[swift-users] Bridging [String] to const char * const *

[Kenny Leung]

Thanks! At least it’s good to know there wasn’t something trivial I was missing.

-Kenny


> On Aug 18, 2016, at 2:33 PM, Jordan Rose <jordan_rose at apple.com> wrote:
> 
> Here's how we do it for internal testing purposes: withArrayOfCStrings. The callback is great because you don't have to worry about lifetimes.
> 
> Jordan
> 
>> On Aug 18, 2016, at 13:04, Kenny Leung via swift-users <swift-users at swift.org> wrote:
>> 
>> Hi All.
>> 
>> When interfacing with C, Swift does some magic to auto-convert Swift strings to char *. This is great, but passing an array of string pointers gets much more involved. The type translates to UnsafePointer<UnsafePointer<CChar>> in Swift.
>> 
>> So I originally tried to get pointers to the individual strings by using cString(using:), and putting them into an Array, but then I found out that there is nothing holding on to the cStrings, so they go away before they can be used. I finally wound up with this hack:
>> 
>> public extension Array {
>>    public func cStringArray() throws -> ArrayBridge<Element,CChar> {
>>        return try ArrayBridge<Element,CChar>(array:self) {
>>            guard let item = $0 as? String,
>>                  let translated = item.cString(using: .utf8) else {
>>                throw hexdreamsCocoa.Errors.InvalidArgumentError
>>            }
>>            return translated
>>        }
>>    }
>> }
>> 
>> /*
>> We need to have this intermediate object around to hold on to the translated objects, otherwise they will go away.
>> The UnsafePointer won't hold on to the objects that it's pointing to.
>> */
>> public struct ArrayBridge<SwiftType,CType> {
>> 
>>    let originals  :[SwiftType]
>>    let translated :[[CType]]
>>    let pointers   :[UnsafePointer<CType>?]
>>    public let pointer    :UnsafePointer<UnsafePointer<CType>?>
>> 
>>    init(array :[SwiftType], transform: (SwiftType) throws -> [CType]) throws {
>>        self.originals = array
>>        self.translated = try array.map(transform)
>> 
>>        var pointers = [UnsafePointer<CType>?]()
>>        for item in translated {
>>            pointers.append(UnsafePointer<CType>(item))
>>        }
>>        pointers.append(nil)
>>        self.pointers = pointers
>>        self.pointer = UnsafePointer(self.pointers)
>>    }
>> }
>> 
>> And then to use it you would do something like
>> 
>> try stringArray.cStringArray().pointer
>> 
>> This all seems pretty ugly. So my question is: Is this the right way to handle this problem? Is there a simpler way? It would be awesome if Swift auto-converted arrays of Strings to const char * const *, since it’s a construct used so much in C.
>> 
>> Thanks!
>> 
>> -Kenny
>> 
>> 
>> 
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