Concept of Constructor and their types in C++

By Akansh Upadhyay

In this guide we will learn about constructor, their types and how to use them in C++.

What is Constructor?

A constructor is a special member function with the same name as that of a class, which is used to initialize the object of the class. When we create an object, the constructor is automatically called.

Example:

Types of the constructor:-

1. Default Constructor

2. Parameterized Constructor

3. Copy Constructor

1. Default Constructor

A constructor with no parameter/arguments passed is called a default constructor. It is called automatically even if the constructor is not defined.

Syntax:

class xyz
{
    xyz()
    {
    }
};

Example:

#include 
using namespace std; 
class Rectangle 
{ 
public: 
int length,breadth; 
Rectangle() 
{ 
length=100; 
breadth=50;
} 
}; 
int main() 
{ 
Rectangle l;
Rectangle b; 
cout<<"\nLength of rectangle is "<<l.length; 
cout<<"\nBreadth of rectangle is "<<l.breadth; 
}

Output:

2. Parameterized Constructor

A constructor where an argument/parameter is passed at the time of object creation is called a parameterized constructor.

Syntax:

class Marks
{
  Marks(int m,int n) //Parameterized Constructor
  {
    //Statements
  }
};

Example:

#include
using namespace std;
class Student{
public:
int marks,rollno;
Student(int r,int m)
{
rollno=r;
marks=m;
}
int roll()
{
return rollno;
}
int mark()
{
return marks;
}

};
int main()
{ 
Student s1(1,98);
Student s2(2,80);
cout<<"\nStudent 1 roll no. is "<<s1.rollno;cout<<" and marks is "<<s1.marks; 
cout<<"\nStudent 2 roll no. is "<<s2.rollno;cout<<" and marks is "<<s2.marks; 
}

Output:

3. Copy Constructor

It is a special type of constructor which is used to copy data members from one object into another object.

Syntax:

class Class_name{
  Constructor_name(class_name &obj_name)
  {
    
  }
};

Example:

#include
using namespace std;
class Item
{		
    public:
    int ino;
    string iname;
    public:
    Item(int p,string q)
    {
      ino=p;
      iname=q;
    }
    int getino()
    {
      cout<<ino; 	
    }
    int getiname()
    {
      cout<<", "<<iname;
    }
};
int main()
{
  Item S1(4,"Item 1");
  Item S2=S1;
  cout<<"\nList in Object 1: ";
  S1.getino();
  S1.getiname();
  cout<<"\n\nList in Object 2: ";
  S2.getino();
  S2.getiname();
  
}

Output: