需要澄清 Swift 中的类型转换运算符

Question

为什么在此 switch 语句中使用类型转换运算符 (as) 而不是其条件形式 (as?)？

我认为类型运算符只能是 (as?) 或 (as!)...？ Apple Swift 文档对此没有提供充分的解释。

这是 Swift 文档中的示例：

var things = [Any]()

things.append(0)
things.append(0.0)
things.append(42)
things.append(3.14159)
things.append("hello")
things.append((3.0, 5.0))
things.append(Movie(name: "Ghostbusters", director: "Ivan Reitman"))
things.append({ (name: String) -> String in "Hello, \(name)" }) 

for thing in things {
        switch thing {
        case 0 as Int:
            println("zero as an Int")
        case 0 as Double:
            println("zero as a Double")
        case let someInt as Int:
            println("an integer value of \(someInt)")
        case let someDouble as Double where someDouble > 0:
            println("a positive double value of \(someDouble)")
        case is Double:
            println("some other double value that I don't want to print")
        case let someString as String:
            println("a string value of \"\(someString)\"")
        case let (x, y) as (Double, Double):
            println("an (x, y) point at \(x), \(y)")
        case let movie as Movie:
            println("a movie called '\(movie.name)', dir. \(movie.director)")
        case let stringConverter as String -> String:
            println(stringConverter("Michael"))
        default:
            println("something else")
        }
    }

这里是link到Apple Swift documentation on Type Casting

Answer 1

如果您阅读底部的 note，您可能会自己找到答案：

The cases of a switch statement use the forced version of the type cast operator (as, not as?) to check and cast to a specific type. This check is always safe within the context of a switch case statement.

（强调我的）

Here 是一个 Apple 博客 post，它详细说明了 as?、as 和 as! 之间的区别。

Answer 2

case 0 as Int:或case let someInt as Int:中的as 是其一部分一个 类型的铸造模式 。在Swift Language Reference switch 语句的case 标签中定义为

case-label → case case-item-list:
case-item-list → pattern guard-clause_opt | pattern guard-clause_opt , case-item-list

模式可以在哪里（在其他情况下）

pattern → value-binding-pattern
pattern → type-casting-pattern
pattern → expression-pattern

类型转换模式是

type-casting-pattern → is-pattern | as-pattern
is-pattern → is type
as-pattern → pattern as type

例如你有

case let someInt as Int:
╰──────────────────────╯ case-label
     ╰────────────────╯  case-item-list -> type-casting pattern
                    ╰─╯  type
                 ╰╯      `as` keyword
     ╰─────────╯         value-binding pattern

Answer 3

Swift-博客：

It may be easiest to remember the pattern for these operators in Swift as: ! implies “this might trap,” while ? indicates “this might be nil.”

还有第三种保证转换的可能性，当向上转换完成时。例如 2 as Any 在与 as! 或 as?

一起使用时会收到警告

在 switch 构造的情况下，case let value as Type: 永远不会失败，也不可能，与表达式 value as? Type

相比，该值将是可选类型

需要澄清 Swift 中的类型转换运算符

Need clarification of Type Casting operator in Swift

swift

casting

typecasting-operator