From: Markus Jais <info@...>
Date: 2002-04-10T10:37:36+09:00
Subject: class << self   question

hello

in several libraries I found this:
class << self

now I played with it and generated the following sample
code (does not do anything useful, just for playing with the concept)

class Vector
   def initialize (x, y)
          @x = x
          @y = y
   end

   class << self
          def func
                 puts "in func"
          end
   end

   def my_method
          self.func()  # Error!! why does this not work ???
   end

end

v  = Vector.new(2, 4)
Vector.func   # works
v.my_method   # Error 

$ ./class.rb
in func
../class.rb:17:in `my_method': undefined method `func' for 
#<Vector:0x401dbbf8> (NameError)
        from ./class.rb:24

I understand that
class << self 
in this case generates a class method called "func"

but I also thought that 
class << object
extends an object 
(like in the pickaxe book on page 243)

why does this not work with
class << self ??
why can I not call
self.func() in the method "my_method" above ??

maybe I am just doing something wrong.
If I just want to do some helper methods for my class which I use
only internally it might be simpler to write some private methods.

is there an advante using 
class << self
for generating class internal helper functions instead of just private 
functions

please explain me this. it seems to be a useful and powerful 
concepts but I really like to understand it completely.

markus