D语言this指针

import std.stdio;

class Box { 
   public: 
      // Constructor deFinition 
      this(double l = 2.0, double b = 2.0, double h = 2.0) { 
         writeln(Constructor called.); 
         length = l;
         breadth = b; 
         height = h; 
      }

      double Volume() { 
         return length * breadth * height; 
      }

      int compare(Box Box) { 
         return this.Volume() > Box.Volume(); 
      }

   private: 
      double length;     // Length of a Box 
      double breadth;    // Breadth of a Box 
      double height;     // Height of a Box 
}

void main() { 
   Box Box1 = new Box(3.3, 1.2, 1.5);    // Declare Box1 
   Box Box2 = new Box(8.5, 6.0, 2.0);    // Declare Box2 

   if(Box1.compare(Box2)) { 
      writeln(Box2 is smaller than Box1); 
   } else { 
      writeln(Box2 is equal to or larger than Box1); 
   }
}