[swift-dev] `withUnsafePointer` mutates `self`

[Kevin Ballard]

On Wed, Dec 16, 2015, at 11:49 AM, Joe Groff wrote:
>
>> On Dec 16, 2015, at 11:47 AM, Kevin Ballard <kevin at sb.org> wrote:
>>
>> On Wed, Dec 16, 2015, at 11:42 AM, Joe Groff wrote:
>>>
>>>> On Dec 16, 2015, at 11:24 AM, Kevin Ballard via swift-dev <swift-
>>>> dev at swift.org> wrote:
>>>>
>>>> On Wed, Dec 16, 2015, at 12:12 AM, Dave Abrahams wrote:
>>>>>
>>>>>> Come to think of it, what's the actual use-case for
>>>>>> withUnsafePointer()?
>>>>>
>>>>> I'm not sure we still have one that isn't covered by &x; that's my
>>>>> point.
>>>>>
>>>>>> If a value is mutable, you can already use &x or
>>>>>> withUnsafeMutablePointer(), and if it's immutable, you can't call
>>>>>> withUnsafePointer() today anyway. The proposed change would just
>>>>>> make withUnsafePointer() into the equivalent of `var x = value;
>>>>>> callSomethingWith(&x)`. The only reason to really want a
>>>>>> withUnsafePointer() function is if it can give you an
>>>>>> UnsafePointer to an immutable value without copying it, but we
>>>>>> can't do that. I'm inclined to say we should just get rid of
>>>>>> withUnsafePointer() entirely, at least until such time as Swift
>>>>>> has a way to pass immutable values by-ref.
>>>>>
>>>>> I'm inclined to agree.  Proposal?
>>>>
>>>> Sure, I'll write one up. I suspect that withUnsafePointer() /
>>>> withUnsafeMutablePointer() are likely to be rarely used today, and
>>>> most uses can probably be trivially replaced with just passing a &x
>>>> ref, so this shouldn't be a big deal.
>>>
>>> We can't remove withUnsafe[Mutable]Pointer; as I mentioned to
>>> Dave, it's necessary to persist a pointer for more than one
>>> immediate call.
>>
>> Rare cases like that can be covered by either declaring a nested
>> function taking the pointer and calling it, calling an anonymous
>> local closure of the right type, or even using
>> withExtendedLifetime(&x) { (ptr: UnsafePointer<T>) in ... }.
>
> These are all just withUnsafePointer with different spelling.

That's true, but they all have the benefit of not requiring extra stdlib
functions. swiftdoc.org lists 36 global functions, a full 4 of which are
withUnsafePointer + variants. The existence of the functions also
implies that they're necessary to work with pointers (especially the
documentation). My suspicion is that nearly all current uses of these
functions can be replaced with &x refs with no change in functionality.
Even the stdlib only has a handful of uses of withUnsafeMutablePointer
(and none at all for withUnsafePointer), and looking through them
quickly, it looks like only the ones in public/core/Runtime.swift.gyb
require a workaround.

-Kevin
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