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

[Quinn "The Eskimo!"]

On 24 Jul 2017, at 07:04, somu subscribe via swift-users <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>

And the general background to this in the “Ownership Manifesto"

<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.

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Quinn "The Eskimo!"                    <http://www.apple.com/developer/>
Apple Developer Relations, Developer Technical Support, Core OS/Hardware