为BitStruct添加list类型

class  BitStruct
   def  self.list(name,element)
    module_eval  <<- HERE
      rest :body
       def   # {name.to_s}=(elist)
         e = Object.const_get(: # {element})
          if  elist.respond_to? :each  and  elist.respond_to? :[]  and  elist[0].is_a? e
            elist.each{  | value |  self.body += value}
         elsif elist.is_a? e
            self.body = elist
          else
             raise  ArgumentError,elist.to_s << "  is not a #{element} " ,caller
         end
      end
       def   # {name.to_s}
         result = []
         e = Object.const_get(: # {element})
         size = e.new.size
         num = self.body.size / size - 1
          for  i  in  0..num
            result[i] = e.new(self.body[i * size,size])
         end
         result
      end
    HERE
  end
end

利用Ruby的动态特性，很容易就达到了，使用的话：

class  A  < BitStruct
   unsigned :id, 32
end
class  B < BitStruct
   list :aList,A
end
alist = []
for  i  in  0.. 10
  a = A.new
  a.id = i
  alist.unshift a
end
b = B.new
b.aList = alist
b.aList.each{ | e |  puts e.id}