C++読書ノートの単目演算子重荷マイナス記号-重荷

#include 
using namespace std;

class Distance
{
   private:
      double feet;             // 0 to infinite
      double inches;           // 0 to 12
   public:
      // required constructors
      Distance()
      {
         feet = 0;
         inches = 0;
      }
      Distance(double f, double i)
      {
         feet = f;
         inches = i;
      }
      // method to display distance
      void displayDistance()
      {
         cout << "Feet: " << feet << " Inches:" << inches <