AutoreleasingUnsafeMutablePointer crashes the app - swift

Question

I receive some data from internet and need to guess the encoding if it's not provided, so I use this function stringEncoding(for:encodingOptions:convertedString:usedLossyConversion:), and it requires passing AutoreleasingUnsafeMutablePointer for receiving the converted string, I wrote code like this:
var str = "Hello, playground"
func decode(data: Data) -> String? {
var covertedString = NSString()
let stringPointer = AutoreleasingUnsafeMutablePointer<NSString?>(&covertedString)
guard NSString.stringEncoding(for: data, encodingOptions: nil, convertedString: stringPointer, usedLossyConversion: nil) != 0 else {
return nil
}
return covertedString as String
}
let data = str.data(using: .utf8)!
decode(data: data)
While the covertedString I got out of the function call is correct, the app always crashes. Any idea why AutoreleasingUnsafeMutablePointer is make it crashes? I tried to not passing convertedString, then it's not crashing any more, so looks like it's the root case. Any idea why it's crashing?
I am using Xcode Version 10.1 (10B61), with Swift 4

Answer 1

In your particular case the problem is that you have created an NSString, but then taken a pointer to a NSString?, which is a different thing.
But that doesn't really matter here. You don't create AutoreleasingUnsafeMutablePointer directly (or generally any kind of UnsafePointer). They're not promised to be valid by the time you use them. Instead, you create them implicitly using &.
func decode(data: Data) -> String? {
var convertedString: NSString? = "" // <- Make sure to make this optional
guard NSString.stringEncoding(for: data,
encodingOptions: nil,
convertedString: &convertedString, // <- Use &
usedLossyConversion: nil) != 0
else {
return nil
}
return convertedString as String?
}