[swift-evolution] [swift-evolution-announce] [Review] SE-0127: Cleaning up stdlib Pointer and Buffer Routines

Charlie Monroe charlie at charliemonroe.net
Sun Jul 24 07:51:12 CDT 2016 

>> Ok, I'll update the proposal to withdraw the change of inout for
>> withUnsafePointer.
> 
> We could also add
> 
>    withUnsafePointer(toCopyOf: x)
> 
> but that can also come later if we decide we need it.

I don't think this is correct naming. It might be in case of structs, but in case of a pointer on the heap, I think this is misleading. It would be absolutely valid to write:

let obj = NSObject()
withUnsafePointer(toCopyOf: obj) { ptr in
	print(ptr)
}

and it doesn't create any copy of anything. I'd personally prefer renaming the current behavior to withUnsafePointer(byReferenceTo:) which is more descriptive.

> Under the discussion for the pull request, Bob mentioned possible
>> removal of the multi-pointer variants
>> (i.e. withUnsafe[Mutable]Pointers) - is this something that you'd
>> agree on?
> 
> Absolutely.

I've updated the proposal and created a pull request.

> 
>> If so, I'd add this to the proposal since I agree that this is an API
>> that is rarely used, can be used by nesting two
>> withUnsafe[Mutable]Pointer calls and is limited to three pointers at
>> max anyway. It almost feels like NSAssert1, NSAssert2, etc.
>> 
>>> On Jul 23, 2016, at 3:35 AM, Dave Abrahams via swift-evolution <swift-evolution at swift.org> wrote:
>>> 
>>> 
>>> on Fri Jul 22 2016, Xiaodi Wu <xiaodi.wu-AT-gmail.com> wrote:
>>> 
>>>> On Fri, Jul 22, 2016 at 6:49 PM, Dave Abrahams via swift-evolution <
>>>> swift-evolution at swift.org> wrote:
>>>> 
>>>>> 
>>>>> on Fri Jul 22 2016, Xiaodi Wu <swift-evolution at swift.org> wrote:
>>>>> 
>>>>>> On Fri, Jul 22, 2016 at 5:06 PM, Dave Abrahams via swift-evolution <
>>>>>> swift-evolution at swift.org> wrote:
>>>>>> 
>>>>>>> 
>>>>>>> on Fri Jul 22 2016, Bob Wilson <swift-evolution at swift.org> wrote:
>>>>>>> 
>>>>>>>> It is not so clear what to do about SR-1956. (Charlie and I had some
>>>>>>>> comments on this in https://github.com/apple/swift-evolution/pull/437
>>>>>>>> <https://github.com/apple/swift-evolution/pull/437>.) Jordan raised
>>>>>>>> the objection that when using withUnsafePointer with a global, there
>>>>>>>> is an expectation that you’ll get the same address every
>>>>>>>> time. Removing inout would cause the argument to be passed by value
>>>>>>>> and the address would refer to a copy. Dmitri agreed that this could
>>>>>>>> be a problem. On the other hand, if you don’t care about the address,
>>>>>>>> or if you’re not using a value type, it would indeed be convenient to
>>>>>>>> have a version of withUnsafePointer that does not require an inout
>>>>>>>> argument.
>>>>>>>> 
>>>>>>>> Option 1: Keep inout (not addressing SR-1956). In this case, there’s
>>>>>>>> no reason to have both withUnsafePointer and
>>>>>>>> withUnsafeMutablePointer. If you want to call a function that expects
>>>>>>>> an UnsafePointer, you can give it an UnsafeMutablePointer and there
>>>>>>>> will be an implicit conversion to make it work. I discussed this with
>>>>>>>> Apple’s stdlib team and they recommended that if we have only one
>>>>>>>> function we use the shorter name “withUnsafePointer” and have it use
>>>>>>>> an UnsafeMutablePointer.
>>>>>>> 
>>>>>>> Very much in favor of Option 1.
>>>>>>> 
>>>>>> 
>>>>>> Ditto, except that I think there is some value in keeping both (i.e.
>>>>> doing
>>>>>> nothing): allowing the user to document intent. It would be inconsistent
>>>>>> and potentially confusing to call the function that returns an
>>>>>> `UnsafeMutablePointer` `withUnsafePointer`.
>>>>> 
>>>>> It doesn't return an `UnsafeMutablePointer`, it passes an
>>>>> `UnsafeMutablePointer` to the body of the closure.
>>>>> 
>>>> 
>>>> Brainfart. Yes, that's what I meant to write. Sorry.
>>>> 
>>>>>> It's rarely used enough, and the shorter name needlessly raises the
>>>>>> question of where I'm really "supposed to be" mutating the
>>>>>> pointee.
>>>>> 
>>>>> I don't understand; you only have the pointee inside the closure.
>>>>> That's where you mutate it (obviously?)
>>>>> 
>>>> 
>>>> If my closure does not mutate the pointee, `withUnsafePointer(_:)` allows
>>>> me to document that. Everything *works* with
>>>> `withUnsafeMutablePointer(_:)`, but I cannot read the code and understand
>>>> that no mutation has happened within the body of the closure. [Am I wrong
>>>> on this?]
>>> 
>>> No, you're right.
>>> 
>>>> For instance, I've been working with some of the Accelerate.framework
>>>> functions and the arguments are often cryptic. Take this call:
>>>> 
>>>> ```
>>>> cblas_sgemm(CblasColMajor, CblasNoTrans, CblasNoTrans, m, n, k, 1, matrix,
>>>> m, b, k, 1, &c, m)
>>>> ```
>>>> 
>>>> There are times when I'd want to
>>>> call `cblas_sgemm(_:_:_:_:_:_:_:_:_:_:_:_:_:_:)` inside an
>>>> `withUnsafe[Mutable]Pointer(_:)` closure. Distinguishing
>>>> `withUnsafePointer(_:)` and `withUnsafeMutablePointer(_:)` would allow a
>>>> reader to know from the outset if `$0.pointee is mutated without having to
>>>> know that the second-from-last argument is the one that stores the result
>>>> (it is not consistently second-from-last; for vDSP_* functions, it's often
>>>> the third-from-last argument, and for others it can be the first argument).
>>>> Removing the current `withUnsafePointer(_:)` would decrease clarity for the
>>>> reader here.
>>> 
>>> Okay, fair enough.
>>> 
>>>>> I've not had to use these functions much, but the distinction between
>>>>>> `Array.withUnsafeBufferPointer(_:)` and
>>>>>> `Array.withUnsafeMutableBufferPointer(_:)` has conditioned me to
>>>>>> mutate the pointee using only "mutable" functions.
>>>>> 
>>>>> Not sure if you're just drawing an analogy,
>>>> 
>>>> I was trying to. I guess ineffectively.
>>>> 
>>>>> but if not, those two
>>>>> methods are not under discussion here.  They are meaningfully different,
>>>>> whereas the existing functions are not, and the one currently called
>>>>> withUnsafePointer is always going to cause people to complain about
>>>>> having to pass a mutable variable.
>>>>> 
>>>>> As a fallback position, I would suggest we only provide the mutating
>>>>> one, but with its existing name.  But I still prefer the shorter name.
>>>>> 
>>>>>> 
>>>>>>>> 
>>>>>>>> Option 2: Fix SR-1956 and have two functions, one with inout and the
>>>>>>>> other not. This would address the inconvenience of not being able to
>>>>>>>> use withUnsafePointer with immutable values, while still supporting
>>>>>>>> the existing behavior. The question then would be what to call these
>>>>>>>> two functions.
>>>>>>> 
>>>>>>> We do not need to support new use-cases in this release, and this would
>>>>>>> be unsatisfying because the “address of a global” property that Jordan
>>>>>>> argued for would not hold for the immutable version.
>>>>>>> 
>>>>>>>> - Option 2a. Combine the two existing functions as in Option 1 and use
>>>>>>>> a new name for the non-inout version, e.g.,
>>>>>>>> withUnsafePointer(toCopyOf:), so that it won’t be confused with the
>>>>>>>> old function. (That particular name doesn’t work very well when
>>>>>>>> dealing with references to objects, since the object itself would not
>>>>>>>> be copied. I haven’t yet come up with a better name, though.) One
>>>>>>>> advantage of this approach is that we would not need to rush the new
>>>>>>>> function into Swift 3 since it would be an additive change.
>>>>>>> 
>>>>>>> Not rushing that into Swift 3 is the same as Option 1.
>>>>>>> 
>>>>>>>> - Option 2b. Switch to use withUnsafeMutablePointer for all the cases
>>>>>>>> where you care about the getting the same address. Change
>>>>>>>> withUnsafePointer to be the non-inout version. Charlie suggested that
>>>>>>>> we could have the migrator convert all existing uses on
>>>>>>>> withUnsafePointer in Swift 2 code to use withUnsafeMutablePointer in
>>>>>>>> Swift 3, but I’m not sure how well that would work.
>>>>>>> 
>>>>>>> That's exactly the same outcome, with respect to the language/library
>>>>>>> surface, as Option 2 AFAICT.  Can we simplify this list of options?
>>>>>>> 
>>>>>>> --
>>>>>>> Dave
>>>>>>> 
>>>>>>> _______________________________________________
>>>>>>> swift-evolution mailing list
>>>>>>> swift-evolution at swift.org
>>>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>>>>> 
>>>>>> _______________________________________________
>>>>>> swift-evolution mailing list
>>>>>> swift-evolution at swift.org
>>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>>>> 
>>>>> 
>>>>> --
>>>>> Dave
>>>>> 
>>>>> _______________________________________________
>>>>> swift-evolution mailing list
>>>>> swift-evolution at swift.org
>>>>> https://lists.swift.org/mailman/listinfo/swift-evolution
>>>>> 
>>> 
>>> -- 
>>> Dave
>>> _______________________________________________
>>> swift-evolution mailing list
>>> swift-evolution at swift.org
>>> https://lists.swift.org/mailman/listinfo/swift-evolution

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