C++: 6. Constructors

A constructor is a special type of member function that is called automatically when an object is created.

In C++, a constructor has the same name as that of the class and it does not have a return type.

C++ Default Constructor

A constructor which has no argument is known as default constructor. It is invoked at the time of creating object.

Example:- 

#include <iostream>  

using namespace std;  

class Employee  

 {  

   public:  

        Employee()    

        {    

            cout<<"Default Constructor Invoked"<<endl;    

        }    

};  

int main(void)   

{  

    Employee e1; //creating an object of Employee   

    Employee e2;   

    return 0;  

}  

Output:- 

Default Constructor Invoked 

Default Constructor Invoked 

C++ Parameterized Constructor

A constructor which has parameters is called the parameterized constructor. It is used to provide different values to distinct objects.

Example:- 

#include <iostream>

using namespace std;

class Employee {

   public:

       int id;//data member (also instance variable)    

       string name;//data member(also instance variable)

       float salary;

       Employee(int i, string n, float s)  

        {  

            id = i;  

            name = n;  

            salary = s;

        }  

       void display()  

        {  

            cout<<id<<"  "<<name<<"  "<<salary<<endl;  

        }  

};

int main(void) {

    Employee e1 =Employee(101, "Sonoo", 890000); //creating an object of Employee 

    Employee e2=Employee(102, "Nakul", 59000); 

    e1.display();  

    e2.display();  

    return 0;

}

Output:- 

101  Sonoo  890000

102  Nakul  59000

Copy Constructor in C++

The copy constructor in C++ is used to copy data of one object to another.

Example:- 

#include<iostream>

using namespace std;

class Demo {

   private:

   int num1, num2;

   public:

   Demo(int n1, int n2) {

      num1 = n1;

      num2 = n2;

   }

   Demo(const Demo &n) {

      num1 = n.num1;

      num2 = n.num2;

   }

   void display() {

      cout<<"num1 = "<< num1 <<endl;

      cout<<"num2 = "<< num2 <<endl;

   }

};

int main() {

   Demo obj1(10, 20);

   Demo obj2 = obj1;

   obj1.display();

   obj2.display();

   return 0;

}

Output:- 

num1 = 10

num2 = 20

num1 = 10

num2 = 20