Classes And Objects, Types of Constructors, Utility Functions, Destructors

<< Classes and Objects, Structure of a class, Constructor

Memory Allocation in C++, Operator and Classes, Structures, Function in C++, >>

CS201 Introduction to Programming

Lecture Handout

Introduction to Programming

Lecture No. 27

Reading Material

Deitel & Deitel - C++ How to Program

Chapter. 6

6.9, 6.10, 6.11, 6.12, 6.13, 6.14

Summary

Classes And Objects

Constructors

Types of Constructors

Utility Functions

Destructors

Classes and Objects

This lecture is a sequel of the previous discussion on 'Classes' and 'Objects'. The use of

'classes and objects' has changed our way of thinking. Instead of having function-oriented

programs i.e. getting data and performing functions with it, we have now data that knows

how to manipulate itself. This way, now the programming is object-oriented. It means

that our programs revolve around data and the objects. Therefore, it would be nice to

have some building blocks for programs so that these can be combined to write a

program. We have so far talked about simple variables like integer, double and char,

followed by strings of characters and arrays of different data types. But now, in the form

of an object, we have a block which knows itself about contents and the behavior. The

upcoming discussion will further explain it. We have used cout for displaying many

things like integers, doubles and strings. Here integer did not know how it is going to

display itself. However, cout knows how to display an integer on the screen. Now we

want to see that an integer should know how to display itself. So it will be a different

process. Now the question arises whether it will be good to see an integer knowing how

to display itself? For this purpose, we will have to expand the scope of thinking.

While engaged in the process of programming, we try to solve a real-world problem. The

real world is not only of integers, floats, doubles and chars, but there are other things like

cycles, cars, buildings, schools and people. We perceive all these things as objects. Each

object has a behavior associated with it. Consider the example of a man who can talk,

walk, sit and stand etc. Similarly, we can think of a vehicle that has many functions.

Page 336

CS201 Introduction to Programming

These objects also have attributes. For example, a man has height, weight, color of eyes

and hair and so on. These all are his attributes. His actions will be referred as functions or

methods. This principle may be applicable to vehicles, aero planes and all other real-

world things. An aero plane has attributes like its height, width, number of seats and

number of engines etc. These are attributes called data members. Its actions include

takeoff, flying and landing. These actions are its functions or methods.

In terms of language, the attributes and actions may be equated with nouns and verbs.

The verbs are actions which are also called methods in the programming terminology.

These methods should be included in the object in such a way that the object itself knows

how to achieve this function or method. Now consider this all in terms of data. Is it

always in terms of salary, payroll, amount and numbers? Actually, data comes in

different varieties. Now a day, a computer is multimedia equipment. We see that there are

not only alphabets and digits displayed on the screen, but also pictures, images, windows,

dialogue boxes, color and so many other things. These are numbers, letters and graphics.

Other than this, we can see videos on our computer. It is just another type of media.

Similarly, we find audio material, which can be played on the computer. Thus in the

expanded terms of data, we come across numbers, pictures, audio and video while

dealing with multimedia.

Now we think about the concept that an integer should know how to display itself. With

the enhanced scope of data, we can also have an audio, which knows how to play itself.

The same applies to video.

Class

A class is a way of defining a user-defined data type. In a class, one may find data

members and functions that can manipulate that data. In the previous lectures, we have

talked about the concept of data hiding i.e. encapsulation that means that the data of a

class cannot be accessed from outside. However, it can be done through some defined

functions (methods). These are the member functions of the class. To hide the data, we

declare it private. If a data is private, it will be available only to member functions of the

class. No other function outside the class (except friend functions) can access the private

data. Normally in a class we divide the private part which is normally what we called

implementation of the class, from the functions that manipulate that private data which is

called the interface (which is the front end).

The example of a room can help us understand private and public parts of a class. A class

is a room having a curtain in its middle. The things behind the curtain (private) are visible

to the residents (insiders) of the room. They know about every thing present in the room.

When the door opens, the outsiders see only the things in front of the curtain. This is the

public interface of the class while behind the curtain is the private interface. A function

inside the class (i.e. a member function) can access and manipulate all things in the class.

A function outside the class can only access and manipulate its public interface part. A

constructor has to be in the public section of the class. There should also be a public

interface so that it can be called from outside.

Page 337

CS201 Introduction to Programming

Constructors

Constructor is a special function, called whenever we instantiate an object of a class. If

we do not define a constructor function in a class, the C++ provides a default constructor.

It is executed at the time of instantiating an object.

To understand the basic function of constructor, we have to go back. While writing c++

Stroustrup noticed that the majority of programming problems, which we call bugs, occur

due to the use of uninitialized data. That is, we declare variables and use them without

providing them any value. For example, we declare an integer as int i ; And it is not

initialized with a value like i= 0; or i = 5; And then somewhere in the program, we write,

say, j = 2 * i ;. This is the usage of an uninitialized data variable. This technique has a

demerit that despite having no syntax error, it may cause a logical error, which is difficult

to find. Thus, initialization of data is a very critical activity. The constructors give us an

opportunity to initialize the data members of an object in such a way that when our

program gets an object, the data part of the object is in a known state. Being in a valid

state, it can be used. The constructors are used to initialize the data members of an object.

A class is a user defined data type it does not take space in the memory unless we create

an object from it. The constructors create space for data members and put values in them.

We want these values to be there when an object is instantiated. Thus initialization is a

good reason for using constructors.

Types of Constructors

Compiler Generated Constructor

If a constructor is not defined by the use the compiler generates it automatically. This

constructor has no parameter. It does nothing. Although the compiler will create a default

constructor for us, the behavior of the compiler-synthesized constructor is rarely what we

want. Thus the default constructor provided by the compiler does no initialization for us.

Simple Constructor

We have earlier discussed that we can write a constructor that takes no argument. The

user defined constructor, that takes no argument is called a simple constructor. We know

that when a compiler generated default constructor is called, it does no initialization. It

does not know whether to put a value in data members like day, month in our previous

class Date. We can avoid this problem by not writing a class without having its

constructor.

A simple constructor can do initialization without any need to take any argument. So we

can write a constructor of Date class like Date ();. When we write such a constructor, it

automatically assumes the roll of the default-constructor. The compiler will not call the

default constructor. Rather, the constructor written by the programmer will be called

whenever an object will be instantiated. It is also a good programming practice to provide

a default constructor (i.e. a constructor wit no argument).

Parameterized constructors

We may define a constructor, which takes arguments as well. This constructor will be

automatically called when the required number of arguments are passed to it. Through

Page 338

CS201 Introduction to Programming

this, we can easily assign the passed values to our class data members for that particular

object.

In our previous example of class Date, we have written a constructor as follows

Date (int, int, int);

This is a parameterized constructor which takes three arguments of type int.

Constructor Overloading

We can provide more than one constructors by using function overloading. The rules for

function overloading are that the name of the function remains the same. However, its

argument list may be different. There are two ways to change the argument list. It can

either vary in the number or type of the arguments. We cannot have two functions with

the same number and type of arguments. In such a case, these will be identical. So it will

not be function overloading. The function overloading requires the argument list to be

different. The same concept of function overloading applies to constructors. If we supply

more than one constructor for a class, it can be called one or the other depending on the

way of calling.

As the constructor does not return any thing, so it has no return type. It means that the

body of the construct function cannot have any return statement. Otherwise, the compiler

will give a syntax error.

Explanation of Constructors

The main purpose of the constructor is to initialize the object in such a manner that it is in

a known valid state. Consider the example of Date class again. In that example, there

were three data members i.e. day, month and year of type int. What values will we give to

these data variables by default if we create an object of Date? There may be any valid

date. We can give a value 01 to day, month and 1901 to year or what ever we want. It

will be a known state despite being meaningless. We can write a constructor of class

Date which takes three arguments int day, int month and int year, and puts values in the

data members of the object, being created. Now the question arises when does this

happen? It happens when we instanciate an object of class Date by writing Date

myDate; When this line executes in the program, some space for 'myDate' is reserved in

the memory. This space contains the space for day, month and year variables. Then

control goes to the constructor that assigns values to day, month and year. Being a

member of the class, the constructor can write values to the data members of the class

that is private. .

In C++ language, we can provide default arguments to functions. As a function, the

constructor can take default arguments. Suppose we have written a constructor of class

date with the arguments by providing default values to its arguments. We can write a

constructor as

Date (int day=1, int month=1, int year=1);

and create an object of class Date as

Date myDate;

This creates an object of type Date. Which constructor will be called? A constructor with

no arguments or the parameterized constructor in which each argument has given a

value? If we provide a constructor which has default values for all the arguments, it will

become the default constructor for the class. Two constructors cannot be considered

Page 339

CS201 Introduction to Programming

same. So it will be better not to write a constructor Date (); (constructor with no

argument) in case of providing a fully qualified constructor (with default values for all

arguments).

Now suppose, we want to initialize the Date object properly by passing a character string

to its constructor. Is it possible to write such a constructor? Yes, we can write such a

constructor. This constructor will take date as a string, say, 01-jan-1999. And in the

constructor, we can split up this string with 'string manipulation functions' and assign

respective values to day, month and year.

Now we recapture the concept of constructors with special reference to their

characteristics.

A constructor is a function which has the same name as the class.

It has no return type, so it contains no return statement.

Whenever an instance of a class comes into scope, the constructor is executed.

The constructors can be overloaded. We can write as many constructors as we require.

At one time, the compiler will call the correct version of the constructor".

Utility Functions

The second issue, we usually come across while dealing with the concepts of 'classes and

objects' is that a class has a data on one side (normally private part) and functions on the

other (normally public part). The functions (methods) are normally written in public part

of the class. Are there functions which are private to a class? Answer is yes. The

functions of a class may be of two categories. One category contains the member

functions which manipulate the data or extract the data and display it. Through these, we

can set and get values to manipulate data. These are the functions which are in public

interface of the class and manipulate the data in the object. But sometimes, we need such

functions that is the requirement of these member functions. Suppose we write a setDate

function. This function is given an argument and it does the same thing as done by the

constructor. In other words, it sets a value of date. Now that function can be public so

that it can be called from outside the class. Now we want that the member functions of

the class can call this function. But it should not be called from outside. In this case, we

put this function in private section of the class. These functions are called utility

functions. These are a utility used by other methods of the class. However, they are not

functions, supposed to be accessed from outside the class. So they are kept private.

Destructors

The name of the destructor is the same as that of a class with a preceding tilde sign (~).

The ~ and name of the class is written as a single word without any space between them.

So the name of the destructor of class Date will be ~Date. The destructor can not be

overloaded. This means that there will be only one destructor for a class.

A destructor is automatically called when an object is destroyed. When does an object

gets destroyed? When we create an object in a function, this is local to that function.

When the function exits the life of the object also comes to end. It means that the object

is also destroyed. What happens if we declare an object in the main program? When the

main program ends, its objects also comes to end and the destructor will be called.

Page 340

CS201 Introduction to Programming

The destructor is normally used for memory manipulation purposes. Suppose we have

such a class that when we create an object of it then its constructor has allocated some

memory. As we know that we have to free the allocated memory to ensure its utilization

for some other program. The destructor is used normally for this purpose to make sure

that any allocated memory is de-allocated and returned to free store (heap).

The destructors can be summarized as the following.

The destructors cannot be overloaded.

The destructors take no arguments.

The destructors don't return a value. So they don't have a return type and no return

statement in the body.

Now we come to the previously defined class Date. Let's see what can we further put in

it. We have put in it constructors. We have provided a parameterized constructor without

default arguments. So the constructor with no arguments will become default one. We

have another constructor with three parameters so that we can pass it the values for day,

month and year. There is also provided a destructor. We have written some methods to

set day, month and year. These were setDay, setMonth and setYear respectively. Each

one of them takes one parameter, a simple integer. Then we have get functions. The

functions getDay, getMonth and getYear return a simple integer. There is also a

function setDate, which takes three parameters (i.e. day, month and year) and sets them.

In set function, we do not simply assign the values to the data members. This can be done

through a constructor. Whenever we put data into an object, it is necessary to make it sure

that valid values should be stored. For example, if we say Date myDate ; and give it

values like 35 for day, 13 for month and 2000 for year. The constructor will set these

values. But these are invalid values for a date. Here we want that these values should be

validated before being assigned to data members. So we write some code for error

checking of the values and store only valid values in data members i.e. day, month and

year. We do the same thing in set function. Then what is the advantage of using set

functions. The set functions are public part of the class and can be called from outside the

class and also by the constructor. So write all the code for error checking and to validate

the data in set function and call this set function in the constructor. Thus when we create

an object of class date, it is written as the following

Date myDate (12,10,2000);

Then an object of Date class is created and the constructor of the class that takes three

arguments, is executed by passing these three values. In the constructor, we call the set

function which sets the values of the data members properly. Thus we get a fine

initialization of the data members.

What an Object is ? An object is an instance of a class. When we say an instance that

means that this object exists and takes space in the memory. What happens when we

create an object i.e. take an instance of the class. A class contains data and methods. Are

these methods reproduced for every object? Every object has data of its own as every

object is distinct from the other. For example, in case of the date class, there may be

objects date1, date2 and date3. These are three different objects having their own value of

Page 341

CS201 Introduction to Programming

date. Being distinct objects, they must have distinct space in memory. What about

functions inside the class?

Whenever we create an object of a class, the functions of the class take a space in the

memory and remain there. There is only one copy of these functions in the memory. The

data part of the class takes individual locations in the memory. So if we create three

objects of a class, say date, there will be one copy of the functions in the memory at the

time of execution of the program. The data will have allocated three spaces in the

memory with different values. Now suppose, we want to change the data of date1, there

is need of setting month of date1 to 3. So we call setMonth function for the object date1.

We use dot operator (.) to call the function of an object. We write this as

date1.setMonth(3); The setMonth function is called from the copy of the functions that

is in the memory. The object name with dot operator makes sure that the function will

operate on the data of that object. Thus only the value of the month of date1 will be set to

3. The values of date2 and date3 will remain untouched. Similarly if we say

date2.setDay(23); the setDay function will be called for object date2 and day of date2

will be set to 23. Thus it is clear that which object calls the function the data of that object

is visible to the function and it manipulates only that data. Thus we have not wasted the

memory by making separate copy of the functions for each object. All objects of one

class share the common functions. On the other hand, every object has its own data space.

The overloaded functions and constructors are also found in this single copy and called

whenever needed. In the overloaded functions, the appropriate function to be called is

resolved by the parameter list (type and number of the arguments to be passed).

In our class Date, we need no functionality for the destructor. We write the destructor

~Date and a cout statement in it. That displays the message `The object has destroyed'

just to demonstrate the execution of the destructor. Similarly we can display a message

like `Date object created' in our constructor function. By this, we can see when the

constructor is called. By seeing these messages on the screen we know that the object is

being created and destroyed properly. If the constructor function is overloaded, we can

put appropriate message in each constructor to know which constructor is called while

creating an object. For example in default constructor, we can display a message `Default

constructor is called'.

The following program demonstrates the execution of constructors and destructors. It is

the previous example of Date class. It displays appropriate messages according to the

constructor called. You will see that the constructor is called depending upon the

parameter list provided when the object is being created.

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

A message is displayed to show which one constructor is called

#include <iostream.h>

//#include <stdlib.h>

// defining the Date class

class Date{

// interface of the class

Page 342

CS201 Introduction to Programming

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);

Date(int, int, int);

// Destructor of the class

~Date ();

// 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;

cout << "The default constructor is called" << endl;

}

// Constructors with two arguments

Date::Date(int theDay, int theMonth)

{

day = theDay;

month = theMonth;

year = 2002;

cout << "The constructor with two arguments is called" << endl ;

}

// Constructors with three arguments

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

{

day = theDay;

month = theMonth;

Page 343

CS201 Introduction to Programming

year = theYear;

cout << "The constructor with three arguments is called" << endl;

}

//Destructor

Date::~Date()

{

cout << "The object has destroyed" << endl;

}

// 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;

}

// getting the value of the year

int Date::getYear()

{

Page 344

CS201 Introduction to Programming

return year;

}

/* Main program. We will take three date objects using the three constructors

(default, two arguments and three arguments and display the date.

int main()

{

Date date1, date2(12,12), date3(25,12,2002); // taking objects of Date class

// displaying the dates on the screen

date1.display();

date2.display();

date3.display();

}

Following is the output of the above program.

The default constructor is called

The constructor with two arguments is called

The constructor with three arguments is called

The date is 1-1-1900

The date is 12-12-2002

The date is 25-12-2002

The object has destroyed

Page 345

Table of Contents: