D语言运算符重载

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 opAdd(Box b) { 
         Box Box = new Box(); 
         Box.length = this.length + b.length; 
         Box.breadth = this.breadth + b.breadth; 
         Box.height = this.height + b.height; 
         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 
   Box Box3 = new Box();    // Declare Box3 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);

   // Box 2 specification 
   Box2.setLength(12.0); 
   Box2.setBreadth(13.0); 
   Box2.setHeight(10.0); 

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

   // volume of Box 2 
   volume = Box2.getVolume(); 
   writeln(Volume of Box2 : , volume); 

   // Add two object as follows: 
   Box3 = Box1 + Box2; 

   // volume of Box 3 
   volume = Box3.getVolume(); 
   writeln(Volume of Box3 : , volume);  
}