ch06 objects and classes

8/6/2019 Ch06 Objects and Classes

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INTRODUCTION

Previously, programmers starting a project would sitPreviously, programmers starting a project would sitdown almost immediately and start writing code.down almost immediately and start writing code.

As programming projects became large and moreAs programming projects became large and more

complicated, it was found that this approach did notcomplicated, it was found that this approach did not

work very well. The problem was complexity.work very well. The problem was complexity.

Large programs are probably the most complicatedLarge programs are probably the most complicated

entities ever created by humans.entities ever created by humans.

Because of this complexity, programs are prone to error,Because of this complexity, programs are prone to error,

and software errors can be expensive and even lifeand software errors can be expensive and even life

threatening (in air traffic control, for example).threatening (in air traffic control, for example).

Three major innovations in programming have beenThree major innovations in programming have been

devised to cope with the problem of complexity. They aredevised to cope with the problem of complexity. They are2


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INTRODUCTION

Object

Object--oriented programming (

OOP)oriented programming (

OOP)

The Unified Modeling Language (UML)The Unified Modeling Language (UML)

Improved software development processesImproved software development processes

You will be taught the C++ language with these developYou will be taught the C++ language with these develop--

mentsments in mind.in mind. You will not only learn a computer language, but newYou will not only learn a computer language, but new

ways of conceptualizing software development.ways of conceptualizing software development.

Other famousObject Oriented languages are JAVA, C# (COther famousObject Oriented languages are JAVA, C# (C

CharpCharp), VB.NET (Visual Basic Dot Net).), VB.NET (Visual Basic Dot Net).

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OBJECT ORIENTED PROGRAMMING

Object Oriented Programming offers a new and powerfulObject Oriented Programming offers a new and powerfulway to cope with complexity.way to cope with complexity.

Instead of viewing a program as a series of steps to beInstead of viewing a program as a series of steps to be

carried out, it views it as a group of objects that havecarried out, it views it as a group of objects that have

certain properties and can take certain actions.certain properties and can take certain actions.

This may sound obscure until you learn more about it,This may sound obscure until you learn more about it,

but it results in programs that are clearer, more reliable,but it results in programs that are clearer, more reliable,

and more easily maintained.and more easily maintained.

A major goal of this course is to teach objectA major goal of this course is to teach object--orientedoriented

programming using C++ and cover all its major features.programming using C++ and cover all its major features.

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THE UNIFIED MODELING LANGUAGE

The Unified Modeling Language (UML) is a graphicalThe Unified Modeling Language (UML) is a graphicallanguage consisting of many kinds of diagrams.language consisting of many kinds of diagrams.

It helps program analysts figure out what a programIt helps program analysts figure out what a program

should do, and helps programmers design andshould do, and helps programmers design and

understand how a program works.understand how a program works.

The UML is a powerful tool that can make programmingThe UML is a powerful tool that can make programming

easier and more effective.easier and more effective.

We introduce each UML feature where it will help toWe introduce each UML feature where it will help to

clarify the OOP topic being discussed.clarify the OOP topic being discussed.

In this way you learn the UML painlessly at the same timeIn this way you learn the UML painlessly at the same time

the UML helps you to learn C++.the UML helps you to learn C++.

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CLASS

AA classclass serves as a plan, or template. It specifies whatserves as a plan, or template. It specifies whatdata and what functions will be included indata and what functions will be included in objectsobjects ofof

thatthat classclass..

Defining theDefining the classclass doesnt create anydoesnt create any objectsobjects, just as the, just as the

typetype intintdoesnt create any variables.doesnt create any variables.

AA classclass is thus a description of a number of similaris thus a description of a number of similar

objects.objects.

PrincePrince,, StingSting, and, and MadonnaMadonna areare membersmembers of the class ofof the class of

rock musiciansrock musicians..

There is no one person called There is no one person called rock musicianrock musician but specific but specific

people with specific names are members of this class ifpeople with specific names are members of this class if

they possess certainthey possess certain characteristicscharacteristics.. 6


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CLASS

Picture a programming objectPicture a programming objectjust like any normal object injust like any normal object in

the real world.the real world.

Each real world object has itsEach real world object has its

own properties and specificown properties and specificthings that you can do with it.things that you can do with it.

For example, a bow has specificFor example, a bow has specific

properties such as color,properties such as color,

number of arrows, and weightnumber of arrows, and weight

and specific capabilities such asand specific capabilities such as

the ability to fire .the ability to fire .

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CLASS

two Bow objects are declared using the Bow class, thetwo Bow objects are declared using the Bow class, the

Bow class describes the properties of a bow, each of theBow class describes the properties of a bow, each of the

two Bow objects has distinct values for these propertiestwo Bow objects has distinct values for these properties 8


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A SIMPLE CLASS

// smallobj.cpp demonstrates a small, simple object#include

using namespace std;

class smallobj // define a class (a concept)

{

private:

int somedata; // class data (attributes)

public:

voidsetdata(int d){ // member function (method)to

somedata = d; // set data

}

voidshowdata(){ // member function to display data

cout


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A SIMPLE CLASS

intmain(){

smallobj s1, s2; // define two objects of class smallobj

s1.setdata(1066); // call member function to set data

s2.setdata(1776);

s1.showdata(); // call member function to display data

s2.showdata();

system("PAUSE");

return 0;

}

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CLASSES AND OBJECTS

AnAn objectobjecthas the same relationship to ahas the same relationship to a classclass that athat avariable has to a data type.variable has to a data type.

AnAn objectobjectis said to be anis said to be an instanceinstance of a class, in the sameof a class, in the same

way my 1954 Chevrolet is anway my 1954 Chevrolet is an instanceinstance of a vehicle.of a vehicle.

In the program theIn the program the classclass , whose name is, whose name is smallobjsmallobj, is, is

defined in the first part of the program.defined in the first part of the program.

In main(), we define twoIn main(), we define two objectsobjects s1 and s2 that ares1 and s2 that are

instancesinstances of that class.of that class.

Each of the twoEach of the two objectsobjects is given a value, and each displaysis given a value, and each displays

its value.its value.

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DEFINING THE CLASS

Syntax for class definitionSyntax for class definition12


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DEFINING THE CLASS

The definition starts with the keyword class, followed byThe definition starts with the keyword class, followed bythe class namethe class namesmallobjsmallobj in this Example.in this Example.

The body of the class is delimited by braces andThe body of the class is delimited by braces and

terminated by a semicolon.terminated by a semicolon.

AnAn objectobjecthas the same relationship to ahas the same relationship to a classclass that athat avariable has to a data type.variable has to a data type.

A key feature of objectA key feature of object--oriented programming is dataoriented programming is data

hiding. it means that data is concealed within a class sohiding. it means that data is concealed within a class so

that it cannot be accessed mistakenly by functionsthat it cannot be accessed mistakenly by functions

outside the class.outside the class.

The primary mechanism for hiding data is to put it in aThe primary mechanism for hiding data is to put it in a

class and make itclass and make it privateprivate.. 13


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DEFINING THE CLASS

PrivatePrivate data or functions can only be accessed fromdata or functions can only be accessed fromwithin the class.within the class.

PublicPublic data or functions, on the other hand, aredata or functions, on the other hand, are

accessible from outside the class.accessible from outside the class.

The data memberThe data member somedatasomedata follows the keywordfollows the keywordprivateprivate,,so it can be accessed from within theso it can be accessed from within the classclass, but not from, but not from

outside.outside.

Member functionsMember functions (also called(also called methodsmethods oror messagesmessages) are) are

functions that are included within a class.functions that are included within a class.

There are two member functions inThere are two member functions in smallobjsmallobj:: setdatasetdata()()

andand showdatashowdata().().

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DEFINING THE CLASS

BecauseBecause setdatasetdata()()andand showdatashowdata()()

follow the keywordfollow the keyword

publicpublic, they can be, they can be

accessed fromaccessed fromoutside the class.outside the class.

In a class theIn a class the

functions do notfunctions do not

occupy memoryoccupy memoryuntil an object ofuntil an object of

the class is created.the class is created.

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DEFINING THE CLASS

Remember that the definition of theRemember that the definition of the classclass smallobjsmallobj doesdoesnot create anynot create any objectsobjects. It only describes how they will. It only describes how they will

look when they are created.look when they are created.

Defining objectsDefining objects means creating them. This is also calledmeans creating them. This is also called

instantiatinginstantiating them, because anthem, because an instanceinstance of theof the classclass isiscreated. An object is ancreated. An object is an instanceinstance of a class.of a class.

s1.setdata(1066);s1.setdata(1066); This syntax is used to call aThis syntax is used to call a membermember

functionfunction that is associated with a specific object s1.that is associated with a specific object s1.

Member functionsMember functions of aof a classclass can be accessed only by ancan be accessed only by an

objectobjectof thatof that classclass..

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WIDGET PARTS AS OBJECTS

(1/2)

// objpart.cpp widget part as an object#include


classpart{ // define class

private:

intmodelnumber; // ID number of widget

intpartnumber; // ID number of widget part

float cost; // cost of part

public:

voidsetpart(intmn, intpn, float c){ // set data

modelnumber = mn;

partnumber = pn;

cost = c;

}

voidshowpart(){ // display data

cout


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WIDGET PARTS AS OBJECTS

(1/2)

cout


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CIRCLES AS OBJECTS

// circles.cpp circles as graphics objects#include "msoftcon.h" // for graphics functions

class circle{ // graphics circle

protected:

int xCo, yCo; // coordinates of center

int radius;

color fillcolor; // color

fstyle fillstyle; // fill pattern

public: // sets circle attributes

voidset(int x, int y, int r, color fc, fstyle fs){

xCo = x; yCo = y; radius = r;

fillcolor = fc; fillstyle = fs;

}

voiddraw(){ // draws the circle

set_color(fillcolor); // set color

set_fill_style(fillstyle); // set fill

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CIRCLES AS OBJECTS

draw_circle(xCo, yCo, radius);// draw solid circle}

};

intmain(){

init_graphics(); // initialize graphics system

circle c1, c2, c3; // create circles

//set circle attributes

c1.set(15, 7, 5, cBLUE, X_FILL);

c2.set(41, 12, 7, cRED, O_FILL);

c3.set(65, 18, 4, cGREEN, MEDIUM_FILL);

c1.draw(); //draw circles

c2.draw();

c3.draw();

set_cursor_pos(1, 25); // lower left corner

system("PAUSE");

return 0;

} 20


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USING CLASS TO REPRESENT

DISTANCES

// englobj.cpp objects using English measurements#include


class Distance{ // English Distance class

private:

int feet;

float inches;

public:

voidsetdist(int ft, float in) // set Distance to args

{ feet = ft; inches = in; }

voidgetdist(){ // get length from user

cout > feet;

cout > inches;

}

voidshowdist() // display distance

{ cout


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USING CLASS TO REPRESENT

DISTANCES

intmain(){

Distance dist1, dist2; // define two lengths

dist1.setdist(11, 6.25); // set dist1

dist2.getdist(); // get dist2 from user

// display lengths

cout

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