Types of Constructors in C++

September 29, 2019

Constructors in C++

Constructors are special class functions which performs initialization of every object.
Whenever an object is created the Compiler calls the Constructor.
Constructors initialize values to object members after storage is allocated to the object.

Syntax

class C
{
    public:
    int a;
    // constructor
    C()
    {
        // object initialization
    }
};

Points to Remember

The name of constructor will be same as the name of the class.
Constructors will never have a return type.

There are 3 types of constructors

Default Constructor
Parameterized Constructor
Copy Constructor

Default Constructor

Default constructor is the constructor which doesn’t take any argument.
It has no parameter.
As soon as the object is created the constructor is called which initializes its data members.
A default constructor is so important for initialization of object members, that even if we do not define a constructor explicitly, the compiler will provide a default constructor implicitly

Syntax

class_name()
{ 
    // constructor Definition 
}

Example:

class Square
{
    public:
    int side;
    Square()
    {
        side = 5;
    }
};
int main()
{
    Square s;
    cout << s.side;
}

Output

Parameterized Constructors

These are the constructors with parameters.
Using this constructor we can provide different values to data members of different objects, by passing the appropriate values as argument.

Syntax

class_name(parameter1, parameter2, ...)
{ 
    // constructor Definition 
}

Example

class Square
{
    public:
    int side;
    Square(int a)
    {
        side=a;
    }
};
int main()
{
    Square s1(5);
    Square s2(10);
    cout << s1.side;
    cout << s2.side;
}

Output

5
10

In the above example, we have initialized 2 objects with user defined values.
We can have any number of parameters in a constructor.

Copy Constructor

Copy Constructor is a type of constructor which is used to create a copy of an already existing object of a class type.
It is usually of the form C (C&), where C is the class name.
A default Copy Constructor is provided to all the classes by compiler.

Syntax

Classname(const classname & objectname)
{
    . . . .
}

Example

#include<iostream>
using namespace std;
class Sample_copy_constructor
{
    private:
    int a, b;   //data members
    
    public:
    Sample_copy_constructor(int a1, int b1)
    {
        a = a1;
        b = b1;
    }
    
    /* Copy constructor */
    Sample_copy_constructor (const Sample_copy_constructor &sample)
    {
        a = sample.a;
        b = sample.b;
    }
    
    void display()
    {
        cout<<a<<" "<<b<<endl;
    }
};
/* main function */
int main()
{
    Sample_copy_constructor obj1(10, 20);     // Normal constructor
    Sample_copy_constructor obj2 = obj1;      // Copy constructor
    cout<<"Normal constructor : ";
    obj1.display();
    cout<<"Copy constructor : ";
    obj2.display();
    return 0;
}

Output

Normal constructor : 10 15 
Copy constructor : 10 15