[swift-users] Simultaneous accesses, but modification requires exclusive access

[somu subscribe]

Thanks a lot Zhao, using a serial queue works.

So both the solutions works, thanks.


> On 24 Jul 2017, at 5:39 PM, Zhao Xin via swift-users <swift-users at swift.org> wrote:
> 
> You can use serial queue.
> 
> class Car {
>     var helper = Helper()
>     lazy private var queue = DispatchQueue(label: "my queue")
>     
>     func test() {
>         helper.doSomething(f1: f1)
>     }
>     
>     func f1() {
>         queue.async {
>             _ = self.helper.v1 //Crash - Simultaneous accesses to <memory address>, but modification requires exclusive access.
>         }
>     }
> }
> 
> Zhao Xin
> 
> On Mon, Jul 24, 2017 at 4:22 PM, Quinn "The Eskimo!" via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
> 
> On 24 Jul 2017, at 07:04, somu subscribe via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
> 
> > - Is there a bug in my code which is being detected in Xcode 9 ?
> 
> Yes.  The problem here is that `doSomething(f1:)` is a mutating function, so it acts like it takes an `inout` reference to `self.helper`.  That’s one mutable reference.  It then calls `Car.f1()`, which tries to get a non-mutating reference to exactly the same struct.  This is outlawed in Swift 4 as part of the memory ownership effort.
> 
> You can read more about the specific change in SE-0176 “Enforce Exclusive Access to Memory”.
> 
> <https://github.com/apple/swift-evolution/blob/master/proposals/0176-enforce-exclusive-access-to-memory.md <https://github.com/apple/swift-evolution/blob/master/proposals/0176-enforce-exclusive-access-to-memory.md>>
> 
> And the general background to this in the “Ownership Manifesto"
> 
> <https://github.com/apple/swift/blob/master/docs/OwnershipManifesto.md <https://github.com/apple/swift/blob/master/docs/OwnershipManifesto.md>>
> 
> > If so could you please explain and suggest an alternate approach / fix ?
> 
> It’s hard to offer concrete suggestions without knowing more about your high-level goals.  One option is for `doSomething(f1:)` to pass the `inout` reference through to `f1`.  For example:
> 
>     mutating func doSomething(f1: (inout Helper) -> ()) {
>         f1(&self)
>     }
> 
>     func f1(h: inout Helper) {
>         _ = h.v1  // no crash
>     }
> 
> but whether that makes sense in your code is for you to decide.
> 
> Share and Enjoy
> --
> Quinn "The Eskimo!"                    <http://www.apple.com/developer/ <http://www.apple.com/developer/>>
> Apple Developer Relations, Developer Technical Support, Core OS/Hardware
> 
> 
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