Accessing a class object outside the function it (the object) was declared in?

[Church]

I'm still pretty new to C++, sorry if this question is stupid... although I've run into a lot of roadblocks this is the first time I can't find the answer through google online. So, I'm using a piece of code off of a tutorial on classes from https://www.tutorialspoint.com/cplusplus/cpp_classes_objects.htm

#include <iostream>

using namespace std;

class Box {
   public:
      double length;   // Length of a box
      double breadth;  // Breadth of a box
      double height;   // Height of a box
};

int main() {
   Box Box1;        // Declare Box1 of type Box
   Box Box2;        // Declare Box2 of type Box
   double volume = 0.0;     // Store the volume of a box here
 
   // box 1 specification
   Box1.height = 5.0; 
   Box1.length = 6.0; 
   Box1.breadth = 7.0;

   // box 2 specification
   Box2.height = 10.0;
   Box2.length = 12.0;
   Box2.breadth = 13.0;
   
   // volume of box 1
   volume = Box1.height * Box1.length * Box1.breadth;
   cout << "Volume of Box1 : " << volume <<endl;

   // volume of box 2
   volume = Box2.height * Box2.length * Box2.breadth;
   cout << "Volume of Box2 : " << volume <<endl;
   return 0;
}

So Box1 has dimensions and Box2 has dimensions, and all is well and good within the main() function but as soon as I try to expand on this code and access either object's variables from another function outside of main() I'm getting errors that the object is undefined?

Is there a way I can access the variables within the class object from somewhere else in the program? Thanks for the help in advance.

[tpb]

There is actually more than one way to pass objects to functions depending on what you want to do. But the most common is to use a reference:

double calcVolume(const Box &b) {  // use a const reference if you aren't modifying the object
    return b.length * b.breadth * b.height;
}

// call it like this:
cout << calcVolume(box) << '\n';

Although in C++ you can make it a class method instead of a stand-alone function, in which case it would look like this:

#include <iostream>

class Box {
    double length, breadth, height;
public:
    Box(double l, double b, double h) : length(l), breadth(b), height(h) {}
    double volume() { return length * breadth * height; }
};

int main() {
    Box b(12, 13, 10);
    std::cout << b.volume() << '\n';
}

[mbozzi]

Although in C++ you can make it a class method instead of a stand-alone function, in which case it would look like this:

To add: if the function can be implemented strictly in terms of the public interface of the class, it should be written as a non-member. The first way is better.