From: Dave Thomas <dave@...>
Date: 2008-09-01T11:29:26+09:00
Subject: Re: instance_eval/class_eval including/extending modules


On Aug 31, 2008, at 9:15 PM, Patrick Li wrote:

> I must say that I also find the difference between
> instance_eval/class_eval and include/extend extremely confusing.
>
> If someone clearly knows the fundamental rules, please please share.
>
> Here's my understanding:
>
> def myMethod; end
>  -creates a method "myMethod" callable by instances of self.
>
> Here's a list of situations that I'm having trouble with:
>
> 1) class Klass
>     def myMethod; end
>   end
>
>  -self is equal to Klass
>  -so myMethod() is callable by Klass.new
>
> 2) Klass.instance_eval {def myMethod; end}
>
> -self is equal to Klass
> -so myMethod() is callable by Klass.new  <-- this isn't true
>
> What's wrong with my understanding? I think I'm not understanding def
> correctly.

Ruby has two separate concepts: self, the current object, and 'the  
current class', which isn't represented by an externally visible  
variable. Methods are stored in the later, independent of the value of  
self. It _is_ confusing, but you can always remember that class_eval  
creates instance methods and instance_eval creates class methods.  
There are actually good reasons why this is so.

If you want to get deeper into this, <pimp>I could recommend a certain  
set of screencasts...</pimp>


Dave