[swift-dev] Question: memory management on Stack with Structs ?

[Thibault Wittemberg via swift-dev]

Hi everyone,

I am a member of the RxSwiftCommunity, and I am facing an issue on a PR that 
I've made (https://github.com/RxSwiftCommunity/NSObject-Rx/pull/49 
<https://github.com/RxSwiftCommunity/NSObject-Rx/pull/49>).

The big picture is:
I have a protocol that declares a computed var (get set) with a default 
implementation that uses the "objc_setAssociatedObject(self, 
&disposeBagContext, disposeObject, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)" 
primitive to store and retrieve its value.
Here is the complete code:
public protocol HasDisposeBag {
    /// a unique RxSwift DisposeBag instance
    var disposeBag: DisposeBag { get set }
}
extension HasDisposeBag {
    func synchronizedBag<T>( _ action: () -> T) -> T {
        objc_sync_enter(self)
        let result = action()
        objc_sync_exit(self)
        return result
    }
    public var disposeBag: DisposeBag {
        get {
            return synchronizedBag {
                if let disposeObject = objc_getAssociatedObject(self, 
&disposeBagContext) as? DisposeBag {
                    return disposeObject
                }
                let disposeObject = DisposeBag()
                objc_setAssociatedObject(self, &disposeBagContext, 
disposeObject, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
                return disposeObject
            }
        }
        set {
            synchronizedBag {
                objc_setAssociatedObject(self, &disposeBagContext, newValue, 
.OBJC_ASSOCIATION_RETAIN_NONATOMIC)
            }
        }
    }
}

The question is: what happens when it is a struct that conforms to this 
protocol ?

In Swift, struct is a value type which means memory is handled in stack, not in 
heap, which indicate a specific life cycle and reusability (because of swift 
compiler optimizations). When compiler is going to reuse an instance of this 
Struct, what will happen with the DisposeBag computed property as it is an 
Object reference dynamically stored with this "objc_setAssociatedObject" 
primitive ?

Let's consider we create an instance of this Struct which has an associated 
DisposeBag. That bag is used in some RxSwift subscriptions. When the lifetime 
of the Struct finishes, Swift compiler can provide the same stack place to 
create another instance because of reusability feature. Will the already 
associated DisposeBag be persisted or will it be cleared for the new Struct ?

Thanks for you help.

Thibault Wittemberg.
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