From: "David A. Black" <dblack@...>
Date: 2004-08-11T08:15:34+09:00
Subject: Re: Singleton class methods

Hi --

On Wed, 11 Aug 2004, Gennady wrote:

> David A. Black wrote:
> > 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.
> 
> irb(main):001:0> class Foo
> irb(main):002:1>   A=10
> irb(main):003:1> end
> => 10
> irb(main):004:0> class Foo
> irb(main):005:1>   def self.f1
> irb(main):006:2>     puts A
> irb(main):007:2>   end
> irb(main):008:1> end
> => nil
> irb(main):009:0> class << Foo
> irb(main):010:1>   A=20
> irb(main):011:1> end
> => 20
> irb(main):012:0> class << Foo
> irb(main):013:1>   def f2
> irb(main):014:2>     puts A
> irb(main):015:2>   end
> irb(main):016:1> end
> => nil
> irb(main):017:0> Foo.f1
> 10
> => nil
> irb(main):018:0> Foo.f2
> 20
> 
> At least for "def Foo.f1" and "class << Foo" it is not the same one: one 
> holds "A=10" and another -- "A=20".

But only one of them is a singleton class; the other is simply Foo
itself :-) When you do def Foo.m, 'self' is Foo; but when you do class
<< Foo, 'self' is the anonymous singleton class of Foo.  Thus the
constants are different.


David

-- 
David A. Black
dblack@wobblini.net