Object oriented programming with C++ - Session 1 - Basic Object Oriented Concepts doc

Session Objectives Contd. Compare Classes with Structures  Describe Private and Public sections of Classes...  Define Member functions  Use the Objects and Member functions of a C

Basic Object Oriented

Concepts

Session 1

Session Objectives

 Discuss the following:

• The Object-Oriented approach

• Drawbacks of traditional programming

Session Objectives (Contd.)

 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

SalesPersonnel Accounts

Traditional programming:

Drawbacks

• 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

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

Data and Functions of an

 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

Different Objects

Name: Jack Age: 28 Weight: 65 kgs

Model: Ferrari Colour: Red Year: 1995

Actions:

Walk Talk Sleep

Actions:

Start Stop Accelerate

 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

 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

 An action required of an object or entity when represented in a class

is called a method

"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 departmental memos or verbal instructions are the messages between objects

inter-• 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

• 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

 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

 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

 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

Make sounds Eat/drink Hunt prey

Cats Actions:

Mew Drink milk Hunt mice

Roar Eat flesh Hunt big game

 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 promotes encapsulation The Draw

method is implemented for different cases but how it

Class:

Shape

Methods:

Draw Move Initialise

Subclasses

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

Private and Public (Contd.)

 Private data is not available outside the class

Class

Data or functions

Data or functions

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

Using the class (Contd.)

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

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 200

object_data 200

object1

object_data 350

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)

Passing Objects (Contd.)

using a function call such as,

int variable =

object1.function1(object2);

actual argument

it is given access to the object of

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.)

but it sacrifices some flexibility in

order to remain efficient

Provides high run-time efficiency

and small code size, but it trades

off some of the power to reuse

classes

 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

 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