From: "David A. Black" <dblack@...>
Date: 2004-08-11T08:27:29+09:00
Subject: Re: Singleton class methods

Hi --

On Wed, 11 Aug 2004, Kyle Putnam wrote:

> On Tue, 2004-08-10 at 17:59, Gennady wrote:
> > Robert Klemme wrote:
> > 
> > <<snip>>
> > 
> > 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.
> > 
> > [sparc.titan:13]prod_1> irb
> > irb(main):001:0> class Test
> > irb(main):002:1>   A=45
> > irb(main):003:1> end
> > => 45
> > irb(main):004:0> class Test
> > irb(main):005:1>   A=88
> > irb(main):006:1> end
> > (irb):5: warning: already initialized constant A
> > => 88
> > irb(main):007:0> class Test
> > irb(main):008:1>   class << self
> > irb(main):009:2>     A=55
> > irb(main):010:2>   end
> > irb(main):011:1> end
> > => 55
> > 
> > NOTE: no warning "already initialized constant A" here.
> > 
> > irb(main):012:0> Test::A
> > => 88
> > irb(main):013:0> class Test
> > irb(main):014:1>   class << self
> > irb(main):015:2>     A=99
> > irb(main):016:2>   end
> > irb(main):017:1> end
> > (irb):15: warning: already initialized constant A
> > => 99
> > 
> > NOTE: and here we have the warning again.
> > 
> > irb(main):018:0> class Test
> > irb(main):019:1>   def self.f
> > irb(main):020:2>     puts A
> > irb(main):021:2>   end
> > irb(main):022:1> end
> > => nil
> > irb(main):023:0> Test.f
> > 88
> > => nil
> > irb(main):024:0> class Test
> > irb(main):025:1>   class << self
> > irb(main):026:2>     def f2
> > irb(main):027:3>       puts A
> > irb(main):028:3>     end
> > irb(main):029:2>   end
> > irb(main):030:1> end
> > => nil
> > irb(main):031:0> Test.f2
> > 99
> > => nil
> 
> Your example clears most of this up, for me anyway. However, is there a
> way to access this other scope? Obviously Test::A accesses the
> non-singleton(?) class's constant. Is this other scope even accessable
> from outside the singleton class?
> 
> irb(main):001:0> class << self
> irb(main):002:1>   CONSTANT = 50
> irb(main):003:1> end
> => 50
> irb(main):005:0> self.class::CONSTANT
> NameError: uninitialized constant CONSTANT
>         from (irb):5
> irb(main):006:0> ::CONSTANT
> NameError: uninitialized constant CONSTANT
>         from (irb):6
> irb(main):007:0> class << self
> irb(main):008:1>   CONSTANT
> irb(main):009:1> end
> => 50

It's the same as with any class: you can open its scope with the
'class' keyword (using "<< obj" to indicate the anonymous class you
want), or you can capture it in a constant or variable and class_eval
from there, and so on.  For example (using a non-Class object, just
for a change of pace :-)

  x = "hi"
  class << x
    C = 1
  end

  xclass = (class << x; self; end)

  puts xclass::C    # =>  1

This (class << x; self; end) thing is rather unwieldy, but there's an
RCR pending for Kernel#singleton_class, which would then do for an
object's singleton class what #class does for its class of origin.
Here's that method, plus the above example rewritten to use it:

  module Kernel
    def singleton_class; class << self; self; end; end
  end

  x = "hi"
  x.singleton_class::C = 1     # (or const_set if you prefer)
  p x.singleton_class::C       # =>  1


David

-- 
David A. Black
dblack@wobblini.net