From: Gennady <gfb@...>
Date: 2004-08-11T07:52:35+09:00
Subject: Re: Singleton class methods

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".

Nevermind, my only point in a reply to the original post was that 2 
syntactic forms provide same functionality, but differ in 
implementation. Looks like I am wrong, implementation is quite similar ;-).

> 
> (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
>