Class variables! What are they, even.

Class variables are variables whose value changes across instances! You can recognize them because they are prepended with @@.

But that definition is pretty opaque. Let’s use an example of a time when information gets uniformly (and rapidly) changed!

With apologies to anyone who’s read 1984 more recently than – aw geez – almost 20 years ago, here’s a spoiler-less summary of what you need to know to follow along:

In 1984 there’s this concept called Doublethink, wherein the people manage to basically avoid contradictory thought. The government fights a perpetual war alongside or against one of two other states – Eurasia and Eastasia. The history books are re-written every time an alliance changes, hence the doublethink-inspired idiom, “We have always been at war with Eastasia.”

We can imagine this rewriting of history with Ruby’s class variables because changing the value of a class variable changes across all instances of that class. You see:

class Doublethink
  @@always_at_war_with = "Eurasia"

  # returns the value of @@always_at_war_with
  # when we begin, it's Eurasia
  def always_at_war_with()

  # sets the value of @@always_at_war_with
  # we can change this arbitrarily,
  # as you will see
  def enemy(them)
    @@always_at_war_with = them
# let's write a history book
history_book =
history_book.always_at_war_with # => "Eurasia"

# what do the newspapers say?
newspaper =
newspaper.always_at_war_with # => "Eurasia"

# oh but wait

# now who are we always at war with?
# let's ask the newspaper
newspaper.always_at_war_with # => "Eastasia"

# and what does our history book tell us?
history_book.always_at_war_with # => "Eastasia"

# what about a new history book we just wrote?
new_history_book =
new_history_book.always_at_war_with # => "Eastasia"

# and if our new history book determines
# we've always been at war with Eurasia, guess what?
new_history_book.always_at_war_with # => "Eurasia"
history_book.always_at_war_with # => "Eurasia"
newspaper.always_at_war_with # => "Eurasia"

We can go back and forth like this indefinitely. (It’s a perpetual war, after all.) There isn’t a single instance of the class Doublethink that won’t have a uniform value for the class variable @@always_at_war_with. Cool, right?

You can also do some Doublethink with instance variables, but that’s for another day:

class Doublethink
  attr_reader :twoplustwo

  def initialize()
    @twoplustwo = 5

Thanks to Richo for helping me understand class variables (finally)