What's a Class Variable in Ruby?

In Ruby, a class variable is a variable declared inside a class that's shared between the class and all its subclasses (i.e. each class instance has the same copy of a class variable). It is:

• Static — i.e. only a single copy of the variable exists, regardless of however many instances of the class you create;
• Created by adding a double at sign (@@) before a variable name (e.g. @@variable_name);
• Always private to outside access;
• Potentially problematic when it comes to inheritance.

For example, consider the following class where a static counter is incremented each time a new object is instantiated:

class Entity
    @@counter = 0

    def initialize
        @@counter += 1
    end

    def counter
        @@counter
    end
end

a = Entity.new
b = Entity.new
c = Entity.new

puts a.counter #=> 3
puts b.counter #=> 3
puts c.counter #=> 3

As you can see from the example above, all instances of the class return the same value from counter() method, because @@counter is a class variable that's shared across all instances of the Entity class.

Consider another example, where a delimiter is declared as a class variable:

class Collection
    @@delimiter = ", "

    def self.from_str(str)
        self.new(str.split(@@delimiter))
    end
end

This can be inherited by other classes, where the same delimiter would be used across all subclasses:

class Fruits < Collection
    def initialize(fruits)
      @fruits = fruits
    end

    attr_reader :fruits
end

a = Fruits.from_str('orange, apple, grapes')
b = Fruits.from_str('banana, mango')

print a.fruits #=> ["orange", "apple", "grapes"]
print b.fruits #=> ["banana", "mango"]

You should use class variables with caution as all subclasses inherit the same class variables from its superclass, which can lead to unexpected/confusing results.