From: Rick DeNatale <rick.denatale@...>
Date: 2006-10-27T23:06:10+09:00
Subject: Re: Singleton Methods

On 10/27/06, Max Lapshin <max@maxidoors.ru> wrote:
>
> On Oct 27, 2006, at 10:15 AM, bparanj@gmail.com wrote:
>
> > What is the purpose of singleton method? Why is singleton method
> > defined on individual object instead of a class? TIA
> >
> >
>
> You call Article.new. new is the singleton method of object Article.
> It is not "class method", there is no such thing in Ruby. There is
> singleton method of object Article

Not really.

irb(main):001:0> class Article
irb(main):002:1>   end
=> nil
irb(main):003:0> Article.method(:new)
=> #<Method: Class#new>

   new is an instance method of Class.

irb(main):004:0> Class.instance_methods.include?("new")
=> true

  Article (or any Class object) is an instance of Class and gets the
new method by normal inheritance.

irb(main):005:0> Article.class
=> Class

irb(main):006:0> Article.method(:new)
=> #<Method: Class#new>

   Each class object does have a singleton class where any class methods live.

irb(main):008:0> Article.singleton_methods
=> []
irb(main):009:0> class Article
irb(main):010:1>   def self.foo
irb(main):011:2>     end
irb(main):012:1>   end
=> nil
irb(main):013:0> Article.singleton_methods
=> ["foo"]

Ruby uses singleton classes of classes instead of explicit
metaclasses, but unlike "normal" singleton classes, singleton classes
of classes get shared with subclasses

irb(main):014:0> class JournalArticle < Article
irb(main):015:1>   end
=> nil
irb(main):016:0> JournalArticle.singleton_methods
=> ["foo"]
irb(main):017:0>

The class and singleton class of class hierarchies parallel each other.

irb(main):017:0> Article.instance_of?(Class)
=> true
irb(main):018:0> Article.class.instance_of?(Class.class)
=> true
irb(main):019:0> JournalArticle.class.instance_of?(Article.class)
=> true
irb(main):020:0> JournalArticle.class.instance_of?(Class.class)
=> true

In general, you can't subclass 'normal' singleton classes. The ruby
implementation does this for singleton classes of classes 'under the
covers.'

irb(main):021:0> a = "foo"
=> "foo"
irb(main):022:0> def a.singleton_method
irb(main):023:1>   "Oh sole mio!"
irb(main):024:1>   end
=> nil
irb(main):025:0> aclass = class << a; self;end
=> #<Class:#<String:0xb7c1ac88>>
irb(main):026:0> class B < aclass
irb(main):027:1>   end
TypeError: can't make subclass of virtual class
        from (irb):26


This double use of singleton classes can be confusing to some who
think that the 'singleton' in 'singleton class' means that it's not
shared with any other object.

Of course all this has probably gotten a tad deep for the context of
the OPs question.


-- 
Rick DeNatale

My blog on Ruby
http://talklikeaduck.denhaven2.com/