Ruby class variables aren't auto-vivified?

Question

Working on a Ruby program I was looking to move some state data from instance variables to class variables, it dawned on me that while instance variables are auto-vivified (if you try to read them "without initializing" them, they are automatically initialized to nil), class variables are not - and this looks very inconsistent to me (compared to most Ruby syntax which is very consistent).

Sample program:

class Test
  def id
    @id.to_i
  end
  def id=(i)
    @id = i
  end
  def nextid
    self.id = id + 1
  end
end

t = Test.new
puts t.nextid #=> 1
puts t.nextid #=> 2

In which case, when calling Test::id, if @id was not initialized, Ruby will auto-vivify it to nil (after which I to_i it to get 0).

Now I decide that I want the running ID to be shared across Test instance, so I rewrite it like this:

class Test
  def id
    @@id.to_i
  end
  def id=(i)
    @@id = i
  end
  def nextid
    self.id = id + 1
  end
end

t = Test.new
puts t.nextid
puts t.nextid

Should work the same, I thought, but no:

NameError: uninitialized class variable @@id in Test

But this workaround works (!?) :

class Test
  def id
    (@@id ||= 0).to_i
  end
  def id=(i)
    @@id = i
  end
  def nextid
    self.id = id + 1
  end
end

t = Test.new
puts t.nextid #=> 1
puts t.nextid #=> 2

(granted, after doing lazy init to 0 I can drop the to_i, but I left it for consistency).

It looks like Ruby understands "lazy initialization" and treats it as the magic needed to not throw NameError - even though ||= is supposedly just syntactic sugar to x = x || val (which BTW doesn't work for initing class variables, thanks for asking).

How come?

Answer 1

Class variables initialization

Here's a possible explanation why @a is nil but @@a is a NameError.

But if you want to use class variables, you should initialize them inside the class, not inside instance methods :

class Test
  @@id = 0

  def id
    @@id
  end

  def id=(i)
    @@id = i
  end

  def nextid
    self.id = id + 1
  end
end

t = Test.new
puts t.nextid
puts t.nextid

Please note that it doesn't make much sense to have an instance setter method for a class variable.

Class instance variables

To avoid mixing instance methods and class variables, you could define everything at the class level with a "class instance variable". It's an instance variable defined at the class level:

class Test
  @id = 0
  class << self
    def id
      @id
    end

    def id=(i)
      @id = i
    end

    def nextid
      self.id = id + 1
    end
  end
end

puts Test.id
# 0
puts Test.nextid
# 1
puts Test.nextid
# 2

It means you could just use attr_accesor:

class Test
  @id = 0
  class << self
    attr_accessor :id
    def nextid
      self.id = id + 1
    end
  end
end