22 Jan C++ Constructors

A Constructor in C++ is always public and has the same name as the class name. Constructor gets called automatically when we create an object of a class. It does not have a return value. In this lesson, we will learn what are Constructors with examples and how to set parameters.

How to create a Constructor

In C++, to create a Constructor, the same class name is used followed by parentheses. Let’s say the class name is Studyopedia, therefore the Constructor would be the following:

Studyopedia() {     // Constructor
   // Code comes here
}

Let us see an example to create a Constructor in C++:

#include <iostream>
using namespace std;

class Rectangle {    
  public:   // Access specifier
  
    // Constructor name is the same as the class name
    Rectangle() {     
      cout << "Our Constructor!!";
      cout << "\nA rectangle has 4 sides, 4 corners, and 4 right angles";
    }
};

int main() {
  
  /* Constructor gets called automatically when we 
  create an object of a class */
  Rectangle rct;   
  
  return 0;
}

Output

Our Constructor!!
A rectangle has 4 sides, 4 corners and 4 right angles

Constructor Parameters

Parameterized Constructor is a constructor with a particular number of parameters. This is useful if you have different objects and you want to provide different values.

First, let us see what a parameterized constructor looks like with 2 parameters:

#include <iostream>
using namespace std;

class Rectangle {    
  public:   // Access specifier
     double length;
     double width;
  
    // Constructor name is the same as the class name
    Rectangle(double len, double wid) { 
      length = len;
      width = wid;
   }
};

int main() {
  
  /* Constructor gets called automatically when we 
     create an object of a class */
  Rectangle rct1(5, 10);
  Rectangle rct2(8, 20);
  Rectangle rct3(10, 25);
   
  // Display
  cout<<"Area of Rectangle1\n";
  cout <<"Length = "<<rct1.length << ", Width = " << rct1.width <<"\n";
  
  cout<<"\nArea of Rectangle2\n";
  cout <<"Length = "<<rct2.length << ", Width = " << rct2.width <<"\n";
  
  cout<<"\nArea of Rectangle3\n";
  cout <<"Length = "<<rct3.length << ", Width = " << rct3.width <<"\n";
  
  return 0;
}

Output

Area of Rectangle1
Length = 5, Width = 10

Area of Rectangle2
Length = 8, Width = 20

Area of Rectangle3
Length = 10, Width = 25

Now, let us understand the flow of parameterized constructors in the above program.

In the above code, we created 3 objects of the Rectangle class. As the parameters, we passed the length and width of the Rectangle to the parameterized constructor:

Rectangle rct1 = new Rectangle(5, 10);
Rectangle rct2 = new Rectangle(8, 20);
Rectangle rct3 = new Rectangle(10, 25);

The length and width values pass to the parameterized constructor:

// parameterized constructor
Rectangle (double len, double wid) {
   length = len;
   width = wid;
}