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< a class = "header" href = "#boxt用于已知大小的堆上数据" name = "boxt用于已知大小的堆上数据" > < h2 > < code > Box< T> < / code > 用于已知大小的堆上数据< / h2 > < / a >
< blockquote >
< p > < a href = "https://github.com/rust-lang/book/blob/master/second-edition/src/ch15-01-box.md" > ch15-01-box.md< / a >
< br >
commit 85b2c9ac704c9dc4bbedb97209d336afb9809dc1< / p >
< / blockquote >
< p > 最简单直接的智能指针是 < em > box< / em > ，它的类型是< code > Box< T> < / code > 。 box 允许你将一个单独的值放在堆上（第四章介绍或栈与堆）。列表 15-1 展示了如何使用 box 在堆上储存一个< code > i32< / code > ： < / p >
< p > < span class = "filename" > Filename: src/main.rs< / span > < / p >
< pre > < code class = "language-rust" > fn main() {
let b = Box::new(5);
println!(" b = {}" , b);
}
< / code > < / pre >
< p > < span class = "caption" > Listing 15-1: Storing an < code > i32< / code > value on the heap using a
box< / span > < / p >
< p > 这会打印出< code > b = 5< / code > 。在这个例子中，我们可以像数据是储存在栈上的那样访问 box 中的数据。正如任何拥有数据所有权的值那样，当像< code > b< / code > 这样的 box 在< code > main< / code > 的末尾离开作用域时，它将被释放。这个释放过程作用于 box 本身（位于栈上）和它所指向的数据（位于堆上）。< / p >
< p > 将一个单独的值存放在堆上并不是很有意义，所以像列表 15-1 这样单独使用 box 并不常见。一个 box 的实用场景是当你希望确保类型有一个已知大小的时候。例如，考虑一下列表 15-2， < em > cons list< / em > 的枚举定义，这是一个来源于函数式编程的数据结构类型。注意它还不能编译：< / p >
< p > < span class = "filename" > Filename: src/main.rs< / span > < / p >
< pre > < code class = "language-rust,ignore" > enum List {
Cons(i32, List),
Nil,
}
< / code > < / pre >
< p > < span class = "caption" > Listing 15-2: The first attempt of defining an enum to
represent a cons list data structure of < code > i32< / code > values< / span > < / p >
< p > 我们实现了一个只存放< code > i32< / code > 值的 cons list。也可以选择实用第十章介绍的泛型来实现一个类型无关的 cons list。< / p >
< blockquote >
< a class = "header" href = "#cons-list-的更多内容" name = "cons-list-的更多内容" > < h4 > cons list 的更多内容< / h4 > < / a >
< p > < em > cons list< / em > 是一个来源于 Lisp 编程语言及其方言的数据结构。在 Lisp 中，< code > cons< / code > 函数（" construct function" 的缩写）利用两个参数来构造一个新的列表，他们通常是一个单独的值和另一个列表。< / p >
< p > cons 函数的概念涉及到更通用的函数式编程术语；“将 x 与 y 连接”通常意味着构建一个新的容器而将 x 的元素放在新容器的开头，其后则是容器 y 的元素。< / p >
< p > cons list 通过递归调用< code > cons< / code > 函数产生。代表递归的 base case 的规范名称是< code > Nil< / code > ，它宣布列表的终止。注意这不同于第六章中的" null" 或" nil" 的概念，他们代表无效或缺失的值。< / p >
< / blockquote >
< p > cons list 是一个每个元素和之后的其余部分都只包含一个值的列表。列表的其余部分由嵌套的 cons list 定义。其结尾由值< code > Nil< / code > 表示。cons list 在 Rust 中并不常见；通常< code > Vec< T> < / code > 是一个更好的选择。实现这个数据结构是< code > Box< T> < / code > 实用性的一个好的例子。让我们看看为什么！< / p >
< p > 使用 cons list 来储存列表< code > 1, 2, 3< / code > 将看起来像这样：< / p >
< pre > < code class = "language-rust,ignore" > use List::{Cons, Nil};
fn main() {
let list = Cons(1, Cons(2, Cons(3, Nil)));
}
< / code > < / pre >
< p > 第一个< code > Cons< / code > 储存了< code > 1< / code > 和另一个< code > List< / code > 值。这个< code > List< / code > 是另一个包含< code > 2< / code > 的< code > Cons< / code > 值和下一个< code > List< / code > 值。这又是另一个存放了< code > 3< / code > 的< code > Cons< / code > 值和最后一个值为< code > Nil< / code > 的< code > List< / code > ，非递归成员代表了列表的结尾。< / p >
< p > 如果尝试编译上面的代码，会得到如列表 15-3 所示的错误：< / p >
< pre > < code > error[E0072]: recursive type `List` has infinite size
-->
|
1 | enum List {
| _^ starting here...
2 | | Cons(i32, List),
3 | | Nil,
4 | | }
| |_^ ...ending here: recursive type has infinite size
|
= help: insert indirection (e.g., a `Box`, `Rc`, or `& `) at some point to
make `List` representable
< / code > < / pre >
< p > < span class = "caption" > Listing 15-3: The error we get when attempting to define
a recursive enum< / span > < / p >
< p > 错误表明这个类型“有无限的大小”。为什么呢？因为< code > List< / code > 的一个成员被定义为递归的：它存放了另一个相同类型的值。这意味着 Rust 无法计算为了存放< code > List< / code > 值到底需要多少空间。让我们一点一点的看：首先了解一下 Rust 如何决定需要多少空间来存放一个非递归类型。回忆一下第六章讨论枚举定义时的列表 6-2 中定义的< code > Message< / code > 枚举：< / p >
< pre > < code class = "language-rust" > enum Message {
Quit,
Move { x: i32, y: i32 },
Write(String),
ChangeColor(i32, i32, i32),
}
< / code > < / pre >
< p > 当 Rust 需要知道需要为< code > Message< / code > 值分配多少空间时，它可以检查每一个成员并发现< code > Message::Quit< / code > 并不需要任何空间，< code > Message::Move< / code > 需要足够储存两个< code > i32< / code > 值的空间，依此类推。因此，< code > Message< / code > 值所需的最大空间等于储存其最大成员的空间大小。< / p >
< p > 与此相对当 Rust 编译器检查像列表 15-2 中的< code > List< / code > 这样的递归类型时会发生什么呢。编译器尝试计算出储存一个< code > List< / code > 枚举需要多少内存，并开始检查< code > Cons< / code > 成员，那么< code > Cons< / code > 需要的空间等于< code > i32< / code > 的大小加上< code > List< / code > 的大小。为了计算< code > List< / code > 需要多少内存，它检查其成员，从< code > Cons< / code > 成员开始。< code > Cons< / code > 成员储存了一个< code > i32< / code > 值和一个< code > List< / code > 值，这样的计算将无限进行下去，如图 15-4 所示：< / p >
< p > < img alt = "An infinite Cons list" src = "img/trpl15-01.svg" class = "center" style = "width: 50%;" / > < / p >
< p > < span class = "caption" > Figure 15-4: An infinite < code > List< / code > consisting of infinite
< code > Cons< / code > variants< / span > < / p >
< p > Rust 无法计算出要为定义为递归的类型分配多少空间，所以编译器给出了列表 15-3 中的错误。这个错误也包括了有用的建议：< / p >
< pre > < code class = "language-text" > = help: insert indirection (e.g., a `Box`, `Rc`, or `& `) at some point to
make `List` representable
< / code > < / pre >
< p > 因为< code > Box< T> < / code > 是一个指针，我们总是知道它需要多少空间：指针需要一个< code > usize< / code > 大小的空间。这个< code > usize< / code > 的值将是堆数据的地址。而堆数据可以是任意大小，不过开始这个堆数据的地址总是能放进一个< code > usize< / code > 中。所以如果将列表 15-2 的定义修改为像这里列表 15-5 中的定义，并修改< code > main< / code > 函数为< code > Cons< / code > 成员中的值使用< code > Box::new< / code > ： < / p >
< p > < span class = "filename" > Filename: src/main.rs< / span > < / p >
< pre > < code class = "language-rust" > enum List {
Cons(i32, Box< List> ),
Nil,
}
use List::{Cons, Nil};
fn main() {
let list = Cons(1,
Box::new(Cons(2,
Box::new(Cons(3,
Box::new(Nil))))));
}
< / code > < / pre >
< p > < span class = "caption" > Listing 15-5: Definition of < code > List< / code > that uses < code > Box< T> < / code > in
order to have a known size< / span > < / p >
< p > 这样编译器就能够计算出储存一个< code > List< / code > 值需要的大小了。Rust 将会检查< code > List< / code > ，同样的从< code > Cons< / code > 成员开始检查。< code > Cons< / code > 成员需要< code > i32< / code > 的大小加上一个< code > usize< / code > 的大小，因为 box 总是< code > usize< / code > 大小的，不管它指向的是什么。接着 Rust 检查< code > Nil< / code > 成员，它并储存一个值，所以< code > Nil< / code > 并不需要任何空间。我们通过 box 打破了这无限递归的连锁。图 15-6 展示了现在< code > Cons< / code > 成员看起来像什么：< / p >
< p > < img alt = "A finite Cons list" src = "img/trpl15-02.svg" class = "center" / > < / p >
< p > < span class = "caption" > Figure 15-6: A < code > List< / code > that is not infinitely sized since
< code > Cons< / code > holds a < code > Box< / code > < / span > < / p >
< p > 这就是 box 主要应用场景：打破无限循环的数据结构以便编译器可以知道其大小。第十七章讨论 trait 对象时我们将了解另一个 Rust 中会出现未知大小数据的情况。< / p >
< p > 虽然我们并不经常使用 box， < code > Box< T> < / code > 作为智能指针经常被使用的两个方面是他们< code > Deref< / code > 和< code > Drop< / code > trait 的实现。让我们研究这些 trait 如何工作以及智能指针如何利用他们。< / p >
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