[swift-users] Using withUnsafePointer on char arrays within C structs

[Hooman Mehr]

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 <swift-users at swift.org> 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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