do_not_use_cv.rb

module TestCV
  def value=(val)
    @@value = val
  end

  def value
    @@value
  end
end

class Foo
  extend TestCV

  self.value = :foo
end

puts 'Foo.value: %s' % Foo.value

class Bar
  extend TestCV

  self.value = :bar
end

puts 'Bar.value: %s' % Bar.value
puts 'Foo.value: %s' % Foo.value

# Foo.value: foo
# Bar.value: bar
# Foo.value: bar