From: Mark Hubbart <discord@...>
Date: 2004-03-18T03:06:37+09:00
Subject: Re: Can subclass have superclass method return instance of subclass


On Mar 17, 2004, at 8:41 AM, walter@mwsewall.com wrote:

> Can find_all defined in Array be coerced into returning
> a QArray and not a plain Array?

The simplest solution: add a to_qa method to your QArray class. Then 
call it like this:

   qArray.find_all{|item| item.e < 50}.to_qa.sum_e

A slightly less-simple solution: Don't subclass Array, wrap it. Then, 
wrap the array methods so it coerces the returned value if an Array is 
returned.

   class QArray
     def initialize(ary)
       @ary = ary
     end
     def sum_e
       @ary.inject(0){|sum, item| sum+=item.e}
     end
     def method_missing(*args)
       if @ary.responds_to? args.first
         response = @ary.send(*args)
         if response.kind_of? Array
           QArray.new( response )
         else
           response
         end
       else
         raise NoMethodError
       end
     end
   end

I didn't test the code, but I'm pretty sure the logic is sound.

--Mark