> On Dec 15, 2015, at 6:46 PM, Kevin Ballard <[email protected]> wrote:
>
> On Tue, Dec 15, 2015, at 06:39 PM, Dave Abrahams wrote:
>>
>>> On Dec 15, 2015, at 6:33 PM, Kevin Ballard via swift-dev
>>> <[email protected]> wrote:
>>>
>>> On Tue, Dec 15, 2015, at 03:03 PM, Joe Groff via swift-dev wrote:
>>>>
>>>> Yeah, it seems to me like a reasonable refinement for 'withUnsafePointer'
>>>> to take an immutable parameter. Since this is a stdlib API change, you
>>>> should suggest that on swift-evolution.
>>>
>>> A change like that is going to break any code that relies on the inout
>>> optimization (where it uses call-by-reference instead of copy-in copy-out
>>> when possible). Yes, such code is in violation of Swift semantics today,
>>> but it does work.
>>
>> Two questions:
>>
>> 1. Don’t we want a withUnsafeMutablePointer for the mutating cases (where
>> the inout optimization can take effect) anyway?
>
> I'm thinking here of cases like passing a context pointer to KVO. You're not
> actually mutating it, you just need a pointer that's the same every time you
> call the code.
Well, it is not possible to code a version of withUnsafePointer that makes that
guarantee in Swift.
>
>> 2. Joe, these APIs predate many of your changes that make &x transparently
>> convert to Unsafe[Mutable]Pointer arguments. Are they obsolete? Can we
>> replace them with { x: Unsafe[Mutable]Pointer in … }(&y) ?
>
> Interesting idea. I rarely use withUnsafePointer/withUnsafeMutablePointer
> because the &x shorthand exists. If withUnsafePointer() is modified to take
> its argument by-value and create a pointer from it, I expect people will
> start using it in place of &x shorthand so they can use it with immutable
> values, without realizing that they're getting a different pointer back each
> time (whether they notice this depends on what they're using the pointer for).
>
> If { x: UnsafePointer<Int> in ... }(&y) works, that's a cute replacement.
> Although it does move the &y reference to after the scope instead of before,
> which is a little unfortunate.
>
> ...huh. On a whim I tested the following and it shocked me by working.
>
> var x: Int = 42
> withExtendedLifetime(&x) { (x: UnsafeMutablePointer<Int>) in
> x.memory += 1
> }
> print(x) // prints 43
>
> If we want to replace withUnsafe[Mutable]Pointer() with a common idiom, I'd
> recommend using withExtendedLifetime like that, because a) it guarantees the
> pointed-to value exists for the duration of the scope, and b) it already
> works.
>
> -Kevin Ballard
-Dave
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