[swift-evolution] SE-184 Improved Pointers

Andrew Trick atrick at apple.com
Sat Aug 19 17:02:35 CDT 2017

> On Aug 15, 2017, at 9:47 PM, Taylor Swift via swift-evolution <swift-evolution at swift.org> wrote:
> Implementation is here: https://github.com/apple/swift/pull/11464 <https://github.com/apple/swift/pull/11464>
> On Sat, Aug 12, 2017 at 8:23 PM, Taylor Swift <kelvin13ma at gmail.com <mailto:kelvin13ma at gmail.com>> wrote:
> I’ve revised the proposal based on what I learned from trying to implement these changes. I think it’s worth tacking the existing methods that take Sequences at the same time as this actually makes the design a bit simpler.
> <https://gist.github.com/kelvin13/5edaf43dcd3d6d9ed24f303fc941214c <https://gist.github.com/kelvin13/5edaf43dcd3d6d9ed24f303fc941214c>>
> The previous version <https://gist.github.com/kelvin13/1b8ae906be23dff22f7a7c4767f0c907> of this document ignored the generic initialization methods on UnsafeMutableBufferPointer and UnsafeMutableRawBufferPointer, leaving them to be overhauled at a later date, in a separate proposal. Instead, this version of the proposal leverages those existing methods to inform a more compact API design which has less surface area, and is more future-proof since it obviates the need to design and add another (redundant) set of protocol-oriented pointer APIs later.
> On Tue, Aug 8, 2017 at 12:52 PM, Taylor Swift <kelvin13ma at gmail.com <mailto:kelvin13ma at gmail.com>> wrote:
> Since Swift 5 just got opened up for proposals, SE-184 Improved Pointers is ready for community review, and I encourage everyone to look it over and provide feedback. Thank you!
> <https://github.com/apple/swift-evolution/blob/master/proposals/0184-improved-pointers.md <https://github.com/apple/swift-evolution/blob/master/proposals/0184-improved-pointers.md>>

Thanks for continuing to improve this proposal. It’s in great shape now.

Upon rereading it today I have to say I strongly object to the `count = 1` default in the following two cases:

+ UnsafeMutablePointer.withMemoryRebound(to: count: Int = 1)
+ UnsafeMutableRawPointer.bindMemory<T>(to:T.Type, count:Int = 1)
  -> UnsafeMutablePointer<T>

To aid understanding, it needs to be clear at the call-site that binding memory only applies to the specified number of elements. It's a common mistake for users to think they can obtain a pointer to a different type, then use that pointer as a base to access other elements. These APIs are dangerous expert interfaces. We certainly don't want to make their usage more concise at the expense of clarity.

In general, I think there's very little value in the `count=1` default, and it creates potential confusion on the caller side between the `BufferPointer` API and the `Pointer` API. For example:

+ initialize(repeating:Pointee, count:Int = 1)

Seeing `p.initialize(repeating: x)`, the user may think `p` refers to the buffer instead of a pointer into the buffer and misunderstand the behavior.

+ UnsafeMutablePointer.deinitialize(count: Int = 1)

Again, `p.deinitialize()` looks to me like it might be deinitializing an entire buffer.

If the `count` label is always explicit, then there's a clear distinction between the low-level `pointer` APIs and the `buffer` APIs.

The pointer-to-single-element case never seemed interesting enough to me to worry about making convenient. If I'm wrong about that, is there some real-world code you can point to where the count=1 default significantly improves clarity?

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