dict[“key”]?.change() // Copy

在语义上等效于：

if var value = dict["key"] {
    value.change() // Copy
    dict["key"] = value
}

将该值从字典中拉出，展开为临时变量，进行突变，然后放回字典中。

因为现在有 两个
对基础缓冲区的引用（一个来自我们的本地临时引用value，一个来自COWStruct字典本身的实例）–我们将强制执行一个基础Buffer实例的副本，因为它不再被唯一引用。

所以，为什么不

array[0].change() // No Copy

做同样的事情？当然应该将元素从数组中拉出，进行突变然后重新插入，以替换先前的值吗？

区别在于，与Dictionary下标包含getter和setter
Array的下标不同，下标包含getter和称为的特殊访问器mutableAddressWithPinnedNativeOwner。

这个特殊的访问器所做的是返回一个 指向 数组基础缓冲区中的元素的 指针 ，以及一个所有者对象，以确保不会从调用者下方释放该缓冲区。这样的访问器称为
地址器 ，因为它处理地址。

因此，当您说：

array[0].change()

您实际上是在 直接 改变数组中的实际元素，而不是临时的。

此类地址不能直接应用于Dictionary的下标，因为它返回Optional，并且基础值未存储为可选值。因此，由于我们无法返回指向存储中值的指针，因此当前必须使用临时包对其进行解包。

在Swift 3中，可以通过在突变临时变量之前从字典中删除值来避免复制您COWStruct的基础Buffer：

if var value = dict["key"] {
    dict["key"] = nil
    value.change() // No Copy
    dict["key"] = value
}

现在， 只有 临时人员可以查看基础Buffer实例。

而且，正如@dfri在评论中指出的那样，可以将其简化为：

if var value = dict.removeValue(forKey: "key") {
    value.change() // No Copy
    dict["key"] = value
}

节省哈希操作。

此外，为方便起见，您可能需要考虑将其作为扩展方法：

extension Dictionary {
  mutating func withValue<R>(
    forKey key: Key, mutations: (inout Value) throws -> R
  ) rethrows -> R? {
    guard var value = removeValue(forKey: key) else { return nil }
    defer {
      updateValue(value, forKey: key)
    }
    return try mutations(&value)
  }
}

// ...

dict.withValue(forKey: "key") {
  $0.change() // No copy
}

在Swift 4中，您 应该
能够使用values属性Dictionary来执行值的直接突变：

if let index = dict.index(forKey: "key") {
    dict.values[index].change()
}

As the values property now returns a special
Dictionary.Values
mutable collection that has a
subscript
with an addressor (see SE-0154
for more info on this change).

However, currently (with the version of Swift 4 that ships with Xcode 9 beta
5), this still makes a copy. This is due to the fact that both the
Dictionary and Dictionary.Values instances have a view onto the underlying
buffer – as the values computed property is just
implemented
with a getter and setter that passes around a reference to the dictionary’s
buffer.

So when calling the addressor, a copy of the dictionary’s buffer is triggered,
therefore leading to two views onto COWStruct‘s Buffer instance, therefore
triggering a copy of it upon change() being called.

I have filed a bug over this here. (
Edit: This has now been fixed
on master with the unofficial introduction of generalised accessors using
coroutines, so will be fixed in Swift 5 – see below for more info).

In Swift 4.1, Dictionary‘s subscript(_:default:) now uses an
addressor, so we can efficiently
mutate values so long as we supply a default value to use in the mutation.

For example:

dict["key", default: COWStruct()].change() // No copy

The default: parameter uses @autoclosure such that the default value isn’t
evaluated if it isn’t needed (such as in this case where we know there’s a
value for the key).

Swift 5 and beyond

With the unofficial introduction
of generalised
accessors in Swift 5, two new underscored accessors have been introduced,
_read and _modify which use coroutines in order to yield a value back to
the caller. For _modify, this can be an arbitrary mutable expression.

The use of coroutines is exciting because it means that a _modify accessor
can now perform logic both before and after the mutation. This allows them to
be much more efficient when it comes to copy-on-write types, as they can for
example deinitialise the value in storage while yielding a temporary mutable
copy of the value that’s uniquely referenced to the caller (and then
reinitialising the value in storage upon control returning to the callee).

标准库已更新，很多以前低效的API来使用新的_modify访问-
这包括Dictionary的subscript(_:)现在可以产生独特的参考价值主叫（使用deinitialisation招我上面提到的）。

这些变化的结果意味着：

dict["key"]?.change() // No copy

无需在Swift
5中进行复制就可以执行值的突变（您甚至可以使用主快照亲自尝试一下）。