[swift-users] Weak references in generic types

[Howard Lovatt]

Playing around I found that if you make the protocol @objc instead of
AnyObject then it works :). EG:

struct WeakReference<T: AnyObject> {
    weak var value: T?
}
@objc protocol P { // Note @objc, class or AnyObject does not work
    var i: Int { get }
}
class CP: P {
    var i: Int = 0
}
let weakPs: [WeakReference<P>] = [WeakReference(value: cP)] // Note typed
as `[WeakReference<P>]`
print("P: \(weakPs[0].value!.i)") // 0

Not a 'pure' Swift solution :(, but OK in my case.

  -- Howard.

On 29 August 2016 at 16:21, Howard Lovatt <howard.lovatt at gmail.com> wrote:

> Hi,
>
> I am wanting to use weak references in generic data structures; in the
> example below Array, but in general any generic type. I can almost get it
> to work :(
>
> My experiments started off well; the following works:
>
> // Array of weak references OK
> struct WeakReference<T: AnyObject> {
>     weak var value: T?
> }
> class C {
>     var i: Int = 0
> }
> let c = C() // Strong reference to prevent collection
> let weakCs = [WeakReference(value: c)] // OK
> print("C: \(weakCs[0].value!.i)") // 0
>
>
> I can add a protocol:
>
> // Array of weak references that implements a protocol OK
> protocol P: AnyObject { // Note AnyObject
>     var i: Int { get }
> }
> class CP: P {
>     var i: Int = 0
> }
> let cP = CP() // Strong reference to prevent collection
> let weakCPs = [WeakReference(value: cP)] // OK
> print("CP: \(weakCPs[0].value!.i)") // 0
>
>
> But when I want an array of weak references to the protocol I get an error:
>
> // Array of weak references of a protocol not OK
> let weakPs: [WeakReference<P>] = [WeakReference(value: cP)] // Using 'P'
> as a concrete type conforming to protocol 'AnyObject' is not supported
> print("P: \(weakPs[0].value!.i)") // 0
>
>
> Is there something I have missed?
>
> The error message, "Using 'P' as a concrete type conforming to protocol
> 'AnyObject' is not supported", implies that it is a temporary limitation of
> the compiler; is this going to be fixed? Should I lodge a bug report?
>
> Thanks in advance for any advice,
>
>   -- Howard.
>
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