> On Jul 28, 2017, at 12:06 AM, David Hart via swift-users
> <swift-users@swift.org> wrote:
>
> Hello,
>
> Indeed, I had reduced the code too much. John McCall was kind enough to have
> a look and here’s the offending code:
>
> func layoutHorizontally(leftRect: inout CGRect, rightRect: inout CGRect) {
> let totalWidth = imageRect.size.width + titleRect.size.width +
> contentSpacing
> rightRect.origin.x = leftRect.maxX + contentSpacing
> }
>
> The problem is that imageRect and titleRect are referenced both directly and
> in the inout parameters.
>
> But there’s something I’m not understanding. I went back to the ownership
> manifesto and re-read the law of exclusivity to make sure I understand it:
>
> If a storage reference expression evaluates to a storage reference that is
> implemented by a variable, then the formal access duration of that access may
> not overlap the formal access duration of any other access to the same
> variable unless both accesses are reads.
>
> So I tried to write a test to trigger the runtime error, but was unsuccessful:
>
> class MyClass {
> var p: CGRect = .zero
> }
>
> func trigger(a: MyClass, b: MyClass) {
> a.p = b.p
> }
>
> let m = MyClass()
> trigger(a: m, b: m)
>
> Here, a storage reference expression (a.p) evaluates to a storage reference
> that is implemented by a variable (the p property of an instance m of
> MyClass) and its formal access duration (the trigger function) overlaps the
> formal access duration of another access to the same variable (through the
> b.p storage reference expression) and both accesses are not reads (a.p is on
> the LHS of an assignment).
>
> Why does this not trigger the Law of Exclusivity?
`b.p` is loaded before `a.p` is written to, so the accesses do not overlap even
when a === b. Something like swap(&a.p, &b.p) (using the Swift 3 global
definition of 'swap') would trigger an exclusivity trap when a === b, since the
access to storage passed as an inout argument needs to last for the duration of
the call.
-Joe
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