[swift-users] Can't initialise using a UTF8 string from within 'withUnsafeBufferPointer' expression

Martin R martinr448 at gmail.com
Mon Jul 18 07:51:53 CDT 2016

This is not an answer to your question, but note that you can pass a Swift String to functions expecting an UnsafePointer<Int8> (C String) parameter, and the compiler will generate a temporary NUL-terminated UTF-8 representation for you:

  let io = DispatchIO(type: .stream, path: "/path/to/file", ...)

So you don't need a convenience initializer for _this_ purpose.

> On 18 Jul 2016, at 07:17, Karl via swift-users <swift-users at swift.org> wrote:
> As I understand it, we are supposed to use withUnsafe{Mutable/Buffer}Pointer or withExtendedLifetime to guarantee that objects we take pointers of will exist and won’t be optimised out by the compiler.
> However, this leads to a problem when trying to initialise something which expects a UTF8 string using only the standard library (not Foundation’s equivalent cStringUsingEncoding):
>> extension DispatchIO {
>> 	convenience init(type: DispatchIO.StreamType, path: String, oflag: Int32, mode: mode_t, queue: DispatchQueue, cleanupHandler: (error: Int32) -> Void) {
>> 		let utf8Path = path.nulTerminatedUTF8
>> 		utf8Path.withUnsafeBufferPointer {
>> 			self.init(type: type, path: UnsafePointer($0.baseAddress!), oflag: oflag, mode: mode, queue: queue, cleanupHandler: cleanupHandler)
>> 		}
>> 	}
>> }
> ERROR: Initializer delegation ('self.init') cannot be nested in another expression
> I don’t really understand why that isn’t allowed for a non-capturing closure, but if we accept that, how do I actually use those pointer/extended lifetime functions to initialise things?
> Thanks
> Karl
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