From: The Higgs bozo <higgs.bozo@...>
Date: 2008-12-03T05:12:51+09:00
Subject: Re: Class atrr_accessor

Oliver Saunders wrote:
> Why does this not work?
> 
>>> class Foo
>>>   self.class.attr_accessor :baz
>>>   end
> TypeError: Foo is not a class
>         from (irb):19

You must have miscopied your error message.  I obtain

  NoMethodError: private method `attr_accessor' called for Class:Class

attr_accessor is a private method of Module, and as such cannot be 
called directly with the dot notation.  You can call it with send(), or 
like this:

  class Foo
    self.class.instance_eval { attr_accessor :baz }
  end

But nevermind, because this is not what you want!

  Foo.baz = :qux
  p Foo.baz # => :qux

  class Bar
  end

  Bar.baz = 4
  p Bar.baz # => 4

You've just added methods #baz and #baz= to every class.  This is 
because

  class Foo
    p(self.class == Class) # => true
  end

You want the self << class construct that you mentioned, which is the 
singleton class,

  class Foo
    class << self
      attr_accessor :baz
    end
  end

When you said self.class, you were calling Object#class which tells you 
the birth canal of an object.  But many things can happen after birth, 
and kids can end up really different from their parents!  This 
difference is described in the singleton class.  It's unique to the 
object.  Above, we are adding a method to Foo's singleton class.

BTW there's nothing special about "class << self".  You could also say,

class Foo
end
class << Foo
  attr_accessor :baz
end

which is the same thing.  "class << x" grabs the singleton class of x.

Hope this helps.
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