object oriented programming with c++/ session 1/ 1 of 50 basic object oriented concepts session 1

Object Oriented Programming with C++/ Session 1/ 1 of 50

Basic Object Oriented Concepts

Session 1


Session Objectives

Discuss the following:

• The Object-Oriented approach

• Drawbacks of traditional programming

• Object-Oriented programming

Discuss basic Object-Oriented concepts such as:

• Objects

• Classes

• Properties


Session Objectives (Contd.)

• Methods

• Abstraction

• Inheritance

• Encapsulation

• Polymorphism

Compare Classes with Structures Describe Private and Public sections

of Classes


Session Objectives (Contd.)

Define Member functions Use the Objects and Member

functions of a Class

• Define Objects

• Access Member Functions

• Pass and return Objects

Discuss briefly the features of C++ and another OO language (Smalltalk)


The Object-Oriented approach

Can classify living beings as objects just as we classify things we use as objects of different types.

Any application can be defined in terms of entities or objects so that the process replicates human thought process as closely as possible.


Departments as objects in an organisation

People in each department control and

operate on that department's data.

Sales

Personnel Accounts


Traditional programming: Drawbacks

Unmanageable programs

• Traditional programs: List of instructions written in a language that tells the computer to perform some actions.

• When programs become larger they become unmanageable.

• Functions/procedures/subroutines adopted to make programs more comprehensible.

• As programs grow larger and more complex, even use of functions can create problems.


Problems in modification of data

Data plays a crucial role in traditional programming techniques.

Adding new data items means modifying all the tasks or functions that access the data. • As program size increases it is difficult to locate

and to modify all functions which use the data

Restrict access to the data so that only a few critical functions act upon it.

Next to impossible to separate parts of a program from revisions made in another part.


Difficulty in implementation

Focus of traditional programming approach: On implementation details.

Focus of a person’s thinking: In terms of things or entities, their properties and actions.

OOP techniques help to correspond real-world entities and actions to functions and data at the programming level.


Introduction to OOP

OOP allows for analysis and design of an application in terms of entities or objects.

Process replicates the human thought process as closely as possible.

Code and data are merged into a single indivisible thing -- an object.

Close match between objects in the programming sense and objects in the real world.


Data and Functions of an Object

AccountsData:Number of employeesSalary statementsBillsVouchersReceiptsPetty cash recordsBanking data

Functions:Calculate salaryPay salary Pay billsTally accountsTransact with banks


Objects

Represent an entity in the real world. A concept or thing with defined

boundaries that is relevant to the problem we are dealing with.

Objects serve two purposes:

• They help to understand the real world

• They provide a practical basis for a computer application


Objects (Contd.)

Each object has its own properties or characteristics that describe what it is or does. • Vehicles in a traffic-monitoring application

• Menus

• The mouse and the keyboard

• A personnel file

• A table of marks relating to an examination

• Time

• Complex numbers


Different Objects

Name: JackAge: 28Weight: 65 kgs

Model: FerrariColour: RedYear: 1995

Actions:WalkTalkSleep

Actions:StartStopAccelerate


Classes

Grouping of objects that have the same properties, common behaviour and common relationships.

The term class is an abbreviation of “class of objects”.

• Example, A class of persons, class of animals, class of processes.

Each object is said to be an instance of its class.


Objects and Classes

Abstract

into

Polygon class

Properties: Vertices Border colour Fill colour

Methods: Draw Erase Move

Polygon objects


Property/Attribute

A characteristic required of an object or entity when represented in a class is called a property.

A class is a prototype and not an actual specimen of the entity.

Each instance of the class or object has its own value for each of its properties but it shares the property names or operations with other instances of the class.


Method

An action required of an object or entity when represented in a class is called a method.

• In a polygon class "draw", "erase" and "move" are examples of the methods that are part of the class.

Object is a "black box" which receives and sends messages.


Method (Contd.)

• The black box actually contains code (sequences of computer instructions) and data (information which the instructions operates on).

• Information passed to and retrieved from each department either by inter-departmental memos or verbal instructions are the messages between objects.

• These messages can be translated to function calls in a program.

Sales

Accounts

What is the salary of Jack?

What is the salary of Jack?

Jack's salary is $2000

Jack's salary is $2000


Abstraction

Process of examining certain aspects of a problem.

An abstract specification tells us what an object does independent of how it works.

Data Abstraction

• Process of examining all the available information about an entity to identify information that is relevant to the application.


Data Abstraction

Used to identify properties and methods of each object as relevant to the application at hand.

By grouping objects into classes, we are doing data abstraction of a problem.

Common definitions are stored once per class rather than once per instance of the class.

Methods can be written once for a class, so that all the objects in a class benefit from code reuse.


Inheritance

It is the property that allows the reuse of an existing class to build a new class.

• A class of animals can be divided into mammals, amphibians, insects, reptiles, etc.

The superclass is the class from which another class inherits its behaviour.

The class that inherits the properties and methods of another class is called the subclass.


Inheritance (Contd.)

Each subclass shares common properties with the class from which it is derived.

• For example, all vehicles in a class may share similar properties of having wheels and a motor

Subclass may have its own particular characteristics.

• For example, a bus may have seats for people, while trucks have space for carrying goods.


Class Animals and its subclasses

Animals

Insects Mammals Reptiles Amphibians


Encapsulation

Providing access to an object only through its messages, while keeping the details private is called information hiding. An equivalent buzzword is encapsulation.

• A class has many properties and methods. It is not necessary for a user to have access to all of them.

• All communication to an object is done via messages. Messages define the interface to the object.


Encapsulation (Contd.)

When we properly encapsulate some code we achieve two objectives:

• We build an impenetrable wall to protect the code from accidental corruption due to the silly little errors that we are all prone to make.

• We also isolate errors to small sections of code to make them easier to find and fix.


Reusability

Object-oriented development allows:

• Information to be shared within an application

• Reuse of designs and code on future projects

Programs are broken down into reusable objects. These objects can then be grouped together in different ways to form new programs.

Inheritance also promotes reuse.


Felines and Subclasses

Felines Actions:Make soundsEat/drinkHunt prey

Cats Actions: Mew Drink milk Hunt mice

Lions Actions: Roar Eat flesh Hunt big game


Polymorphism

Polymorphism means that the same functions may behave differently on different classes.

The existing object stays the same, and any changes made are only additions to it.


Polymorphism

Polymorphism promotes encapsulation. The Draw method is implemented for different cases but how it is to be implemented need not be visible to the user.

Class:ShapeMethods:DrawMoveInitialise

Subclasses


Class Construct

The general syntax of the class construct is:

class user_defined name{

private:

data_type members;

implementation operations;

public:

data_type members;

implementation operations;

};


Classes and Structures

A structure contains one or more data items (called members) which are grouped together as a single unit.

Class consists of not only data elements but also functions, which operate on the data elements.

All the variables and the functions declared in a class, whether in the public or private section, are the members of the class.

In a structure all elements are public by default.


Public and Private

Class members can be declared in the public or the private sections of a class.

To prevent data from unrestricted access, the data members of a class are normally declared in the private section.

The public part constitutes the interface to objects of the class.

The data members and functions declared in the public section, can be accessed by any function in the outside world (outside of the class).


Private and Public (Contd.)

Private data is not available outside the class to any program

Also, private data of other classes is hidden and not available within the functions of this class.

Class

Data or functions

Data or functions

Private

Public

Not accessible from outside class

Accessible from outside class


Member functions

In C++, a function declared as a member of a class is called a member function.

• Member functions are usually put in the public part of the class because they have to be called outside the class either in a program or in a function.

Declaration of a member function must define its return value as well as the list of its arguments.

• For example, the member function

void setdate(int, int, int) has no return value or a void return value, and has three integer arguments.


Member functions (Contd.)

Member functions can be more complex as they can have local variable, parameters etc.

Should not have the same name as a data member.


Using the class

begin program

class exampleclass{ // specify a class

private:

object_data is an integer; // class data

public:

member_function1(parameter1, parameter2…)

{assign value to object_data}

member_function2(){display data}

};


Using the class (Contd.)

main program{

//define the objects of class exampleclass exampleclass object1,object2;

//call member fn to assign value 200

//to object_data

object1.member_function1(200);

//call member function to display data

object1.member_function2();

object2.member_function1(350);


}


Defining Objects

exampleclass object1,object2; defines two objects, object1 and object2, of class exampleclass.

The definition actually creates objects that can be used by the program.

When we define an object, space is set aside for it in memory.


Calling member functions

We communicate with objects by calling their member functions. object1.member_function1(200);


A member function is always called to act on a specific object, not on the class in general.

Associated with a specific object with the dot operator ( the period).


Calling member functions

The general syntax for accessing a member function of a class is

class_object.function_member()

The dot operator is called the class member operator.


Two objects with different values

exampleclass class specifier

object_data 200object_data 200

object1

object_data 350

object2

Objects of the class exampleclass

object_data

Specifications for exampleclass objects


Passing Objects

Objects can be passed to a function and returned back just like normal variables.

The compiler creates another object as a formal variable in the called function. It copies all the data members from the actual object.

int function1(exampleclass obj1)

{ return object_data;} Formal argument


Passing Objects (Contd.)

The above function can be invoked using a function call such as,

int variable = object1.function1(object2);

actual argument

When a member function is called, it is given access to the object of which the function is a member.


Returning Objects

A return statement in a function is considered to initialise a variable of the returned type.

exampleclass object3 = object1.function1(object2);

Passing and returning of objects is not very efficient since it involves passing and returning a copy of the data members.


Object-Oriented Languages

C++ , Smalltalk , Eiffel, CLOS, Java Vary in their support of object-

oriented concepts. No single language that matches

all styles and meets all needs.


An introduction to C++

Developed by Bjarne Stroustrup at AT&T Bell Laboratories.

Derived from the C language. It is compatible with C (it is actually

a superset). Most important elements added to C to create C++ are concerned with classes, objects and object-oriented programming.


An introduction to C++ (Contd.)

C++ programs are fast and efficient but it sacrifices some flexibility in order to remain efficient.

C++ uses compile-time binding. Provides high run-time efficiency and small code size, but it trades off some of the power to reuse classes.


Smalltalk

Pure object-oriented language. While C++ makes some practical

compromises to ensure fast execution and small code size, Smalltalk makes none.

Uses run-time binding, which means that nothing about the type of an object need be known before a Smalltalk program is run.

Significantly faster to develop than C++ programs.


Smalltalk

Has a rich class library that can be easily reused via inheritance

Has a dynamic development environment.

It is not explicitly compiled, like C++. Smalltalk generally takes longer to

master than C++. It is syntactically very simple.

object oriented programming with c++/ session 1/ 1 of 50 basic object oriented concepts session 1

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