xcode - Swift String 有索引类型吗？

Question

据我所知，我了解到 Swift 字符串不能按整数值索引。但我很好奇索引的类型是什么，或者有没有。我在 Xcode palyground 中写了以下代码:

let greeting = "Hello! Tom"
let index = greeting.startIndex

通过点击键并点击索引值，Xcode推断索引类型为Index(请见下图)。但是，当我将第二行更改为

 let index: Index = greeting.startIndex

我从 Xcode 收到了两个错误警告

 error: use of undeclared type 'Index'
 error: could not infer type for 'index'

Xcode不是告诉我它推断索引类型是Index吗，为什么我明确声明类型时会出现错误？谁能给我解释一下？

Answer 1

字符串类型有一个索引:

extension String {
    public typealias Index = String.CharacterView.Index
    /// The position of the first `Character` in `self.characters` if
    /// `self` is non-empty; identical to `endIndex` otherwise.
    public var startIndex: Index { get }
    /// The "past the end" position in `self.characters`.
    ///
    /// `endIndex` is not a valid argument to `subscript`, and is always
    /// reachable from `startIndex` by zero or more applications of
    /// `successor()`.
    public var endIndex: Index { get }
    /// Access the `Character` at `position`.
    ///
    /// - Requires: `position` is a valid position in `self.characters`
    ///   and `position != endIndex`.
    public subscript (i: Index) -> Character { get }
}

声明 Index 在当前上下文中是不明确的。使用 String.Index 显式引用 String 扩展中定义的类型别名:

let index: String.Index = greeting.startIndex // 0

如果你真的想发疯，你可以在顶级范围内创建另一个类型别名:

public typealias Index = String.Index
let index: Index = greeting.startIndex // 0

虽然不确定我会推荐这个。