strange behaviour of class variables.rb

class Object
  def metaclass
    class << self; self; end
  end
end

# - Normal cases -

class Foo
  @@bar = 0
  def self.set_bar(x)
    @@bar = x
  end
end

p Foo.send(:class_variable_get, :@@bar) # => 0
Foo.set_bar(5)
p Foo.send(:class_variable_get, :@@bar) # => 5

# --

class Foo; end

def Foo.set_bar(x)
  @@bar = x
end

p Foo.send(:class_variable_get, :@@bar) rescue nil # => nil (NameError)
Foo.set_bar(5)
p Foo.send(:class_variable_get, :@@bar) # => 5
p Foo.metaclass.send(:class_variable_get, :@@bar) # => 5

# - Strange case -

class Foo
  @@bar = 0
end

def Foo.set_bar(x)
  @@bar = x
end

p Foo.send(:class_variable_get, :@@bar) # => 0
Foo.set_bar(5)
p Foo.send(:class_variable_get, :@@bar) # => 0
p Foo.metaclass.send(:class_variable_get, :@@bar) # => 5

# - Work-around -

class Foo
  @@bar = 0
end

def Foo.set_bar(x)
  class_variable_set(:@@bar, x)
end

p Foo.send(:class_variable_get, :@@bar) # => 0
Foo.set_bar(5)
p Foo.send(:class_variable_get, :@@bar) # => 5
p Foo.metaclass.send(:class_variable_get, :@@bar) rescue nil # => nil (NameError)