From: Pat Maddox <pergesu@...>
Date: 2006-05-14T20:05:18+09:00
Subject: Re: Making sense of the various Ruby "eval"s

On 5/14/06, dblack@wobblini.net <dblack@wobblini.net> wrote:
> Hi --
>
> On Sun, 14 May 2006, Pat Maddox wrote:
>
> > I genuinely have no clue what the difference is between class_eval and
> > instance_eval.  Check out this quick irb session:
> >
> > irb(main):111:0> class Foo; end
> > => nil
> > irb(main):112:0> Foo.instance_eval { define_method(:foo) { "foo" } }
> > => #<Proc:0x00318b00@(irb):112>
> > irb(main):113:0> Foo.foo
> > NoMethodError: undefined method `foo' for Foo:Class
> >       from (irb):113
> >       from :0
> > irb(main):114:0> Foo.new.foo
> > => "foo"
> > irb(main):115:0> class Bar; end
> > => nil
> > irb(main):116:0> Bar.class_eval { define_method(:bar) { "bar" } }
> > => #<Proc:0x003078a0@(irb):116>
> > irb(main):117:0> Bar.bar
> > NoMethodError: undefined method `bar' for Bar:Class
> >       from (irb):117
> >       from :0
> > irb(main):118:0> Bar.new.bar
> > => "bar"
> >
> > I would think that since I used class_eval in one and instance_eval in
> > the other, SOMETHING should be different..
>
> Fear not; it is :-)  Look at what happens when you use def:
>
> >> class C; end
> => nil
> >> C.instance_eval { def x; 1; end }  # singleton method on C
> => nil
> >> C.x
> => 1
> >> C.new.x
> NoMethodError: undefined method `x' for #<C:0x35b16c>
>          from (irb):4
>          from :0
> >> C.class_eval { def y; 1; end } # instance method of C
> => nil
> >> C.y
> NoMethodError: undefined method `y' for C:Class
>          from (irb):6
>          from :0
> >> C.new.y
> => 1
>
> C.instance_eval handles the business of C itself, as an object.  You'd
> get similar results with any_object.instance_eval { def x; 1; end }
>
> C.class_eval takes you into a class definition block for C, where def
> is an instance method of C.  So it's much more class (and module)
> specific in its engineering.
>
>
> David
>
> --
> David A. Black (dblack@wobblini.net)
> * Ruby Power and Light, LLC (http://www.rubypowerandlight.com)
>    > Ruby and Rails consultancy and training
> * Author of "Ruby for Rails" from Manning Publications!
>    > http://www.manning.com/black
>
>

Okay so I guess the problem is that I'm using define_method?  I don't
understand the difference between define_method and def then.

define_method: "Defines an instance method in the receiver...This
block is evaluated using instance_eval..."

After first reading that, I thought it explained why there was no
difference between using instance_eval or class_eval with
define_method, because define_method uses instance_eval itself.
However isn't self different when I use instance_eval vs class_eval,
so then the inner instance_eval call would operate on a different
self, making the results different?

Pat