C++ Constructor and Destructor

In this tutorial, I will discuss about two special types of member functions in C++ namely constructor and destructor.

 

Constructor in C++

A constructor in C++ is a special type of member function that gets called automatically (i.e. implicitly) when an object of the class is created.

A constructor has the same name as the name of the class to which it belongs and it is always public.

A constructor does not have a return type (not even void), since it never returns a value.

A class constructor example is shown below:

class Rectangle {

   public:
     // data members 
     int length;  
     int breadth;

     // class constructor 
     Rectangle() { 
        length = 10; 
        breadth = 10; 
     } 
};

 

C++ constructors are of three types —

1. Default constructor
2. Parameterized constructor
3. Copy constructor

We will describe each of them one by one.

 

1) Default constructor

Below is an example of a default constructor.

#include <iostream>
using namespace std;

class Rectangle {

   public:
     // data members 
     int length;
     int breadth;

     // default constructor 
     Rectangle() { 
        length = 10; 
        breadth = 10; 
     } 

     // member function
     int computeArea() {
        return length * breadth;
     }
};

int main() {
    // creating object of Rectangle class
    Rectangle rect;
    
    // computing area for rect 
    cout << "AREA : " << rect.computeArea();

    return 0;
}

Output:

AREA: 100

 

2) Parameterized constructor

Below is an example of a parameterized constructor. 

This helps us to assign initial value to an object at the time of its creation as shown in the example below −

#include <iostream>
using namespace std;

class Rectangle {

   private:
     // data members 
     int length;
     int breadth;

   public:
     // parameterized constructor 
     Rectangle(int l, int b) { 
        length = l;
        breadth = b; 
     }

     // member function
     int computeArea() {
        return length * breadth;
     }
};

int main() {
    // creating objects of Rectangle class
    Rectangle rect1(20, 15);
    Rectangle rect2(30, 20);

    // computing area for rect1 
    cout << "AREA1 : " << rect1.computeArea() << endl;

    // computing area for rect2
    cout << "AREA2 : " << rect2.computeArea() << endl;
   
    return 0;
}

Output:

AREA1: 300
AREA2: 600

 

3) Copy constructor

Below is an example of a copy constructor. It is used to copy data of one object to another.

See the following example −

#include <iostream>
using namespace std;

class Rectangle {

   private:
     // data members 
     int length;
     int breadth;

   public:
     // parameterized constructor 
     Rectangle(int l, int b) { 
        length = l;
        breadth = b; 
     }

     // copy constructor with a Rectangle object as parameter
     Rectangle(Rectangle &obj) { 
        length = obj.length;
        breadth = obj.breadth; 
     }

     // member function 
     int computeArea() { 
        return length * breadth; 
     }
};

int main() {
    // creating an object of Rectangle class 
    Rectangle rect1(20, 15);
 
    // computing area for rect1 
    cout << "AREA1 : " << rect1.computeArea() << endl;

    // copy contents of rect1 to another object rect2
    Rectangle rect2 = rect1;

    // computing area for rect2
    cout << "AREA2 : " << rect2.computeArea() << endl; 

    return 0; 
}

Output:

AREA1: 300
AREA2: 300

 

 

Destructor in C++

A destructor in C++ is a special member function that is executed whenever an object of it’s class goes out of scope.

A destructor is having the same name as the class prefixed with a tilde (~)

It doesn’t take any parameter and never returns a value.

It can be very useful for releasing resources (such as closing files, releasing memory pointers etc.) before the program ends.

Following example explains the concept of destructor −

#include <iostream>
using namespace std;

class Rectangle {

   private:
      // data members
      int length; 
      int breadth;

   public:
      Rectangle(int l, int b); // constructor declaration 
      ~Rectangle(); // destructor declaration 
      int computeArea(); // member function declaration
};

// constructor definition
Rectangle :: Rectangle(int l, int b) {
   cout << "constructor called" << endl;
   length = l;
   breadth = b;
}

// destructor definition
Rectangle :: ~Rectangle() {
   cout << "destructor called" << endl;
}

// member function definition
int Rectangle :: computeArea() { 
   return length * breadth; 
}

int main() {
   // creating object of Rectangle class
   Rectangle rect(20, 10);

   // computing area for rect
   cout << "AREA : " << rect.computeArea() << endl;

   return 0; 
}

Output:

constructor called
AREA : 200
destructor called