From: Ben Lipton <ben.lipton@...>
Date: 2007-12-22T11:59:35+09:00
Subject: Re: Singleton Method

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I think the answer is No, there isn't a difference, but it's something I've
been wondering about.
From http://blog.jayfields.com/2007/04/ruby-class-methods.html we have:

"In fact, you can define a class method various ways, but the result is
always an instance method on the singleton class."

class Parser
  def self.self_process(script)
    # ...
  end

  def Parser.parser_process(script)
    # ...
  end

  class << self
    def singleton_process(script)
      # ...
    end
  end

  def self.singleton
    class << self; self; end
  end
end

Now, this is talking about class methods, not singleton methods on arbitrary
objects, but I think the idea still applies because class methods are
methods on a singleton, in this case the object Parser (which happens to be
a Class). So, the second and the third examples are the ones you're asking
about, because inside the "class Parser ... end", self is equal to Parser.
So, he's doing

def Parser.parser_process(script)
end

and

class << Parser
  def singleton_process(script)
  end
end

just like your example.


On Dec 21, 2007 8:24 PM, Joon You <joon@rubyhead.com> wrote:

> Just out of curiosity, is there a difference between
>
> def a.second
> ...
> end
>
> and
>
> class << a
>
> ?
>
> Gary Wright wrote:
> > On Dec 21, 2007, at 6:51 PM, Ryan Lewis wrote:
> >
> >> What's the difference between a regular method and a singleton method?
> >
> >  From the perspective of a particular object, singleton methods are
> > those methods that are defined within the singleton class of
> > the object.
> >
> > Regular methods are methods that are defined within the normal
> > inheritance tree of the object's class.
> >
> > Method lookup is done by first checking the singleton class of an
> > object and if not found there by checking the normal inheritance tree.
> >
> > By default objects don't have a singleton class.  The singleton class
> > is created when it is referenced:
> >
> > $ irb
> >  >> a = [1,2,3]
> > => [1, 2, 3]
> >  >> a.second
> > NoMethodError: undefined method `second' for [1, 2, 3]:Array
> >          from (irb):2
> >          from :0
> >  >> class <<a
> >  >>   def second
> >  >>     self[1]
> >  >>     end
> >  >>   end
> > => nil
> >  >> a.second
> > => 2
> >  >> [4, 5, 6].second
> > NoMethodError: undefined method `second' for [4, 5, 6]:Array
> >          from (irb):9
> >          from :0
> >  >>
> >
> > In this example, I opened up the singleton class associated
> > with 'a' and defined a new method, second, that returns the
> > second element of the array.  As you can see, this method
> > is only available to that one particular array and not to
> > all array's in general.
> >
> > Gary Wright
>
> --
> Posted via http://www.ruby-forum.com/.
>
>

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