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< h1 id = "特征-trait" > < a class = "header" href = "#特征-trait" > 特征 Trait< / a > < / h1 >
< p > 如果我们想定义一个文件系统，那么把该系统跟底层存储解耦是很重要的。文件操作主要包含四个：< code > open< / code > 、< code > write< / code > 、< code > read< / code > 、< code > close< / code > ，这些操作可以发生在硬盘，可以发生在内存，还可以发生在网络 IO 甚至（…我实在编不下去了，大家来帮帮我）。总之如果你要为每一种情况都单独实现一套代码，那这种实现将过于繁杂，而且也没那个必要。< / p >
< p > 要解决上述问题，需要把这些行为抽象出来，就要使用 Rust 中的特征 < code > trait< / code > 概念。可能你是第一次听说这个名词，但是不要怕，如果学过其他语言，那么大概率你听说过接口，没错，特征跟接口很类似。< / p >
< p > 在之前的代码中，我们也多次见过特征的使用，例如 < code > #[derive(Debug)]< / code > ，它在我们定义的类型(< code > struct< / code > )上自动派生 < code > Debug< / code > 特征，接着可以使用 < code > println!("{:?}", x)< / code > 打印这个类型；再例如：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > fn add< T: std::ops::Add< Output = T> > (a:T, b:T) -> T {
a + b
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 通过 < code > std::ops::Add< / code > 特征来限制 < code > T< / code > ，只有 < code > T< / code > 实现了 < code > std::ops::Add< / code > 才能进行合法的加法操作，毕竟不是所有的类型都能进行相加。< / p >
< p > 这些都说明一个道理，特征定义了< strong > 一组可以被共享的行为，只要实现了特征，你就能使用这组行为< / strong > 。< / p >
< h2 id = "定义特征" > < a class = "header" href = "#定义特征" > 定义特征< / a > < / h2 >
< p > 如果不同的类型具有相同的行为，那么我们就可以定义一个特征，然后为这些类型实现该特征。< strong > 定义特征< / strong > 是把一些方法组合在一起，目的是定义一个实现某些目标所必需的行为的集合。< / p >
< p > 例如，我们现在有文章 < code > Post< / code > 和微博 < code > Weibo< / code > 两种内容载体，而我们想对相应的内容进行总结，也就是无论是文章内容，还是微博内容，都可以在某个时间点进行总结，那么总结这个行为就是共享的，因此可以用特征来定义：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > pub trait Summary {
fn summarize(& self) -> String;
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 这里使用 < code > trait< / code > 关键字来声明一个特征，< code > Summary< / code > 是特征名。在大括号中定义了该特征的所有方法，在这个例子中是： < code > fn summarize(& self) -> String< / code > 。< / p >
< p > 特征只定义行为看起来是什么样的，而不定义行为具体是怎么样的。因此，我们只定义特征方法的签名，而不进行实现，此时方法签名结尾是 < code > ;< / code > ，而不是一个 < code > {}< / code > 。< / p >
< p > 接下来，每一个实现这个特征的类型都需要具体实现该特征的相应方法，编译器也会确保任何实现 < code > Summary< / code > 特征的类型都拥有与这个签名的定义完全一致的 < code > summarize< / code > 方法。< / p >
< h2 id = "为类型实现特征" > < a class = "header" href = "#为类型实现特征" > 为类型实现特征< / a > < / h2 >
< p > 因为特征只定义行为看起来是什么样的，因此我们需要为类型实现具体的特征，定义行为具体是怎么样的。< / p >
< p > 首先来为 < code > Post< / code > 和 < code > Weibo< / code > 实现 < code > Summary< / code > 特征：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > pub trait Summary {
fn summarize(& self) -> String;
}
pub struct Post {
pub title: String, // 标题
pub author: String, // 作者
pub content: String, // 内容
}
impl Summary for Post {
fn summarize(& self) -> String {
format!("文章{}, 作者是{}", self.title, self.author)
}
}
pub struct Weibo {
pub username: String,
pub content: String
}
impl Summary for Weibo {
fn summarize(& self) -> String {
format!("{}发表了微博{}", self.username, self.content)
}
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 实现特征的语法与为结构体、枚举实现方法很像：< code > impl Summary for Post< / code > ，读作“为 < code > Post< / code > 类型实现 < code > Summary< / code > 特征”，然后在 < code > impl< / code > 的花括号中实现该特征的具体方法。< / p >
< p > 接下来就可以在这个类型上调用特征的方法：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > fn main() {
let post = Post{title: "Rust语言简介".to_string(),author: "Sunface".to_string(), content: "Rust棒极了!".to_string()};
let weibo = Weibo{username: "sunface".to_string(),content: "好像微博没Tweet好用".to_string()};
println!("{}",post.summarize());
println!("{}",weibo.summarize());
}< / code > < / pre >
< p > 运行输出：< / p >
< pre > < code class = "language-console" > 文章 Rust 语言简介, 作者是Sunface
sunface发表了微博好像微博没Tweet好用
< / code > < / pre >
< p > 说实话，如果特征仅仅如此，你可能会觉得花里胡哨没啥用，接下来就让你见识下 < code > trait< / code > 真正的威力。< / p >
< h4 id = "特征定义与实现的位置孤儿规则" > < a class = "header" href = "#特征定义与实现的位置孤儿规则" > 特征定义与实现的位置(孤儿规则)< / a > < / h4 >
< p > 上面我们将 < code > Summary< / code > 定义成了 < code > pub< / code > 公开的。这样，如果他人想要使用我们的 < code > Summary< / code > 特征，则可以引入到他们的包中，然后再进行实现。< / p >
< p > 关于特征实现与定义的位置，有一条非常重要的原则：< strong > 如果你想要为类型< / strong > < code > A< / code > < strong > 实现特征< / strong > < code > T< / code > < strong > ，那么< / strong > < code > A< / code > < strong > 或者< / strong > < code > T< / code > < strong > 至少有一个是在当前作用域中定义的！< / strong > 例如我们可以为上面的 < code > Post< / code > 类型实现标准库中的 < code > Display< / code > 特征，这是因为 < code > Post< / code > 类型定义在当前的作用域中。同时，我们也可以在当前包中为 < code > String< / code > 类型实现 < code > Summary< / code > 特征，因为 < code > Summary< / code > 定义在当前作用域中。< / p >
< p > 但是你无法在当前作用域中，为 < code > String< / code > 类型实现 < code > Display< / code > 特征，因为它们俩都定义在标准库中，其定义所在的位置都不在当前作用域，跟你半毛钱关系都没有，看看就行了。< / p >
< p > 该规则被称为< strong > 孤儿规则< / strong > ，可以确保其它人编写的代码不会破坏你的代码，也确保了你不会莫名其妙就破坏了风马牛不相及的代码。< / p >
< h4 id = "默认实现" > < a class = "header" href = "#默认实现" > 默认实现< / a > < / h4 >
< p > 你可以在特征中定义具有< strong > 默认实现< / strong > 的方法，这样其它类型无需再实现该方法，或者也可以选择重写该方法：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > pub trait Summary {
fn summarize(& self) -> String {
String::from("(Read more...)")
}
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 上面为 < code > Summary< / code > 定义了一个默认实现，下面我们编写段代码来测试下：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > impl Summary for Post {}
impl Summary for Weibo {
fn summarize(& self) -> String {
format!("{}发表了微博{}", self.username, self.content)
}
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 可以看到，< code > Post< / code > 选择了默认实现，而 < code > Weibo< / code > 重写了该方法，调用和输出如下：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > println!("{}",post.summarize());
println!("{}",weibo.summarize());
< span class = "boring" > }< / span > < / code > < / pre >
< pre > < code class = "language-console" > (Read more...)
sunface发表了微博好像微博没Tweet好用
< / code > < / pre >
< p > 默认实现允许调用相同特征中的其他方法，哪怕这些方法没有默认实现。如此，特征可以提供很多有用的功能而只需要实现指定的一小部分内容。例如，我们可以定义 < code > Summary< / code > 特征，使其具有一个需要实现的 < code > summarize_author< / code > 方法，然后定义一个 < code > summarize< / code > 方法，此方法的默认实现调用 < code > summarize_author< / code > 方法：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > pub trait Summary {
fn summarize_author(& self) -> String;
fn summarize(& self) -> String {
format!("(Read more from {}...)", self.summarize_author())
}
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 为了使用 < code > Summary< / code > ，只需要实现 < code > summarize_author< / code > 方法即可：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > impl Summary for Weibo {
fn summarize_author(& self) -> String {
format!("@{}", self.username)
}
}
println!("1 new weibo: {}", weibo.summarize());
< span class = "boring" > }< / span > < / code > < / pre >
< p > < code > weibo.summarize()< / code > 会先调用 < code > Summary< / code > 特征默认实现的 < code > summarize< / code > 方法，通过该方法进而调用 < code > Weibo< / code > 为 < code > Summary< / code > 实现的 < code > summarize_author< / code > 方法，最终输出：< code > 1 new weibo: (Read more from @horse_ebooks...)< / code > 。< / p >
< h2 id = "使用特征作为函数参数" > < a class = "header" href = "#使用特征作为函数参数" > 使用特征作为函数参数< / a > < / h2 >
< p > 之前提到过，特征如果仅仅是用来实现方法，那真的有些大材小用，现在我们来讲下，真正可以让特征大放光彩的地方。< / p >
< p > 现在，先定义一个函数，使用特征作为函数参数：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > pub fn notify(item: & impl Summary) {
println!("Breaking news! {}", item.summarize());
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > < code > impl Summary< / code > ，只能说想出这个类型的人真的是起名鬼才，简直太贴切了，顾名思义，它的意思是 < strong > 实现了< code > Summary< / code > 特征< / strong > 的 < code > item< / code > 参数。< / p >
< p > 你可以使用任何实现了 < code > Summary< / code > 特征的类型作为该函数的参数，同时在函数体内，还可以调用该特征的方法，例如 < code > summarize< / code > 方法。具体的说，可以传递 < code > Post< / code > 或 < code > Weibo< / code > 的实例来作为参数，而其它类如 < code > String< / code > 或者 < code > i32< / code > 的类型则不能用做该函数的参数，因为它们没有实现 < code > Summary< / code > 特征。< / p >
< h2 id = "特征约束trait-bound" > < a class = "header" href = "#特征约束trait-bound" > 特征约束(trait bound)< / a > < / h2 >
< p > 虽然 < code > impl Trait< / code > 这种语法非常好理解，但是实际上它只是一个语法糖：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > pub fn notify< T: Summary> (item: & T) {
println!("Breaking news! {}", item.summarize());
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 真正的完整书写形式如上所述，形如 < code > T: Summary< / code > 被称为< strong > 特征约束< / strong > 。< / p >
< p > 在简单的场景下 < code > impl Trait< / code > 这种语法糖就足够使用，但是对于复杂的场景，特征约束可以让我们拥有更大的灵活性和语法表现能力，例如一个函数接受两个 < code > impl Summary< / code > 的参数：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > pub fn notify(item1: & impl Summary, item2: & impl Summary) {}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 如果函数两个参数是不同的类型，那么上面的方法很好，只要这两个类型都实现了 < code > Summary< / code > 特征即可。但是如果我们想要强制函数的两个参数是同一类型呢？上面的语法就无法做到这种限制，此时我们只能使特征约束来实现：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > pub fn notify< T: Summary> (item1: & T, item2: & T) {}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 泛型类型 < code > T< / code > 说明了 < code > item1< / code > 和 < code > item2< / code > 必须拥有同样的类型，同时 < code > T: Summary< / code > 说明了 < code > T< / code > 必须实现 < code > Summary< / code > 特征。< / p >
< h4 id = "多重约束" > < a class = "header" href = "#多重约束" > 多重约束< / a > < / h4 >
< p > 除了单个约束条件，我们还可以指定多个约束条件，例如除了让参数实现 < code > Summary< / code > 特征外，还可以让参数实现 < code > Display< / code > 特征以控制它的格式化输出：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > pub fn notify(item: & (impl Summary + Display)) {}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 除了上述的语法糖形式，还能使用特征约束的形式：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > pub fn notify< T: Summary + Display> (item: & T) {}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 通过这两个特征，就可以使用 < code > item.summarize< / code > 方法，以及通过 < code > println!("{}", item)< / code > 来格式化输出 < code > item< / code > 。< / p >
< h4 id = "where-约束" > < a class = "header" href = "#where-约束" > Where 约束< / a > < / h4 >
< p > 当特征约束变得很多时，函数的签名将变得很复杂：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > fn some_function< T: Display + Clone, U: Clone + Debug> (t: & T, u: & U) -> i32 {}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 严格来说，上面的例子还是不够复杂，但是我们还是能对其做一些形式上的改进，通过 < code > where< / code > ： < / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > fn some_function< T, U> (t: & T, u: & U) -> i32
where T: Display + Clone,
U: Clone + Debug
{}
< span class = "boring" > }< / span > < / code > < / pre >
< h4 id = "使用特征约束有条件地实现方法或特征" > < a class = "header" href = "#使用特征约束有条件地实现方法或特征" > 使用特征约束有条件地实现方法或特征< / a > < / h4 >
< p > 特征约束，可以让我们在指定类型 + 指定特征的条件下去实现方法，例如：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use std::fmt::Display;
struct Pair< T> {
x: T,
y: T,
}
impl< T> Pair< T> {
fn new(x: T, y: T) -> Self {
Self {
x,
y,
}
}
}
impl< T: Display + PartialOrd> Pair< T> {
fn cmp_display(& self) {
if self.x > = self.y {
println!("The largest member is x = {}", self.x);
} else {
println!("The largest member is y = {}", self.y);
}
}
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > < code > cmp_display< / code > 方法，并不是所有的 < code > Pair< T> < / code > 结构体对象都可以拥有，只有 < code > T< / code > 同时实现了 < code > Display + PartialOrd< / code > 的 < code > Pair< T> < / code > 才可以拥有此方法。
该函数可读性会更好，因为泛型参数、参数、返回值都在一起，可以快速的阅读，同时每个泛型参数的特征也在新的代码行中通过< strong > 特征约束< / strong > 进行了约束。< / p >
< p > < strong > 也可以有条件地实现特征< / strong > ，例如，标准库为任何实现了 < code > Display< / code > 特征的类型实现了 < code > ToString< / code > 特征：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > impl< T: Display> ToString for T {
// --snip--
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 我们可以对任何实现了 < code > Display< / code > 特征的类型调用由 < code > ToString< / code > 定义的 < code > to_string< / code > 方法。例如，可以将整型转换为对应的 < code > String< / code > 值，因为整型实现了 < code > Display< / code > ： < / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > let s = 3.to_string();
< span class = "boring" > }< / span > < / code > < / pre >
< h2 id = "函数返回中的-impl-trait" > < a class = "header" href = "#函数返回中的-impl-trait" > 函数返回中的 < code > impl Trait< / code > < / a > < / h2 >
< p > 可以通过 < code > impl Trait< / code > 来说明一个函数返回了一个类型，该类型实现了某个特征：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > fn returns_summarizable() -> impl Summary {
Weibo {
username: String::from("sunface"),
content: String::from(
"m1 max太厉害了，
)
}
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 因为 < code > Weibo< / code > 实现了 < code > Summary< / code > ，因此这里可以用它来作为返回值。要注意的是，虽然我们知道这里是一个 < code > Weibo< / code > 类型，但是对于 < code > returns_summarizable< / code > 的调用者而言，他只知道返回了一个实现了 < code > Summary< / code > 特征的对象，但是并不知道返回了一个 < code > Weibo< / code > 类型。< / p >
< p > 这种 < code > impl Trait< / code > 形式的返回值，在一种场景下非常非常有用，那就是返回的真实类型非常复杂，你不知道该怎么声明时（毕竟 Rust 要求你必须标出所有的类型），此时就可以用 < code > impl Trait< / code > 的方式简单返回。例如，闭包和迭代器就是很复杂，只有编译器才知道那玩意的真实类型，如果让你写出来它们的具体类型，估计内心有一万只草泥马奔腾，好在你可以用 < code > impl Iterator< / code > 来告诉调用者，返回了一个迭代器，因为所有迭代器都会实现 < code > Iterator< / code > 特征。< / p >
< p > 但是这种返回值方式有一个很大的限制：只能有一个具体的类型，例如：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > fn returns_summarizable(switch: bool) -> impl Summary {
if switch {
Post {
title: String::from(
"Penguins win the Stanley Cup Championship!",
),
author: String::from("Iceburgh"),
content: String::from(
"The Pittsburgh Penguins once again are the best \
hockey team in the NHL.",
),
}
} else {
Weibo {
username: String::from("horse_ebooks"),
content: String::from(
"of course, as you probably already know, people",
),
}
}
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 以上的代码就无法通过编译，因为它返回了两个不同的类型 < code > Post< / code > 和 < code > Weibo< / code > 。< / p >
< pre > < code class = "language-console" > `if` and `else` have incompatible types
expected struct `Post`, found struct `Weibo`
< / code > < / pre >
< p > 报错提示我们 < code > if< / code > 和 < code > else< / code > 返回了不同的类型。如果想要实现返回不同的类型，需要使用下一章节中的< a href = "https://course.rs/basic/trait/trait-object.html" > 特征对象< / a > 。< / p >
< h2 id = "修复上一节中的-largest-函数" > < a class = "header" href = "#修复上一节中的-largest-函数" > 修复上一节中的 < code > largest< / code > 函数< / a > < / h2 >
< p > 还记得上一节中的< a href = "https://course.rs/basic/trait/generic.html#泛型详解" > 例子< / a > 吧，当时留下一个疑问，该如何解决编译报错：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > error[E0369]: binary operation `> ` cannot be applied to type `T` // 无法在 `T` 类型上应用`> `运算符
--> src/main.rs:5:17
|
5 | if item > largest {
| ---- ^ ------- T
| |
| T
|
help: consider restricting type parameter `T` // 考虑使用以下的特征来约束 `T`
|
1 | fn largest< T: std::cmp::PartialOrd> (list: & [T]) -> T {
| ^^^^^^^^^^^^^^^^^^^^^^
< span class = "boring" > }< / span > < / code > < / pre >
< p > 在 < code > largest< / code > 函数体中我们想要使用大于运算符（< code > > < / code > ）比较两个 < code > T< / code > 类型的值。这个运算符是标准库中特征 < code > std::cmp::PartialOrd< / code > 的一个默认方法。所以需要在 < code > T< / code > 的特征约束中指定 < code > PartialOrd< / code > ，这样 < code > largest< / code > 函数可以用于内部元素类型可比较大小的数组切片。< / p >
< p > 由于 < code > PartialOrd< / code > 位于 < code > prelude< / code > 中所以并不需要通过 < code > std::cmp< / code > 手动将其引入作用域。所以可以将 < code > largest< / code > 的签名修改为如下：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > fn largest< T: PartialOrd> (list: & [T]) -> T {}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 但是此时编译，又会出现新的错误：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > error[E0508]: cannot move out of type `[T]`, a non-copy slice
--> src/main.rs:2:23
|
2 | let mut largest = list[0];
| ^^^^^^^
| |
| cannot move out of here
| help: consider using a reference instead: `& list[0]`
error[E0507]: cannot move out of borrowed content
--> src/main.rs:4:9
|
4 | for & item in list.iter() {
| ^----
| ||
| |hint: to prevent move, use `ref item` or `ref mut item`
| cannot move out of borrowed content
< span class = "boring" > }< / span > < / code > < / pre >
< p > 错误的核心是 < code > cannot move out of type [T], a non-copy slice< / code > ，原因是 < code > T< / code > 没有< a href = "https://course.rs/basic/ownership/ownership.html#拷贝浅拷贝" > 实现 < code > Copy< / code > 特性< / a > ，因此我们只能把所有权进行转移，毕竟只有 < code > i32< / code > 等基础类型才实现了 < code > Copy< / code > 特性，可以存储在栈上，而 < code > T< / code > 可以指代任何类型（严格来说是实现了 < code > PartialOrd< / code > 特征的所有类型）。< / p >
< p > 因此，为了让 < code > T< / code > 拥有 < code > Copy< / code > 特性，我们可以增加特征约束：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > fn largest< T: PartialOrd + Copy> (list: & [T]) -> T {
let mut largest = list[0];
for & item in list.iter() {
if item > largest {
largest = item;
}
}
largest
}
fn main() {
let number_list = vec![34, 50, 25, 100, 65];
let result = largest(& number_list);
println!("The largest number is {}", result);
let char_list = vec!['y', 'm', 'a', 'q'];
let result = largest(& char_list);
println!("The largest char is {}", result);
}< / code > < / pre >
< p > 如果并不希望限制 < code > largest< / code > 函数只能用于实现了 < code > Copy< / code > 特征的类型，我们可以在 < code > T< / code > 的特征约束中指定 < a href = "https://course.rs/basic/ownership/ownership.html#克隆深拷贝" > < code > Clone< / code > 特征< / a > 而不是 < code > Copy< / code > 特征。并克隆 < code > list< / code > 中的每一个值使得 < code > largest< / code > 函数拥有其所有权。使用 < code > clone< / code > 函数意味着对于类似 < code > String< / code > 这样拥有堆上数据的类型，会潜在地分配更多堆上空间，而堆分配在涉及大量数据时可能会相当缓慢。< / p >
< p > 另一种 < code > largest< / code > 的实现方式是返回在 < code > list< / code > 中 < code > T< / code > 值的引用。如果我们将函数返回值从 < code > T< / code > 改为 < code > & T< / code > 并改变函数体使其能够返回一个引用，我们将不需要任何 < code > Clone< / code > 或 < code > Copy< / code > 的特征约束而且也不会有任何的堆分配。尝试自己实现这种替代解决方式吧！< / p >
< h2 id = "通过-derive-派生特征" > < a class = "header" href = "#通过-derive-派生特征" > 通过 < code > derive< / code > 派生特征< / a > < / h2 >
< p > 在本书中，形如 < code > #[derive(Debug)]< / code > 的代码已经出现了很多次，这种是一种特征派生语法，被 < code > derive< / code > 标记的对象会自动实现对应的默认特征代码，继承相应的功能。< / p >
< p > 例如 < code > Debug< / code > 特征，它有一套自动实现的默认代码，当你给一个结构体标记后，就可以使用 < code > println!("{:?}", s)< / code > 的形式打印该结构体的对象。< / p >
< p > 再如 < code > Copy< / code > 特征，它也有一套自动实现的默认代码，当标记到一个类型上时，可以让这个类型自动实现 < code > Copy< / code > 特征，进而可以调用 < code > copy< / code > 方法，进行自我复制。< / p >
< p > 总之，< code > derive< / code > 派生出来的是 Rust 默认给我们提供的特征，在开发过程中极大的简化了自己手动实现相应特征的需求，当然，如果你有特殊的需求，还可以自己手动重写该实现。< / p >
< p > 详细的 < code > derive< / code > 列表参见< a href = "https://course.rs/appendix/derive.html" > 附录-派生特征< / a > 。< / p >
< h2 id = "调用方法需要引入特征" > < a class = "header" href = "#调用方法需要引入特征" > 调用方法需要引入特征< / a > < / h2 >
< p > 在一些场景中，使用 < code > as< / code > 关键字做类型转换会有比较大的限制，因为你想要在类型转换上拥有完全的控制，例如处理转换错误，那么你将需要 < code > TryInto< / code > ： < / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use std::convert::TryInto;
fn main() {
let a: i32 = 10;
let b: u16 = 100;
let b_ = b.try_into()
.unwrap();
if a < b_ {
println!("Ten is less than one hundred.");
}
}< / code > < / pre >
< p > 上面代码中引入了 < code > std::convert::TryInto< / code > 特征，但是却没有使用它，可能有些同学会为此困惑，主要原因在于< strong > 如果你要使用一个特征的方法，那么你需要将该特征引入当前的作用域中< / strong > ，我们在上面用到了 < code > try_into< / code > 方法，因此需要引入对应的特征。< / p >
< p > 但是 Rust 又提供了一个非常便利的办法，即把最常用的标准库中的特征通过 < a href = "https://course.rs/appendix/prelude.html" > < code > std::prelude< / code > < / a > 模块提前引入到当前作用域中，其中包括了 < code > std::convert::TryInto< / code > ，你可以尝试删除第一行的代码 < code > use ...< / code > ，看看是否会报错。< / p >
< h2 id = "几个综合例子" > < a class = "header" href = "#几个综合例子" > 几个综合例子< / a > < / h2 >
< h4 id = "为自定义类型实现--操作" > < a class = "header" href = "#为自定义类型实现--操作" > 为自定义类型实现 < code > +< / code > 操作< / a > < / h4 >
< p > 在 Rust 中除了数值类型的加法，< code > String< / code > 也可以做< a href = "https://course.rs/basic/compound-type/string-slice.html#操作字符串" > 加法< / a > ，因为 Rust 为该类型实现了 < code > std::ops::Add< / code > 特征，同理，如果我们为自定义类型实现了该特征，那就可以自己实现 < code > Point1 + Point2< / code > 的操作:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use std::ops::Add;
// 为Point结构体派生Debug特征，
#[derive(Debug)]
struct Point< T: Add< T, Output = T> > { //限制类型T必须实现了Add特征，
x: T,
y: T,
}
impl< T: Add< T, Output = T> > Add for Point< T> {
type Output = Point< T> ;
fn add(self, p: Point< T> ) -> Point< T> {
Point{
x: self.x + p.x,
y: self.y + p.y,
}
}
}
fn add< T: Add< T, Output=T> > (a:T, b:T) -> T {
a + b
}
fn main() {
let p1 = Point{x: 1.1f32, y: 1.1f32};
let p2 = Point{x: 2.1f32, y: 2.1f32};
println!("{:?}", add(p1, p2));
let p3 = Point{x: 1i32, y: 1i32};
let p4 = Point{x: 2i32, y: 2i32};
println!("{:?}", add(p3, p4));
}< / code > < / pre >
< h4 id = "自定义类型的打印输出" > < a class = "header" href = "#自定义类型的打印输出" > 自定义类型的打印输出< / a > < / h4 >
< p > 在开发过程中，往往只要使用 < code > #[derive(Debug)]< / code > 对我们的自定义类型进行标注，即可实现打印输出的功能：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > #[derive(Debug)]
struct Point{
x: i32,
y: i32
}
fn main() {
let p = Point{x:3,y:3};
println!("{:?}",p);
}< / code > < / pre >
< p > 但是在实际项目中，往往需要对我们的自定义类型进行自定义的格式化输出，以让用户更好的阅读理解我们的类型，此时就要为自定义类型实现 < code > std::fmt::Display< / code > 特征：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > #![allow(dead_code)]
use std::fmt;
use std::fmt::{Display};
#[derive(Debug,PartialEq)]
enum FileState {
Open,
Closed,
}
#[derive(Debug)]
struct File {
name: String,
data: Vec< u8> ,
state: FileState,
}
impl Display for FileState {
fn fmt(& self, f: & mut fmt::Formatter) -> fmt::Result {
match *self {
FileState::Open => write!(f, "OPEN"),
FileState::Closed => write!(f, "CLOSED"),
}
}
}
impl Display for File {
fn fmt(& self, f: & mut fmt::Formatter) -> fmt::Result {
write!(f, "< {} ({})> ",
self.name, self.state)
}
}
impl File {
fn new(name: & str) -> File {
File {
name: String::from(name),
data: Vec::new(),
state: FileState::Closed,
}
}
}
fn main() {
let f6 = File::new("f6.txt");
//...
println!("{:?}", f6);
println!("{}", f6);
}< / code > < / pre >
< p > 以上两个例子较为复杂，目的是为读者展示下真实的使用场景长什么样，因此需要读者细细阅读，最终消化这些知识对于你的 Rust 之路会有莫大的帮助。< / p >
< p > 最后，特征和特征约束，是 Rust 中极其重要的概念，如果你还是没搞懂，强烈建议回头再看一遍，或者寻找相关的资料进行补充学习。如果已经觉得掌握了，那么就可以进入下一节的学习。< / p >
< h2 id = "课后练习" > < a class = "header" href = "#课后练习" > 课后练习< / a > < / h2 >
< blockquote >
< p > < a href = "https://practice-zh.course.rs/generics-traits/traits.html" > Rust By Practice< / a > ，支持代码在线编辑和运行，并提供详细的< a href = "https://github.com/sunface/rust-by-practice/blob/master/solutions/generics-traits/traits.md" > 习题解答< / a > 。< / p >
< / blockquote >
< / main >
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