Ruby class variables

# B class redefine class variable

class A
  @@class_var = 'first'

  def first
    @@class_var
  end
end

class B < A
  @@class_var = 'second'

  def second
    @@class_var
  end
end

p B.new.first           # => second
p B.new.second          # => second
p B.singleton_methods   # => []


# A and B class have different variable
# In this case we have instance(singleton) variable for A and B

class A
  @class_var = 'first'

  class << self
    attr_accessor :class_var
  end

  def first
    A.class_var
  end
end

class B < A
  @class_var = 'second'

  class << self
    attr_accessor :class_var
  end

  def second
    B.class_var
  end
end

p B.new.first           # => first
p B.new.second          # => second
p B.singleton_methods   # => [:class_var, :class_var=]