改变 ArraySlice 会实例化一个新的数组实例吗？

Question

var absences = [0, 2, 0, 4, 0, 3, 1, 0]
let midpoint = absences.count / 2

var firstHalf = absences.prefix(upTo: midpoint)
let secondHalf = absences.suffix(from: midpoint)

来自 Apple 的引述：

Neither the firstHalf nor secondHalf slices allocate any new storage of their own. Instead, each presents a view onto the storage of the absences array.

当我尝试按如下方式改变 firstHalf 时：

firstHalf[1] = 19

firstHalf 的值发生变化，但原始数组 absences 保持不变（firstHalf[1] 等于 19 而 absences[1] 等于 2）那么在后台发生了什么。我是否通过改变数组切片来实例化一个新数组？提前致谢。

Answer 1

是的，标准库的集合类型，包括 Array 和 ArraySlice，都具有写时复制行为。这意味着它们可以 与其他集合共享 存储它们的元素，直到它们发生变异，在这种情况下，它们将获取它们自己的副本。

在您的情况下，切片 firstHalf 对其具有视图的底层数组缓冲区是非唯一引用的（因为 absences 和 secondHalf 也对其具有视图).因此，当你去改变 firstHalf 时，一个副本被触发——创建一个包含切片元素的 new 缓冲区（但不一定是 entire数组）。

firstHalf 现在对这个新缓冲区有独特的看法，absences 和 secondHalf 都共享对旧数组缓冲区的看法。因此 firstHalf 现在可以在不影响原始数组元素的情况下改变其缓冲区的元素，从而保留值语义。

Answer 2

这是 Swift 集合在幕后采用的标准写时复制行为，在您尝试改变集合之前，它不会复制集合。有关写时复制的讨论，请参阅 WWDC 2015 视频 Building Better Apps with Value Types in Swift。

评论 in the code 为我们澄清了这一点：

/// Slices Share Indices
/// --------------------
///
/// A collection and its slices share the same indices. An element of a
/// collection is located under the same index in a slice as in the base
/// collection, as long as neither the collection nor the slice has been
/// mutated since the slice was created.

...

/// Slices Inherit Collection Semantics
/// -----------------------------------
///
/// A slice inherits the value or reference semantics of its base collection.
/// That is, when working with a slice of a mutable
/// collection that has value semantics, such as an array, mutating the
/// original collection triggers a copy of that collection, and does not
/// affect the contents of the slice.

改变 ArraySlice 会实例化一个新的数组实例吗？

Would mutating an ArraySlice instantiate a new array instance?

arrays

copy-on-write

swift