Swift 3：将String Array转换为C Char Array的编译错误

Question

I have the following C struct:

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

My Swift array has the following values:

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

I want to populate the C Struct field 'categories' with 'Weekday' & 'Weekend'. To do this I call my 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
}

I keep getting the following XCode 8 error:

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

Any suggestions? I don't understand why there is the optional and '!' in the C-Struct definition implicitly.

Answer 1

As the compiler emits as an error, you need to unwrap after Int8 w/ " ? " as follows.

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
}

then,

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 "?"
}

the code above works w/o error. Martin's solution gives a compile error at

mycat.categories = &cargs (see the link)

I don't know why.

Answer 2

Check the reference of utf8String property of NSString :

Discussion

This C string is a pointer to a structure inside the string object, which may have a lifetime shorter than the string object and will certainly not have a longer lifetime. Therefore, you should copy the C string if it needs to be stored outside of the memory context in which you use this property.

The term memory context is not well-defined, but one thing sure is that you cannot expect the allocated region for the C string would live forever. When the member categories in the category_fmc_s_type is accessed, the pointers may be pointing to the already freed regions.

Applying the suggestion from Martin R to your code, your code would be like this:

fileprivate static func toPointer(_ args: [String]) -> UnsafeMutablePointer<UnsafeMutablePointer<Int8>?> {
    let buffer = UnsafeMutablePointer<UnsafeMutablePointer<Int8>?>.allocate(capacity: args.count)
    buffer.initialize(from: args.lazy.map{strdup($0)})
    return buffer
}

And remember, after you finish using the category_fmc_s_type , you need to deallocate the regions allocated by strdup(_:) and UnsafeMutablePointer.allocate(capacity:) :

fileprivate static func freePointer(_ pointers: UnsafeMutablePointer<UnsafeMutablePointer<Int8>?>, count: Int) {
    for i in 0..<count {
        free(pointers[i])
    }
    pointers.deinitialize(count: count)
    pointers.deallocate(capacity: count)
}