From: Robert Klemme <bob.news@...>
Date: 2004-10-01T17:05:00+09:00
Subject: Re: Can you rewrite this in a better way?


"Bob Sidebotham" <bob@windsong.bc.ca> schrieb im Newsbeitrag
news:c427d.152105$%S.128398@pd7tw2no...
> I would like to define a method that can be used to compactly define
> some items of interest (here called "fields") for subclasses. It's used
> like this:
>
> class B < A
>    fields :a, :b, :c
> end
>
> class C < A
>    fields :x, :y
> end
>
> p B.new.fields                => [:a, :b, :c]
> p C.new.fields                => [:x, :y]
>
> Here are two definitions for class A that support these semantics:
>
> ---#1------------------------------------------------
>
> class A
>    def self.fields(*fields)
>      @fields = fields
>      def fields
>        self.class.getFields
>      end
>    end
>
>    private
>    def self.getFields
>      @fields
>    end
> end
>
> ---#2------------------------------------------------
>
> class A
>    def self.fields(*fields)
>      eval "def fields
>        [#{fields.collect{|f| ':' + f.to_s}.join(',')}]
>      end"
>    end
> end
>
> -----------------------------------------------------
>
> Is there a better way (more efficient, shorter and/or clearer) to do
this?

There are numerous alternatives:

class B
  attr_accessor :a, :b, :c
end

class B < Struct.new(:a, :b, :c)
end

Also you can use Class#inherited(cl) to do some magic on a sub class.  For
an example (not exactly what you want, but you get the picture):

module WithFields
  def fields(*a)
    @fields = a unless a.empty? ; @fields
  end
end

class Base
  extend WithFields

  def self.inherited(cl)
    cl.extend WithFields
    def cl.inherited(cl2) superclass.inherited(cl2) end
  end
end

class Sub < Base
  fields :a, :b, :c
end

class Sub2 < Sub
  fields :x, :y
end

p Sub.fields
p Sub2.fields

Kind regards

    robert