[swift-evolution] [Pitch] Normalize Slice Types for Unsafe Buffers

[Ben Cohen]

So now that I look at it, it appears UnsafeRawBufferPointer(copyBytes:) has the same problems we are trying to solve on initialize(as:from) i.e. it is completely at the mercy of the passed-in collection's count being accurate, and if it isn't it'll scribble over memory. We should probably apply similar fixes and change it to take a sequence.

I realize it's a sticking plaster on this particular issue though, so still doesn't answer whether it's better for UnsafeRawBufferPointer to be a collection, just created more work...

> On Dec 8, 2016, at 17:06, Andrew Trick <atrick at apple.com> wrote:
> 
> 
>>> On Dec 8, 2016, at 4:54 PM, Jordan Rose <jordan_rose at apple.com> wrote:
>>> 
>>> 
>>>> On Dec 8, 2016, at 16:53, Ben Cohen <ben_cohen at apple.com> wrote:
>>>> 
>>>> 
>>>> On Dec 8, 2016, at 4:35 PM, Jordan Rose via swift-evolution <swift-evolution at swift.org> wrote:
>>>> 
>>>> Um, Sequence doesn’t have a subscript (or indexes). Sequences are single-pass. So if this is important, it needs to stay a Collection.
>>>> 
>>> 
>>> Just because something fulfills one of the requirements of a Collection does not mean it should be one. It needs to tick all the boxes before its allowed to be elevated.
>>> 
>>> But it’s still allowed to have subscripts (UnsafePointer has subscripting but isn’t a collection) or be multi-pass (strides are multiples but are only sequences). That’s OK
>>> 
>>> In this case, yes it’s multi-pass, yes it has a subscript, but no it isn’t a collection because it doesn’t meet the requirements for slicing i.e. that indices of the slice be indices of the parent.
>>> (relatedly… it appears this requirement is documented on the concrete Slice type rather than on Collection… which is a documentation bug we should fix).
>> 
>> Ah, right, thank you. Retracted.
> 
> Let me restate, because I think Jordan's question was valid given my statement.
> 
> It would be *nice* for raw buffers to be Collection<UInt8> because they’re meant to be a replacement for code that is typically written for [UInt8], and anything you can do with an array that applies to raw buffers is covered by Collection<UInt8>.
> 
> However, I don’t expect the raw buffer to be used in a generic context except being passed to utilities that copy the bytes out. That will either be done by directly iterating over the collection or invoking some other API that could take a Sequence. The most important is probably Array.append(contentsOf:), which is moving over to Sequence. However, we would also need to change UnsafeRawBufferPointer(copyBytes:), NSData(replaceSubrange:), and whatever else I haven't thought of. That's a small disadvantage to this solution.
> 
> I'm also a little concerned that Sequence is immutable, so generic code has no way to copy bytes into the buffer.
> 
> My bigger concern is still that the range subscript’s inconsistent behavior may still lead to bugs in practice in nongeneric code.
> 
> -Andy
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-evolution/attachments/20161208/732d2285/attachment.html>