From: Mark Hubbart <discordantus@...>
Date: 2005-04-29T15:46:02+09:00
Subject: Re: determining when inside 'class << self'

On 4/28/05, Assaph Mehr <assaph@gmail.com> wrote:
> 
> Ara.T.Howard wrote:
> > i like a method like this
> >
> >    def inside_metaclass?
> >      # to be implemented
> >    end
> >
> >    class C
> >      class << self
> >        p inside_metaclass? #=> true
> >      end
> >
> >      p inside_metaclass? #=> false
> >    end
> >
> >    p inside_metaclass? #=> false
> >
> >    class << []
> >      p inside_metaclass? #=> true
> >    end
> >
> > can this be done?
> 
> Would you settle for testing if you're inside an anonymous class?
> Anonymous classes have names that look like:
> #<Class:...>
> 
> So you can define your method as:
> 
> def inside_metaclass?
>   Integer === (self.to_s =~ /^#<Class:/)
> end
> 
> Which will evaluate to true for anonymous classes. Since (I believe)
> all meta-classes are anonymous that can give you a pretty good guess
> (or at least has the same result as your sample).

To expand on that... metaclasses have a peculiar inspect value. This
code is breakable, but probably only if you try to:

class Class
  def metaclass?
    id = inspect[/\A\#<Class:\#<.+?\:0x(.+?)>>\Z/, 1]
    ObjectSpace._id2ref(id.to_i(16)/2) if id
  end
end

The metaclass? method will return nil if the receiver is not a
metaclass, or it's instance if it *is* a metaclass. Use it like this:

class << (a="test")
  p self.metaclass?  #==> prints "test"
end

class Foo
  p self.metaclass? #==> prints nil
end

There's probably a safer way to do this using ruby/dl, or evil.rb. I
don't know, though.

cheers,
Mark

> Out of curiosity I've run:
> ObjectSpace.each_object { |o| p [o, o.class] if o.to_s =~ /^#<Class:/ }
> 
> Which returned ~30 such objects (anonymous classes, unless I'm
> mistaken). So unless you're opening an anonymous class, you should be
> OK (and I'm not even sure you can reopen an anonymous class).
> 
> HTH,
> Assaph
> 
>