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

[Hooman Mehr]

> On Jul 24, 2017, at 2:38 AM, somu subscribe via swift-users <swift-users at swift.org> wrote:
> 
> 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 ?

It is not by chance. It illustrates the key difference between classes and structs: class instances are passed by reference because they have identity. The identity of an object never mutates: If you change `Helper` to a class, then you can also change `helper` property from `var` to `let` and still change the value of `v1` inside `helper`. If this is not clear to you, I recommend you first get a better understanding of differences between value types and reference types and especially learn about object aliasing before deciding which approach is the right approach for you. 

> 
> 
> 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 ?

It depends on what you mean by hiding and whether those classes sharing common implementation details also share a common public API (protocol) or ancestry (shared superclass). 

Generally, what you call `Helper` is the correct way to go if the following is true: 

Your `Helper` makes logical sense as something coherent with its own meaningful properties, so that it can be given a specific name that makes sense (besides a very broad name such as your example `Helper`).

> 
> Thanks and regards,
> Muthu
> 
> 
>> On 24 Jul 2017, at 4:14 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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