From: Sean O'Halpin <sean.ohalpin@...>
Date: 2007-11-17T04:13:46+09:00
Subject: Re: What is the correct way to check for existence of virtual class?

On Nov 16, 2007 5:56 PM, Voroztsov Artem <artem.voroztsov@gmail.com> wrote:
> 2007/11/16, Rick DeNatale <rick.denatale@gmail.com>:
>
> > On Nov 16, 2007 8:47 AM, Voroztsov Artem <artem.voroztsov@gmail.com> wrote:
> > > One can write:
> > >
> > >   class Object
> > >     def singleton_class
> > >       class <<self;  self; end rescue nil
> > >     end
> > >   end
> > >
> > >   puts "aaa".singleton_class
> > >   puts 8.singleton_class
> > >
> > > Is it the assumed way to do the job?
> >
> > There's a bit of a Heisenberg problem here, in that the act of
> > measuring alters the result.
> >
> > Singleton instance classes aren't created until they are needed, but
> > one of the things which creates them is class <<x;end so you're really
> > not checking whether or not the singleton class already exists this
> > way.
> >
> > Another way would be
> >
> >    !var.singleton_methods.empty?
> >
> > BTW, your 8.singleton_class will fail, Fixnums can't have singleton classes.
> >
> > --
> > Rick DeNatale
> >
> > My blog on Ruby
> > http://talklikeaduck.denhaven2.com/
> >
> >
>
> Thanks!
>
> I see.
>
> In fact I want to create singleton class  if it is possible.
>
> I guess checking if singleton class already exists should be tricky
> coding or impossible.
> Let's look at the code:
>
>   class Object
>     def singleton_class
>       class <<self;  self; end rescue nil
>     end
>   end
>
>   ["abc", 8].each do |a|
>     puts "\nObject: #{a}"
>
>     puts !a.singleton_methods.empty?
>
>     puts a.singleton_class
>
>     puts !a.singleton_methods.empty?
>   end
>
>
> Output is
>
>   Object: abc
>   false
>   #<Class:#<String:0x2b28c50>>
>   false
>
>   Object: 8
>   false
>   nil
>   false
>
>
> So it looks like expression
>
>   !a.singleton_methods.empty?
>
> can miss existence of singleton class.
>
> But in fact these two problems have pragmatic sense:
> 1) Get singleton class if it is possible
> 2) Check if objects has singleton methods
>
> While this one -- not :
> 3) Check if singleton class exists
> (now I understand that there is no use case for it)
>
> Artem
>
>
FWIW, these are my experiments with trying to find a way to detect if
I'm currently executing within a singleton scope:

def in_singleton?
  if self.respond_to?(:superclass)
    if superclass && superclass.superclass
      # the mystery of that which is its own parent <#Class:Class> (&
grandparent, ad infinitum)
      superclass.superclass.superclass == superclass.superclass
    else
      false
    end
  else
    false
  end
end
public :in_singleton?

class O
  in_singleton? # => false
end
class O
  self.in_singleton? # => false
end
class O
  class << self
    in_singleton? # => true
  end
end

class P
end

o = P.new
o.in_singleton? # => false
class << o
  in_singleton? # => true
end
o.in_singleton? # => false
nil.in_singleton? # => false
NilClass.in_singleton? # => false
Object.in_singleton? # => false
Class.in_singleton? # => false
class Class
  in_singleton? # => false
  class << self
    in_singleton? # => true
  end
end
class Qux
  in_singleton? # => false
  class << self
    in_singleton? # => true
  end
end
# but...
class Object
  in_singleton? # => false
  class << self
    in_singleton? # => false
  end
end

Regards,
Sean