c - swift 3 : Compilation error converting String Array to C Char Array

Question

我有以下 C 结构:

typedef struct {
  char** categories;
  int category_size;
} category_fmc_s_type;

我的 Swift 数组具有以下值:

let categories = ["Weekday", "Weekend"]

我想用“工作日”和“周末”填充 C 结构字段“类别”。为此，我调用了 toPointer():

fileprivate static func toPointer(_ args: [String]) -> UnsafeMutablePointer<UnsafeMutablePointer<Int8>> {
  let buffer = UnsafeMutablePointer<UnsafeMutablePointer<Int8>>.allocate(capacity: args.count)
  for (index, value) in args.enumerated() {
    buffer[index] = UnsafeMutablePointer<Int8>(mutating: (value as NSString).utf8String!)
  }
  return buffer
}

我不断收到以下 XCode 8 错误:

Cannot convert value of type 'UnsafeMutablePointer<UnsafeMutablePointer<Int8>>' to expected argument type 'UnsafeMutablePointer<UnsafeMutablePointer<Int8>?>!'

有什么建议吗？我不明白为什么有可选的和'!'在 C-Struct 定义中隐含。

Answer 1

由于编译器作为错误发出，您需要在 Int8 w/"?"之后解包，如下所示。

fileprivate func toPointer(_ args: [String]) -> UnsafeMutablePointer<UnsafeMutablePointer<Int8>?> {
  let buffer = UnsafeMutablePointer<UnsafeMutablePointer<Int8>?>.allocate(capacity: args.count)
  for (index, value) in args.enumerated() {
    buffer[index] = UnsafeMutablePointer<Int8>(mutating: (value as NSString).utf8String!)
  }
  return buffer
}

然后，

func testMyCat() {
  let categories = ["Weekday", "Weekend"]
  let buffer = toPointer(categories)
  var mycat = category_fmc_s_type()
  mycat.categories = buffer  // you would see compile error w/o "?"
}

上面的代码没有错误。 Martin's solution在

处给出编译错误

mycat.categories = &cargs(查看链接)

我不知道为什么。