From: Brian Candler <b.candler@...>
Date: 2009-12-14T19:51:13+09:00
Subject: Re: ruby eval magic

unknown wrote:
> It seems that we don't need class_eval method. Because if we use a
> Module or Class directly, we can use:
> SomeClass.instance_eval { ... }
> 
> and if we have an object and we need to work on its class, we can use:
> some_object.class.instance_eval { ... }
> 
> So, do we need class_eval/module_eval?

Yes, we do. But it took me a while until I found out why.

When Ruby code is executing, there are two bits of state which are 
maintained:
- the current object, which is exposed as 'self'
- the current class, where instance methods are defined. This is pretty 
well hidden, but sometimes it matters.

Try the following code:

    class Foo; end

    Foo.class_eval "def bar; puts 'Hello!'; end"

    Foo.new.bar

If you change class_eval to instance_eval, it won't work as shown. In 
fact you'll make a class method on Foo (Foo.bar)

AFAIK, class_eval and module_eval are the same thing though.
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