Understanding Constructors in C++

class Box {
public:
    // Default constructor
    Box() {
        // Initialization code goes here
    }
};

int main() {
    Box obj; // Calls the default constructor
    return 0;
}

#include
using namespace std;
//Declaring a class: Box
class Box {
    double length,width,height;
    public:
        // Default constructor
        Box() {
            // Initialization code goes here
            length=3.0;
            width=4.0;
            height=6.0;
        }
        double volume(){
            return length*width*height;
        }
    };
 
int main() {
    Box b1; // Calls the default constructor
    double vol=b1.volume();
    cout<<"Volume of Box b1: "<<vol;
    return 0;
}

Volume of Box b1: 72

#include<iostream>
using namespace std;

//Declaring a class: Box
class Box {
    double length, width, height; //Private by default
    public:
        /* Default constructor declaration 
        (implementation will be outside teh class)*/
        Box();

        double volume(){
            return length*width*height;
        }
    };
    // Default constructor definition
    Box :: Box(){
        length=4.0;
        width=5.0;
        height=7.0;
    }
int main() {
    Box b1; // Calls the default constructor
    double vol=b1.volume();
    cout<<"Volume of Box b1: "<<vol<<endl;
    return 0;
}

Volume of Box b1: 72