From: Robert Klemme <bob.news@...>
Date: 2004-02-16T20:29:55+09:00
Subject: Re: accessing constants from class methods


"Ara.T.Howard" <Ara.T.Howard@noaa.gov> schrieb im Newsbeitrag
news:Pine.LNX.4.44.0402131650180.4232-100000@fattire.ngdc.noaa.gov...
>
> what i would like to accomplish would be something like this:
>
>
>   class Abstract
>     class << self
>       def x
>         # defined?(self::X)  ? self::X : nil
>       end
>     end
>   end
>
>   class Concrete < Abstract
>     X = 42
>   end
>
>   # p Base.x    # => nil
>   # p Derived.x # => 42
>
>
> does this make sense?  in essence i want an abstract class method with
the
> semantics of "iff a concrete class has defined a certain constant,
return it"
>
> the problem is that 'self::X' above tries to reference 'Class::X', if i
> replace 'self' with 'Abstract' then i will be referencing the incorrect
'X'...
>
> how does one say 'the constant of the current class' from within a class
> method?

Use instance vars of classes:

class Class
  def x
    ancestors.each do |cl|
      val = cl.instance_variable_get("@x")
      return val if val
    end

    nil
  end

  def x=(val); @x=val; end
end

class Abstract; end

class Concrete < Abstract
  self.x = 42
end

class MostConcrete < Concrete
  self.x = "forty two"
end

p Abstract.x
p Concrete.x
p MostConcrete.x

Regards

    robert