[swift-evolution] Proposal: [stdlib] Remove withUnsafe[Mutable]Pointer[s]()

[Jacob Bandes-Storch]

withExtendedLifetime(&x) does extend the lifetime of &x, it just doesn't
extend the lifetime of x.

However, withExtendedLifetime(x) does extend the lifetime of x by holding a
strong reference (if it's a reference type), which can be useful. Maybe
withExtendedLifetime should be changed to <T: AnyObject> rather than <T> ?

Jacob

On Wed, Dec 16, 2015 at 2:07 PM, Michael Gottesman via swift-evolution <
swift-evolution at swift.org> wrote:

>
> > On Dec 16, 2015, at 2:22 PM, Dave Abrahams <dabrahams at apple.com> wrote:
> >
> >
> >> On Dec 16, 2015, at 11:54 AM, Michael Gottesman via swift-evolution <
> swift-evolution at swift.org> wrote:
> >>
> >>>
> >>> On Dec 16, 2015, at 1:49 PM, Kevin Ballard via swift-evolution <
> swift-evolution at swift.org> wrote:
> >>>
> >>> Another replacement for withUnsafe[Mutable]Pointer is declaring a
> nested function of the appropriate type (this is equivalent to the
> anonymous closure, but perhaps more readable):
> >>>
> >>> func foo(ptr: UnsafePointer<Int>) {
> >>>  // ...
> >>> }
> >>> foo(&x)
> >>>
> >>> -Kevin
> >>>
> >>> On Wed, Dec 16, 2015, at 11:38 AM, Kevin Ballard wrote:
> >>>> # Introduction
> >>>>
> >>>> The stdlib provides functions withUnsafePointer() and
> withUnsafeMutablePointer() (and plural variants) that take an inout
> reference and call a block with the UnsafePointer/UnsafeMutablePointer
> created from the reference.
> >>>>
> >>>> # Problem
> >>>>
> >>>> withUnsafePointer() can only be used with mutable variables, because
> those are the only things that can be used with inout &refs. Both functions
> are also fairly useless, as &x refs can be passed directly to functions
> taking an UnsafePointer or UnsafeMutablePointer. The existence of the
> functions mostly just causes people to think they're necessary when they're
> not. The provide no functionality that passing &x refs directly to the
> functions taking a pointer doesn't already fulfill.
> >>>>
> >>>> # Solution
> >>>>
> >>>> Remove the functions from the stdlib. The Swift Book should also be
> updated to talk about passing an &x ref to a function that takes an
> UnsafePointer or UnsafeMutablePointer (but of course changes to the book
> are not covered by the open-source project). Most uses of these functions
> can probably be replaced with a &x ref directly. If any can't, they could
> be replaced with the following equivalent expressions:
> >>>>
> >>>> { (ptr: UnsafePointer<Int>) in
> >>>>  // ...
> >>>> }(&x)
> >>>>
> >>>> or:
> >>>>
> >>>> withExtendedLifetime(&x) { (ptr: UnsafePointer<Int>) in
> >>>>  // ...
> >>>> }
> >>
> >> One thing to keep in mind here is that with*Pointer and friends is also
> meant to enable one to work around issues with the optimizer if they come
> up in a convenient manner. I.e. imagine if one is attempting to process an
> image using a 5d array and for whatever reason, you are not getting the
> performance you need. Hopefully you would file a bug report and then use
> with*Pointer for your image processing loop.
> >>
> >> My fear about withExtendedLifetime is that the name is a misnomer. You
> are not extending the lifetime.
> >
> > What makes you say that?
>
> Let me be more specific.
>
> My issue with the name 'withExtendedLifetime' is that it is suggestive
> that the lifetime of &x is being extended in a way that is different from
> if one just passed off &x to any old function. In reality though, nothing
> special is happening here implying that the name is misleading. A better
> name IMO would be something that drops any such implication.
>
> Michael
>
> >
> > If in fact it is true, shouldn't you file a bug report?
> >
> > -Dave
> >
> >
> >
>
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>
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