Using methods from 2nd-level extended class

Question

I'm trying to use methods from that is extended by the class that this class is extending from. An example of what I'm trying to do:

class A
  def foo
    "Foobar"
  end
end

class B
  extend A
end

class C
  extend B
end

B.foo #=> "Foobar"
C.foo #=> "Foobar"

I'm not sure if this type of functionality is available in Ruby. I know that this is possible by changing extend to include in B, but I'd like the methods available in A as class methods in B as well as C.

Answer 1

extend and include are for modules; you cannot, to my knowledge, use modules with extend and include (in fact Ruby will raise an error). Instead you should define A as a module and then extend B and C with A. See John Nunemaker's RailsTips write-up on extend and include to get a better handle on this design pattern.

Another option to do this is to have B and C inherit from A, like so:

class A
  def self.foo
    "Foobar"
  end
end
class B < A; end
class C < B; end