How can Ruby's attr_accessor produce class variables or class instance variables instead of instance variables?

Question

If I have a class with an attr_accessor, it defaults to creating an instance variable along with the corresponding getters and setters. But instead of creating an instance variable, is there a way to get it to create a class variable or a class instance variable instead?

Answer 1

Like this:

class TYourClass
  class << self
    attr_accessor :class_instance_variable
  end
end

You can look at this as opening the metaclass of the class (of which the class itself is an instance) and adding an attribute to it.

attr_accessor is a method of class Class, it add's two methods to the class, one which reads the instance variable, and other that set's it. Here's a possible implementation:

class Class
  def my_attr_accessor(name)
    define_method name do
      instance_variable_get "@#{name}"
    end 
    define_method "#{name}=" do |new_val|
      instance_variable_set "@#{name}" new_val
    end
  end
end

Completely untested class atribute accessor:

class Class
  def class_attr_accessor(name)
    define_method name do
      class_variable_get "@@#{name}"
    end 
    define_method "#{name}=" do |new_val|
      class_variable_set "@@#{name}" new_val
    end
  end
end

Answer 2

Work your way through _why's seeing metaclasses clearly [mirror] - it's very helpful when trying to understand the way ruby's classes and objects work

Answer 3

In Rails (or anywhere you require 'activesupport') you can use

cattr_accessor :name

to get true class variable accessors.

The class instance variables that others have pointed out are usually more useful. The APIdock cattr_accessor page has some helpful discussion clarifying when you would want one not the other, plus the source to the cattr_accessor/reader/writer functions.