From: Robert Klemme <bob.news@...>
Date: 2004-08-11T06:06:14+09:00
Subject: Re: Singleton class methods


"Gennady" <gfb@tonesoft.com> schrieb im Newsbeitrag
news:411926AD.1050408@tonesoft.com...
> David A. Black wrote:
> > Hi --
> >
> > On Wed, 11 Aug 2004, Gennady wrote:
> >
> >
> >>Kyle Putnam wrote:
> >>
> >>>Hello,
> >>>
> >>>It seems these two syntaxes accomplish the same thing, or are there
> >>>differences?
> >>>
> >>>    class Foo
> >>>        class << self
> >>>            def bar
> >>>                # ...
> >>>            end
> >>>        end
> >>>    end
> >>>
> >>>compared to...
> >>>
> >>>    class Foo
> >>>        def Foo.bar
> >>>            # ...
> >>>        end
> >>>    end
> >>>
> >>>Cheers,
> >>>    Kyle
> >>>
> >>>
> >>
> >>Exactly the same from the functionality point of view (it differs in
> >>implementation, though). I personally prefer the first form, as when it
> >>comes to renaming a class, you can do it in one place. Also with this
> >>form you can do other nice things such as defining class attribute
> >>accessors with attr_reader, attr_writer or attr_accessor.
> >
> >
> > One difference (I believe this was pointed out to me once by Guy
> > Decoux, in response to the same question) is the scope of constants.
> > If you open the new scope with << self, constants visible in method
> > definitions will be those of that singleton class, rather than those
> > of the original class.
>
> That's kind of what I meant. It seems like when you do
>
> class Foo
>    def self.bar
>    end
> end
>
> method "bar" is added to the list of methods of the original class Foo.

We have to be precise here to avoid confusion: did you mean as instance
method?  Then that's wrong.  This method is added to the singleton class the
same way in both approaches.

$ irbs
>> class Foo;end
=> nil
>> class Foo
>> class <<self
>> p( instance_methods.include?( "bar" ) )
>> end
>> end
false
=> nil
>> class Foo
>> def self.bar;end
>> end
=> nil
>> class Foo
>> class <<self
>> p( instance_methods.include?( "bar" ) )
>> end
>> end
true
=> nil


> On the other hand, for
>
> class Foo
>    class << self
>      def bar
>      end
>    end
> end
>
> a singleton class is created for class object "Foo" and method "bar" is
> defined in that singleton class, with all consequences. Hence, the
> difference in implementation.
>
> It is how I see it. I may be completely wrong, though.

I'm afraid you're wrong here.  :-)  A difference is though that you can
define constants for the singleton class using the <<self idiom while you
can't with the direct approach AFAIK.

Kind regards

    robert