Constructors in C++

A constructor is a member function which has the same name as that of the class and does not have a return type. A constructor is called automatically when an object of the class is created.

The following program shows a constructor of the class ‘rectangle‘. The constructor is called automatically when an object of the class ‘obj1‘ is created.

#include<iostream>
using namespace std;

class rectangle {
    public: 

    int length = 4;
    int breadth = 7;

    rectangle() {
        
        cout<<"The length of the rectangle is "<<length<<endl;
        cout<<"The breadth of the rectangle is "<<breadth;

    }

};

int main () {
    
    rectangle obj1;

   return 0;
}

Output:

The length of the rectangle is 4 
The breadth of the rectangle is 7

A constructor can also have parameters. The following program shows a constructor with parameters ‘x‘ and ‘y‘. An object ‘obj1‘ of the class is created and arguments are passed to print the output.

#include<iostream>
using namespace std;

class rectangle {
    public:
    int length;
    int breadth;

    rectangle (int x, int y) {

        length = x;
        breadth = y;

        cout<<"The length of the rectangle is "<<x<<endl;
        cout<<"The breadth of the rectangle is "<<y;

    }
};

int main () {

        rectangle obj1(4 , 7);

        return 0;

}

Output:

The length of the rectangle is 4 
The breadth of the rectangle is 7

A copy constructor is a member function which is used to copy contents of an object of a class. An another object is initialized with an object of the same class.

The following program shows a constructor and a copy constructor in class ‘rectangle‘. An object ‘obj2‘ copies the content of ‘obj1‘. A syntax of a copy constructor is ClassName (ClassName &ObjectName).

#include<iostream>
using namespace std;

class rectangle {
public:

int length;
int breadth;

rectangle (int x, int y) {

        length = x;
        breadth = y;

        }

    rectangle(rectangle &obj) {
        
        length = obj.length;
        breadth = obj.breadth;

        }

};

int main () {

   rectangle obj1(4 , 7);
   
   cout<<"The length of the rectangle is "<<obj1.length<<endl;
   cout<<"The breadth of the rectangle is "<<obj1.breadth<<endl;

   rectangle obj2 = obj1;

   cout<<"The length of the rectangle is "<<obj2.length<<endl;
   cout<<"The breadth of the rectangle is "<<obj2.breadth;
   
   return 0;
 
}

Output:

The length of the rectangle is 4 
The breadth of the rectangle is 7
The length of the rectangle is 4
The breadth of the rectangle is 7