> On 28 Jul 2017, at 18:55, Joe Groff <jgr...@apple.com> wrote: > >> >> On Jul 28, 2017, at 12:06 AM, David Hart via swift-users >> <swift-users@swift.org <mailto: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.
Thanks for the explanation! It’s starting to make sense :) Is it possible to trigger the exclusivity trap without an inout argument? > -Joe
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