[swift-users] [Bug or ByDesign?] unowned optional values not allowed

Jordan Rose jordan_rose at apple.com
Tue Jul 5 11:21:56 CDT 2016

Longstanding bug, rdar://problem/17277899 <rdar://problem/17277899>. Surprisingly few people have asked for it.


> On Jul 3, 2016, at 23:29, Karl via swift-users <swift-users at swift.org> wrote:
> Currently it’s not possible to have an unowned optional value. E.g:
> class A {
>          unowned var parent : A?   // ‘unowned’ may only be applied to class and class-bound protocol types, not ‘A?'
>          deinit { 
>            if let p = parent { print("Bye, mom!") }
>            print("deallocating")
>        }
> }
> Sometimes you want an unowned value, but it doesn’t actually need to be set.
> In a specific case I was working on, we wanted to use a private embedded struct to wrap some functionality away as a unit, but we need a reference back to the enclosing instance. We could have a weak reference, but that comes with additional overheads, and it’s unnecessary because that struct code will all get inlined and the reference will just be a pointer back to ‘self’ (it would be cool if the compiler could detect that and just not emit the variable at all, but I don’t think it’ll do that). In any case, in order to initialise it with the parent that reference needs to be an optional/implicitly-unwrapped-optional (so that it can be initialised to nil and then set after the parent calls super.init).
> Given that weak optionals work, I’m tempted to think it’s a bug, but I’m asking in case it was intentionally omitted.
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