Inherit class-level instance variables in Ruby?

Question

I want a child class to inherit a class-level instance variable from its parent, but I can't seem to figure it out. Basically I'm looking for functionality like this:

class Alpha
  class_instance_inheritable_accessor :foo #
  @foo = [1, 2, 3]
end

class Beta < Alpha
  @foo << 4
  def self.bar
    @foo
  end
end

class Delta < Alpha
  @foo << 5
  def self.bar
    @foo
  end
end

class Gamma < Beta
  @foo << 'a'
  def self.bar
    @foo
  end
end

And then I want this to output like this:

> Alpha.bar
# [1, 2, 3]

> Beta.bar
# [1, 2, 3, 4]

> Delta.bar
# [1, 2, 3, 5]

> Gamma.bar
# [1, 2, 3, 4, 'a']

Obviously, this code doesn't work. Basically I want to define a default value for a class-level instance variables in the parent class, which its subclasses inherit. A change in a subclass will be the default value then for a sub-subclass. I want this all to happen without a change in one class's value affecting its parent or siblings. Class_inheritable_accessor gives exactly the behavior I want... but for a class variable.

I feel like I might be asking too much. Any ideas?

Answer 1

Rails has this built into the framework as a method called class_attribute. You could always check out the source for that method and make your own version or copy it verbatim. The only thing to watch out for is that you don't change the mutable items in place.