the keyword const for parameter

6-cia/src/geo2d-bb.cc

@@ -14,8 +14,9 @@ class Point
   Point(const Point& other); //copy constructor
   ~Point();                 //destructor never with parameters no return value
   static double distance(Point p,Point q);
-  double distance(Point& other);    //non static instance method
-                 //    |------------- c++ reference
+  double distance(const Point& other);    //non static instance method
+                 //|------------------ only read
+                 //     |------------- c++ reference
  private:
   double xc;  //x-coordinate
   double yc;  //y-coordinate
@@ -49,12 +50,10 @@ double Point::distance(Point p,Point q)
                  //  |----|----|----|----------------private
 }
 
-double Point::distance(Point& other)  
+double Point::distance(const Point& other)  
 {
-/* hack
- other.xc=0; like by value but in fact by reference
- other.yc=0;
-*/ 
+// other.xc=0; //error because read only
+// other.yc=0;
  return std::hypot(other.xc - xc,
                    other.yc - yc);
 }