From: "David A. Black" <dblack@...>
Date: 2004-08-11T07:20:49+09:00
Subject: Re: Singleton class methods

Hi --

On Wed, 11 Aug 2004, Gennady wrote:

> I meant that "class << self" creates a separate anonymous singleton 
> class that is associated (?) with an instance of class "Class" 
> representing class "Foo", that is a singleton by itself, of course. That 
> was my understanding of "class <<" idiom.
>
> The more I think about it, the more it seems like the same is happening 
> when you do "def Foo.bar" as well.
>
> The only difference is that "def Foo.bar" reopens the original singleton 
> class for "Foo", where the first occurrence of "class << self" in Foo 
> definition creates a new anonymous singleton class (see bellow).

I'm not clear on your original/new distinction here.  Each object can
have a maximum of one singleton class; so if Foo doesn't have one, it
gets created (when you do "def Foo.meth" or "class Foo; class << self"
or class << Foo).  So there's no original vs. new singleton class for
a given object; it's always the same one.

(And of course the object doesn't have to be a Class object -- it can
be any object.)

> When you have
> 
> class Test
> end
> 
> You can create constants both in
> 
> class Test
>    A=45
>    def self.f
>    end
> end
> 
> and in
> 
> class Test
>    class << self
>      A=45
>      def f
>      end
>    end
> end
> 
> cases. The difference is only where they are visible.

Yes -- see my earlier post on this point.  This is just a case of the
fact that you can define constants in any class scope; and in your
example, both Test and Test's singleton class are Class objects.  


David

-- 
David A. Black
dblack@wobblini.net