Constructor

Constructor is a special method which invoked automatically when we create a new instance. It is used to automatically initiate the values of fields present inside class. Constructor must always present inside a class and constructor name should be same as that of class name. There are 2 types of constructor when it comes to C++ programming language, let us see both with its definition below

Default constructor - Constructor without any parameters(we'll learn more about this in Constructor Overloading chapter).
Parameterized constructor - Constructor with parameters.


Syntax
className() {
    //Field Intializers;
}

CODE

EG 1 . Creating Rectangle class with constructor.

#include <iostream>
using namespace std;

class Rectangle {
  public:
    double length, width, height, density;
    //Constructor
    Rectangle(double l,double w,double h,double d) {
        length = l;
        width = w;
        height = h;
        density = d;
    }
    double Area() {
        return length * width;
    }
    double Perimeter() {
        return 2 * (length+width);
    }
    double Volume() {
        return Area() * height;
    }
    double Weight() {
        return Volume() * density;
    }
    void RectangleCalculation() {
        cout << Area() << endl;
        cout << Perimeter() << endl;
        cout << Volume() << endl;
        cout << Weight() << endl;
    }
};

int main() {
    //Invoking constructor with 4 arguments
    Rectangle wall(5, 6, 7, 2.8);
    wall.RectangleCalculation();
}

OUTPUT
30
22
210
588

CODE EXPLANATION
In the above code we are going to automatically assign the values of length, width, height, density so inside Rectangle class, we created a Rectangle constructor with 4 parameters. In main method while creating instance of Rectangle class we are giving 4 values inside () which lies after object name. Since we are giving 4 arguments there, system will invoke Rectangle constructor with 4 parameters. Each value will be assigned to its respective parameters then System will go inside constructor and assign each parameter value to the fields as mentioned above. Now we automatically initiated the values of length, width, height and density.


Exercises
1. Create a Traingle class with a constructor.


(Click Here, For more exercises)