[swift-evolution] SE-184 Improved Pointers

[Taylor Swift]

On Sat, Aug 19, 2017 at 6:05 PM, Andrew Trick <atrick at apple.com> wrote:

>
> On Aug 9, 2017, at 8:51 AM, Taylor Swift <kelvin13ma at gmail.com> wrote:
>
>
>
> On Wed, Aug 9, 2017 at 2:34 AM, Andrew Trick <atrick at apple.com> wrote:
>
>>
>> On Aug 8, 2017, at 11:10 PM, Taylor Swift <kelvin13ma at gmail.com> wrote:
>>
>>
>> On Wed, Aug 9, 2017 at 1:51 AM, Andrew Trick <atrick at apple.com> wrote:
>>
>>>
>>> On Aug 8, 2017, at 8:44 PM, Taylor Swift <kelvin13ma at gmail.com> wrote:
>>>
>>> cool,, as for UnsafeMutableRawBufferPointer.copy(from:bytes:), I cannot
>>> find such a function anywhere in the API. There is copyBytes(from:)
>>> <https://developer.apple.com/documentation/swift/unsafemutablerawbufferpointer/2635415-copybytes>,
>>> but the documentation is messed up and mentions a nonexistent count: argument
>>> over and over again. The documentation also doesn’t mention what happens if
>>> there is a length mismatch, so users are effectively relying on an
>>> implementation detail. I don’t know how to best resolve this.
>>>
>>>
>>> We currently have `UnsafeMutableRawBufferPointer.copyBytes(from:)`. I
>>> don’t think your proposal changes that. The current docs refer to the
>>> `source` parameter, which is correct. Docs refer to the parameter name, not
>>> the label name. So `source.count` is the size of the input. I was pointing
>>> out that it has the semantics: `debugAssert(source.count <= self.count)`.
>>>
>>> Your proposal changes `UnsafeRawPointer.copyBytes(from:count:)` to
>>> `UnsafeRawPointer.copy(from:bytes:)`. Originally we wanted to those API
>>> names to match, but I’m fine with your change. What is more important is
>>> that the semantics are the same as `copyBytes(from:)`. Furthermore, any new
>>> methods that you add that copy into a raw buffer (e.g.
>>> initializeMemory(as:from:count:)) should have similar behavior.
>>>
>>>
>> I’m fine with switching to taking the count from the source, though I
>> think taking the count from the destination is slightly better because
>> 1) the use cases I mentioned in the other email, and 2) all the other
>> memorystate functions use self.count instead of source.count, if they
>> take a source argument. But being consistent with the raw pointer
>> version is more important.
>>
>>
>> If it’s copying from a buffer it should not take a count, if it’s copying
>> from a pointer it obviously needs to take a count. What I mean by the two
>> versions being named consistently is simply that they’re both named
>> `copyBytes`. That really isn’t important though. The overflow/underflow
>> semantics being consistent are important.
>>
>> (Incidentally, the reason “bytes” needs to be in the somewhere name is
>> because this method isn’t capable of copying nontrivial values)
>>
>> Should the methods that don’t deal with raw buffers also be modified to
>> use the source argument (i.e. UnsafeMutableBufferPointer.ini
>> tialize(from:))?
>>
>>
>> I’m not sure what you mean by this. It also allows the destination to be
>> larger than the source. Initializing from a sequence does not trap on
>> overflow because we can’t guarantee the size of the sequence ahead of time.
>> When I talk about consistent overflow/underflow semantics, I’m only talking
>> about initializing one unsafe buffer/pointer from another unsafe
>> buffer/pointer.
>>
>> Also, was there a reason why UnsafeMutableRawBufferPoin
>> ter.copyBytes(from:) uses the source’s count instead of its own? Right
>> now this behavior is “technically” undocumented behavior (as the public
>> docs haven’t been updated) so if there was ever a time to change it, now
>> would be it.
>>
>>
>> Mainly because partial initialization is more expected than dropping data
>> on the floor. Ultimately, this should be whatever typical developers would
>> expect the behavior to be. I would be very hesitant to change the behavior
>> now though.
>>
>> -Andy
>>
>
> The problem is I would expect to be able to safely call deinitialize() and
> friends after calling initialize(from:). If Element is a class type and
> initialize doesn’t fill the entire buffer range, calling deinitialize()
> will crash. That being said, since copy(from:bytes:) and copyBytes(from:)
> don’t do any initialization and have no direct counterparts in
> UnsafeMutableBufferPointer, it’s okay if they have different behavior than
> the other methods.
>
>
> You astutely pointed out that the UnsafeMutableBufferPointer.deinitialize()
> method is dangerous, and I asked you to add a warning to its comments.
> However, given the danger, I think we need to justify adding the method to
> begin with. Are there real use cases that greatly benefit from it?
>

I agree that’s a problem, which is why i was iffy on supporting partial
initialization to begin with. The use case is for things like growing
collections where you have to periodically move to larger storage. However,
deinitialize is no more dangerous than moveInitialize,
assign(repeating:count:), or moveAssign; they all deinitialize at least one
entire buffer. If deinitialize is to be omitted, so must a majority of the
unsafe pointer API.
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