From: "Marcin Mielżyński" <lopexx@...>
Date: 2006-01-25T00:41:50+09:00
Subject: Re: "self.class" vs. "(class << self; self; end)"

ssmoot@gmail.com wrote:

Yes there is a difference.
In the first example You simply reopen a class a define a method for it.
In the second example You reopen an eigenclass (which is a singleton 
instance of class object) and then define a method for it.

Ruby returns last evaluated statement which in this case '(class << 
self; self; end)' is eigenclass...

The eigneclass pattern (a metaclass) is a method of creating single 
class instances per instances of them. (I dont know if I made it clear :) )


consider:

def test1()
   msg = 'bar'
   self.class.class_eval do
     p self
     define_method('foo') { msg }
   end
end


def test2
   msg = 'baz'
   (class << self; self; end).class_eval do
     p self
     define_method('foo') { msg }
   end
end


test1
p foo

test2
p foo

output:

Object
"bar"
#<Class:#<Object:0x2859258>>
"baz"

lopex