Classes and Objects, Structure of a class, Constructor

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CS201 Introduction to Programming

Lecture Handout

Introduction to Programming

Lecture No. 26

Reading Material

Deitel & Deitel - C++ How to Program

Chapter. 6

6.5, 6.7, 6.8, 6.10, 6.11, 6.14

Summary

Classes and Objects

Definition of a class

Separation of Interface from the Implementation

Structure of a class

Sample program

Constructor

Default arguments with constructors

Tips

Classes and Objects

In today's lecture, we will try to learn about the concepts of `classes' and `objects'.

However, we are not going to formally cover the object-oriented programming but only

the ways to manipulate the classes and objects.

We had talked about structures in our previous lectures. In structures, some data variables

are gathered, grouped and named as a single entity. Class and structure are very closely

related. In classes, we group some data variables and functions. These functions normally

manipulate these variables.

Before going ahead, it is better to understand what a class is:

"A class includes both data members as well as functions to manipulate that data"

These functions are called `member functions'. We also call them methods. So a class has

data (the variables) and functions to manipulate that data. A class is a `user defined' data

type. This way, we expand the language by creating a new data type. When we create

variables of a class, a special name is used for them i.e. Objects.

"Instances of a class are called objects"

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With the definition of class, we have a new data type like int, char etc. Here int i; means

`i' is an instance of data type int. When we take a variable of a class, it becomes the

instance of that class, called object.

Definition of a class

Let's have a look on the structure of a class. It is very similar to the struct keyword.

Keyword class is used and the braces enclose the definition of the class i.e.

class name_of_class{

// definition of class

}

The new data type i.e. classes helps us to have grouped data members and member

functions to manipulate the data. Consider a structure of Date having data members i.e.

year, month and day. Now we can declare a variable of structure Date and use dot

operator to access its members i.e.

Date myDate;

myDate.month=3;

We have to use the name of the object, a dot operator and the data member of structure to

be accessed. The data members are of normal data types like int, float, char etc. Other

data types can also be used.

Let's consider an example of Date Class shown in the following statement.

class Date{

int Day;

int month;

int year;

};

Now we will take its object in the fashion given below:

Date myDate;

Separation of Interface from the Implementation

To access the data members of the class, we will again use dot operator. Before going

ahead, we will see what is the difference between struct and class. It is the visibility of

the data members that differentiates between struct and class. What does the word

`visibility' mean? Consider an example of payroll system. We have stored the tax rate i.e.

5% in a variable i of type int. Later, we used the same i in a loop and changed the value

of tax rate unintentionally. Now the calculation of the pay in the end will not provide the

correct results. To avoid this problem, we can tag the tax rate variable as int tax_rate;.

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But this variable again is visible in the whole program and anyone can change its value.

The data is open and visible to every part of the program, creating a big problem.

In normal programming, we will like to see the data encapsulated. It means that data is

hidden somewhere. However, it can be used. Let's consider a real world problem to

understand it. Most of us have wrist-watches. To have accuracy, it is necessary to adjust

the time. How can we do that? We can change the time by using the button that is

provided on one side of the watch. This is a kind of encapsulation. We can see the hands

of the watch but cannot touch them. To change their position we used the button.

Whenever we talk about the class, we have to think of this concept that data is available

somewhere. We don't need to know about the exact structure i.e. what is inside the

watch. All we know is that its internal structure is defined somewhere that cannot be seen

or touched. We can only see its interface. If we need to adjust the time, a button may be

used. It is a nice separation of implementation and interface. Classes allow us to do that.

Structure of a class

Let's have a look inside a class. Consider the example of class Date. Can we set the

values of the data members of the object `myDate' i.e. day, month or year. We cannot say

like myDate.month = 11;. Try to do this. The compiler will give error and stop compiling

the program. It will not recognize the variable `month'. In other words, it cannot see

`month'. The default visibility for the data members of the class is called `private'. These

can only be used within the class and are not visible outside.

"The default visibility of all the data members and member function of a class is

hidden and private"

`private' is also a keyword. What will be the opposite of the private? What we will have

to do to use the data members and manipulate them. The keyword for this purpose is

public. In the class definition, if you do not mention the visibility and start defining the

data and functions, these will be by default private. As a good programmer, we should

always write the keyword private with a colon as:

private:

Now all the data and functions following this statement will have the private visibility.

To define the public data, we need to write the keyword public with a colon as:

public:

Now all the data and functions following the public keyword will have the public

visibility. These will be visible from outside the class. We can have multiple public and

private parts in the class definition but it becomes confusing. So normally we have only

one public and one private part. Again consider the Date example. By making the data

members as private, we will write functions to set and get the date. As this is needed to be

visible from outside the class, these functions will be defined as public.

class Date

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{

private:

// private data and functions

public:

// public data and functions

};

Normally, the data in a class is kept private. If we make the data public, it is same as

structure and anyone can access this data. On the other hand, the functions which we

have written to manipulate this data, are kept as public. These methods can be called

from outside the class i.e. from the main program. These are the member functions of the

class. The difference between these and the ordinary functions is that they are part of

class. Moreover, they can see the private data members of the class and also manipulate

them.

We have made the data members private in the Date class. In the program, we take an

object of Date class as Data myDate;. myDate is a variable of type Date. Now if we say

myDate.month = 3; this statement will be illegal as the month is a private data member of

the Date class. Now try to understand this concept. You can think class as a box having

different things in it. How can we touch inside the box? We have a window and can see

only those things that are visible through this window. Those things which we cannot see

from the window, can not be accessed from outside. Day, month and year are somewhere

inside the box and are not visible through the window. Now we want to assign some

values to these data members. For this purpose, we will define a member function in the

class in the public section. Being present in public section, it will be visible through the

window. As this is the member function, it can see and manipulate the private data of the

class. Now it's a two-step process. We can see the public functions and public functions

can view the private data members of the class. We will write a function to set the value

to the month. We cannot write it as myDate.month = 10;. So our function prototype will

be as:

void setMonth(int month)

and we may call this function as:

myDate.setMonth(10);

Now the function setMonth will assign the value 10 to month data member of the object

myDate. The same thing will be applicable if we want to print the date. We can write a

public function print and can access it as:

myDate.print();

The function print can see the private data members. So it will format the date and print

it. In structures, the data members are public by default. It means that these are visible to

all and anyone can change them. Is there any disadvantage of this? Think about the date.

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What may be the valid values of the day? Can we have a day less than zero or greater

than 32. So the minimum and maximum values of the day are 1 and 31 respectively.

Similarly, in case of month, the minimum and maximum values may be 1 and 12. We can

assign different values to year like 1900, 2002 etc. If we are using Date structure instead

of a class, we can write in the program as myDate.month=13; and the month will be set

to 13. So the date will become invalid. We may want that other programmers also use this

structure. But other programmers may put invalid values to the data-member as these are

publicly accessible. Similarly in structures, everything is visible i.e. what are the names

of the data members. How are these manipulated?

Now we want that only those things should be visible which we want to show and those

things which we want to hide should not be visible We can get this by using the private

and public in the classes. Public becomes the interface of the class, what we want to show

to others. With the use of public interface, the objects can be manipulated. Private

becomes the inside of the class i.e. the data members, the implementation. We don't want

to show the implementation of our classes to others. This is the concept of separation of

interface from implementation. It is a crucially important concept in modern

programming. We have separated the interface from the implementation. As long as the

interface remains the same, the implementation can be changed. Let's think about it in

real world. The example from the automobiles sector can help us understand further. The

production of cars in the world started in the late 18th century and early 19th century. Let's

compare these early or prototype cars with today's modern ones. There is a big difference

between the old and new cars. Technology has changed. Now what is still common in

both the types. Steering, clutch, brakes and accelerator pads are still the basic components

of a car. So the interface is same. The internal functionality can be changed. To turn the

car, old cars used rod mechanisms and modern cars have the microprocessor to do this

job. Our physical action is same in both the cases. The interface i.e. steering is same and

also the effect that wheels have turned to right is the same too. The internal

implementation has completely changed. The old combustion engine cannot be compared

with the state-of-the technology based modern engines. But the interface is the same i.e.

we turn the key to start an engine. This concept of separation of implementation from

interface comes into our programming. We have written a program today to calculate the

orbital time of moon around the earth. In today's physics, we have formula to calculate

this. We have defined the interface calculateOrbitalTime(). This is a function that will

calculate the orbital time of moon around earth. This formula may prove wrong after

some time. Now what can we do? Despite the change in the implementation, interface

remains the same i.e. the name of the function is same. Now when the program will use

this function, it gets the correct result as we have implemented the new formula inside the

function. Moreover, the main program does not need to be changed at all. Being a very

neat concept, it can be used while dealing with objects and classes.

Sample program

Let's see the example of Date class in detail.

class Date

{

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public:

void display();

Date(int, int, int);

private:

int day, month, year;

};

Date is the name of new user defined data type. After the braces, we have written the

keyword public. In this section, we will define the interface of the class. We have

declared a function display() which will print the date on the screen. Another function

Date(int day, int month, int year) is declared. The name of this function is same as the

name of the class, having no return type. This function is called constructor. Then we

write the keyword private and define the implementation of the class. Here we have three

variables i.e. day, month and year of type int. In the end closing braces and the semi-

colon. This is the definition of user defined data type i.e. class. It will not occupy any

memory space as it has no data currently. It is the same as we write in case of `int'. It

does not occupy any memory but when we say int I, the memory is reserved for i. The

class is a `user defined data' type. Now in our program, when we write Date myDate; an

instance of the class is created i.e. an object. Object reserves space in the memory. Object

will have these data members. What about the `functions'? For a moment, we can say

that functions are also in the memory.

We want to use this class in our program and display the date using the display()

function. We have written the prototype of the display() function in the class without

defining the display() function yet. A special way is used to define these functions. We

will write the name of the class, followed by two colons and the name of the function.

The rest is same as we used to do with ordinary functions.

Date::display()

{

// the definition of the function

cout << "The date is " << day << "-" << month << "-" << year << endl;

}

You might have noted the difference in the first line. The double colon is called scope

resolution operator. It resolves the scope and tells that this function belongs to whom. In

this case, the ( Date::display()) ) tells that the display() function belongs to the Date class.

So the scope resolution is required. In a way, consider it as function is defined inside the

class. If you have private function, even then the definition mechanism is same. We will

define the function outside of the class. Even then it will not be visible as its visibility is

private. The way to define the member functions is, class name, double colon, name of

the function including arguments and then the body of the function. Can we define the

function inside the class? Yes we can. When we write the function inside the class, the

compiler tries to treat that function as inline function. As a good programming practice,

we define the functions outside of the class. So to make sure that the function belongs to

the class, the scope resolution operator is used.

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We have so far tried to discuss Date class at a rudimentary level. That is we can create

objects of Date class and display the date using its functions. We can do a lot of other

things with this class. When we say int i; and ask to print its value. The answer is that we

have not assigned any value to it yet and don't know what will be there at that memory

location. Similarly, when we declare an object of the Date class as Date myDate; an

object is created. But we don't know about the values of day, month and year. Now if we

call its public function display() using the dot operator as myDate.display(). It will print

whatever the value is in the data members. We need functions to set/change the date.

Suppose we want to set the day, the month and the year separately. For this purpose, we

need three more public functions. We can name these functions as setDay(int ),

setMonth(int) and setYear(int). These functions may be called inside the program as

myDate.setDay(15), myDate.setMonth(12) and setYear(2002). These functions will

change the value of day, month and year. As these are member functions, so scope

resolution operator is being used.

void Date::setDay(int i)

{

day = i;

}

void Date::setMonth(int i)

{

month = i;

}

void Date::setYear(int i)

{

year = i;

}

The question arises, which objects data members are being set. In the setDay function, we

are assigning value to the day. But this day belongs to which object. The answer is, we

have just defined the function, it is not called yet. The functions are called by the objects,

not by the class. When we say Date myDate; it means that we have an object of type

Date. Now we can say myDate.setDay(10). The value of day of myDate object will be set

to 10. When we create objects, these will reserve space in memory. Suppose, the objects

are date1, date2, date3. These will be created at different memory locations having there

own data members. When we call a member function with the object name, this function

will manipulate the data of this object. Let's consider the following code snippet to

understand it.

Date date1, date2, date3;

// Manipulating date1 object

date1.setDay(10);

date1.setMonth(12);

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date1.setYear(2002);

date1.display();

// Manipulating date2 object

date2.setDay(15);

date2.setMonth(1);

date2.setYear(2003);

date2.display();

We have declared three objects of type Date. All these objects have data members day,

month and year. When we call a function, that is defined in class, with some object name,

it uses the data of that object which is calling the function. Suppose, when we write

date1.setMonth(12); it will manipulate the data of object date1. Similarly when we say

date2.display(), the function is defined inside the class. However, it will use the data of

date2 object. Remember that we will always call these member functions by referring to

some specific object. We can call these functions with date1, date2 or date3 respectively.

We will never call these functions referring to a class that is we cannot say

Date.display(); It is illegal. The functions of getting data from objects and setting data of

objects are standard. So we normally use the word `set' for setting the data and `get' for

getting the data. It is a matter of style. You can call it whatever you want. But it will be a

bad idea to name a function print() and it is setting the value of month. It will work but in

a very confused manner. If we want to write a function to set month, the logical choice of

the function name is setMonth(int). Similarly, setDay(int) and setYear(int) will be used to

set the day and year respectively. If we want to get the values of these data members, the

logical choice will be getDay(), getMonth() and getYear(). The names are self-

explanatory. These functions are defined as member functions of the class. They are put

in the public section of the class and constitute the public interface of the class. These

will be visible from outside the class. Normally they manipulate the data that is hidden

inside the class i.e. in the private section of the class. No need to show the working of the

functions only its name, argument and the return type is told to the user. User of the class

is our program.

Here is the complete code of the Date class.

/* A sample program with the Date class. Set methods are given to set the day, month and

year.The date is also diplayed on the screen using member function. */

#include <iostream.h>

// defining the Date class

class Date{

// interface of the class

public:

void display();

// to display the date on the screen

void setDay(int i); // setting the day

void setMonth(int i); // setting the month

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void setYear(int i); // setting the year

// hidden part of the class

private:

int day, month, year;

};

// The display function of the class date

void Date::display()

{

cout << "The date is " << day << "-" << month << "-" << year << endl;

}

// setting the value of the day

void Date::setDay(int i)

{

day = i;

}

// setting the value of the month

void Date::setMonth(int i)

{

month = i;

}

// setting the value of the year

void Date::setYear(int i)

{

year = i;

}

// Main program. We will take two date objects, set day, month, year and display the date.

int main()

{

Date date1,date2; // taking objects of Date class

// setting the values and displaying

date1.setDay(1);

date1.setMonth(1);

date1.setYear(2000);

date1.display();

// setting the values and displaying

date1.setDay(10);

date1.setMonth(12);

date1.setYear(2002);

date1.display();

}

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The output of the program is:

The date is 1-1-2000

The date is 10-12-2002

Constructors

We have written a function named Date(int, int, int) in our class. This is in the public

section of our class. It has no return type, having the name as that of class. Such functions

are called constructors. When we create an object by writing Date myDate; A function is

invisibly called which does something with this object. This function is constructor. If we

do not write a constructor, C++ writes a default constructor for us. By and large, we want

that the object should be created in a certain state. When our object myDate is created its

data members-day, month and year have some value. We can initialize these data

members with zero or with some specific date. How can we do that? Native data types

can be initialized as:

int i;

i = 10;

int i = 10;

Generally, a constructor initializes the object into a state that is recognizable and

acceptable. The default constructor does not take any parameter. We can have many

constructors of a class by overloading them. The constructor for Date class is:

Date(int, int, int);

This is the prototype of the constructor that is defined in the class. The definition of

constructor is same as we used with the member functions.

Date::Date(int theDay, int theMonth, int theYear)

{

day = theDay;

month = theMonth;

year = theYear;

}

How can we call this constructor? We know that constructor is automatically called when

an object is created. To use this constructor, we will take an object as:

Date myDate(1, 1 , 2003);

Here two things have taken place. 1) An object is created 2) The data members are

initialized. This is happening in the memory at run time. Nothing will happen at compile

time. The constructor will be called after the object creation and before the control given

back to the program. Here the value of day of the myDate object is 1, the value of month

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is 1 and the value of year is 2003. It has created and initialized an object. Now if we call

the display() function. These values will be displayed. Constructor is used to initialized

an object and put it into a consistent and valid state.

Default arguments with constructors

We can also use the default arguments with the constructors. In the case of Date,

normally the days and months are changing and the year remains same for one year. So

we can give the default value to year.

Date::Date(int theDay, int theMonth, int theYear = 2002)

{

// The body of the constructor

}

Now we have different ways of creating objects of class Date.

Date myDate;

In this case, the default constructor will be called while the data members remain un-

initialized.

Date myDate(1, 1, 2000);

The constructor will be called and initialized. The day will be 1, month will be 1 and the

year will be 2000.

Date myDate(1, 1);

The constructor will be called and initialized. The day will be 1, month will be 1 and the

year will be initialized to the default value i.e. 2002.

There are some complications. Constructor is itself a function of C++ and can be

overloaded. We can have many constructors. Suppose, we are asked to write the date i.e.

1, 1, 2000. Some of us may write it as 1, 1, 2000. Some will write it as 1/1/2000. A

considerable number may write as 1-1-2000. One can write date as 1 Jan. 2000. There

may have many formats of dates. It will be nice if we can initialize the object using any

of these formats. So we may have a constructor which takes a character string. The date

format is '01-Jan-2003'. So the constructor should parse the string. The string before the

hyphen is day (i.e. 01) convert it into an integer and assign it to day. Again get the strings

before the 2nd hyphen (i.e. Jan), check which month is it (i.e. 1) and assign it to month.

Rest of the string is year so convert it into integer and assign it to year. We are doing a lot

of horizontal integration here. The good thing is that the rules of simple functions

overloading applies to constructors also. The rules of default arguments also apply while

we are using default arguments with constructors. The idea is to make the class as

friendly as possible for the users. We have two constructors. Of these, one takes three ints

and the other takes the date as a character string. We may want to add more constructors.

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But we don't want to add too many constructors in the class as there is a limit of

everything. Within limits and the reasons, provision of two to three alternatives to the

users of the class for object creation is nice. May be the program that is using our class, is

applying months as character strings. We should provide a constructor that deals with

this. We will further explain this subject in the coming lectures. A constructor is a special

kind of function having same name as that of a class. It has no return type. Declare it

without return type. Constructor can take arguments. The default constructor takes no

argument.

Here is the code of the Date class using the different constructors.

A sample program with the Date class. Use of constructors is shown here.

#include <iostream.h>

//#include <stdlib.h>

// defining the Date class

class Date{

// interface of the class

public:

void display();

// to display the date on the screen

void setDay(int i); // setting the day

void setMonth(int i); // setting the month

void setYear(int i); // setting the year

int getDay();

// getting the value of day

int getMonth();

// getting the value of month

int getYear();

// getting the value of year

// Constructors of the class

Date();

Date(int, int, int);

// hidden part of the class

private:

int day, month, year;

};

// defining the constructor

// default constructor. setting the date to a default date

Date::Date()

{

day = 1;

month = 1;

year = 1900;

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}

// Constructors with default arguments

Date::Date(int theDay, int theMonth, int theYear = 2002)

{

day = theDay;

month = theMonth;

year = theYear;

}

// The display function of the class date

void Date::display()

{

cout << "The date is " << getDay() << "-" << getMonth() << "-" << getYear() <<

endl;

}

// setting the value of the day

void Date::setDay(int i)

{

day = i;

}

// setting the value of the month

void Date::setMonth(int i)

{

month = i;

}

// setting the value of the year

void Date::setYear(int i)

{

year = i;

}

// getting the value of the day

int Date::getDay()

{

return day;

}

// getting the value of the month

int Date::getMonth()

{

return month;

}

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// getting the value of the year

int Date::getYear()

{

return year;

}

// Main program. We will take three date objects using constructors, and display the date.

int main()

{

Date date1, date2(1, 1, 2000), date3(10,12); // taking objects of Date class

// displaying the dates on the screen

date1.display();

date2.display();

date3.display();

}

The output of the program is:

The date is 1-1-1900

The date is 1-1-2000

The date is 10-12-2002

Summary

A class is a user defined data type. It has data members and member functions. Normally

member functions are called methods. Data members are generally kept as private. The

member functions, used to manipulate the data members, are kept public so that these are

visible from outside the class. The public part of the class is known as the interface of the

class. It may contain data members and functions but normally we put functions as

public. The member functions can manipulate the data members (public and private) of

the class. Non-member functions can not see or access the private part of the class. We

try to separate the implementation of the class from its interface.

Tips

Explicitly write keyword private in the class definition

Separate the interface and implementation

The default constructor has no arguments

Constructor has the same name as of class

The data members of the class are initialized at runtime

Initializing the data members in the definition of the class is a syntax error

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