# Instance Variable Names Available from Class? **URL:** \ **Category:** Ruby\ **Created:** [February 26, 2009, 5:44pm UTC](https://www.ruby-forum.com/t/instance-variable-names-available-from-class/161455 "2009-02-26T17:44:24Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![farfignugen](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/f17d59/32.png) [@farfignugen](https://www.ruby-forum.com/u/farfignugen)\ **Post date:** [February 26, 2009, 5:44pm UTC](https://www.ruby-forum.com/t/instance-variable-names-available-from-class/161455/1 "2009-02-26T17:44:24Z")
In metaprogramming, I know how to get the instance variable names from an Object (through instance\_variables). Is there a way to get them from a Class? ---
**Author:** ![farfignugen](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/f17d59/32.png) [@farfignugen](https://www.ruby-forum.com/u/farfignugen)\ **Post date:** [February 26, 2009, 6:28pm UTC](https://www.ruby-forum.com/t/instance-variable-names-available-from-class/161455/2 "2009-02-26T18:28:27Z")
On Thu, Feb 26, 2009 at 10:13 PM, Chris Gardner [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > In metaprogramming, I know how to get the instance variable names from > an Object (through instance\_variables). Is there a way to get them from > a Class? Hello Chris, before answering your question, let me ask: does #instance\_variables really work the way you think it does? Consider: $: irb 01\> class Foo 02\> def initialize 03\> @bar = ‘bar’ 04\> end 05\> def baz=(arg) 06\> @baz = arg 07\> end 08\> end → nil 09\> Foo.new.instance\_variables → [:@bar] Why isn’t @baz listed? What would you have to do before it would be listed? Ponder on that, then read over this previous thread: [http://www.ruby-forum.com/topic/107601](http://www.ruby-forum.com/topic/107601) Hope that gives you some new perspectives (and eventually, the answer to your original question!) Cheers, lasitha ---
**Author:** ![farfignugen](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/f17d59/32.png) [@farfignugen](https://www.ruby-forum.com/u/farfignugen)\ **Post date:** [February 26, 2009, 8:51pm UTC](https://www.ruby-forum.com/t/instance-variable-names-available-from-class/161455/3 "2009-02-26T20:51:01Z")
Chris Gardner wrote: > In metaprogramming, I know how to get the instance variable names from > an Object (through instance\_variables). Is there a way to get them from > a Class? A class _is_ an object, with its own instance variables, and you can see them in the same way: class Foo @bar = 123 def self.flurble @bar end end p Foo.instance\_variables # ["@bar"] p Foo.flurble # 123 Or did you mean something else? ---
**Author:** ![farfignugen](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/f17d59/32.png) [@farfignugen](https://www.ruby-forum.com/u/farfignugen)\ **Post date:** [February 26, 2009, 9:00pm UTC](https://www.ruby-forum.com/t/instance-variable-names-available-from-class/161455/4 "2009-02-26T21:00:46Z")
What I mean is that I would like to ask a class for the names of instance variables of its objects, which may have yet to be instantiated. class Foo def intialize(bar, baz) @bar = bar @baz = baz end end I was trying to ask class Foo for the names @bar and @baz. I’ve found out that these are accessible only through Foo instances. I’m a Ruby newbie and am just today learning about Ruby’s approach to metaprogramming. Brian C. wrote: > Chris Gardner wrote: > > > In metaprogramming, I know how to get the instance variable names from > > an Object (through instance\_variables). Is there a way to get them from > > a Class? > > A class _is_ an object, with its own instance variables, and you can see > them in the same way: > > class Foo > @bar = 123 > > def self.flurble > @bar > end > end > > p Foo.instance\_variables # ["@bar"] > > p Foo.flurble # 123 > > Or did you mean something else? ---
**Author:** ![farfignugen](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/f17d59/32.png) [@farfignugen](https://www.ruby-forum.com/u/farfignugen)\ **Post date:** [February 26, 2009, 9:30pm UTC](https://www.ruby-forum.com/t/instance-variable-names-available-from-class/161455/5 "2009-02-26T21:30:33Z")
On Feb 26, 2009, at 2:59 PM, Chris Gardner wrote: > What I mean is that I would like to ask a class for the names of > instance variables of its objects, which may have yet to be > instantiated. The question doesn’t apply in Ruby since instance variables are dynamically created on a per object basis. Even if you use standard helpers such as attr\_accessor and company, those methods simply create instance methods that manipulate instance variables–they don’t actually create the instance variables. Just to be clear, there is no language-supported way to introspect a Ruby class and determine what instance variables may or may not be supported by all instances of an object. You can’t even introspect to find out what methods were created by the attr\* family of methods and what methods were simply defined explicitly. Sure you could come up with heuristics such as: If there is a method called x and another called x= then there is probably an instance variable named @x. But that is just a guess, not something supported by the language semantics. The idea that each instance of a class can have its own behavior and state that is independent of its class definition and that can evolve over time is something that must be understood before a programmer can fully grasp Ruby’s programming and object model. Gary W. ---
**Author:** ![farfignugen](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/f/f17d59/32.png) [@farfignugen](https://www.ruby-forum.com/u/farfignugen)\ **Post date:** [February 27, 2009, 2:55am UTC](https://www.ruby-forum.com/t/instance-variable-names-available-from-class/161455/6 "2009-02-27T02:55:14Z")
On Thu, Feb 26, 2009 at 2:59 PM, Chris Gardner [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)wrote: > I was trying to ask class Foo for the names @bar and @baz. I’ve found > out that these are accessible only through Foo instances. > > I’m a Ruby newbie and am just today learning about Ruby’s approach to > metaprogramming. [http://talklikeaduck.denhaven2.com/articles/2008/02/08/whose-variable-is-it-anyway](http://talklikeaduck.denhaven2.com/articles/2008/02/08/whose-variable-is-it-anyway) – Rick DeNatale Blog: [http://talklikeaduck.denhaven2.com/](http://talklikeaduck.denhaven2.com/) Twitter: [http://twitter.com/RickDeNatale](http://twitter.com/RickDeNatale) WWR: [http://www.workingwithrails.com/person/9021-rick-denatale](http://www.workingwithrails.com/person/9021-rick-denatale) LinkedIn: [http://www.linkedin.com/in/rickdenatale](http://www.linkedin.com/in/rickdenatale)