From: Robert Klemme <bob.news@...>
Date: 2004-09-15T17:09:52+09:00
Subject: Re: Singleton method on object via define_method?


"Mauricio Fern�ndez" <batsman.geo@yahoo.com> schrieb im Newsbeitrag
news:20040914113158.GA12061@student.ei.uni-stuttgart.de...
> On Tue, Sep 14, 2004 at 07:39:57PM +0900, Robert Klemme wrote:
> > > There's at least one case where you can know if the singleton class
has
> > > been created or not (singletons of singletons).
> >
> > How come?  What's special about singletons here?
>
> >> o = Object.new
> => #<Object:0x401fdd20>
> >> class << o; class << self; def foo; "foo" end end end
> => nil
> >> class << o; class << self; foo end end
> => "foo"
> >> class << o; class << self; class << self; end end end
> => nil
> >> class << o; class << self; foo end end
> NameError: undefined local variable or method `foo' for
#<Class:#<Class:#<Object:0x401fdd20>>>
>         from (irb):5

Hm, but what about:

>> o = Object.new
=> #<Object:0x10184d40>
>> class << o; class << self; def to_s ; "foo" end end end
=> nil
>> class << o; class << self;to_s end end
=> "foo"
>> class << o; class << self; class << self; end end end
=> nil
>> class << o; class << self; to_s end end
=> "#<Class:foo>"

> > > And you can always know
> > > if you're using evil.rb, of course :-)
> >
> > How exactly would you want to do it?  I don't see how eval helps here.
>
> eval is evil, but I really meant evil.rb :-)

Oops.  I wasn't aware of evil.rb and assumed an evil pun. :-)  Shame on
me...

> >> require 'evil'
> => true
> >> o = Object.new
> => #<Object:0x4032e35c>
> >> o.internal.klass.to_i
> => 1075690784
> >> class << o; end
> => nil
> >> o.internal.klass.to_i
> => 1076982108
> >> class << o; def foo; end end
> => nil
> >>
ObjectSpace._id2ref(o.internal.klass.to_i/2).instance_methods.include?
"foo"
> => true
> >> klass = ObjectSpace._id2ref(o.internal.klass.to_i/2)
> => #<Class:#<Object:0x4032e35c>>
> >> class << o; self end
> => #<Class:#<Object:0x4032e35c>>
>
> hence
>
> >>  def has_singleton?(obj); obj.class.id * 2 != obj.internal.klass.to_i
end
> => nil
> >> o = ""
> => ""
> >> has_singleton? o
> => false
> >> class << o; end
> => nil
> >> has_singleton? o
> => true

Hm, well ok.  But is there an equivalent without evil.rb, i.e. with
"normal" Ruby code?  I know that you can access all kinds of internals if
you write an extension, but AFAIK you can't know whether there is a
singleton class or not.  Please correct me if I'm wrong here...

Kind regards

    robert