example.rb

# This has issues with inheritance because @@thing isn't idependant to each class
class Parent
  @@thing = []

  def self.thing
    @@thing
  end
end

class Child < Parent
end

Parent.thing << :a
Child.thing
# => :a


# This has issues with inheritance because @thing isn't set for child classes
class Parent
  @thing = []

  def self.thing
    @thing
  end
end

class Child < Parent
end

Parent.thing << :a
Child.thing
# => nil


# This one has no issues with inheritance

class Parent
  def self.thing
    @thing ||= []
  end
end

class Child << Parent
end

Parent.thing << :a
Child.thing
# => []