From: Justin Collins <justincollins@...>
Date: 2011-07-14T10:34:53+09:00
Subject: Re: Issue with: instance_variable_defined?

On 07/13/2011 06:03 PM, Hal Fulton wrote:
> I expect the third line to be true rather than false.
>
> Comments?
>
> Hal
>
> class SomeClass
>        attr_reader :a1, :a2    # Creates @a1, a1, @a2, a2
>        attr_writer :b1, :b2    # Creates @b1, b1=, @b2, b2=
>        attr_accessor :c1, :c2  # Creates @c1, c1, c1=, @c2, c2, c2=
>        # ...
>    def initialize
>      @c2 = 99
>    end
> end
>
> obj = SomeClass.new
> p obj.instance_variable_defined?(:@c2)   # true
> p obj.instance_variable_defined?(:@d2)   # false
> p obj.instance_variable_defined?(:@a2)   # false  :-/
>

I was going to say that the attr_* methods don't create instance 
variables, only the methods to read/write them, as that has been my 
understanding. But actually, the documentation (at least for attr_reader 
and attr_accessor) says they _do_ create instance variables.

http://rdoc.info/stdlib/core/1.9.2/Module:attr_accessor

So, I don't know. Perhaps there is a difference in semantics between 
"create" and "define".

-Justin