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<h2 id="trait定义共同行为"><a class="header" href="#trait定义共同行为">Trait：定义共同行为</a></h2>
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<!-- https://github.com/rust-lang/book/blob/main/src/ch10-02-traits.md -->
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<!-- commit e5c8a0c99d5d3d830ad35e67f203a3378b4de16f -->
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<p><em>trait</em> 定义了某个特定类型拥有可能与其他类型共享的功能。可以通过 trait 以一种抽象的方式定义共同行为。可以使用 <em>trait bounds</em> 指定泛型是任何拥有特定行为的类型。</p>
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<blockquote>
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<p>注意：<em>trait</em> 类似于其他语言中的常被称为 <strong>接口</strong>（<em>interfaces</em>）的功能，虽然有一些不同。</p>
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</blockquote>
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<h3 id="定义-trait"><a class="header" href="#定义-trait">定义 trait</a></h3>
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<p>一个类型的行为由其可供调用的方法构成。如果可以对不同类型调用相同的方法的话，这些类型就可以共享相同的行为了。trait 定义是一种将方法签名组合起来的方法，目的是定义一个实现某些目的所必需的行为的集合。</p>
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<p>例如，这里有多个存放了不同类型和属性文本的结构体：结构体 <code>NewsArticle</code> 用于存放发生于世界各地的新闻故事，而结构体 <code>SocialPost</code> 最多只能存放 280 个字符的内容，以及指示该帖子是新发布的、转发的还是对另一条帖子的回复的元数据。</p>
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<p>我们想要创建一个名为 <code>aggregator</code> 的多媒体聚合库用来显示可能储存在 <code>NewsArticle</code> 或 <code>SocialPost</code> 实例中的数据摘要。为了实现功能，每个结构体都要能够获取摘要，这样的话就可以调用实例的 <code>summarize</code> 方法来请求摘要。示例 10-12 中展示了一个表现这个概念的公有 <code>Summary</code> trait 的定义：</p>
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<p><span class="filename">文件名：src/lib.rs</span></p>
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<pre><code class="language-rust noplayground">pub trait Summary {
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fn summarize(&self) -> String;
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}</code></pre>
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<p><span class="caption">示例 10-12：<code>Summary</code> trait 定义，它包含由 <code>summarize</code> 方法提供的行为</span></p>
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<p>这里使用 <code>trait</code> 关键字来声明一个 trait，后面是 trait 的名字，在这个例子中是 <code>Summary</code>。我们也声明 <code>trait</code> 为 <code>pub</code> 以便依赖这个 crate 的其它 crate 也可以使用这个 trait，正如我们见过的一些示例一样。在大括号中声明描述实现这个 trait 的类型所需要的行为的方法签名，在这个例子中是 <code>fn summarize(&self) -> String</code>。</p>
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<p>在方法签名后跟分号，而不是在大括号中提供其实现。接着每一个实现这个 trait 的类型都需要提供其自定义行为的方法体，编译器也会确保任何实现 <code>Summary</code> trait 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>
|
|
|
<p>trait 体中可以有多个方法：一行一个方法签名且都以分号结尾。</p>
|
|
|
<h3 id="为类型实现-trait"><a class="header" href="#为类型实现-trait">为类型实现 trait</a></h3>
|
|
|
<p>现在我们定义了 <code>Summary</code> trait 的签名，接着就可以在多媒体聚合库中实现这个类型了。示例 10-13 中展示了 <code>NewsArticle</code> 结构体上 <code>Summary</code> trait 的一个实现，它使用标题、作者和创建的位置作为 <code>summarize</code> 的返回值。对于 <code>SocialPost</code> 结构体，我们选择将 <code>summarize</code> 定义为用户名后跟帖子全文作为返回值，并假设帖子内容已经被限制为 280 字符以内。</p>
|
|
|
<p><span class="filename">文件名：src/lib.rs</span></p>
|
|
|
<pre><code class="language-rust noplayground"><span class="boring">pub trait Summary {
|
|
|
</span><span class="boring"> fn summarize(&self) -> String;
|
|
|
</span><span class="boring">}
|
|
|
</span><span class="boring">
|
|
|
</span>pub struct NewsArticle {
|
|
|
pub headline: String,
|
|
|
pub location: String,
|
|
|
pub author: String,
|
|
|
pub content: String,
|
|
|
}
|
|
|
|
|
|
impl Summary for NewsArticle {
|
|
|
fn summarize(&self) -> String {
|
|
|
format!("{}, by {} ({})", self.headline, self.author, self.location)
|
|
|
}
|
|
|
}
|
|
|
|
|
|
pub struct SocialPost {
|
|
|
pub username: String,
|
|
|
pub content: String,
|
|
|
pub reply: bool,
|
|
|
pub repost: bool,
|
|
|
}
|
|
|
|
|
|
impl Summary for SocialPost {
|
|
|
fn summarize(&self) -> String {
|
|
|
format!("{}: {}", self.username, self.content)
|
|
|
}
|
|
|
}</code></pre>
|
|
|
<p><span class="caption">示例 10-13：在 <code>NewsArticle</code> 和 <code>SocialPost</code> 类型上实现 <code>Summary</code> trait</span></p>
|
|
|
<p>在类型上实现 trait 类似于实现常规方法。区别在于 <code>impl</code> 关键字之后，我们提供需要实现 trait 的名称，接着是 <code>for</code> 和需要实现 trait 的类型的名称。在 <code>impl</code> 块中，使用 trait 定义中的方法签名，不过不再后跟分号，而是需要在大括号中编写函数体来为特定类型实现 trait 方法所拥有的行为。</p>
|
|
|
<p>现在库在 <code>NewsArticle</code> 和 <code>SocialPost</code> 上实现了<code>Summary</code> trait，crate 的用户可以像调用常规方法一样调用 <code>NewsArticle</code> 和 <code>SocialPost</code> 实例的 trait 方法了。唯一的区别是 trait 必须和类型一起引入作用域以便使用额外的 trait 方法。这是一个二进制 crate 如何利用 <code>aggregator</code> 库 crate 的例子：</p>
|
|
|
<pre><code class="language-rust ignore">use aggregator::{SocialPost, Summary};
|
|
|
|
|
|
fn main() {
|
|
|
let post = SocialPost {
|
|
|
username: String::from("horse_ebooks"),
|
|
|
content: String::from(
|
|
|
"of course, as you probably already know, people",
|
|
|
),
|
|
|
reply: false,
|
|
|
repost: false,
|
|
|
};
|
|
|
|
|
|
println!("1 new post: {}", post.summarize());
|
|
|
}</code></pre>
|
|
|
<p>这会打印出 <code>1 new post: horse_ebooks: of course, as you probably already know, people</code>。</p>
|
|
|
<p>其他依赖 <code>aggregator</code> crate 的 crate 也可以将 <code>Summary</code> 引入作用域以便为其自己的类型实现该 trait。需要注意的限制是，只有在 trait 或类型至少有一个属于当前 crate 时，我们才能对类型实现该 trait。例如，可以为 <code>aggregator</code> crate 的自定义类型 <code>SocialPost</code> 实现如标准库中的 <code>Display</code> trait，这是因为 <code>SocialPost</code> 类型位于 <code>aggregator</code> crate 本地的作用域中。类似地，也可以在 <code>aggregator</code> crate 中为 <code>Vec<T></code> 实现 <code>Summary</code>，这是因为 <code>Summary</code> trait 位于 <code>aggregator</code> crate 本地作用域中。</p>
|
|
|
<p>但是不能为外部类型实现外部 trait。例如，不能在 <code>aggregator</code> crate 中为 <code>Vec<T></code> 实现 <code>Display</code> trait。这是因为 <code>Display</code> 和 <code>Vec<T></code> 都定义于标准库中，它们并不位于 <code>aggregator</code> crate 本地作用域中。这个限制是被称为<strong>相干性</strong>（<em>coherence</em>）的程序属性的一部分，或者更具体的说是 <strong>孤儿规则</strong>（<em>orphan rule</em>），其得名于不存在父类型。这条规则确保了其他人编写的代码不会破坏你的代码，反之亦然。没有这条规则的话，两个 crate 可以分别对相同类型实现相同的 trait，而 Rust 将无从得知应该使用哪一个实现。</p>
|
|
|
<h3 id="默认实现"><a class="header" href="#默认实现">默认实现</a></h3>
|
|
|
<p>有时为 trait 中的某些或全部方法提供默认的行为，而不是在每个类型的每个实现中都定义自己的行为是很有用的。这样当为某个特定类型实现 trait 时，可以选择保留或重载每个方法的默认行为。</p>
|
|
|
<p>示例 10-14 中我们为 <code>Summary</code> trait 的 <code>summarize</code> 方法指定一个默认的字符串值，而不是像示例 10-12 中那样只是定义方法签名：</p>
|
|
|
<p><span class="filename">文件名：src/lib.rs</span></p>
|
|
|
<pre><code class="language-rust noplayground">pub trait Summary {
|
|
|
fn summarize(&self) -> String {
|
|
|
String::from("(Read more...)")
|
|
|
}
|
|
|
}
|
|
|
<span class="boring">
|
|
|
</span><span class="boring">pub struct NewsArticle {
|
|
|
</span><span class="boring"> pub headline: String,
|
|
|
</span><span class="boring"> pub location: String,
|
|
|
</span><span class="boring"> pub author: String,
|
|
|
</span><span class="boring"> pub content: String,
|
|
|
</span><span class="boring">}
|
|
|
</span><span class="boring">
|
|
|
</span><span class="boring">impl Summary for NewsArticle {}
|
|
|
</span><span class="boring">
|
|
|
</span><span class="boring">pub struct SocialPost {
|
|
|
</span><span class="boring"> pub username: String,
|
|
|
</span><span class="boring"> pub content: String,
|
|
|
</span><span class="boring"> pub reply: bool,
|
|
|
</span><span class="boring"> pub repost: bool,
|
|
|
</span><span class="boring">}
|
|
|
</span><span class="boring">
|
|
|
</span><span class="boring">impl Summary for SocialPost {
|
|
|
</span><span class="boring"> fn summarize(&self) -> String {
|
|
|
</span><span class="boring"> format!("{}: {}", self.username, self.content)
|
|
|
</span><span class="boring"> }
|
|
|
</span><span class="boring">}</span></code></pre>
|
|
|
<p><span class="caption">示例 10-14：<code>Summary</code> trait 的定义，带有一个 <code>summarize</code> 方法的默认实现</span></p>
|
|
|
<p>如果想要对 <code>NewsArticle</code> 实例使用这个默认实现，可以通过 <code>impl Summary for NewsArticle {}</code> 指定一个空的 <code>impl</code> 块。</p>
|
|
|
<p>虽然我们不再直接为 <code>NewsArticle</code> 定义 <code>summarize</code> 方法了，但是我们提供了一个默认实现并且指定 <code>NewsArticle</code> 实现 <code>Summary</code> trait。因此，我们仍然可以对 <code>NewsArticle</code> 实例调用 <code>summarize</code> 方法，如下所示：</p>
|
|
|
<pre><code class="language-rust ignore"><span class="boring">use aggregator::{self, NewsArticle, Summary};
|
|
|
</span><span class="boring">
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span> let article = NewsArticle {
|
|
|
headline: String::from("Penguins win the Stanley Cup Championship!"),
|
|
|
location: String::from("Pittsburgh, PA, USA"),
|
|
|
author: String::from("Iceburgh"),
|
|
|
content: String::from(
|
|
|
"The Pittsburgh Penguins once again are the best \
|
|
|
hockey team in the NHL.",
|
|
|
),
|
|
|
};
|
|
|
|
|
|
println!("New article available! {}", article.summarize());
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>这段代码会打印 <code>New article available! (Read more...)</code>。</p>
|
|
|
<p>为 <code>summarize</code> 创建默认实现并不要求对示例 10-13 中 <code>SocialPost</code> 上的 <code>Summary</code> 实现做任何改变。其原因是重载一个默认实现的语法与实现没有默认实现的 trait 方法的语法一样。</p>
|
|
|
<p>默认实现允许调用相同 trait 中的其他方法，哪怕这些方法没有默认实现。如此，trait 可以提供很多有用的功能而只需要实现指定一小部分内容。例如，我们可以定义 <code>Summary</code> trait，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><code class="language-rust noplayground">pub trait Summary {
|
|
|
fn summarize_author(&self) -> String;
|
|
|
|
|
|
fn summarize(&self) -> String {
|
|
|
format!("(Read more from {}...)", self.summarize_author())
|
|
|
}
|
|
|
}
|
|
|
<span class="boring">
|
|
|
</span><span class="boring">pub struct SocialPost {
|
|
|
</span><span class="boring"> pub username: String,
|
|
|
</span><span class="boring"> pub content: String,
|
|
|
</span><span class="boring"> pub reply: bool,
|
|
|
</span><span class="boring"> pub repost: bool,
|
|
|
</span><span class="boring">}
|
|
|
</span><span class="boring">
|
|
|
</span><span class="boring">impl Summary for SocialPost {
|
|
|
</span><span class="boring"> fn summarize_author(&self) -> String {
|
|
|
</span><span class="boring"> format!("@{}", self.username)
|
|
|
</span><span class="boring"> }
|
|
|
</span><span class="boring">}</span></code></pre>
|
|
|
<p>为了使用这个版本的 <code>Summary</code>，只需在为类型实现 trait 时定义 <code>summarize_author</code> 即可：</p>
|
|
|
<pre><code class="language-rust ignore"><span class="boring">pub trait Summary {
|
|
|
</span><span class="boring"> fn summarize_author(&self) -> String;
|
|
|
</span><span class="boring">
|
|
|
</span><span class="boring"> fn summarize(&self) -> String {
|
|
|
</span><span class="boring"> format!("(Read more from {}...)", self.summarize_author())
|
|
|
</span><span class="boring"> }
|
|
|
</span><span class="boring">}
|
|
|
</span><span class="boring">
|
|
|
</span><span class="boring">pub struct SocialPost {
|
|
|
</span><span class="boring"> pub username: String,
|
|
|
</span><span class="boring"> pub content: String,
|
|
|
</span><span class="boring"> pub reply: bool,
|
|
|
</span><span class="boring"> pub repost: bool,
|
|
|
</span><span class="boring">}
|
|
|
</span><span class="boring">
|
|
|
</span>impl Summary for SocialPost {
|
|
|
fn summarize_author(&self) -> String {
|
|
|
format!("@{}", self.username)
|
|
|
}
|
|
|
}</code></pre>
|
|
|
<p>一旦定义了 <code>summarize_author</code>，我们就可以对 <code>SocialPost</code> 结构体的实例调用 <code>summarize</code> 了，而 <code>summarize</code> 的默认实现会调用我们提供的 <code>summarize_author</code> 定义。因为实现了 <code>summarize_author</code>，<code>Summary</code> trait 就提供了 <code>summarize</code> 方法的功能，且无需编写更多的代码。</p>
|
|
|
<pre><code class="language-rust ignore"><span class="boring">use aggregator::{self, SocialPost, Summary};
|
|
|
</span><span class="boring">
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span> let post = SocialPost {
|
|
|
username: String::from("horse_ebooks"),
|
|
|
content: String::from(
|
|
|
"of course, as you probably already know, people",
|
|
|
),
|
|
|
reply: false,
|
|
|
repost: false,
|
|
|
};
|
|
|
|
|
|
println!("1 new social post: {}", post.summarize());
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>这会打印出 <code>1 new post: (Read more from @horse_ebooks...)</code>。</p>
|
|
|
<p>请注意，无法从一个方法的重写实现中调用与其同名的默认实现。</p>
|
|
|
<h3 id="trait-作为参数"><a class="header" href="#trait-作为参数">trait 作为参数</a></h3>
|
|
|
<p>知道了如何定义 trait 和在类型上实现这些 trait 之后，我们可以探索一下如何使用 trait 来接受多种不同类型的参数。示例 10-13 中为 <code>NewsArticle</code> 和 <code>SocialPost</code> 类型实现了 <code>Summary</code> trait，用其来定义了一个函数 <code>notify</code> 来调用其参数 <code>item</code> 上的 <code>summarize</code> 方法，该参数是实现了 <code>Summary</code> trait 的某种类型。为此可以使用 <code>impl Trait</code> 语法，像这样：</p>
|
|
|
<pre><code class="language-rust ignore"><span class="boring">pub trait Summary {
|
|
|
</span><span class="boring"> fn summarize(&self) -> String;
|
|
|
</span><span class="boring">}
|
|
|
</span><span class="boring">
|
|
|
</span><span class="boring">pub struct NewsArticle {
|
|
|
</span><span class="boring"> pub headline: String,
|
|
|
</span><span class="boring"> pub location: String,
|
|
|
</span><span class="boring"> pub author: String,
|
|
|
</span><span class="boring"> pub content: String,
|
|
|
</span><span class="boring">}
|
|
|
</span><span class="boring">
|
|
|
</span><span class="boring">impl Summary for NewsArticle {
|
|
|
</span><span class="boring"> fn summarize(&self) -> String {
|
|
|
</span><span class="boring"> format!("{}, by {} ({})", self.headline, self.author, self.location)
|
|
|
</span><span class="boring"> }
|
|
|
</span><span class="boring">}
|
|
|
</span><span class="boring">
|
|
|
</span><span class="boring">pub struct SocialPost {
|
|
|
</span><span class="boring"> pub username: String,
|
|
|
</span><span class="boring"> pub content: String,
|
|
|
</span><span class="boring"> pub reply: bool,
|
|
|
</span><span class="boring"> pub repost: bool,
|
|
|
</span><span class="boring">}
|
|
|
</span><span class="boring">
|
|
|
</span><span class="boring">impl Summary for SocialPost {
|
|
|
</span><span class="boring"> fn summarize(&self) -> String {
|
|
|
</span><span class="boring"> format!("{}: {}", self.username, self.content)
|
|
|
</span><span class="boring"> }
|
|
|
</span><span class="boring">}
|
|
|
</span><span class="boring">
|
|
|
</span>pub fn notify(item: &impl Summary) {
|
|
|
println!("Breaking news! {}", item.summarize());
|
|
|
}</code></pre>
|
|
|
<p>对于 <code>item</code> 参数，我们指定了 <code>impl</code> 关键字和 trait 名称，而不是具体的类型。该参数支持任何实现了指定 trait 的类型。在 <code>notify</code> 函数体中，可以调用任何来自 <code>Summary</code> trait 的方法，比如 <code>summarize</code>。我们可以传递任何 <code>NewsArticle</code> 或 <code>SocialPost</code> 的实例来调用 <code>notify</code>。任何用其它如 <code>String</code> 或 <code>i32</code> 的类型调用该函数的代码都不能编译，因为它们没有实现 <code>Summary</code>。</p>
|
|
|
<h4 id="trait-bound-语法"><a class="header" href="#trait-bound-语法">Trait Bound 语法</a></h4>
|
|
|
<p><code>impl Trait</code> 语法更直观，但它实际上是更长形式的 <em>trait bound</em> 语法的语法糖。它看起来像：</p>
|
|
|
<pre><code class="language-rust ignore">pub fn notify<T: Summary>(item: &T) {
|
|
|
println!("Breaking news! {}", item.summarize());
|
|
|
}</code></pre>
|
|
|
<p>这种更冗长的写法与上一节的示例等价，但更为冗长。trait bound 与泛型参数声明在一起，位于尖括号中的冒号后面。</p>
|
|
|
<p><code>impl Trait</code> 很方便，适用于短小的例子。更长的 trait bound 则适用于更复杂的场景。例如，可以获取两个实现了 <code>Summary</code> 的参数。使用 <code>impl Trait</code> 的语法看起来像这样：</p>
|
|
|
<pre><code class="language-rust ignore">pub fn notify(item1: &impl Summary, item2: &impl Summary) {</code></pre>
|
|
|
<p>这适用于 <code>item1</code> 和 <code>item2</code> 允许是不同类型的情况（只要它们都实现了 <code>Summary</code>）。不过如果你希望强制它们都是相同类型呢？但如果我们希望强制两个参数必须具有相同类型，则必须使用 trait bound，如下所示：</p>
|
|
|
<pre><code class="language-rust ignore">pub fn notify<T: Summary>(item1: &T, item2: &T) {</code></pre>
|
|
|
<p>泛型 <code>T</code> 被指定为 <code>item1</code> 和 <code>item2</code> 的参数限制，如此传递给参数 <code>item1</code> 和 <code>item2</code> 值的具体类型必须一致。</p>
|
|
|
<h4 id="通过--指定多个-trait-bound"><a class="header" href="#通过--指定多个-trait-bound">通过 <code>+</code> 指定多个 trait bound</a></h4>
|
|
|
<p>我们也可以指定多个 trait bound。假设我们希望 <code>notify</code> 在 <code>item</code> 上既能使用格式化显示，又能使用 <code>summarize</code> 方法：在 <code>notify</code> 的定义中，指定 <code>item</code> 必须同时实现 <code>Display</code> 和 <code>Summary</code> 两个 trait。这可以通过 <code>+</code> 语法实现：</p>
|
|
|
<pre><code class="language-rust ignore">pub fn notify(item: &(impl Summary + Display)) {</code></pre>
|
|
|
<p><code>+</code> 语法也适用于泛型的 trait bound：</p>
|
|
|
<pre><code class="language-rust ignore">pub fn notify<T: Summary + Display>(item: &T) {</code></pre>
|
|
|
<p>通过指定这两个 trait bound，<code>notify</code> 的函数体可以调用 <code>summarize</code> 并使用 <code>{}</code> 来格式化 <code>item</code>。</p>
|
|
|
<h4 id="通过-where-简化-trait-bound"><a class="header" href="#通过-where-简化-trait-bound">通过 <code>where</code> 简化 trait bound</a></h4>
|
|
|
<p>然而，使用过多的 trait bound 也有缺点。每个泛型有其自己的 trait bound，所以有多个泛型参数的函数在名称和参数列表之间会有很长的 trait bound 信息，这使得函数签名难以阅读。为此，Rust 有另一个在函数签名之后的 <code>where</code> 从句中指定 trait bound 的语法。所以除了这么写：</p>
|
|
|
<pre><code class="language-rust ignore">fn some_function<T: Display + Clone, U: Clone + Debug>(t: &T, u: &U) -> i32 {</code></pre>
|
|
|
<p>还可以像这样使用 <code>where</code> 从句：</p>
|
|
|
<pre><code class="language-rust ignore">fn some_function<T, U>(t: &T, u: &U) -> i32
|
|
|
where
|
|
|
T: Display + Clone,
|
|
|
U: Clone + Debug,
|
|
|
{
|
|
|
<span class="boring"> unimplemented!()
|
|
|
</span><span class="boring">}</span></code></pre>
|
|
|
<p>这个函数签名就显得不那么杂乱，函数名、参数列表和返回值类型都离得很近，看起来跟没有那么多 trait bounds 的函数很像。</p>
|
|
|
<h3 id="返回实现了-trait-的类型"><a class="header" href="#返回实现了-trait-的类型">返回实现了 trait 的类型</a></h3>
|
|
|
<p>也可以在返回值中使用 <code>impl Trait</code> 语法，来返回实现了某个 trait 的类型：</p>
|
|
|
<pre><code class="language-rust ignore"><span class="boring">pub trait Summary {
|
|
|
</span><span class="boring"> fn summarize(&self) -> String;
|
|
|
</span><span class="boring">}
|
|
|
</span><span class="boring">
|
|
|
</span><span class="boring">pub struct NewsArticle {
|
|
|
</span><span class="boring"> pub headline: String,
|
|
|
</span><span class="boring"> pub location: String,
|
|
|
</span><span class="boring"> pub author: String,
|
|
|
</span><span class="boring"> pub content: String,
|
|
|
</span><span class="boring">}
|
|
|
</span><span class="boring">
|
|
|
</span><span class="boring">impl Summary for NewsArticle {
|
|
|
</span><span class="boring"> fn summarize(&self) -> String {
|
|
|
</span><span class="boring"> format!("{}, by {} ({})", self.headline, self.author, self.location)
|
|
|
</span><span class="boring"> }
|
|
|
</span><span class="boring">}
|
|
|
</span><span class="boring">
|
|
|
</span><span class="boring">pub struct SocialPost {
|
|
|
</span><span class="boring"> pub username: String,
|
|
|
</span><span class="boring"> pub content: String,
|
|
|
</span><span class="boring"> pub reply: bool,
|
|
|
</span><span class="boring"> pub repost: bool,
|
|
|
</span><span class="boring">}
|
|
|
</span><span class="boring">
|
|
|
</span><span class="boring">impl Summary for SocialPost {
|
|
|
</span><span class="boring"> fn summarize(&self) -> String {
|
|
|
</span><span class="boring"> format!("{}: {}", self.username, self.content)
|
|
|
</span><span class="boring"> }
|
|
|
</span><span class="boring">}
|
|
|
</span><span class="boring">
|
|
|
</span>fn returns_summarizable() -> impl Summary {
|
|
|
SocialPost {
|
|
|
username: String::from("horse_ebooks"),
|
|
|
content: String::from(
|
|
|
"of course, as you probably already know, people",
|
|
|
),
|
|
|
reply: false,
|
|
|
repost: false,
|
|
|
}
|
|
|
}</code></pre>
|
|
|
<p>通过使用 <code>impl Summary</code> 作为返回值类型，我们指定了 <code>returns_summarizable</code> 函数返回某个实现了 <code>Summary</code> trait 的类型，但是不确定其具体的类型。在这个例子中 <code>returns_summarizable</code> 返回了一个 <code>SocialPost</code>，不过调用方并不知情。</p>
|
|
|
<p>返回一个只是指定了需要实现的 trait 的类型的能力在闭包和迭代器场景十分的有用，第十三章会介绍它们。闭包和迭代器创建只有编译器知道的类型，或者是非常非常长的类型。<code>impl Trait</code> 允许你简单的指定函数返回一个 <code>Iterator</code> 而无需写出实际的冗长的类型。</p>
|
|
|
<p>不过这只适用于返回单一类型的情况。例如，这段代码的返回值类型指定为返回 <code>impl Summary</code>，但是返回了 <code>NewsArticle</code> 或 <code>SocialPost</code> 就行不通：</p>
|
|
|
<pre><code class="language-rust ignore does_not_compile"><span class="boring">pub trait Summary {
|
|
|
</span><span class="boring"> fn summarize(&self) -> String;
|
|
|
</span><span class="boring">}
|
|
|
</span><span class="boring">
|
|
|
</span><span class="boring">pub struct NewsArticle {
|
|
|
</span><span class="boring"> pub headline: String,
|
|
|
</span><span class="boring"> pub location: String,
|
|
|
</span><span class="boring"> pub author: String,
|
|
|
</span><span class="boring"> pub content: String,
|
|
|
</span><span class="boring">}
|
|
|
</span><span class="boring">
|
|
|
</span><span class="boring">impl Summary for NewsArticle {
|
|
|
</span><span class="boring"> fn summarize(&self) -> String {
|
|
|
</span><span class="boring"> format!("{}, by {} ({})", self.headline, self.author, self.location)
|
|
|
</span><span class="boring"> }
|
|
|
</span><span class="boring">}
|
|
|
</span><span class="boring">
|
|
|
</span><span class="boring">pub struct SocialPost {
|
|
|
</span><span class="boring"> pub username: String,
|
|
|
</span><span class="boring"> pub content: String,
|
|
|
</span><span class="boring"> pub reply: bool,
|
|
|
</span><span class="boring"> pub repost: bool,
|
|
|
</span><span class="boring">}
|
|
|
</span><span class="boring">
|
|
|
</span><span class="boring">impl Summary for SocialPost {
|
|
|
</span><span class="boring"> fn summarize(&self) -> String {
|
|
|
</span><span class="boring"> format!("{}: {}", self.username, self.content)
|
|
|
</span><span class="boring"> }
|
|
|
</span><span class="boring">}
|
|
|
</span><span class="boring">
|
|
|
</span>fn returns_summarizable(switch: bool) -> impl Summary {
|
|
|
if switch {
|
|
|
NewsArticle {
|
|
|
headline: String::from(
|
|
|
"Penguins win the Stanley Cup Championship!",
|
|
|
),
|
|
|
location: String::from("Pittsburgh, PA, USA"),
|
|
|
author: String::from("Iceburgh"),
|
|
|
content: String::from(
|
|
|
"The Pittsburgh Penguins once again are the best \
|
|
|
hockey team in the NHL.",
|
|
|
),
|
|
|
}
|
|
|
} else {
|
|
|
SocialPost {
|
|
|
username: String::from("horse_ebooks"),
|
|
|
content: String::from(
|
|
|
"of course, as you probably already know, people",
|
|
|
),
|
|
|
reply: false,
|
|
|
repost: false,
|
|
|
}
|
|
|
}
|
|
|
}</code></pre>
|
|
|
<p>这里尝试返回 <code>NewsArticle</code> 或 <code>SocialPost</code> 是不被允许的，原因在于编译器中 <code>impl Trait</code> 语法的实现限制。第十八章的 <a href="ch18-02-trait-objects.html#顾及不同类型值的-trait-对象">“顾及不同类型值的 trait 对象”</a> 部分会介绍如何编写这样一个函数。</p>
|
|
|
<h3 id="使用-trait-bound-有条件地实现方法"><a class="header" href="#使用-trait-bound-有条件地实现方法">使用 trait bound 有条件地实现方法</a></h3>
|
|
|
<p>通过使用带有 trait bound 的泛型参数的 <code>impl</code> 块，可以有条件地只为那些实现了特定 trait 的类型实现方法。例如，示例 10-15 中的类型 <code>Pair<T></code> 总是实现了 <code>new</code> 方法并返回一个 <code>Pair<T></code> 的实例（回忆一下第五章的 <a href="ch05-03-method-syntax.html#定义方法">“定义方法”</a> 部分，<code>Self</code> 是一个 <code>impl</code> 块类型的类型别名（type alias），在这里是 <code>Pair<T></code>）。不过在下一个 <code>impl</code> 块中，只有那些为 <code>T</code> 类型实现了 <code>PartialOrd</code> trait（来允许比较） <strong>和</strong> <code>Display</code> trait（来启用打印）的 <code>Pair<T></code> 才会实现 <code>cmp_display</code> 方法：</p>
|
|
|
<pre><code class="language-rust noplayground">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);
|
|
|
}
|
|
|
}
|
|
|
}</code></pre>
|
|
|
<p><span class="caption">示例 10-15：根据 trait bound 在泛型上有条件的实现方法</span></p>
|
|
|
<p>也可以对任何实现了特定 trait 的类型有条件地实现 trait。对任何满足特定 trait bound 的类型实现 trait 被称为 <em>blanket implementations</em>，它们被广泛的用于 Rust 标准库中。例如，标准库为任何实现了 <code>Display</code> trait 的类型实现了 <code>ToString</code> trait。这个 <code>impl</code> 块看起来像这样：</p>
|
|
|
<pre><code class="language-rust ignore">impl<T: Display> ToString for T {
|
|
|
// --snip--
|
|
|
}</code></pre>
|
|
|
<p>因为标准库有了这些 blanket implementation，我们可以对任何实现了 <code>Display</code> trait 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 edition2024"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>let s = 3.to_string();
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>Blanket implementation 会出现在 trait 文档的 “Implementers” 部分。</p>
|
|
|
<p>Trait 和 trait bound 让我们能够使用泛型类型参数来减少重复，而且能够向编译器明确指定泛型类型需要拥有哪些行为。然后编译器可以利用 trait bound 信息检查代码中所用到的具体类型是否提供了正确的行为。在动态类型语言中，如果我们调用了一个未定义的方法，会在运行时出现错误。Rust 将这些错误移动到了编译时，甚至在代码能够运行之前就强迫我们修复问题。另外，我们也无需编写运行时检查行为的代码，因为在编译时就已经检查过了。这样既提升了性能又不必放弃泛型的灵活性。</p>
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