Are Ruby class variables bad?

Question

When I implemented an "instance of"/singleton type pattern, RubyMine notified that using class variables were considered bad form.

The only information I came across is that using class variables can make inheritance a bit squirrelly. Is there any other reason why the following code would give me problems?

class Settings
  private_class_method :new
  attr_accessor :prop1
  attr_accessor :prop2

  @@instance = nil

  def Settings.instance_of
    @@instance = new unless @@instance
    @@instance
  end
  def initialize
    @prop2 = "random"
  end
end

Also, is there a better way, Ruby-wise, to achieve the same objective to ensure only a single instance?

Answer 1

The problem with class variables in Ruby is that when you inherit from a class then the new class does not get a new copy of its own class variable but uses the same one that it inherited from its superclass.

For example:

class Car
  @@default_max_speed = 100
  def self.default_max_speed
    @@default_max_speed
  end
end

class SuperCar < Car
  @@default_max_speed = 200 # and all cars in the world become turbo-charged
end

SuperCar.default_max_speed # returns 200, makes sense!
Car.default_max_speed # returns 200, oops!

The recommended practice is to use class instance variables (remember that classes are simply objects of class Class in Ruby). I highly recommend reading Chapter 14 of Eloquent Ruby by Russ Olsen, which covers this topic in detail.