Ruby class variables in metaclass

Question

I have a ruby class like this:

class C
  @@v = 1

  class << self
    p @@v  # everything goes well here
  end
end

class << C
  # here I get an exception 
  # `singletonclass': uninitialized class variable @@v in Object (NameError)
  # from a.rb:5:in `<main>'

  p @@v
end

My question is: the exception said 'uninitialized class variable @@v in Object (NameError)', but why ruby lookups class variables in Object(main)?

It seems like ruby always lookups metaclass' class variables outside it.

Answer 1

You have defined the class variable @@v for the class C. We shouldn't be surprised that this:

class << C
  p @@v
end

raises an exception, because here we are dealing with the singleton class of C, for which we have not defined a class variable @@v.

The real question is, why does it work when you do it this way?

class C
  @@v = 1

  class << self
    p @@v
  end
end

The answer appears to be that, for your convenience, whenever you access the singleton of a class from inside that class, Ruby transparently gives you access to that class's class variables.

Note that this works consistently when you define methods as well, whether you use the class << self; def method_name syntax or the def self.method_name syntax:

# assuming @@v in C is 1
class C
  def self.print_v_from_inside
    p @@v
  end
end

def C.print_v_from_outside
  p @@v
end

C.print_v_from_inside
# => 1

C.print_v_from_outside
# => NameError: uninitialized class variable @@v in Object