From: "Ara.T.Howard" <ahoward@...>
Date: 2004-02-14T23:54:56+09:00
Subject: Re: accessing constants from class methods

On Sat, 14 Feb 2004, daz wrote:

> Date: Sat, 14 Feb 2004 08:24:08 -0000
> From: daz <dooby@d10.karoo.co.uk>
> Newsgroups: comp.lang.ruby
> Subject: Re: accessing constants from class methods
> 
> 
> "Ara.T.Howard" wrote:
> >
> > 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?
> >
> 
> #-----
> 
> class A
>   class << self
>     def x
>       const_defined?(:X) ? const_get(:X) : nil
>     end
>   end
> end
> 
> class C < A
>   X = 42
> end
> 
> p A.x    # => nil
> p C.x    # => 42
> 
> #-----
> 
> 
> daz

perfect!

-a
-- 
===============================================================================
| EMAIL   :: Ara [dot] T [dot] Howard [at] noaa [dot] gov
| PHONE   :: 303.497.6469
| ADDRESS :: E/GC2 325 Broadway, Boulder, CO 80305-3328
| URL     :: http://www.ngdc.noaa.gov/stp/
| TRY     :: for l in ruby perl;do $l -e "print \"\x3a\x2d\x29\x0a\"";done 
===============================================================================