objects and classes

5/27/2018 Objects and Classes

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Introduction

Previously, programmers starting a project would sit

down almost immediately and start writing code.

As programming projects became large and more

complicated, it was found that this approach did not

work very well. The problem was complexity. Large programs are probably the most complicated

entities ever created by humans.

Because of this complexity, programs are prone to error,

and software errors can be expensive and even life

threatening (in air traffic control, for example).

Three major innovations in programming have been

devised to cope with the problem of complexity. 2


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Object Oriented Programming

Object Oriented Programming offers a new and powerful

way to cope with complexity.

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

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

certain properties and can take certain actions. This may sound obscure until you learn more about it,

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

and more easily maintained.

A major goal of this course is to teach object-oriented

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

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The Unified Modeling Language

The Unified Modeling Language (UML) is a graphical

language consisting of many kinds of diagrams.

It helps program analysts figure out what a program

should do, and helps programmers design and

understand how a program works. The UML is a powerful tool that can make programming

easier and more effective.

We introduce each UML feature where it will help to

clarify the OOP topic being discussed.

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

the UML helps you to learn C++.

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Class

A classserves as a plan, or template. It specifies what

data and what functions will be included in objectsof

that class.

Defining the classdoesnt create any objects, just as the

type intdoesnt create any variables. A classis thus a description of a number of similar

objects.

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#include

#includeusingnamespacestd;

classstudent // define a class (a concept){

private:intage; // class data (attributes)

public:voidset_age(intg) // member function (method){ // to set ageage = g;

}voidshow_age() // member function to display age{cout


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intmain()

{ student Ali, Usman; // define two objects of class studentAli.set_age(21); // call member function to set ageUsman.set_age(18);

Ali.show_age(); // call member function to display ageUsman.show_age();

system("PAUSE");return0;

}

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A Simple Class (2/2)


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An objecthas the same relationship to a classthat a

variable has to a data type.

An objectis said to be an instanceof a class, in the same

way my Corolla is an instanceof a vehicle.

In the program the class, whose name is student, isdefined in the first part of the program.

In main(), we define two objectsAli and Usman that are

instancesof that class.

Each of the two objectsis given a value, and each displays

its value.

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Classes and Objects


9/70Syntax for class definition 9

Defining the Class


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Defining the Class

The definition starts with the keyword class, followed by

the class namestudentin this Example.

The body of the class is delimited by braces and

terminated by a semicolon.

An objecthas the same relationship to a classthat avariable has to a data type.

A key feature of object-oriented programming is data

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

that it cannot be accessed mistakenly by functions

outside the class.

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

class and make itprivate. 10


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Defining the Class

Privatedata or functions can only be accessed from

within the class.

Publicdata or functions, on the other hand, are

accessible from outside the class.

The data member agefollows the keywordprivate, so itcan be accessed from within the class, but not from

outside.

Member functions (also called methodsor messages) are

functions that are included within a class.

There are two member functions in studentclass:

set_age() and show_age().

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Defining the Class

Because

set_age()and

show_age()

follow the keyword

public, they can

be accessed from

outside the class.

In a class the functions

do not occupy memory until an object of the class is

created.

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Defining the Class

Remember that the definition of the classstudent

does not create any objects. It only describes how they

will look when they are created.

Defining objects means creating them. This is also called

instantiatingthem, because an instanceof the classiscreated. An object is an instanceof a class.

ali.set_age(21);This syntax is used to call amember functionthat is associated with a specific object

ali.Member functions of a classcan be accessed only by an

objectof that class.

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// mobile.cpp demonstrates mobile phone as an object

#include#include

usingnamespacestd;classmobile // class name should be the name of a concept{

private:

string company_name;string model_number;

string emi_number;

floatcost;

public:

voidset_info(string cn,string mn, string emi, floatprice){ // set data

company_name = cn;

model_number = mn;

emi_number = emi;

cost = price;

} 14

Using Mobile Phone as an Object (1/2)


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voidshow_info()

{ // display datacout


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If you were designing an inventory program you might

actually want to create a class something like mobile.

Its an example of a C++ object representing a physical

object in the real worlda mobile phone.

Standard C++ includes a new class called string. Using stringobject, you no longer need to worry about

creating an array of the right size to hold string variables.

The string class assumes all the responsibility for memory

management.

Using Mobile Phone as an Object

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#include


classHeight{ // A Height class

private:

intfeet;

floatinches;

public:voidset(intft, floatin){

feet = ft; inches = in;

}

voidget(){ // get height information from user

cout


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};

intmain()

{

Height H1, H2; // define two height objects

H1.set(4, 9.35F); // set feet = 4 and inches = 9.35 in H1 object

H2.get(); // get feet and inches info from user for H2

// display height information from H1 and H2 Objects

cout


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Constructors

We see that member functionscan be used to give values

to the data items in an object.

Sometimes, however, its convenient if an object can

initializeitself when its first created, without requiring a

separate call to a member function. Automatic initialization is carried out using a special

member function called a constructor.

A constructoris a member function that is executed

automatically whenever an object is created.

Constructor has the same name of class and it has no

return type.

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// A Constructor Example


classHouse{

private:

doublearea; // area in square feet

public:

House() : area(1000){ // a zero (no) argument constructorcout area;

}

}; 20

Constructors: House Example (1/2)


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intmain()

{ House H1, H2; // define objects and calls constructor

cout


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#include // for cin and cout

#include // for rand(), srand() and system()#include // for time()

usingnamespacestd;

classDice{

private:

intnumber;

public:Dice():number(1){ // zero (no) argument constructor

// Sets the default number rolled by a dice to 1

// seed random number generator with current system time

srand(time(0));

}

introll() //Function to roll a dice.{ // This function uses a random number generator to randomly

// generate a number between 1 and 6, and stores the number

// in the instance variable number and returns the number.

number = rand() % 6 + 1;

returnnumber;

} 22

Constructors: A Dice Example (1/2)


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intget_number() // Function to return the number on the top face

{// of the die. It returns the value of the variable number.returnnumber;

}

};

intmain(){Dice d1,d2;

cout


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Destructors

You might guess that another function is called

automatically when an object is destroyed. This is indeedthe case. Such a function is called a destructor.

A destructoralso has the same name as the class name

but is preceded by a tilde (~) sign: Like constructors, destructors do not have a return value.

They also take no arguments.

The most common use of destructors is to de-allocate

memorythat was allocated for the object by the

constructor.

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#include

usingnamespacestd;

classHouse{

private: doublearea; // area in square feet

public:

House() : area(1000) // a zero argument const.

{ cout


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// A Constructor Example

#include

usingnamespacestd;

classHouse{

private: doublearea; // area in square feet

public:

House() : area(1000) // a zero (no) argument constructor

{ cout


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Objects as Function Arguments

Next program demonstrates some new aspects of

classes, which are:

Constructor Overloading

Defining Member Functions Outside The Class

Objects as Function Arguments.

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// englcon.cpp constructors, adds objects using member function

#include

#include

usingnamespacestd;

classDistance // English Distance class

{

private:

intfeet; floatinches;

public:

Distance() : feet(0), inches(0.0)

{ cout inches;

} 28

Objects as Function Arguments (1/3)


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voidshowdist(){ // display distance

cout


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intmain()

{

Distance dist1, dist3; // Calls no args. constructor

Distance dist2(11, 6.25); // Calls two args. constructor

dist1.getdist(); // get dist1 from user

dist3.add_dist(dist1, dist2); // dist3 = dist1 + dist2

// display all lengths

cout


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Constructors Overloading

Since there are now two explicit constructors with the

same name, Distance(), we say the constructor isoverloaded.

Which of the two constructors is executed when an

object is created depends on how many arguments areused in the definition:

Distance length;

// calls first constructor

Distance width(11, 6.0);

// calls second constructor

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Member Functions Defined Outside the Class

void add_dist( Distance, Distance );

This tells the compiler that this function is a member of

the class but that it will be defined outside the class

declaration, someplace else in the listing.

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Objects as Arguments

Since add_dist()is a member functionof the Distanceclass,

it can access the privatedata in any object of classDistance supplied to it as an argument, using names like

dist1.inchesand dist2.feet.

In the following statement dist3.add_dist(dist1, dist2); add_dist() can access dist3, the object for which it was

called, it can also access dist1and dist2, because they are

supplied as arguments.

When the variables feetand inchesare referred to within

this function, they refer to dist3.feetand dist3.inches.

Notice that the result is not returned by the function. The

return typeof add_dist()is void. 33


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Objects as Arguments

The result is stored automatically in the dist3object.

To summarize, every callto a member functionis

associated with a particular object(unless its a static

function; well get to that later).

Using the member namesalone (feetand inches), thefunction has direct access to all the members, whether

privateor public, of that object.

member functionsalso have indirect access, using the

objectname and the membername, connected with the

dot operator (dist1.inchesor dist2.feet) to other objects of

the same classthat are passed as arguments.

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The Default Copy Constructor

Weve seen two ways to initialize objects.

A no-argument constructor can initialize data members to

constant values

A multi-argument constructor can initialize data members to

values passed as arguments.

You can also initialize one object with another object of

the same type. Surprisingly, you dont need to create a

special constructor for this; one is already built into all

classes. Its called the default copy constructor. Its a one

argument constructor whose argument is an object of

the same class as the constructor. The next program

shows how this constructor is used. 36


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// ecopycon.cpp initialize objects using default copy constr.

#include

#include

usingnamespacestd;

classDistance

{ // English Distance class

private:


public:

Distance() : feet(0), inches(0.0) // constructor (no args)

{ cout inches;

} 37

The Default Copy Constructor (1/2)


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voidshowdist(){ //display distance

cout


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// englret.cpp function returns value of type Distance

#include

#include

usingnamespacestd;

classDistance{ // English Distance class

private:


public:Distance() : feet(0), inches(0.0) // constructor (no args)

{ cout inches;

}

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Returning Objects from Functions (1/3)

Returning Objects from Functions


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voidshowdist() //display distance

{ cout


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intmain()

{

Distance dist1, dist3; // define two lengths

Distance dist2(11, 6.25); // define, initialize dist2


dist3 = dist1.add_dist(dist2); // dist3 = dist1 + dist2

cout


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To execute the statement dist3 = dist1.add_dist(dist2);

A temporary object of class Distanceis created to store

the sum.

The sum is calculated by adding two distances.

The first is the object dist1, of which add_dist()is a member. Itsmember data is accessed in the function as feetand

inches.

The second is the object passed as an argument, dist2.

Its member data is accessed as d2.feetand d2.inches.

The result is stored in tempand accessed as temp.feetand

temp.inches.

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The tempobject is then returned by the function using

the statement returntemp;

The statement in main() assigns it to dist3.

Notice that dist1is not modified; it simply supplies data

to add_dist(). Figure on next slide shows how this looks.

In the topic, Operator Overloading, well see how to

use the arithmetic + operator to achieve the even more

natural expression like dist3 = dist1 + dist2;

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8

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Structures and Classes

In fact, you can use structures in almost exactly the same

way that you use classes. The only formal differencebetween classand structis that in a class the members

are private by default, while in a structure they are public

by default. You can just as well write

classfoo

{

intdata1;public:

voidfunc();

}; //and the data1 will still be private.

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Structures and Classes

If you want to use a structure to accomplish the same

thing as this class, you can dispense with the keywordpublic, provided you put the public members before the

private ones

structfoo{voidfunc();

private:

intdata1;

}; // since public is the default.

However, in most situations programmers dont use a

structthis way. They use structures to group only data,

and classes to group both data and functions.46


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Classes, Objects and Memory

you might have the impression that each objectcreated

from a class contains separatecopiesof that classs dataand member functions.

Its true that each objecthas its own separate data items

But all the objectsin a given classuse the same memberfunctions.

The member functions are createdandplacedin memory

onlyoncewhen they are definedin the classdefinition.

Since the functions for each object are identical. The data

items, however, will hold different values.

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Static Class Data

If a data item in a class is declared as static, only one such

item is created for the entire class, no matter how manyobjectsthere are.

A staticdata item is useful when all objects of the same

class must sharea common item of information.

A member variable definedas statichas characteristics

similar to a normal static variable: It is visibleonly within

the class, but its lifetimeis the entireprogram. It conti-

nues to exist even if there are no objectsof the class. a normal static variable is used to retain information

between calls to a function, static class member data is

used to share information among the objects of a class.49


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Static Class Data

Why would you want to use staticmember data? As an

example, suppose an object needed to know how manyother objects of its class were in the program.

In a road-racing game, for example, a race car might want

to know how many other cars are still in the race.

In this case a staticvariable Total_Carscould be included

as a member of the class. All the objects would have

access to this variable. It would be the same variable for

all of them; they would all see the same number ofTotal_Cars.

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Static Data Class


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Static Data Class

(1/2)// statdata.cpp demonstrates a simple static data member

#include

usingnamespacestd;

classCar

{

private:

staticintTotal_Cars; // only one data item for all objects

// note: "declaration" only!

public:

Car() // increments count when object created

{ Total_Cars++; }

intHow_Many() // returns count

{ returnTotal_Cars; }

~Car()

{ Total_Cars--; }

};

intCar::Total_Cars = 0; // *definition* of count 51

Static Data Class


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intmain()

{

Car Toyota, Honda, Suzuki; // create three objects

cout


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Static Class Data

Ordinary variablesare usually declared (the compiler is

told about their name and type) and defined (thecompiler sets aside memory to hold the variable) in the

same statement. e.g. int a;

Static member data, on the other hand, requires two

separate statements.

The variables declarationappears in the classdefinition,

but the variable is actually definedoutsidethe class, in much

the same way as a global variable. If staticmember data were defined inside the class, it

would violate the idea that a class definition is only a

blueprint and does not set aside any memory.53

t M b F ti


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//constfu.cpp demonstrates const member functions

classaClass

{

private:

intalpha;public:

voidnonFunc() // non-const member function

{

alpha = 99; // OK

}

voidconFunc() const // const member function

{

alpha = 99; // ERROR: cant modify a member

}

};

constMember Function

A constmember function guarantees that it will never

modify any of its classs member data.

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Distance Class and Use of const


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// const member functions & const arguments to member functions

#include

usingnamespacestd;

classDistance // English Distance class

{

private:

intfeet;

floatinches;

public: // constructor (no args)

Distance() : feet(0), inches(0.0)

{ } // constructor (two args)

Distance(intft, floatin) : feet(ft), inches(in){ }

voidgetdist(){ // get length from user

cout > feet;

cout > inches;

} 55


(1/3)



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voidshowdist() const // display distance

{ cout


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intmain()

{

Distance dist1, dist3; // define two lengthsDistance dist2(11, 6.25); // define, initialize dist2


dist3 = dist1.add_dist(dist2); // dist3 = dist1 + dist2

//display all lengths

cout


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constMember Function Arguments

if an argument is passed to an ordinary function by

reference, and you dont want the function to modify it,the argument should be made const in the function

declaration (and definition). This is true of member

functions as well.Distance Distance::add_dist(constDistance& d2) const{

In above line, argument to add_dist() is passed by

reference, and we want to make sure that wont modify

this variable, which is dist2 in main().

Therefore we make the argument d2 to add_dist() const

in both declaration and definition.

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t Obj t


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constObjects

In several example programs, weve seen that we can

apply constto variables of basic types such as intto keepthem from being modified.

In a similar way, we can apply constto objectsof classes.

When an objectis declared as const, you cant modify it.

It follows that you can use only const member functions

with it, because theyre the only ones that guarantee not

to modify it. e.g. A football field (for American-style

football) is exactly 300 feet long. If we were to use thelength of a football field in a program, it would make

sense to make it const, because changing it would

represent the end of the world for football fans.59

constObjects


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// constObj.cpp constant Distance objects

#include

usingnamespacestd;classDistance // English Distance class

{

private:

intfeet;

floatinches;

public:

// 2-arg constructor

Distance(intft, floatin) : feet(ft), inches(in)

{ }

voidgetdist() // user input; non-const function{

cout > feet;

cout > inches;

}

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j

(1/2)

constObjects


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voidshowdist() const // display distance; const function

{

cout


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Implement a Circleclass. Each object of this class will

represent a circle, storing its radiusand the x and ycoordinates of its center as floats.

One constructorshould initialize it data to 0, and another

constructor should initialize it to fixed values given by user.

Make void setValues(float, float, float) functions to set x,y

and radius.

Make float area()function, and a float circumference()

function to returnareaand circumference.

Make void print() function to display xycoordinates and

radiusof a circle.

Call these functions in main() to display their working.

g

No.1

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Assignment Question


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Create a class Rectanglewith attributes lengthand width,

each of which defaults to 1.Provide member functions that calculate the perimeterand

the areaof the rectangle.

Also, provide setand getfunctions for the lengthand width

attributes.

The setfunctions should verify that lengthand widthare

each floating-pointnumbers larger than 0.0 and less than

20.0.

Make a Drawfunction that makes a rectangle using a

character *on console.

g

No.2

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Assignment Question


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Create a class called time

that has separate intmember data for hours, minutes, andseconds.

One constructorshould initialize this data to 0, and another

constructor should initialize it to fixed values.

Make void print()to display time in 23:59:59 format.

Make void setHour( int ) to set hours.

Make void setminute (int ) to set minutes.

Make void setSecond( int ) to set seconds.

Make void setTime( int, int, int ) to set hour, minute, second

Make int getHour(); int getMinute(); int getSecond();

to return hours, minuteand secondsrespectively.

g

No.3

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Assignment Question


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include a tickmember function that increments the

time stored in a Time object by one second. An addmember function should add two objects of

type time passed as arguments.

Be sure to test the following cases:1. Incrementing into the next minute.

2. Incrementing into the next hour.

3. Incrementing into the next day (i.e., 23:59:59 to 00:00:00).

Make 1000 times loop in a main function. Call tick and

printTimefunctions in that loop for an object. Also make twoobjects and add them to a third object and print their values.

g

No.3

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Assignment Question


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Create a class called Rationalfor performing arithmetic

with fractions. Write a program to test your class. Use integer variables to represent the privatedata of

the class the numeratorand the denominator.

Provide a constructorthat enables an object of this classto be initialized when it is declared. The constructor

should contain default values in case no initializers are

provided and should store the fraction in reduced form.

For example, the fraction 2/4would be stored in theobject as 1in the numeratorand 2in the denominator.

Provide publicmember functions that perform each of

the following tasks:

g

No.4

66

Assignment Question


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1. Adding two Rational numbers.

The result should be stored in reduced form.

1. Subtracting two Rational numbers.


3. Multiplying two Rational numbers.


3. Dividing two Rational numbers.

The result should be stored in reduced form.3. Printing Rational numbers in the form a/b, where a is

the numerator and b is the denominator.

4. Printing Rational numbers in floating-point format.

g

No.4

67

Assignment Question


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The equation of a line in standard form is ax + by = c, where

both aand bcannot be zero, and a, b, and care real numbers. If bis not equal to zero, then a/b is the slopeof the line.

If ais equal to zero, then it is a horizontalline

if b is equal to zero, then it is a verticalline.

The slope of a vertical line is undefined.

Two lines are parallelif they have the sameslopeor both

are vertical lines.

Two lines are perpendicularif either one of the lines is

horizontal and the other is vertical or the productof theirslopesis 1

Design the class lineTypeto store a line. To store a line, you

need to store the values of a (coefficient of x), b(coefficient of

y), and c.

No.5

68

Assignment Question


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Your class must contain the following operations.

If a line is nonvertical, then determine its slope. Determine if two lines are equal. Two lines a1x + b1y = c1

and a2x +b2y = c2 are equal if

either a1 = a2, b1 = b2, and c1= c2

or a1 = ka2, b1= kb2, and c1 = kc2 for some real numberk.)

Determine if two lines are parallel.

Determine if two lines are perpendicular.

If two lines are not parallel, then find the point of

intersection.

Add appropriate constructorsto initialize variables of

lineType.

Also write a ro ram to test our class.

No.5

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Assignment Question


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Create a class TicTacToeto play the game of tic-tac-toe

The class contains as private data a 3-by-3 two-dimensional arrayof integers.

The constructorshould initialize the empty board to all

zeros.Allow two human players.

place a 1in the specified square, wherever the first player moves, .

Place a 2wherever the second player moves.

Each move must be to an empty square.

After each move, determine whether the game has been

wonor is a draw.

No.6

objects and classes

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