[swift-evolution] [RFC] UnsafeBytePointer API for In-Memory Layout

[Geordie Jay]

So what's in it for us as Swift devs?

It may be technically undefined behaviour (by that I think you mean there's
no real knowing what could happen), but it seems to be rampant throughout
pretty much all the C code I've come in contact with (I'm less familiar
with C++).

If we lose type information by calling a C API that takes a void pointer,
how can we hope to retrieve it in any safe way, other than saying "we
assume with good reason and hope to hell that this is what we say it is".
And if we can't do that, what advantage does this proposal provide over
what we already have?
Joe Groff <jgroff at apple.com> schrieb am Mo., 9. Mai 2016 um 22:16:

>
> > On May 9, 2016, at 12:38 PM, Geordie Jay via swift-evolution <
> swift-evolution at swift.org> wrote:
> >
> > I read this proposal and I'm a bit unsure what its purpose would be:
> >
> > Basically you want to prevent UnsafePointer<XYZ>(UnsafePointer<Void>)
> conversions and/or vice-versa? And you'd achieve this by replacing
> UnsafePointer<Void> with UnsafeBytePointer that has no bound pointer type?
> >
> > In one sense the change seems fine to me, but as someone who uses a lot
> of C APIs and a lot of CoreAudio/CoreMIDI in Swift already I can't really
> see what benefit it'd bring. Presumably we'd still want an option of
> converting UnsafeBytePointer to UnsafePointer<SomeActualType> for things
> like C function pointer callback "context"/"userInfo" uses, so it's not
> like we'd be preventing programmer error in that way.
> >
> > Call me conservative but to me the current system seems to work as well
> as it can. If anything it's already enough boilerplate going through hoops
> converting an UnsafeMutablePointer<Void> into a [Float] even when I know
> and the C API knows perfectly well what it actually contains... Would
> happily be convinced otherwise about this proposal though, I'm pretty new
> at all this.
> >
> > Geordie
>
> > On May 9, 2016, at 12:57 PM, Guillaume Lessard via swift-evolution <
> swift-evolution at swift.org> wrote:
> >
> > I’m sympathetic to the elimination of UnsafePointer<Void> as general
> shorthand for an arbitrary pointer, but I lose the plot of this very long
> proposal. It seems to me that this increases API surface, yet everything I
> could do before, I could still do; it just involves more typing. What
> exactly does this make better?
> >
> > Cheers,
> > Guillaume Lessard
>
> Andy, I think it's worth clarifying the primary purpose of this proposal.
> Our main goal here is to provide a legal means for "type-punning" memory
> access. Like C and C++, it's technically undefined behavior in Swift to
> cast an UnsafePointer<T> to an UnsafePointer<U> of a different type and
> load a value out of memory that's of a different type from what was stored
> there. We don't take much advantage of this yet in Swift's optimizer, since
> we don't have good alternative API. UnsafeBytePointer seeks to fill this
> gap by providing a type that can safely do type-punned loads and stores.
>
> -Joe
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