Ruby class variables aren't auto-vivified? - ruby

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