From: nobu.nokada@...
Date: 2004-01-15T10:49:25+09:00
Subject: Re: Nested scopes and the Singleton pattern

Hi,

At Thu, 15 Jan 2004 06:40:07 +0900,
Hal Fulton wrote:
> By the way, my last change was to implement a simple version of the
> Singleton pattern (without using singleton.rb).
> 
> 
>    # Note: *Reopening* existing classes
> 
>    class ABC
>      class XYZ
>        @@instance = nil
>        def initialize(foo)
>          # Do some stuff...
>          # and then:
>          def XYZ.new(foo)
>            @@instance
>          end
>        end
>      end
>    end
> 
> 
> This works fine.
> 
> First question: Is this a valid way to implement Singleton, or am I
> overlooking something?

Have you considered about race conditions?

> So I did:
> 
>    # Note: *Reopening* existing classes
> 
>    class ABC::XYZ
>      @@instance = nil
>      def initialize(foo)
>        # Do some stuff...
>        # and then:
>        def ABC::XYZ.new(foo)
>          @@instance
>        end
>      end
>    end
> 
> But it didn't work.

def ABC::XYZ means singleton method XYZ of ABC by itself, so
trailing . is superfluous.  Try

       def (ABC::XYZ).new(foo)

-- 
Nobu Nakada