From: Kyle Putnam <kyle@...>
Date: 2004-08-11T07:51:43+09:00
Subject: Re: Singleton class methods

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