t.rb

class C

  attr_accessor :b

  def foo
    b ? b.foo : raise(NoMethodError.new)
  end

  alias gob foo

  def self.from_b b
    new.tap {|c| c.instance_variable_set '@b', b }
  end
end

class B
  attr_accessor :foo
 def c
   C.from_b(self)
 end
end

require 'test/unit'

class T < Test::Unit::TestCase
 def test_this
   b = B.new
   b.foo = 2
   assert_equal b.foo, b.c.foo
   assert_equal b.foo, b.c.gob
   b.foo = 3
   assert_equal b.foo, b.c.foo
   assert_equal b.foo, b.c.gob

   assert_raises(NoMethodError) { C.new.foo }
 end
end