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OBJECT ORIENTED PROGRAMMING
LAB 02
CONSTRUCTORS & DESTRUCTORS
Summary
- Classes and Objects
- Constructors
- Types of Constructors
- Utility Functions
- Destructors
- Sample Program
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. 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 realworld 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.
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
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.
Copy Constructor
It allows to instantiate an object as an exact copy of another in terms of its attribute values. It takes as an argument a reference to an object of the same class and creates a new object that is a copy.
For example, consider the following,
Date date1; Date date2 (date1);
Then, date2 will be a copy of date1.
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 put 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 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
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 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
- 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 date 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 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
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() { 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- The date is 12-12- The date is 25-12- The object has destroyed The object has destroyed The object has destroyed
Exercises:
1- Using classes write a C++ program that can display a set of numbers. The user is asked for a start/end values for the set and the difference between two consecutive numbers. E.g.
Range: 1 – 10 Difference: 2
Output: 1 3 5 7 9
2- Using classes write a C++ program that can calculate the area of a triangle. The formula for finding the area is: ½ * base * altitude.
3- using copy constructor write a C++ program that makes the copy of the date object and displays the result as follows:
Jan 27, 2004 Jan 27, 2004
Note: first line is the actual data whereas, second output is the copy of the first
