[swift-evolution] [Draft] UnsafeRawPointer API

[Andrew Trick]

I agree, this syntax works best, all considered:

UnsafeRawPointer.cast(to: UnsafePointer<B>.Type)


> On Jun 26, 2016, at 11:39 PM, Dave Abrahams <dabrahams at apple.com> wrote:
> 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)

FWIW: I prefer to avoid the angle brackets, but it's not a battle that needs to be fought over this feature. When experimenting with cast syntax, I found that passing type parameters as function arguments was much more readable than using generic type parameters, which I think obscure the signature. This will be especially true when '.self' goes away.

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

Casting from a raw pointer to a typed pointer is only more dangerous
than other raw pointer operations because it is the first step in this
sequence of operations, which is undefined:

ptrA = rawPtr.cast(to: UnsafePointer<A>.self)
ptrA.initialize(with: A())
ptrA.deinitialize()

ptrB = rawPtr.cast(to: UnsafePointer<B>.self)
ptrB.initialize(with: B())

>  Also, how do you get an
> UnsafeMutablePointer?

UnsafeRawPointer casts to UnsafePointer<T>
UnsafeMutableRawPointer casts to UnsafeMutablePointer<T>

>> 
>> Examples:
>> 
>> ---
>> Case 1: casting a raw pointer as an argument
> 
> 
> 
> foo(rawPtr.cast(to: UnsafePointer<A>.self))
> 
>> ---
>> Case 2: "recasting" a typed pointer argument
>> 
>> 
> 
> 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)
>> 
> 
> 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
>> 
> 
> return UnsafeRawPointer(foo()).cast(to: UnsafePointer<B>.self)
> 
> -- 
> -Dave

Yes, that works.

-Andy