module iostream;
export import iostream;
module vector;
export import vector;
// and so on 

export module box;

import <compare>;

export class Box
{
public:
   Box() = default;
   Box(double l, double w, double h) : m_length { l }, m_width { w }, m_height { h } {}

   double volume() const { return m_length * m_width * m_height; }

   double getLength() const { return m_length; }
   double getWidth()  const { return m_width; }
   double getHeight() const { return m_height; }

   std::partial_ordering operator<=>(const Box& otherBox) const
   {
      return volume() <=> otherBox.volume();
   }
   std::partial_ordering operator<=>(double otherVolume) const
   {
      return volume() <=> otherVolume;
   }

   bool operator==(const Box& otherBox) const
   {
      return m_length == otherBox.m_length
         && m_width == otherBox.m_width
         && m_height == otherBox.m_height;
   }

private:
   double m_length { 1.0 };
   double m_width  { 1.0 };
   double m_height { 1.0 };
};

import <iostream>;
import <format>;
import <string_view>;
import <vector>;

import box;

void show(const Box& box)
{
   std::cout << std::format("Box({:.1f}, {:.1f}, {:.1f})",
                            box.getLength(), box.getWidth(), box.getHeight());
}
void show(const Box& box1, std::string_view relationship, const Box& box2)
{
   show(box1); std::cout << relationship; show(box2); std::cout << std::endl;
}

int main()
{
   const std::vector boxes { Box {2.0, 1.5, 3.0}, Box {1.0, 3.0, 5.0},
                            Box {1.0, 2.0, 1.0}, Box {2.0, 3.0, 2.0} };
   const Box theBox        { 3.0, 1.0, 4.0 };

   for (const auto& box : boxes)
      if (theBox > box) show(theBox, " is greater than ", box);  // > works

   std::cout << std::endl;

   for (const auto& box : boxes)
      if (theBox != box) show(theBox, " is not equal to ", box); // != works

   std::cout << std::endl;

   for (const auto& box : boxes)
      if (6.0 <= box)   // yes, even double <= Box works!!
      {
         std::cout << "6 is less than or equal to "; show(box); std::cout << std::endl;
      }
}