From: "David A. Black" <dblack@...>
Date: 2004-02-15T00:40:32+09:00
Subject: Re: accessing constants from class methods

Hi --

On Sat, 14 Feb 2004, Ara.T.Howard wrote:

> 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!

OK, it's probably something simple but I'm staring and staring at the
original code, and its output (which is the same as daz's code), and I
can't figure out why it wasn't OK.

Explanations welcome :-)


David

-- 
David A. Black
dblack@wobblini.net