Swift 将 Generic 转换为具有 nil 值的 Optional 会导致 fatalError

Question

使用 Swift 2，在我设计的示例中，我将 String 转换为 Int 或更具体地说是 Int 或 Int? 使用泛型。在 Int? 应该为 nil 的情况下，转换将失败并出现 fatalError: fatal error: unwrapping an Optional value

这些看起来可能是相似/重复的问题:

• Swift - casting a nil core data string as an optional value

• Swift: detecting an unexpected nil value in a non-optional at runtime: casting as optional fails

我的问题是:应该如何转换为 nil 的可选值？

例子:

class Thing<T>{
    var item: T

    init(_ item: T){
        self.item = item
    }
}

struct Actions{

    static func convertIntForThing<T>(string: String, thing:Thing<T>) -> T{
        return convertStringToInt(string, to: T.self)
    }

    static func convertStringToInt<T>(string: String, to: T.Type) -> T{
        debugPrint("Converting to ---> \(to)")

        if T.self == Int.self{
            return Int(string)! as! T
        }

        // in the case of "" Int? will be nil, the cast
        // here causes it to blow up with:
        //
        // fatal error: unexpectedly found nil while unwrapping an
        // Optional value even though T in this case is an Optional<Int>
        return Int(string) as! T

    }
}


func testExample() {
    // this WORKS:
    let thing1 = Thing<Int>(0)
    thing1.item = Actions.convertIntForThing("1", thing: thing1)

    // This FAILS:
    // empty string  where value = Int("")
    // will return an Optional<Int> that will be nil
    let thing2 = Thing<Int?>(0)
    thing2.item = Actions.convertIntForThing("", thing: thing2)
}

testExample()

Answer 1

你不能将 nil 转换为 some-kind-of-nil，但是你可以make some-kind-of-nil，正如这个人为的例子所示:

    func test(s:String) -> Int? {
        var which : Bool { return Int(s) != nil }
        if which {
            return (Int(s)! as Int?)
        } else {
            return (Optional<Int>.None)
        }
    }

    print(test("12"))
    print(test("howdy"))