人们如何处理迭代Swift结构的value-type属性? [英] how do people deal with iterating a Swift struct value-type property?

问题描述

在这里，人们必须时时刻刻出现这种明显的情况:

Here's an obvious situation that must arise all the time for people:

struct Foundation {
    var columns : [Column] = [Column(), Column()]
}
struct Column : CustomStringConvertible {
    var cards = [Card]()
    var description : String {
        return String(describing:self.cards)
    }
}
struct Card {}
var f = Foundation()
for var c in f.columns {
    c.cards.append(Card())
}

该代码是合法的，但对f毫无影响，因为var c仍是副本，而f的实际columns不受影响.

That code is legal but of course it has no effect on f, because var c is still a copy — the actual columns of f are unaffected.

我理解为什么会发生为什么没有任何困难.我的问题是人们对此通常做.

I am not having any difficulties understanding why that happens. My question is what people generally do about it.

很显然，我可以通过将Column声明为 class 而不是 struct 来解决整个问题，但这是人们通常会做的事情吗? (我试图遵循一种精神上的严格要求，即在不需要动态调度/多态性/子类化时应该避免使用类;也许我对此进行了过多讨论，或者人们通常会做其他事情，例如使用inout不知何故.)

Clearly I can just make the whole matter go away by declaring Column a class instead of a struct, but is that what people usually do? (I'm trying to follow a mental stricture that one should avoid classes when there's no need for dynamic dispatch / polymorphism / subclassing; maybe I'm carrying that too far, or maybe there's something else people usually do, like using inout somehow.)

推荐答案

从Swift 4开始，一种折衷方法是迭代可变集合的索引，而不是元素本身，以便

As of Swift 4, a compromise is to iterate over the indices of a mutable collection instead of the elements themselves, so that

for elem in mutableCollection {
    // `elem` is immutable ...
}

或

for var elem in mutableCollection {
   // `elem` is mutable, but a _copy_ of the collection element ...
}

成为

for idx in mutableCollection.indices {
    // mutate `mutableCollection[idx]` ...
}

在您的示例中:

for idx in f.columns.indices {
   f.columns[idx].cards.append(Card()) 
}

正如@Hamish在评论中指出的那样，Swift的未来版本可能会实现

As @Hamish pointed out in the comments, a future version of Swift may implement a mutating iteration, making

for inout elem in mutableCollection {
   // mutate `elem` ...
}

可能.

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