[swift-evolution] SE-184 Improved Pointers

Taylor Swift kelvin13ma at gmail.com
Sat Aug 19 20:16:39 CDT 2017

What you’re describing is basically an earlier version of the proposal
which had a slightly weaker precondition (source >= destination) than yours
(source == destination). That one basically ignored the Sequence methods at
the expense of greater API surface area.

On Sat, Aug 19, 2017 at 9:08 PM, Andrew Trick <atrick at apple.com> wrote:

> On Aug 19, 2017, at 6:03 PM, Taylor Swift <kelvin13ma at gmail.com> wrote:
> On Sat, Aug 19, 2017 at 8:52 PM, Andrew Trick <atrick at apple.com> wrote:
>>> 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.
>> Here's an alternative. Impose the precondition(source.count ==
>> self.count) to the following UnsafeMutableBufferPointer convenience methods
>> that you propose adding:
>> +++ func assign(from:UnsafeBufferPointer<Element>)
>> +++ func assign(from:UnsafeMutableBufferPointer<Element>)
>> +++ func moveAssign(from:UnsafeMutableBufferPointer<Element>)
>> +++ func moveInitialize(from:UnsafeMutableBufferPointer<Element>)
>> +++ func initialize(from:UnsafeBufferPointer<Element>)
>> +++ func initialize(from:UnsafeMutableBufferPointer<Element>)
>> I don't that introduces any behavior that is inconsistent with other
>> methods. `copyBytes` is a totally different thing that only works on
>> trivial types. The currently dominant use case for UnsafeBufferPointer,
>> partially initialized backing store, does not need to use your new
>> convenience methods. It can continue dropping down to pointer+count style
>> initialization/deinitialization.
>> -Andy
> the latest draft does not have assign(from:UnsafeMutableBufferPointer<
> Element>) or  initialize(from:UnsafeMutableBufferPointer<Element>), it
> uses the generic Sequence methods that are already there that do not
> require that precondition.
> Sorry, I was pasting from your original proposal. Here are the relevant
> methods from the latest draft:
> https://github.com/kelvin13/swift-evolution/blob/
> 1b7738513c00388b8de3b09769eab773539be386/proposals/0184-
> improved-pointers.md
> +++ func moveInitialize(from:UnsafeMutableBufferPointer<Element>)
> +++ func moveAssign(from:UnsafeMutableBufferPointer<Element>)
> But with the precondition, the `assign` method could be reasonably added
> back, right?
> +++ func assign(from:UnsafeMutableBufferPointer<Element>)
> Likewise, I don’t have a problem with initialize(from:
> UnsafeBufferPointer) where self.count==source.count. The Sequence
> initializer is different. It’s designed for the Array use case and forces
> the caller to deal with partial initialization.
> UnsafeMutableRawBufferPointer.moveInitializeMemory on the other hand
> probably doesn't need that precondition since there's no way to
> deinitialize. It just needs clear comments.
> -Andy
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