MyFunnyDev

class A
  @@foo = "class variable of the class A"
  @foo = "class instance variable of the class A"
 
  def instance_method
   @foo = "instance variable of the class A"
  end
 
  def self.class_method1
    # class variables are visible to and shared by the instance and class methods
    @@foo
  end
 
  def self.class_method2
    # class instance variables are visible to and shared by the class methods
    @foo
  end
end

p A.new.instance_method # instance variable of the class A
p A.class_method1 # class variable of the class A
p A.class_method2 # class instance variable of the class A

class B < A
  @@foo = "class variable of the class B"
  @foo = "class instance variable of the class B"
end

p B.class_method1 # class variable in B
# class variable in A is overwritten by one in B !!!
p A.class_method1 # class variable in B

p B.class_method2 # class instance variable of the class B
# class instance variable in A is NOT overwritten by one in B !!!
p A.class_method2 # class instance variable of the class A