串？ 不能转换为“ NSString”

Question

I'm following an Apple WatchKit course on Udemy, in which I create an currency conversion type app. The following syntax works for the gentleman in the video, but gives me an error:

func parser(parser: NSXMLParser, foundCharacters string: String?) {
....

currencyConversions[getCurrency] = (string as NSString).doubleValue

He states that we need to first convert this object into an NSString, and then convert it into a double. However, I get the following error:

String? is not convertible to 'NSString'

Perhaps he's using a newer version of Swift/Xcode? How can I fix this?

Answer 1

This works with Xcode 6.4 and Xcode 7.1.1 :

Since s is an Optional String , you need to safely unwrap it before casting it as an NSString . Use the nil coalescing operator ?? to do that:

currencyConversions[getCurrency] = ((string ?? "") as NSString).doubleValue

Answer 2

NSString has a constructor that composes it from an ordinary string. Try calling it from NSString(string!), and hopefully then it should work!

Answer 3

You need to unwrap the string object before casting it. Do not forget to check for nil value for optional object before unwrapping.

if (string != nil){
   currencyConversions[getCurrency] = (string! as NSString).doubleValue
}