parent_as_parameter.rb

class A
  def initialize(parent)
    @parent = parent
  end

  def test
    puts "A::test"
  end
end

class B
  def initialize(parent)
    @parent = parent
  end

  def test
    puts "B::test"
    @parent.a.test
  end
end

class C
  attr_reader :a, :b
  def initialize
    @a = A.new self
    @b = B.new self

    @b.test
  end
end

c = C.new