# Issue with: instance\_variable\_defined? **URL:** \ **Category:** ruby-talk\ **Created:** [14 July 2011 01:03 UTC](https://rubytalk.org/t/issue-with-instance-variable-defined/63037 "2011-07-14T01:03:45Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![HAL\_90001](https://yyz1.discourse-cdn.com/flex029/user_avatar/rubytalk.org/hal_90001/32/2046_2.png) [@HAL\_90001](https://rubytalk.org/u/HAL_90001)\ **Post date:** [14 July 2011 01:03 UTC](https://rubytalk.org/t/issue-with-instance-variable-defined/63037/1 "2011-07-14T01:03:45Z")
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 :-/ ---
**Author:** ![Justin\_Collins](https://avatars.discourse-cdn.com/v4/letter/j/3e96dc/32.png) [@Justin\_Collins](https://rubytalk.org/u/Justin_Collins)\ **Post date:** [14 July 2011 01:34 UTC](https://rubytalk.org/t/issue-with-instance-variable-defined/63037/2 "2011-07-14T01:34:53Z")
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](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 > **···** > > 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 :-/ ---
**Author:** ![Tim\_Pease](https://avatars.discourse-cdn.com/v4/letter/t/3e96dc/32.png) [@Tim\_Pease](https://rubytalk.org/u/Tim_Pease)\ **Post date:** [14 July 2011 04:52 UTC](https://rubytalk.org/t/issue-with-instance-variable-defined/63037/3 "2011-07-14T04:52:37Z")
The behavior seems correct. The "rb\_attr" method in eval.c only creates a reader method and a writer method. It cannot initialize an instance variable since it operates at the class level, not the instance level. In order to create the behavior you expect, the attr\_\* methods would have to inject "magic" into the object initialization sequence. That is the only way a class level method could initialize an instance variable for every object instantiated from the class. I do not see any evidence of such magic in the C-level "rb\_attr" method. I believe you are getting a false value correctly.   p obj.instance\_variable\_defined?(:@b2) # false   obj.b2 = 'foo'   p obj.instance\_variable\_defined?(:@b2) # true Since the :a2 accessor is only creating a reader, you will never be able to create an instance variable in the normal fashion ...   obj.instance\_variable\_set(:@a2, 'bar')   p obj.instance\_variable\_defined?(:@a2) # true But that is an obvious example to the point of absurdity. Blessings, TwP > **···** > > On Jul 13, 2011, at 7:03 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 :-/