Default constructor

If you want to customize how a class initializes its members or calls functions when an object of the class is created, you have to define a constructor. Standard constructors have no parameters.

If no constructors are declared in a class, the compiler automatically provides a default constructor. On the other hand, if a class declaration contains only parameterized constructors, no default constructor is generated, and the class file has no parameterless constructor at all.

Syntax

class ClassName {
    public:
        // create a constructor
        ClassName() {
            // Constructor code
        }
};

int main() {
    ClassName obj;

    return 0;
}

Example: Default Constructor

#include <iostream>
using namespace std;

class Cube {
    private:
        int length, width, height;
    public:
        Cube() {
            length = 4;
            width = 3;
            height = 4;
        }

        int getVolume() {
            return length * width * height;
        }
};

int main() {
    Cube c1;

    cout << c1.getVolume() << endl;

    return 0;
}

Output

48