[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 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)
```
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