Swift 是按值传递还是按引用传递

Swift 中的事物类型

规则是:

• 类实例是引用类型(即你对类实例的引用实际上是一个指针)

• Class instances are reference types (i.e. your reference to a class instance is effectively a pointer)

函数是引用类型

其他一切都是值类型；其他一切"仅表示结构实例和枚举实例，因为这就是 Swift 中的全部内容.例如，数组和字符串是结构体实例.正如 newacct 指出的那样，您可以通过使用 inout 并获取地址来传递对其中一个事物的引用(作为函数参数).但是类型本身就是值类型.

Everything else is a value type; "everything else" simply means instances of structs and instances of enums, because that's all there is in Swift. Arrays and strings are struct instances, for example. You can pass a reference to one of those things (as a function argument) by using inout and taking the address, as newacct has pointed out. But the type is itself a value type.

引用类型对象在实践中很特殊，因为:

A reference type object is special in practice because:

• 仅仅赋值或传递给函数就可以产生对同一个对象的多个引用

• Mere assignment or passing to function can yield multiple references to the same object

对象本身是可变的，即使对它的引用是一个常量(let，无论是显式的还是隐式的).

The object itself is mutable even if the reference to it is a constant (let, either explicit or implied).

对对象的更改会影响该对象，正如对它的所有引用所见.

A mutation to the object affects that object as seen by all references to it.

那些可能是危险的，所以要小心.另一方面，传递引用类型显然是高效的，因为只复制和传递一个指针，这是微不足道的.

Those can be dangers, so keep an eye out. On the other hand, passing a reference type is clearly efficient because only a pointer is copied and passed, which is trivial.

显然，传递值类型是更安全的"，而 let 的意思是它所说的:您不能通过 let 引用来改变结构实例或枚举实例.另一方面，安全性是通过制作单独的值副本来实现的，不是吗?这不会使传递值类型的成本变得昂贵吗?

Clearly, passing a value type is "safer", and let means what it says: you can't mutate a struct instance or enum instance through a let reference. On the other hand, that safety is achieved by making a separate copy of the value, isn't it? Doesn't that make passing a value type potentially expensive?

嗯，是和不是.它并不像你想象的那么糟糕.正如 Nate Cook 所说，传递值类型并不必然意味着复制，因为 let(显式或隐式)保证了不变性，因此不需要复制任何东西.甚至传递到 var 引用并不意味着东西将被复制，只是它们在必要时可以被复制(因为有一个突变).文档特别建议您不要弄乱内裤.

Well, yes and no. It isn't as bad as you might think. As Nate Cook has said, passing a value type does not necessarily imply copying, because let (explicit or implied) guarantees immutability so there's no need to copy anything. And even passing into a var reference doesn't mean that things will be copied, only that they can be if necessary (because there's a mutation). The docs specifically advise you not to get your knickers in a twist.