Class variables in Ruby

Question

I\'ve come across the following example from this tutorial:

class Song
  @@plays = 0

  def initialize(name, artist, duration)
    @name = name
    @artist =         


        

Answer 1

Class variables are never really required. But the reason isn't that they're shared state. I mean, it's good to avoid shared state where you can, but that's not the real problem here.

The reason they're recommended against is, as shown in that article, they are really confusing. In particular, a class's class variables are shared by its subclasses and instances of its subclasses. For example:

class Parent
end

class Child1 < Parent
  @@class_var = "Child1's"
end

class Child2 < Parent
  @@class_var = "Child2's"
end

With this code, Child1 and its instances will all see a class variable named @@class_var with the value "Child1's" and Child2 and its instances will all see a class variable named @@class_var with the value "Child2's". But suppose later on we reopen Parent and write this:

class Parent
  @@class_var = "Parent's"
end

Now Parent and the instances it creates will all see a class variable named @@class_var with the value "Parent's". But that's not all. Now that the parent class has this variable, Child1 and Child2 suddenly share the variable, so all of the @@class_vars have the value "Parent's". And if you reassign the variable in Child1, it's still shared, so all of the classes get updated. How confusing!

Instead of class variables, you can just use instance variables of the class, like this:

class Parent
  @class_var = "Parent's"
  def self.class_var
    @class_var
  end
end

class Child1 < Parent
  @class_var = "Child1's"
end

class Child2 < Parent
  @class_var = "Child2's"
end

Now, Parent.class_var will return "Parent's", Child1.class_var will return "Child1's" and Child2.class_var will return "Child2's" — just like you expect.