[swift-evolution] [Draft] UnsafeRawPointer API

[L. Mihalkovic]

Regards
(From mobile)

> On Jun 27, 2016, at 8:39 AM, Dave Abrahams <dabrahams at apple.com> wrote:
> 
> 
> on Fri Jun 24 2016, Andrew Trick <atrick-AT-apple.com> wrote:
> 
>>> On Jun 24, 2016, at 11:22 AM, Andrew Trick via swift-evolution
>>> <swift-evolution at swift.org> wrote:
>>> 
>>>> On Jun 24, 2016, at 11:17 AM, L. Mihalkovic
>>>> <laurent.mihalkovic at gmail.com
>>>> <mailto:laurent.mihalkovic at gmail.com>> wrote:
>>>> 
>>>> I like the watch-what-you-wish-for warning of unsafeCast.
>>> 
>>> I’ll try porting stdlib to the “UnsafeRawPointer.unsafeCast(to:
>>> T.Type)” syntax and see how bad it is.
>> 
>> I don't think there's a clear winner here. Let me enumerate some
>> options.
>> 
>> Option (1) UnsafePointer<T>(cast: UnsafeRawPointer)
> 
> The problem with this one is that T can be deduced based on type
> context.  I think we ought to move away from that for operations like
> this one.
> 
>> Option (2) UnsafePointer<T>(_: UnsafeRawPointer, to: T.self)
> 
> I think you mean T.Type, not T.self, because this looks like a declaration.
> 
> To evaluate, you have to look at the use-site:
> 
>    let p = UnsafePointer(r, to: Int.self)
> 
> I don't find “to” to be descriptive enough.  Maybe

toType

> 
>    let p = UnsafePointer(r, pointee: Int.self)

I find pointee a total aberation :)

> 
> is better.  But I hate that the language doesn't give us a way to say
> “don't deduce generic parameters here.”  This is the only syntax that
> feels right, IMO:
> 
>    let p = UnsafePointer<Int>(r)
> 
>> Option (3) UnsafeRawPointer.unsafeCast<T>(to: T.Type) ->
>> UnsafePointer<T>
> 
>    r.unsafeCast(to: Int.self)
> 
> I don't see adding “unsafe” to the name of the operation as adding
> anything.  It isn't any more unsafe than other UnsafeRawPointer
> operations.  

It is unsafe in the sense that there are no guarantees that it is a sensible thing to do. I guess that means it is more 'noguaranteeexplicitorimpliedapplied' in the sense that it will like mechanically work, even if it produce an aberation as a result

> Also, it reads like we're casting the raw pointer to an
> Int, rather than to an UnsafePointer<Int>.

Really good one... But then instead of 'to' or 'pointee', something along the lines of 'wrappedType', which lookes a little less balerina-ish than pointee.....

>  Also, how do you get an
> UnsafeMutablePointer?
> 
>> Option (4) unsafeCast(rawPointer: UnsafeRawPointer, to: T.self) ->
>> UnsafePointer<T>
> 
> This one won't read correctly for the same reasons as #3.
> 
>    r.cast(to: UnsafePointer<Int>.self)
> 
> works better for me than any of the alternatives given our inability to
> get the One True Syntax.
> 
>> ---
>> Option (3) is the most explicit and searchable, and forces
>> UnsafeRawPointer to be spelled out in the conversion (unless you
>> already have a raw pointer).
> 
> Huh?  I'm confused here.  What you wrote looks like it's intended to be
> a regular method, in which case of course invoking it would require a raw
> pointer and wouldn't force you to write UnsafeRawPointer out anywhere.
> 
> The only way it could force you to write UnsafeRawPointer would be if it
> was a static method, but in that case it has too few arguments.
> 
>> I like this because conceptually, you need to cast to a raw pointer
>> before casting to a new pointee type, and casting a raw pointer to a
>> typed pointer carries important semantics beyond simply converting to
>> a typed pointer. The main problem with Option (3) is that optional raw
>> pointers can't be converted naturally (without using `map`).
> 
>  r ?? someExpressionUsing(r!)
> 
> best I can do.
> 
>> Another thing I'm a little nervous about is confusing a cast of the
>> pointer value with a cast of the pointee type:
>> 
>>  `unsafeBitCast(rawPtr, to: Int.self)`
>> 
>> is very different from
>> 
>>  `rawPtr.unsafeCast(to: Int.self)`
>> 
>> Does this need to be clarified?
> 
> Yes!
> 
>> If so, we can go back to the `toPointee` label that I proposed
>> earlier.
>> 
>> With that in mind, Option(4) is starting to look pretty good.
>> 
>> Examples:
>> 
>> ---
>> Case 1: casting a raw pointer as an argument
> 
> Use sites! (yay)...
> 
>> func foo(_: UnsafePointer<A>)
>> 
>> let rawPtr = UnsafeRawPointer(...)
>> 
>> (1) foo(UnsafePointer(cast: rawPtr))
>> 
>> (2) foo(UnsafePointer(rawPtr, to: A.self))
>> 
>> (3) foo(rawPtr.unsafeCast(to: A.self))
>> 
>> (4) foo(unsafeCast(rawPointer: rawPtr, to: A.self))
> 
> 
> foo(rawPtr.cast(to: UnsafePointer<A>.self))
> 
>> ---
>> Case 2: "recasting" a typed pointer argument
>> 
>> Note that typed pointer arguments are implicitly cast to raw pointer
>> arguments, so the conversion from PtrB to raw is implicit.
>> 
>> func foo(_: UnsafePointer<A>)
>> 
>> let ptrB = UnsafePointer<B>(...)
>> 
>> (1) foo(UnsafePointer(cast: ptrB))
>> 
>> (2) foo(UnsafePointer(ptrB, to: A.self))
>> 
>> (3) foo(UnsafeRawPointer(ptrB).unsafeCast(to: A.self))
>> 
>> (4) foo(unsafeCast(rawPointer: ptrB, to: A.self))
> 
> foo(UnsafeRawPointer(ptrB).cast(to: UnsafePointer<A>.self))
> 
> I don't believe in making these “double-hops” concise.
> 
>> ---
>> Case 3: Optional argument (only Option 3 is affected)
>> 
>> func nullableFoo(_: UnsafePointer<Int>?)
>> 
>> let ptrB: UnsafePointer<UInt>? = ...
>> 
>> (1) nullableFoo(UnsafePointer(cast: ptrB))
>> 
>> (2) nullableFoo(UnsafePointer(ptrB, to: A.self))
>> 
>> (3) nullableFoo(UnsafeRawPointer(ptrB).map { $0.unsafeCast(to: A.self) })
>> 
>> (4) nullableFoo(unsafeCast(rawPointer: ptrB, to: A.self))
> 
> nullableFoo(UnsafeRawPointer(ptrB)?.cast(to: UnsafePointer<A>.self))
> 
> You do the above with a failable init on UnsafeRawPointer that takes an
> optional UnsafePointer.
> 
>> ---
>> Case 4: Return values
>> 
>> func foo() -> UnsafePointer<A>
>> 
>> func caller() -> UnsafePointer<B> { ...
>> 
>> (1) return UnsafePointer(cast: foo())
>> 
>> (2) return UnsafePointer(foo(), to: B.self)
>> 
>> (3) let rawPtr = UnsafeRawPointer(foo())
>>    return rawPtr.unsafeCast(to: B.self)
>> 
>> (4) return unsafeCast(rawPointer: foo(), to: B.self)
> 
> return UnsafeRawPointer(foo()).cast(to: UnsafePointer<B>.self)
> 
> IMO-ly y'rs,
> 
> -- 
> -Dave