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The Swift Programing Language
Automatic Reference Counting / Unowned References
> Unlike a weak reference, however, an unowned reference is used when
the other instance has the same lifetime or a longer lifetime. You indicate
an unowned reference by placing the unowned keyword before a property
or variable declaration.
> An unowned reference is expected to always have a value. As a result,
ARC never sets an unowned reference’s value to nil, which means that
unowned references are deﬁned using nonoptional types.