Class-level instance variables

class_level_instance_variables.rb

class Person
  class << self
    # Make it available from the outside
    attr_reader :traits

    # This removes duplication in this class and subclasses
    def init_variables
      @traits = {}
    end

    # Initialize those variables
    def inherited(subclass)
      subclass.init_variables
    end

    # Regular class method
    def taste_buds(value)
      @traits[:taste_buds] = value
    end
  end

  # Person can use those variables too, but doesn't have to
  # if it's considered "abstract"
  init_variables

  # Let's set some
  taste_buds 30
end

class YoungPerson < Person
  taste_buds 500
end

class OldPerson < Person
  taste_buds 50
end

p Person.traits      # => {:taste_buds=>30}
p YoungPerson.traits # => {:taste_buds=>500}
p OldPerson.traits   # => {:taste_buds=>50}