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. For example,

class  Wall {
   public:

    // create a constructor
    Wall() {
        // code
    }
};

Here, the function Wall() is a constructor of the class Wall. Notice that the constructor

• has the same name as the class,
• does not have a return type, and
• is public

 

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C++ Default Constructor

A constructor with no parameters is known as a default constructor. In the example above, Wall() is a default constructor.

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Example 1: C++ Default Constructor

// C++ program to demonstrate the use of default constructor

#include <iostream>
using namespace std;

// declare a class
class  Wall {

  private:
       double length;

   public:
    // create a constructor
    Wall() {

        // initialize private variables
        length = 5.5;

        cout << "Creating a wall." << endl;
        cout << "Length = " << length << endl;
    }
};

int main(){

    // create an object
    Wall wall1;

    return 0;
}

Output:

Creating a Wall
Length = 5.5

Here, when the wall1 object is created, the Wall() constructor is called. This sets the length variable of the object to 5.5.

Note: If we have not defined a constructor in our class, then the C++ compiler will automatically create a default constructor with an empty code and no parameters.

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C++ Parameterized Constructor

In C++, a constructor with parameters is known as a parameterized constructor. This is the preferred method to initialize member data.

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Example 2: C++ Parameterized Constructor

// C++ program to calculate the area of a wall

#include <iostream>
using namespace std;

// declare a class
class Wall {
   private:
    double length;
    double height;

   public:
    // create parameterized constructor
    Wall(double len, double hgt) {
        // initialize private variables
        length = len;
        height = hgt;
    }

    double calculateArea(){
        return length * height;
    }
};

int main(){
    // create object and initialize data members
    Wall wall1(10.5, 8.6);
    Wall wall2(8.5, 6.3);

    cout << "Area of Wall 1: " << wall1.calculateArea() << endl;
    cout << "Area of Wall 2: " << wall2.calculateArea() << endl;

    return 0;
}

Output:

Area of Wall 1: 90.3
Area of Wall 2: 53.55

Here, we have created a parameterized constructor Wall() that has 2 parameters: double len and double hgt. The values contained in these parameters are used to initialize the member variables length and height.

When we create an object of the Room class, we pass the values for the member variables as arguments. The code for this is:

Wall wall1(10.5, 8.6);
Wall wall2(8.5, 6.3);

With the member variables thus initialized, we can now calculate the area of the wall with the calculateArea() function.

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C++ Copy Constructor

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

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Example 3: C++ Copy Constructor

#include <iostream>
using namespace std;

// declare a class
class Wall {
   private:
    double length;
    double height;

   public:

    // parameterized constructor
    Wall(double len, double hgt) {
        // initialize private variables
        length = len;
        height = hgt;
    }

    // copy constructor with a Wall object as parameter
    Wall(Wall &obj) {
        // initialize private variables
        length = obj.length;
        height = obj.height;
    }
    double calculateArea(){
        return length * height;
    }
};

int main(){

    // create an object of Wall class
    Wall wall1(10.5, 8.6);

    // print area of wall1
    cout << "Area of Room 1: " << wall1.calculateArea() << endl;

    // copy contents of room1 to another object room2
    Wall wall2 = wall1;

    // print area of wall2
    cout << "Area of Room 2: " << wall2.calculateArea() << endl;

    return 0;
}

Output

Area of Room 1: 90.3
Area of Room 2: 90.3

In this program, we have used a copy constructor to copy the contents of one object of the Wall class to another. The code of the copy constructor is:

Room(Room &obj) {
    length = obj.length;
    height = obj.height;
}

Notice that the parameter of this constructor has the address of an object of the Wall class.

We then assign the values of the variables of the first object to the corresponding variables of the second object. This is how the contents of the object is copied.

In main(), we then create two objects wall1 and wall2 and then copy the contents of the first object to the second with the code

Wall wall2 = wall1;

Note: A constructor is primarily used to initialize objects. They are also used to run a default code when an object is created.