Computer Science Notes for Class 12 (ASSEB / AHSEC)
Chapter 4 - Constructor and Destructor
Contents
Introduction to Constructor
In this chapter, we will learn about constructor, need for constructor, its characteristics, types of constructor and constructor overloading
What is a Constructor?
A constructor is a special member function of a class that is automatically called whenever an object of the class is created.
Its main purpose is to initialize the data members (variables) of an object.
Definition
A constructor is a special member function having the same name as the class and no return type. It is automatically executed when an object is created
Why Do We Need Constructors?
Suppose we create a student object
Student s1;
Without a constructor, the variables inside the object may contain garbage values.
A constructor assigns proper initial values automatically.
Example Without constructor
Roll = Garbage
Marks = Garbage
Example With constructor
Roll = 1
Marks = 95
Thus, constructors make programs safer and easier to use
Characteristics of Constructors
- Constructor name must be the same as the class name
- It has no return type, not even void.
- It initializes data members.
- A constructor can be overloaded.
- Constructors may have parameters.
- They are generally declared in the public section.
- Constructors cannot be inherited.
- Constructors cannot be virtual
Syntax of a Constructor
class ClassName
{
public :
ClassName( )
{
// Initialization
}
};
Types of Constructors
There are three types of constructors:
- Default Constructor
- Parameterized Constructor
- Copy Constructor
1.Default Constructor
A constructor that does not take any arguments is called a default constructor.
Syntax
ClassName( )
{
}
Example
Student( )
{
roll = 10;
marks = 85;
}
Advantages of Default Constructor
- Initializes every object automatically
- Reduces program effort
- Avoids garbage values.
- Makes program reliable
2.Parameterized Constructor
A constructor that accepts arguments (parameters) is called a parameterized constructor. Different objects can be initialized with different values.
Syntax
ClassName ( type variable)
{
}
Example
#include<iostream>
using namespace std;
class Student
{
int roll;
int marks;
public:
Student (int r, int m)
{
roll = r;
marks = m;
}
void display( )
{
cout<<”Roll = “<<roll<<endl;
cout<<”Marks = “<<marks<<endl;
}
};
int main( )
{
Student s1(1,90);
Student s2(2,95);
s1.display( );
s2.display( );
return 0;
}
Output
Roll = 1
Marks = 90
Roll = 2
Marks = 95
Explanation
For object s1
r = 1
m = 90
For object s2
r = 2
m = 95
Thus, different objects can have different initial values.
3.Copy Constructor
A constructor that creates one object by copying another object of the same class is called a copy constructor.
Syntax
ClassName(ClassName &obj)
{
}
Example
#include<iostream>
using namespace std;
class Student
{
int roll;
public:
Student (int r)
{
roll = r;
}
Student (Student &obj)
{
roll = obj.roll;
}
void display( )
{
cout <<”Roll =”<<roll<<endl;
}
};
int main( )
{
Student s1(10);
Student s2(s1);
s1.display( );
s2.display( );;
return 0;
}
Output
Roll = 10
Roll = 10
Constructor Overloading
A class can have more than one constructor with different parameter lists. This is called constructor overloading.
Example
#include<iostream>
using namespace std;
class Student
{
int roll;
public:
Student ( )
{
roll = 0;
}
Student ( int r)
{
roll = r;
}
void display( )
{
cout<<”Roll = “<<roll<<endl;
}
};
int main( )
{
Student s1;
Student s2(15);
s1.display( );
s2.display( );
return 0;
}
Output
Roll = 0
Roll = 15
Important Board Examination Points
- Constructor name must be the same as the class name
- Constructor has no return type
- Constructor executes automatically when an object is created
- Constructor is mainly used to initialize objects
- Constructors can be overloaded.