gistfile1.rb

# An A is intended to securely encapsulate a B, so that any attempts to access the B must go via the A
class A
  def initialize
    @b = B.new # This is supposed to be completely encapsulated, and inaccessible to the outside world
  end

  def foo
    puts "Foo"
  end

  def method_missing(name, *args, &block)
    # Maybe implement some sort of authorisation here. This is the motivation for the tight encapsulation.
    @b.send name, *args, &block
  end
end

class B
  def ask
    C.new.ask { foo }
  end

  def foo
    puts "Bar"
  end
end

class C
  def ask
    yield # When the block is yielded, self is the B instance, allowing direct access to B from outside, thus breaking the encapsulation
  end
end

A.new.ask # This returns 'Bar', where I really want 'Foo'