D语言一元运算符重载

import std.stdio;

class Box { 
   public:  
      double getVolume() { 
         return length * breadth * height; 
      }

      void setLength( double len ) { 
         length = len; 
      }

      void setBreadth( double bre ) { 
         breadth = bre; 
      }

      void setHeight( double hei ) { 
         height = hei; 
      }

      Box opUnary(string op)() { 
         if(op == "++") { 
            Box box = new Box(); 
            box.length = this.length + 1; 
            box.breadth = this.breadth + 1 ; 
            box.height = this.height + 1; 
            return box; 
         }
      }

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

// Main function for the program 
void main( ) { 
   Box Box1 = new Box();    // Declare Box1 of type Box 
   Box Box2 = new Box();    // Declare Box2 of type Box 
   double volume = 0.0;     // Store the volume of a box here 

   // box 1 specification 
   Box1.setLength(6.0); 
   Box1.setBreadth(7.0); 
   Box1.setHeight(5.0);

   // volume of box 1 
   volume = Box1.getVolume(); 
   writeln("Volume of Box1 : ", volume); 

   // Add two object as follows: 
   Box2 = ++Box1;

   // volume of box2 
   volume = Box2.getVolume(); 
   writeln("Volume of Box2 : ", volume);  
}