Swift容器的可变性浅吗?还是可以将元素更改为原位? [英] Is the mutability of Swift containers shallow? Or can an element be mutated in place?

问题描述

我在Set中有一个对象集合.对象的类型遵循GeneratorType，所以我有一个变异方法next.尽管我可以通过添加/删除元素来对集合进行变异，但是我不知道如何对元素进行变异.同时使用for-in语句和forEach方法会给出有关元素不可更改的错误. Swift容器的可变性浅吗?

I have a collection of objects in a Set. The objects' type follows GeneratorType, so I have a mutating method next. Although I can mutate the set by adding/removing elements, I don't know how to mutate an element. Using both for-in statement and the forEach method give errors about the element being immutable. Is the mutability of Swift containers shallow?

有没有一种方法可以在包含的元素上调用变异方法?除Set以外，它是否还适用于其他采集/序列类型? (我所做的更改将更改每个对象的哈希，也许不允许我影响这样的集合.)

Is there a way to call a mutating method on a contained element? Does it work for other collection/sequence types besides Set? (My changes would change each object's hash, and maybe I'm not allowed to affect a set like that.)

推荐答案

这种情况类似于对字典值之一进行变异的情况，其中该值是一种值类型.即使您对字典的引用是var引用，也无法做到.通常，您只需拉出值，对其进行变异，然后再放回去:

The situation is similar to that of mutating one of a dictionary's values, where that value is a value type. You can't do it, even if your reference to the dictionary is a var reference. Typically, you'll just pull the value out, mutate it, and put it back again:

var d = ["key":"hell"]
var val = d["key"]!
val.append(Character("o"))
d["key"] = val

您的Set的工作原理类似.

Your Set works similarly.

var s = Set(["hell"])
var val = s.remove("hell")!
val.append(Character("o"))
s.insert(val)

还有其他方法可以指出这一点，但实际上它们等同于同一件事.

There are other ways to notate that, but in effect they amount to the same thing.

这里要记住的是没有值类型，例如一个结构，真正是可变的.我们说的好像是这样，Swift的mutating函数也是如此，但是变异实际上包括将新值分配回引用中.这就是为什么为什么引用必须为var.因此，没有任何值类型的突变.

The thing to keep in mind here is that no value type, e.g. a struct, is truly mutable. We speak of it as if it were, and so does Swift with its mutating functions, but mutation actually consists of assigning a new value back into the reference. That is why the reference must be a var. There is thus no such thing as mutation in place for any value type.

现在当然有了引用类型，例如一堂课，情况就完全不同了:

Now, of course, with a reference type, e.g. a class, the situation is completely different:

let s = Set([NSMutableString(string:"hell")])
s.first!.appendString("o")
s // {"hello"}

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