[swift-evolution] Casting Bug Swift

[James Campbell]

In the actual code, the array with the protocol type i.e [Eatable] is used
as an associated type. So perhaps it normally gets rejected but isn't in
this case ?



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On Thu, Feb 25, 2016 at 5:07 PM, Jordan Rose <jordan_rose at apple.com> wrote:

> I think there are two things going on here:
>
> - The compiler allows a memory-corrupting cast, which should be rejected
> (a bug).
> - But it's a useful thing to do (a language change).
>
> I'm actually surprised about the bug, since we get plenty of questions
> about why the compiler doesn't allow it, implying that the compiler is
> indeed rejecting it. The answer is that protocol values and class values
> don't have the same representation, so converting between [Eatable] and
> [Burger] is an O(N) operation that requires allocating a new array. But
> that may not be a good enough reason not to allow it—conversions from
> NSArray to Array can do the same thing if the NSArray was mutable.
>
> Jordan
>
>
> On Feb 25, 2016, at 8:53 , Radosław Pietruszewski via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> Hey James,
>
> The appropriate place for compiler bugs is https://bugs.swift.org, not
> Swift Evolution.
>
> Best,
> — Radek
>
> On 25 Feb 2016, at 17:44, James Campbell via swift-evolution <
> swift-evolution at swift.org> wrote:
>
> Say I have been passed an array of objects that conform to a protocol
> called "Eatable" i.e [Eatable]
>
> If I then try to cast them to an array of classes who happen to implement
> that protocol i.e [Burger]
>
> The compiler lets me but in the runtime it corrupts the memory.
>
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