D Operator Overloading

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);  
}