[swift-users] inout params seem to have undefined behavior

zh ao owenzx at gmail.com
Sat Jun 11 21:18:40 CDT 2016

inout says it will copy into the function and copy back after the function
is finished. But the order is unknown. So I think the example,

print(acopy)  // prints "[1, 2, 99]" (e.g. a let variable changed!)

​s a bug as the value changed inside of the function, before the function

​Swift doc:

“In-out parameters are passed as follows:

​. ​
When the function is called, the value of the argument is copied.
​. ​
In the body of the function, the copy is modified.
​ .​
When the function returns, the copy’s value is assigned to the original

摘录来自: Apple Inc. “The Swift Programming Language (Swift 2.2)”。 iBooks.


On Sun, Jun 12, 2016 at 9:27 AM, Karl Pickett via swift-users <
swift-users at swift.org> wrote:

> Wow there are some real doozy inout code examples in there, showing
> aliasing much more fun than my snippet.  Unfortunately I can't
> understand anything else the doc is talking about.  I guess I'll just
> say a prayer and throw salt over my shoulder if using inout.
> On Sat, Jun 11, 2016 at 6:05 PM, Brent Royal-Gordon
> <brent at architechies.com> wrote:
> >> My recollection is that in Swift the subscript operator (`arr[2]` in
> this case) can refer to the setter xor the getter, but not both within the
> same statement.
> >
> > Quite to the contrary. Rather than using the setter directly, Swift
> often uses `materializeForSet`, a combined get-and-set operation which is
> much more efficient, particularly when assigning directly into arrays. To
> keep from having to use very slow access all the time, it imposes a rule
> (which is not and cannot be enforced by the compiler) that you can't hold
> two mutable references to overlapping storage simultaneously, or they may
> do strange things like lose some of the writes you make.
> >
> > Here's an old design document discussing some things in this area: <
> https://github.com/apple/swift/blob/73841a643c087e854a2f62c7e073317bd43af310/docs/proposals/Accessors.rst>
> I'm not sure how authoritative it is, but it might give you an idea of
> what's going on.
> >
> > --
> > Brent Royal-Gordon
> > Architechies
> >
> _______________________________________________
> swift-users mailing list
> swift-users at swift.org
> https://lists.swift.org/mailman/listinfo/swift-users
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