Accordingly, a strong reference to an object can be of both Optional and non-Optional type (optionality and reference strength are different, albeit related, concepts). A weak reference is always of type Optional, as the object can be deallocated and the reference automatically be set to nil. Unowned references are like weak references but are not set to nil automatically by ARC. They can be either non-Optional or Optional. An unowned reference is expected to always have a value, so accessing the value of an unowned reference after the referenced instance has been deallocated, we result in a runtime error.
