D语言比较运算符重载

import std.random; 
import std.stdio; 
import std.string; 

struct Box { 
   int volume;  
   int opCmp(const ref Box box) const { 
      return (volume == box.volume ? box.volume - volume: volume - box.volume); 
   }

   string toString() const { 
      return format("Volume:%s/n", volume); 
   } 
} 

void main() { 
   Box[] boxes; 
   int j = 10; 

   foreach (i; 0 .. 10) { 
      boxes ~= Box(j*j*j); 
      j = j-1; 
   } 

   writeln("Unsorted Array"); 
   writeln(boxes);  
   boxes.sort; 
   writeln("Sorted Array"); 
   writeln(boxes); 
   writeln(boxes[0]<boxes[1]); 
   writeln(boxes[0]>boxes[1]); 
   writeln(boxes[0]<=boxes[1]); 
   writeln(boxes[0]>=boxes[1]); 
}