[swift-users] Weak references in generic types

[Howard Lovatt]

@Karl,

You say "In the second example, you’re creating WeakReference<P>. P does
not conform to P or to AnyObject.", but P does conform to AnyObject.

I suspect it is a compiler limitation/ bug.

 -- Howard.

On Thursday, 1 September 2016, Karl <razielim at gmail.com> wrote:

>
> On 1 Sep 2016, at 03:23, Howard Lovatt via swift-users <
> swift-users at swift.org
> <javascript:_e(%7B%7D,'cvml','swift-users at swift.org');>> wrote:
>
> 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
> <javascript:_e(%7B%7D,'cvml','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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>
>
> Your problem is protocol self-conformance. In the first example, you’re
> creating WeakReference<CP>. CP conforms to P and to AnyObject. In the
> second example, you’re creating WeakReference<P>. P does not conform to P
> or to AnyObject.
>
> As for why @objc fixes it? … ¯\_(ツ)_/¯ all bets are off whenever @objc
> gets involved in anything.
>
> Karl
>


-- 
-- Howard.
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