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

[somu subscribe]

Thank a lot Quinn, your solution to use inout works well without crashing.

Question 1:
- Also changing Helper to a class doesn’t seem to crash. Is that a solution that wouldn’t cause a crash or just works by chance ?


Background:
Just a little background into what I was trying to achieve (I could be wrong):

- I have a set of classes C1, C2, C3 which has a lot of common code

- I would like to build something that can be reused without exposing the implementation details. (I can subclass but would expose the underlying functions, same applies to protocol as well)

- I thought I would build helper class / struct which would contain the common code. I can make the helper a private property so that the functions wouldn’t be exposed to the instances of C1, C2, C3. In order to achieve that I had to pass some functions from C1 into the Helper struct.

Question 2:
- Is this problem (hiding implementation details) normally tackled using Helper class (or struct) or is there a more better approach ?

Thanks and regards,
Muthu


> On 24 Jul 2017, at 4:14 PM, Quinn The Eskimo! via swift-users <swift-users at swift.org> wrote:
> 
> 
> 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.
> 
> Share and Enjoy
> --
> Quinn "The Eskimo!"                    <http://www.apple.com/developer/>
> Apple Developer Relations, Developer Technical Support, Core OS/Hardware
> 
> 
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