D语言this别名

import std.stdio;

struct Rectangle { 
   long length; 
   long breadth;  

   double value() const @property { 
      return cast(double) length * breadth; 
   }
   alias value this; 
} 
double volume(double rectangle, double height) {
   return rectangle * height; 
}

void main() { 
   auto rectangle = Rectangle(2, 3);  
   writeln(volume(rectangle, 5)); 
}