“让"到底是怎么回事?关键字在 Swift 中的作用? [英] How exactly does the "let" keyword work in Swift?

问题描述

我在指南中阅读了这个简单的解释:

I've read this simple explanation in the guide:

常量的值不需要在编译时就知道，但你必须给它赋值一次.

The value of a constant doesn’t need to be known at compile time, but you must assign it a value exactly once.

但我想要比这更多的细节.如果常量引用了一个对象，我还能修改它的属性吗?如果它引用了一个集合，我可以从中添加或删除元素吗?我来自 C# 背景；它是否类似于 readonly 的工作方式(除了能够在方法体中使用它)，如果不是，它有什么不同?

But I want a little more detail than this. If the constant references an object, can I still modify its properties? If it references a collection, can I add or remove elements from it? I come from a C# background; is it similar to how readonly works (apart from being able to use it in method bodies), and if it's not, how is it different?

推荐答案

let 有点像 C 中的 const 指针.let，您可以更改对象的属性或调用其上的方法，但不能为该标识符分配不同的对象.

let is a little bit like a const pointer in C. If you reference an object with a let, you can change the object's properties or call methods on it, but you cannot assign a different object to that identifier.

let 也对集合和非对象类型有影响.如果使用 let 引用 struct，则无法更改其属性或调用其任何 mutating func 方法.

let also has implications for collections and non-object types. If you reference a struct with a let, you cannot change its properties or call any of its mutating func methods.

将 let/var 与集合一起使用与可变/不可变 Foundation 集合非常相似:如果将数组分配给 let，则可以'不要改变它的内容.如果您使用 let 引用字典，则无法添加/删除键/值对或为键分配新值——它是真正不可变的.如果你想给数组或字典中的下标赋值、附加或以其他方式改变，你必须用 var 声明它.

Using let/var with collections works much like mutable/immutable Foundation collections: If you assign an array to a let, you can't change its contents. If you reference a dictionary with let, you can't add/remove key/value pairs or assign a new value for a key — it's truly immutable. If you want to assign to subscripts in, append to, or otherwise mutate an array or dictionary, you must declare it with var.

(在 Xcode 6 beta 3 之前，Swift 数组具有奇怪的值和引用语义组合，并且在分配给 let 时是部分可变的——现在已经不存在了.)

(Prior to Xcode 6 beta 3, Swift arrays had a weird mix of value and reference semantics, and were partially mutable when assigned to a let -- that's gone now.)

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