-domain objects, let us separate the object/classes into presentation-specific (user-interface related) and problem-specific classes.

Write the CRC cards for wo problem specific classes.

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Object Oriented Anslysis and Design (CS 212)

Example-Vending Machine

Problem-specific

• Vending Machine
• Snack item
• Price
• Calories
• Selection
• User

Presentation-specific

• Display screen
• Selection Buttons
• Item Dispenser
• Money receptacle

Object Oriented Anslysis and Design (CS 212)

Two Programming Paradigms

Structural (Procedural) PROGRAM

Function calls

Messages passing

Object-Oriented PROGRAM

Object Oriented Anslysis and Design (CS 212)

Classes & Objects

• The class is the cornerstone of C++
• It gives the C++ its identity from C
• It makes possible encapsulation, data hiding and inheritance
• Class:
• Consists of both data and methods
• Defines properties and behavior of a set of entities
• Object:
• An instance of a class
• A variable identified by a unique name

Object Oriented Anslysis and Design (CS 212)

Classes & Objects

class Rectangle

{

private:

int width;

int length;

public:

void set(int w, int l);

int area();

}

Rectangle r1;

Rectangle r2;

Rectangle r3;

​

……

​

int a;

Object Oriented Anslysis and Design (CS 212)

Define a Class Type

​

class class_name

{

permission_label:

member;

permission_label:

member;

...

};

class Rectangle

{

private:

int width;

int length;

public:

void set(int w, int l);

int area();

};

Object Oriented Anslysis and Design (CS 212)

Class Definition-Data Members

• Abstract the common attributes of a group of entities, their values determine the state of an object
• Can be of any type, built-in or user-defined
• non-static data member
• Each class object has its own copy
• Cannot be initialized explicitly in the class body
• Can be initialized with member function, or class constructor
• static data member
• Acts as a global object, part of a class, not part of an object of that class
• One copy per class type, not one copy per object

Object Oriented Anslysis and Design (CS 212)

Static Data Member

class Rectangle

{

private:

int width;

int length;

static int count;

public:

void set(int w, int l);

int area();

}

Rectangle r1;

Rectangle r2;

Rectangle r3;

​

width

length

width

length

width

length

r1

r3

r2

count

Object Oriented Anslysis and Design (CS 212)

Memory Allocation for Objects

Memory created when functions defined

Memory created when objects defined

Object Oriented Anslysis and Design (CS 212)

Class Definition – Member Functions

• Used to
• access the values of the data members (accessor)
• perform operations on the data members (implementor)

​

• Are declared inside the class body, in the same way as declaring a function

​

• Their definition can be placed inside the class body, or outside the class body

​

• Can access both public and private members of the class
• Can be referred to using dot or arrow member access operator

Object Oriented Anslysis and Design (CS 212)

Member Function (Definition)

class Rectangle {

private:

int width, length;

public:

void set (int w, int l);

int area() {return width*length; }

}

void Rectangle :: set (int w, int l) {

width = w;

length = l;

}

​

r1.set(5,8);

​

rp->set(8,10);

​

Object Oriented Anslysis and Design (CS 212)

Member Functions

• static member function
• const member function
• declaration
• return_type func_name (para_list) const;
• definition
• return_type func_name (para_list) const { … }
• return_type class_name :: func_name (para_list) const { … }
• Makes no modification about the data members
• Ensures safety of the member variables

​

Object Oriented Anslysis and Design (CS 212)

Member Function: Can be constant also

​

class Time

{

private :

int hrs, mins, secs ;

​

public :

​

void Write ( ) const ;

​

} ;

void Time :: Write( ) const

{

cout <<hrs << “:” << mins << “:” << secs << endl;

}

function declaration

function definition

Object Oriented Anslysis and Design (CS 212)

Class Definition: Access Control

• Information hiding
• To prevent the direct access from outside the class
• Access Specifiers (optional, repeatable, not ordered)
• Public (Class Interface)
• may be accessible from anywhere within a program
• Private (default access specifier)
• may be accessed only by the member functions, and friends of this class, not open for nonmember functions
• protected
• acts as public for derived classes (virtual)
• behaves as private for the rest of the program
• Difference between classes and structs in C++

Object Oriented Anslysis and Design (CS 212)

Example: Time Class

class Time {

public :

void Set ( int hours , int minutes , int seconds ) ;

void Increment ( ) ;

void Write ( ) const ;

Time ( int initHrs, int initMins, int initSecs ) ; // constructor

Time ( ) ; // default constructor

private :

int hrs ;

int mins ;

int secs ;

} ;

Object Oriented Anslysis and Design (CS 212)

What is an object?

OBJECT (Instance of a class)

Operations

​

Data

set of methods

(public member functions)

​

​

internal state

(values of private data members)

Object Oriented Anslysis and Design (CS 212)

Getters and Setters

• Private access specifier facilitates data hiding

​

• Public set and get function allows clients to access data, but not indirectly

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• Class may store data in one way, however shows to the client in a different way

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• Get and set function helps the client to interact with the object

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• The private data member remains safely encapsulated

Object Oriented Anslysis and Design (CS 212)

Class Interface

• Header files supports reusability

​

• Interface: standardized way of interaction between a pair of objects (e.g. people, systems)

​

• Defines what services a client can use and how to request those services

​

• However, NOT how the class carry out those services

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• A classes interface consists of the public member functions

​

​

Object Oriented Anslysis and Design (CS 212)

Class Interface Diagram

Private data:

​

hrs

​

mins

​

secs

Set

Increment

Write

Time

Time

Time class

Object Oriented Anslysis and Design (CS 212)

Separating Interface from Implementation

• It is better software engineering to define member functions outside the class definition
• Their implementation can be hidden from the clients code
• Client code independent of class’s implementation

​

Object Oriented Anslysis and Design (CS 212)

In a nutshell

• Class can be considered as a user-defined data type

​

• An object is just a variable of certain class.

​

• There are three parts in the definition of a class:
• data members,
• member functions, and
• access control.

Object Oriented Anslysis and Design (CS 212)