From: Gennady <gfb@...>
Date: 2004-08-11T08:28:36+09:00
Subject: Re: Singleton class methods

David A. Black wrote:

> 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

That's where a slight misunderstanding might have occurred: I was 
calling "Foo itself" an original singleton class (isn't it? ;-)), and 
the one created by "class << Foo" -- an anonymous singleton class of Foo 
(i omitted word "anonymous" from my original reply, sorry).

> << Foo, 'self' is the anonymous singleton class of Foo.  Thus the
> constants are different.
> 
> 
> David
>