Swift 1.2 Xcode 6.3 NSString? to String gives Thread 1: EXC_BAD_ACCESS (code=1, address=0x20)

Question

Now apple have handily gotten rid of the NSString and String automatic compatibility, I'm having a bit of a nightmare going between the two. I'm getting a couple of NSStrings out of a dictionary and I can't convert them to regular Strings...

I've tried:

let fileNameString: String = String(format: "%@", filename!)

let fileNameString: String = (filename as! String)

let fileNameString = filename as? String

let fileNameString = (filename as? String) ?? ""

if let fileNameString = filename as? String {
    println("\(fileNameString)")
}

But all produce the error.

I've broken in at the point of conversion and can see neither NSStrings are nil:

But no joy with either. Getting Thread 1: EXC_BAD_ACCESS (code=1, address=0x20) . Am I missing something obvious here?

Even just trying to print the NSString filename before conversion causes the same error..

Posting the code prior to conversion attempt to see if that has anything to do with it...

// First we create a head request as the info I need is in the headers
var newRequest: NSMutableURLRequest = NSMutableURLRequest(URL: request.URL!)
newRequest.HTTPMethod = "HEAD"
var response: NSURLResponse?
NSURLConnection.sendSynchronousRequest(newRequest, returningResponse: &response, error: nil)

// Unwrap response as httpResponse in order to access allHeaderFields        
if let httpResponse = response as? NSHTTPURLResponse {

    let headerString = "sfn-Document-Filename"
    let headerNSString = headerString as NSString
    let filetypeString = "Content-Type"
    let filetypeNSString = filetypeString as NSString

    // This is a dictionary where the keys are NSCFStrings
    // (NSStrings, hence creating the NSStrings above)
    var allHeaders = httpResponse.allHeaderFields

    // Getting the filename out here only works with as? NSString. as? String creates the same error as converting.
    let filename = allHeaders[headerNSString] as? NSString

    // This is a string which contains the type as 'application/pdf' for example.  We only need the part after the /.
    // Again, trying to get this out as a String fails
    let typeString = allHeaders[filetypeNSString] as? NSString
    var typeArray = typeString?.componentsSeparatedByString("/") as! [NSString]
    let filetype = typeArray[1]
}

Answer 1

If this were an NSString, then all you’d need to do is filename as String (no !). But it sounds like the problem is your filename, of optional type NSString?, is nil. (option-click filename to confirm its type)

If there’s a reasonable default (say, an empty string), try

let fileNameString = (filename as? String) ?? ""

Or if you need to handle the nil with specific code:

if let fileNameString = filename as? String {
    // use fileNameString, which will be unwrapped and of type String
}
else {
    // log error or similar
}

Or, if you want to defer unwrapping, but want to change the type inside the possible value, you can do

let fileNameString = filename as? String
// which is a less long-winded way of saying
let fileNameString = filename.map { $0 as String }

Generally speaking, you should try and cut down on the ! usage, since it leads to problems like this. ! is only for those times when you know from the code that the value absolutely positively cannot be nil.

edit: based on your sample code, try the following:

let url = NSURL(string: "http://www.google.com")
let request = url.map { NSMutableURLRequest(URL: $0) }
request?.HTTPMethod = "HEAD"

let response: NSHTTPURLResponse? = request.flatMap {
    var response: NSURLResponse?
    NSURLConnection.sendSynchronousRequest($0, returningResponse: &response, error: nil)
    return response as? NSHTTPURLResponse
}

let headers = response?.allHeaderFields as? [String:String]

// google.com has no such header but still...
let filename = headers?["sfn-Document-Filename"]

// bear in mind someArray[1] will also crash if there's no such entry,
// first/last are better if that's what you want
let type = headers?["Content-Type"]?
              .componentsSeparatedByString(";").first?
              .componentsSeparatedByString("/").last

Everything here is optional, but safe, so you can test at various points for nil for logging/error reporting purposes.