Yes, the easiest way is to rely on compiler magic for ObjC/C interoperability,
but it is also important to know what is really happening.
My preferred version of the compiler magic is this actually:
func allZerosUUID() -> String {
return NSUUID(uuidBytes: [UInt8](repeating: 0, count: 32)).uuidString
}
Which is effectively this:
func allZerosUUID() -> String {
let allZeros = [UInt8](repeating: 0, count: 32)
return NSUUID(uuidBytes: allZeros).uuidString
}
(Note the use of let and absence of &).
> On Mar 2, 2017, at 1:33 PM, Russell Finn <[email protected]> wrote:
>
> I agree that the use of `withUnsafeBufferPointer` to get to the contents of
> an array makes more sense than `withUnsafePointer`, once you discover the
> concept of a "buffer pointer" (which a new Swift programmer may not do at
> first).
>
> However, I note that the following code:
>
> func allZerosUUID() -> String {
> var allZeros = [UInt8](repeating: 0, count: 32)
> return NSUUID(uuidBytes: &allZeros)
> }
>
> also compiles without error and returns the intended value. This appears to
> be supported by the "Constant Pointers" section of the "Interacting with C
> APIs" chapter of "Using Swift with Cocoa and Objective-C": "When a function
> is declared as taking an `UnsafePointer<Type>` argument, it can accept ... a
> `[Type]` value, which is passed as a pointer to the start of the array."
>
> I suspect the availability of `&array` in this context, coupled with an
> awareness of the existence of `withUnsafePointer(to:)`, is what led my
> co-worker down the wrong road (and to a puzzling error message); but since we
> have now discovered two more appropriate ways to write the code in question,
> perhaps it's not worth worrying about too much.
>
> Thanks to everyone for the discussion.
>
>
> On Wed, Mar 1, 2017 at 7:36 PM, Hooman Mehr <[email protected]
> <mailto:[email protected]>> wrote:
> Your co-worker needs to get passed the learning curve of these “unsafe” APIs
> and note that Swift arrays are complex data structures. &allZeros does not
> give you a pointer to a bunch of zero bytes, but a pointer to a struct that
> contains the private implementation details of allZeros array.
>
> Here is the correct way to do it:
>
> func allZerosUUID() -> String {
>
> let allZeros = [UInt8](repeating: 0, count: 32)
>
> return allZeros.withUnsafeBufferPointer { NSUUID(uuidBytes:
> $0.baseAddress).uuidString }
> }
>
>
>> On Mar 1, 2017, at 2:35 PM, Russell Finn via swift-users
>> <[email protected] <mailto:[email protected]>> wrote:
>>
>> Thanks to Joe and Quinn for their answers. I have a related followup — a
>> co-worker learning Swift wrote the following function:
>>
>> func allZerosUUID() -> String {
>> var allZeros = [UInt8](repeating: 0, count: 32)
>> return withUnsafePointer(to: &allZeros) { zerosPtr in
>> return NSUUID(uuidBytes: zerosPtr).uuidString
>> }
>> }
>>
>> but was puzzled that Xcode 8.2.1 gave an error "Cannot convert value of type
>> 'UnsafePointer<_>' to expected argument type 'UnsafePointer<UInt8>!'" on the
>> line with the NSUUID initializer. Their expectation was that `zerosPtr`
>> would be of type `UnsafePointer<UInt8>` because `allZeros` is of type
>> `[UInt8]`.
>>
>> They discovered that they could work around this by adding a call to
>> `withMemoryRebound`:
>>
>> func allZerosUUID() -> String {
>> var allZeros = [UInt8](repeating: 0, count: 32)
>> return withUnsafePointer(to: &allZeros) { zerosPtr in
>> zerosPtr.withMemoryRebound(to: UInt8.self, capacity:
>> allZeros.count) { zerosPtr in
>> return NSUUID(uuidBytes: zerosPtr).uuidString
>> }
>> }
>> }
>>
>> but felt that this should be unnecessary. Perhaps I'm missing something
>> simple, but I was unable to explain this compiler behavior; can anyone on
>> the list do so?
>>
>> (Yes, I did point out that they could pass `&allZeros` directly to
>> `NSUUID(uuidBytes:)`.)
>>
>> Thanks — Russell
>>
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>
>
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