Question

module M

  def f=(x)
    @f= x
  end

  def f
    @f
  end

end

class A
  extend M
end

class B < A
end

A.f= 42
puts A.f
puts B.f

this produces

42
nil

Is @f a class variable to A and B?
How do I share a variable between A and B by only writing this into M?

Answer 1

By not using @@f directly, but rather class_variable_set() and class_variable_get(), class variables of A, B and C can be used from within M.

module M

    def f=(x)
        class_variable_set("@@f", x)
    end

    def f
        class_variable_get("@@f")
    end

end

class A
    extend M
end

class B < A
end

class C
    extend M
end

A.f= 42
C.f= 23
puts A.f
puts B.f
puts C.f

this produces

42
42
23

As you can see, the variables are shared by A and B, but not C.