From: "Christopher J. Bottaro" <cjbottaro@...>
Date: 2008-07-15T10:41:36+09:00
Subject: Re: instance_eval or class_eval on metaclass?

Ahh, thanks for clearing that up for me.

On Jul 11, 10:28 am, "David A. Black" <dbl...@rubypal.com> wrote:
> Hi --
>
>
>
> On Sat, 12 Jul 2008, Christopher J. Bottaro wrote:
> > I was looking at someone's implementation of #meta_eval and I noticed
> > they used #instance_eval.  I've always been using #class_eval with
> > metaclasses and getting my desired effect.  So why use #instance_eval
> > over #class_eval when working with metaclasses?
>
> > Furthermore, I can't even tell a difference, as demonstrated by this
> > code snippet.
>
> > class Object
> >  def metaclass
> >    class << self
> >      self
> >    end
> >  end
> > end
>
> > class Test
>
> >  def f
> >    metaclass.class_eval do
> >      puts self.inspect
> >    end
> >  end
>
> >  def g
> >    metaclass.instance_eval do
> >      puts self.inspect
> >    end
> >  end
>
> > end
>
> > t = Test.new
> > t.f
> > t.g
>
> > #<Class:#<Test:0x8eae4>>
> > #<Class:#<Test:0x8eae4>>
>
> > Using #class_eval and #instance_eval both produce the same result.
> > Can anyone shed some light on this subject?
>
> They give you the same self but they behave differently in the face of
> 'def'.
>
> class Object
>    def singleton_class   # Sorry; it's what makes sense to me.
>      class << self
>        self
>      end
>    end
> end
>
> class TestMe
>
>    def f
>      singleton_class.class_eval do
>        puts self.inspect
>        def x
>          puts "def in class_eval == instance method"
>        end
>      end
>    end
>
>    def g
>      singleton_class.instance_eval do
>        puts self.inspect
>        def y
>          puts "def in obj. instance_eval == singleton method on obj"
>        end
>      end
>    end
>
> end
>
> t = TestMe.new
> t.f
> t.g
> t.x
> t.singleton_class.y
>
> You can see the same thing one level down too:
>
> >> class C; end
> => nil
> >> C.class_eval { def x; end }
> => nil
> >> C.instance_eval { def y; end }
> => nil
> >> C.new.x
> => nil
> >> C.y
>
> => nil
>
> David
>
> --
> Rails training from David A. Black and Ruby Power and Light:
>    Intro to Ruby on Rails  July 21-24      Edison, NJ
>    Advancing With Rails    August 18-21    Edison, NJ
> Seehttp://www.rubypal.comfor details and updates!