From: "David A. Black" <dblack@...>
Date: 2008-07-12T00:28:34+09:00
Subject: Re: instance_eval or class_eval on metaclass?

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

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