[swift-evolution] [Review] SE-0184: Unsafe[Mutable][Raw][Buffer]Pointer: add missing methods, adjust existing labels for clarity, and remove deallocation size

[Xiaodi Wu]

On Sun, Sep 3, 2017 at 21:43 Taylor Swift <kelvin13ma at gmail.com> wrote:

> On Sat, Sep 2, 2017 at 7:53 PM, Andrew Trick <atrick at apple.com> wrote:
>
>>
>> On Sep 2, 2017, at 5:34 PM, Xiaodi Wu <xiaodi.wu at gmail.com> wrote:
>>
>> On Sat, Sep 2, 2017 at 4:41 PM, Andrew Trick <atrick at apple.com> wrote:
>>
>>>
>>> On Sep 2, 2017, at 2:06 PM, Taylor Swift via swift-evolution <
>>> swift-evolution at swift.org> wrote:
>>>
>>> the subscript doesn’t know about the stride that you want to use. If you
>>>>> want to get rid of the `at:` ambiguity, you have to get rid of it
>>>>> everywhere, or users will just wind up having to remember two ways (one
>>>>> ambiguous and one less ambiguous) of doing the same thing, instead of one
>>>>> (ambiguous) way of doing it.
>>>>>
>>>>
>>>> Certainly, that's a good point. On rethink and another re-reading of
>>>> the proposal, it's unclear to me that the addition of `at` arguments to
>>>> UnsafeMutablePointer is sufficiently justified by the proposal text. Is it
>>>> merely that it's shorter than writing `foo + MemoryLayout<T>.stride *
>>>> offset`? With the ambiguity of `at`, it seems that the current way of
>>>> writing it, though longer, is certainly less ambiguous.
>>>>
>>>
>>> Please reread it; UnsafeMutablePointer’s methods do *not* use `at:`.
>>>
>>>
>>> Regarding the typed buffer pointers, I think it is clear by convention,
>>> and will be well documented, that the `at` label refers to a position in
>>> `self`.
>>>
>>> The raw buffer pointer API isn’t so obvious. Since the `at` refers to
>>> `self` it might more logically be a byte offset. Note that `at` as a label
>>> name always refers to a strided index.
>>>
>>> This would be a bit more explicit:
>>> UnsafeRawBufferPointer.initializeMemory(as:T.self, atByteOffset:
>>> position * MemoryLayout<T>.stride, from: bufferOfT)
>>>
>>> But possibly less convenient… Since that `at` label currently on
>>> UnsafeRawPointer.initializeMemory is almost never used, I don’t think we
>>> need to worry too much about convenience.
>>>
>>> That existing `at` label on UnsafeRawPointer.initializeMemory, would
>>> also need to be renamed, which is fine.
>>>
>>
>> If I may suggest one more incremental improvement in clarity, it would be
>> to move `at[ByteOffset]` to be the first argument; this eliminates the
>> possible reading that we are offsetting the source buffer:
>>
>> ```
>> UnsafeRawBufferPointer.initializeMemory(atByteOffset: position *
>> MemoryLayout<T>.stride, as:T.self, from: bufferOfT)
>> ```
>>
>>
>> Sure, that probably makes sense if we decide to go with a byte offset vs.
>> stride index.
>> -Andy
>>
>
> If we use byte offset, then the at parameter in UnsafeMutableRawPointer
> should be removed, since pointer arithmetic can be used instead (just like
> with UnsafeMutablePointer).
>

I agree that it seems quite sensible to remove the ‘at’ parameter
altogether from the UMRP method.

Not convinced moving the at: argument to come before the as: argument is
> worth it in terms of source breakage.
>

Since much of this proposal involves shuffling and relabeling arguments,
I’d argue it’s better to break slight more source in one go for the optimal
API than to break slightly less for a slightly less optimal API, no? (This
is assuming there is agreement that ‘at:as:’ is less prone to
misinterpretation than ‘as:at:’.)

>
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