From: Robert Klemme <shortcutter@...>
Date: 2006-11-19T01:30:10+09:00
Subject: Re: How to call a class method via the singleton if it exists

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On 18.11.2006 16:56, Trans wrote:
> I want to call a class level method from the the instance level, but I
> need it to go through the singeton if it exists. Here is the most basic
> example:
> 
>   class X
>     def self.f ; "A" ; end
>     def g ; self.class.f ; end
>   end
> 
>   x = X.new
>   x.g  #=> "A"
> 
>   def x.f; "B"; end
>   x.g  #=> "A"
> 
> How do I get the last to return "B"? I know I could use
> '(class<<self;self;end)' instead of 'self.class', but that will create
> a singelton even if doesn't exist, which seems very wasteful. Yet I
> don't recall any (non-hack) way to detect if a singleton is defined. Is
> there?
> 
> It would be nice if there was a reference with which we could call up
> through the class level like this.

Two ideas attached.

	robert


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#!/usr/bin/ruby

class Object
  # option 1
  def dispatch(m,*a,&b)
    (respond_to?(m) ? self : self.class).
      send(m,*a,&b)
  end

  # option 2
  class Dispatcher
    def initialize(*a) @a=a end
    def method_missing(m,*a,&b)
      @a.each {|o| return o.send(m,*a,&b) if o.respond_to? m}
      super
    end
  end

  def disp; Dispatcher.new(self, self.class) end
end

  class X
    def self.f ; "A" ; end
    def g ; self.class.f ; end
    def h ; dispatch :f ; end
    def i ; disp.f ; end
  end

  x = X.new
p  x.g  #=> "A"
p  x.h  #=> "A"
p  x.i  #=> "A"

  def x.f; "B"; end
p  x.g  #=> "A"
p  x.h  #=> "B"
p  x.i  #=> "B"



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