Get attr_accessor/instance variables in ruby

Question

So I know that you can get all instance variables in Ruby by calling #instance_variables, but if they haven't yet been set then they don't show up.

Example

class Walrus
  attr_accessor :flippers, :tusks
end

w = Walrus.new
w.instance_variables # => []
w.tusks              # => nil
w.instance_variables # => [:@tusks]

I want to access all of the instance variables defined by attr_accessor immediately.

w = Walrus.new
w.instance_variables # => [:@tusks, :@flippers]

possible duplicate of [How to get attributes that were defined through attr\_reader or attr\_accessor](http://stackoverflow.com/questions/10006889/how-to-get-attributes-that-were-defined-through-attr-reader-or-attr-accessor) — Aidan Feldman, Dec 04 '14 at 22:01

Sergio Tulentsev · Accepted Answer · 2012-05-19T20:18:56.147

15

Well, they don't yet exist. Instance variables spring into existence upon first assignment. If you want them in a brand new instance, then touch them in the constructor.

class Walrus
  attr_accessor :flippers, :tusks

  def initialize
    self.flippers = self.tusks = nil
  end
end

w = Walrus.new
w.instance_variables # => [:@tusks, :@flippers]

edited May 19 '12 at 20:18

answered May 18 '12 at 14:04

Sergio Tulentsev

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score 5 · Answer 2 · answered May 18 '12 at 14:16

Well, attr_accessor creates a pair of methods, a setter and a getter. I'm not sure if there's a built-in way to get a list, but you could look through the instance methods for the resulting pairs:

Walrus.instance_methods.find_all do |method|
  method != :== &&
  method != :! &&
  Walrus.instance_methods.include?(:"#{method}=")
end

score 0 · Answer 3 · answered Feb 15 '15 at 05:23

How about using methods minus the methods from Object or Class? the returning array will contain your defined attributes/methods, and the instance variables defined in getter/accessor.

your_instance_name.methods - Object.methods
your_instance_name.methods - Class.methods

Get attr_accessor/instance variables in ruby

3 Answers3