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<ol class="chapter"><li class="chapter-item expanded affix "><a href="title-page.html">Rust 程序设计语言</a></li><li class="chapter-item expanded affix "><a href="foreword.html">前言</a></li><li class="chapter-item expanded affix "><a href="ch00-00-introduction.html">简介</a></li><li class="chapter-item expanded "><a href="ch01-00-getting-started.html"><strong aria-hidden="true">1.</strong> 入门指南</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch01-01-installation.html"><strong aria-hidden="true">1.1.</strong> 安装</a></li><li class="chapter-item expanded "><a href="ch01-02-hello-world.html"><strong aria-hidden="true">1.2.</strong> Hello, World!</a></li><li class="chapter-item expanded "><a href="ch01-03-hello-cargo.html"><strong aria-hidden="true">1.3.</strong> Hello, Cargo!</a></li></ol></li><li class="chapter-item expanded "><a href="ch02-00-guessing-game-tutorial.html"><strong aria-hidden="true">2.</strong> 写个猜数字游戏</a></li><li class="chapter-item expanded "><a href="ch03-00-common-programming-concepts.html"><strong aria-hidden="true">3.</strong> 常见编程概念</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch03-01-variables-and-mutability.html"><strong aria-hidden="true">3.1.</strong> 变量与可变性</a></li><li class="chapter-item expanded "><a href="ch03-02-data-types.html"><strong aria-hidden="true">3.2.</strong> 数据类型</a></li><li class="chapter-item expanded "><a href="ch03-03-how-functions-work.html"><strong aria-hidden="true">3.3.</strong> 函数</a></li><li class="chapter-item expanded "><a href="ch03-04-comments.html"><strong aria-hidden="true">3.4.</strong> 注释</a></li><li class="chapter-item expanded "><a href="ch03-05-control-flow.html"><strong aria-hidden="true">3.5.</strong> 控制流</a></li></ol></li><li class="chapter-item expanded "><a href="ch04-00-understanding-ownership.html"><strong aria-hidden="true">4.</strong> 认识所有权</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch04-01-what-is-ownership.html"><strong aria-hidden="true">4.1.</strong> 什么是所有权？</a></li><li class="chapter-item expanded "><a href="ch04-02-references-and-borrowing.html"><strong aria-hidden="true">4.2.</strong> 引用与借用</a></li><li class="chapter-item expanded "><a href="ch04-03-slices.html"><strong aria-hidden="true">4.3.</strong> Slice 类型</a></li></ol></li><li class="chapter-item expanded "><a href="ch05-00-structs.html"><strong aria-hidden="true">5.</strong> 使用结构体组织相关联的数据</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch05-01-defining-structs.html"><strong aria-hidden="true">5.1.</strong> 结构体的定义和实例化</a></li><li class="chapter-item expanded "><a href="ch05-02-example-structs.html"><strong aria-hidden="true">5.2.</strong> 结构体示例程序</a></li><li class="chapter-item expanded "><a href="ch05-03-method-syntax.html"><strong aria-hidden="true">5.3.</strong> 方法语法</a></li></ol></li><li class="chapter-item expanded "><a href="ch06-00-enums.html"><strong aria-hidden="true">6.</strong> 枚举和模式匹配</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch06-01-defining-an-enum.html"><strong aria-hidden="true">6.1.</strong> 枚举的定义</a></li><li class="chapter-item expanded "><a href="ch06-02-match.html"><strong aria-hidden="true">6.2.</strong> match 控制流结构</a></li><li class="chapter-item expanded "><a href="ch06-03-if-let.html"><strong aria-hidden="true">6.3.</strong> if let 简洁控制流</a></li></ol></li><li class="chapter-item expanded "><a href="ch07-00-managing-growing-projects-with-packages-crates-and-modules.html"><strong aria-hidden="true">7.</strong> 使用包、Crate 和模块管理不断增长的项目</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch07-01-packages-and-crates.html"><strong aria-hidden="true">7.1.</strong> 包和 Crate</a></li><li class="chapter-item expanded "><a href="ch07-02-defining-modules-to-control-scope-and-privacy.html"><strong aria-hidden="true">7.2.</strong> 定义模块来控制作用域与私有性</a></li><li class="chapter-item expanded "><a href="ch07-03-paths-for-referring-to-an-item-in-the-module-tree.html"><strong aria-hidden="true">7.3.</strong> 引用模块项目的路径</a></li><li class="chapter-item expanded "><a href="ch07-04-bringing-paths-into-scope-with-the-use-keyword.html"><strong aria-hidden="true">7.4.</strong> 使用 use 关键字将路径引入作用域</a></li><li class="chapter-item expanded "><a href="ch07-05-separating-modules-into-different-files.html"><strong aria-hidden="true">7.5.</strong> 将模块拆分成多个文件</a></li></ol></li><li class="chapter-item expanded "><a href="ch08-00-common-collections.html"><strong aria-hidden="true">8.</strong> 常见集合</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch08-01-vectors.html"><strong aria-hidden="true">8.1.</strong> 使用 Vector 储存列表</a></li><li class="chapter-item expanded "><a href="ch08-02-strings.html"><strong aria-hidden="true">8.2.</strong> 使用字符串储存 UTF-8 编码的文本</a></li><li class="chapter-item expanded "><a href="ch08-03-hash-maps.html"><strong aria-hidden="true">8.3.</strong> 使用 Hash Map 储存键值对</a></li></ol></li><li class="chapter-item expanded "><a href="ch09-00-error-handling.html"><strong aria-hidden="true">9.</strong> 错误处理</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch09-01-unrecoverable-errors-with-panic.html"><strong aria-hidden="true">9.1.</strong> 用 panic! 处理不可恢复的错误</a></li><li class="chapter-item expanded "><a href="ch09-02-recoverable-errors-with-result.html"><strong aria-hidden="true">9.2.</strong> 用 Result 处理可恢复的错误</a></li><li class="chapter-item expanded "><a href="ch09-03-to-panic-or-not-to-panic.html"><strong aria-hidden="true">9.3.</strong> 要不要 panic!</a></li></ol></li><li class="chapter-item expanded "><a href="ch10-00-generics.html"><strong aria-hidden="true">10.</strong> 泛型、Trait 和生命周期</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch10-01-syntax.html"><strong aria-hidden="true">10.1.</strong> 泛型数据类型</a></li><li class="chapter-item expanded "><a href="ch10-02-traits.html" class="active"><strong aria-hidden="true">10.2.</strong> Trait：定义共同行为</a></li><li class="chapter-item expanded "><a href="ch10-03-lifetime-syntax.html"><strong aria-hidden="true">10.3.</strong> 生命周期确保引用有效</a></li></ol></li><li class="chapter-item expanded "><a href="ch11-00-testing.html"><strong aria-hidden="true">11.</strong> 编写自动化测试</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch11-01-writing-tests.html"><strong aria-hidden="true">11.1.</strong> 如何编写测试</a></li><li class="chapter-item expanded "><a href="ch11-02-running-tests.html"><strong aria-hidden="true">11.2.</strong> 控制测试如何运行</a></li><li class="chapter-item expanded "><a href="ch11-03-test-organization.html"><strong aria-hidden="true">11.3.</strong> 测试的组织结构</a></li></ol></li><li class="chapter-item expanded "><a href="ch12-00-an-io-project.html"><strong aria-hidden="true">12.</strong> 一个 I/O 项目：构建命令行程序</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch12-01-accepting-command-line-arguments.html"><strong aria-hidden="true">12.1.</strong> 接受命令行参数</a></li><li class="chapter-item expanded "><a href="ch12-02-reading-a-file.html"><strong aria-hidden="true">12.2.</strong> 读取文件</a></li><li class="chapter-item expanded "><a href="ch12-03-improving-error-handling-and-modularity.html"><strong aria-hidden="true">12.3.</strong> 重构以改进模块化与错误处理</a></li><li class="chapter-item expanded "><a href="ch12-04-testing-the-librarys-functionality.html"><strong aria-hidden="true">12.4.</strong> 采用测试驱动开发完善库的功能</a></li><li class="chapter-item expanded "><a href="ch12-05-working-with-environment-variables.html"><strong aria-hidden="true">12.5.</strong> 处理环境变量</a></li><li class="chapter-item expanded "><a href="ch12-06-writing-to-stderr-instead-of-stdout.html"><strong aria-hidden="true">12.6.</strong> 将错误信息输出到标准错误而不是标准输出</a></li></ol></li><li class="chapter-item expanded "><a href="ch13-00-functional-features.html"><strong aria-hidden="true">13.</strong> Rust 中的函数式语言功能：迭代器与闭包</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch13-01-closures.html"><strong aria-hidden="true">13.1.</strong> 闭包：可以捕获其环境的匿名函数</a></li><li class="chapter-item expanded "><a href="ch13-02-iterators.html"><strong aria-hidden="true">13.2.</strong> 使用迭代器处理元素序列</a></li><li class="chapter-item expanded "><a href="ch13-03-improving-our-io-project.html"><strong aria-hidden="true">13.3.</strong> 改进之前的 I/O 项目</a></li><li class="chapter-item expanded "><a href="ch13-04-performance.html"><strong aria-hidden="true">13.4.</strong> 性能比较：循环对迭代器</a></li></ol></li><li class="chapter-item expanded "><a href="ch14-00-more-about-cargo.html"><strong aria-hidden="true">14.</strong> 更多关于 Cargo 和 Crates.io 的内容</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch14-01-release-profiles.html"><strong aria-hidden="true">14.1.</strong> 采用发布配置自定义构建</a></li><li class="chapter-item expanded "><a href="ch14-02-publishing-to-crates-io.html"><strong aria-hidden="true">14.2.</strong> 将 crate 发布到 Crates.io</a></li><li class="chapter-item expanded "><a href="ch14-03-cargo-workspaces.html"><strong aria-hidden="true">14.3.</strong> Cargo 工作空间</a></li><li class="chapter-item expanded "><a href="ch14-04-installing-binaries.html"><strong aria-hidden="true">14.4.</strong> 使用 cargo install 安装二进制文件</a></li><li class="chapter-item expanded "><a href="ch14-05-extending-cargo.html"><strong aria-hidden="true">14.5.</strong> Cargo 自定义扩展命令</a></li></ol></li><li class="chapter-item expanded "><a href="ch15-00-smart-pointers.html"><strong aria-hidden="true">15.</strong> 智能指针</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch15-01-box.html"><strong aria-hidden="true">15.1.</strong> 使用 Box&lt;T&gt; 指向堆上数据</a></li><li class="chapter-item expanded "><a href="ch15-02-deref.html"><strong aria-hidden="true">15.2.</strong> 使用 Deref Trait 将智能指针当作常规引用处理</a></li><li class="chapter-item expanded "><a href="ch15-03-drop.html"><strong aria-hidden="true">15.3.</strong> 使用 Drop Trait 运行清理代码</a></li><li class="chapter-item expanded "><a href="ch15-04-rc.html"><strong aria-hidden="true">15.4.</strong> Rc&lt;T&gt; 引用计数智能指针</a></li><li class="chapter-item expanded "><a href="ch15-05-interior-mutability.html"><strong aria-hidden="true">15.5.</strong> RefCell&lt;T&gt; 与内部可变性模式</a></li><li class="chapter-item expanded "><a href="ch15-06-reference-cycles.html"><strong aria-hidden="true">15.6.</strong> 引用循环会导致内存泄漏</a></li></ol></li><li class="chapter-item expanded "><a href="ch16-00-concurrency.html"><strong aria-hidden="true">16.</strong> 无畏并发</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch16-01-threads.html"><strong aria-hidden="true">16.1.</strong> 使用线程同时地运行代码</a></li><li class="chapter-item expanded "><a href="ch16-02-message-passing.html"><strong aria-hidden="true">16.2.</strong> 使用消息传递在线程间通信</a></li><li class="chapter-item expanded "><a href="ch16-03-shared-state.html"><strong aria-hidden="true">16.3.</strong> 共享状态并发</a></li><li class="chapter-item expanded "><a href="ch16-04-extensible-concurrency-sync-and-send.html"><strong aria-hidden="true">16.4.</strong> 使用 Sync 与 Send Traits 的可扩展并发</a></li></ol></li><li class="chapter-item expanded "><a href="ch17-00-async-await.html"><strong aria-hidden="true">17.</strong> Async 和 await</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch17-01-futures-and-syntax.html"><strong aria-hidden="true">17.1.</strong> Futures 和 async 语法</a></li><li class="chapter-item expanded "><a href="ch17-02-concurrency-with-async.html"><strong aria-hidden="true">17.2.</strong> 并发与 async</a></li><li class="chapter-item expanded "><a href="ch17-03-more-futures.html"><strong aria-hidden="true">17.3.</strong> 使用任意数量的 futures</a></li><li class="chapter-item expanded "><a href="ch17-04-streams.html"><strong aria-hidden="true">17.4.</strong> 流（Streams）</a></li><li class="chapter-item expanded "><a href="ch17-05-traits-for-async.html"><strong aria-hidden="true">17.5.</strong> 深入理解 async 相关的 traits</a></li><li class="chapter-item expanded "><a href="ch17-06-futures-tasks-threads.html"><strong aria-hidden="true">17.6.</strong> Futures，任务（tasks）和线程（threads）</a></li></ol></li><li class="chapter-item expanded "><a href="ch18-00-oop.html"><strong aria-hidden="true">18.</strong> Rust 的面向对象编程特性</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch18-01-what-is-oo.html"><strong aria-hidden="true">18.1.</strong> 面向对象语言的特点</a></li><li class="chapter-item expanded "><a href="ch18-02-trait-objects.html"><strong aria-hidden="true">18.2.</strong> 顾及不同类型值的 trait 对象</a></li><li class="chapter-item expanded "><a href="ch18-03-oo-design-patterns.html"><strong aria-hidden="true">18.3.</strong> 面向对象设计模式的实现</a></li></ol></li><li class="chapter-item expanded "><a href="ch19-00-patterns.html"><strong aria-hidden="true">19.</strong> 模式与模式匹配</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch19-01-all-the-places-for-patterns.html"><strong aria-hidden="true">19.1.</strong> 所有可能会用到模式的位置</a></li><li class="chapter-item expanded "><a href="ch19-02-refutability.html"><strong aria-hidden="true">19.2.</strong> Refutability（可反驳性）: 模式是否会匹配失效</a></li><li class="chapter-item expanded "><a href="ch19-03-pattern-syntax.html"><strong aria-hidden="true">19.3.</strong> 模式语法</a></li></ol></li><li class="chapter-item expanded "><a href="ch20-00-advanced-features.html"><strong aria-hidden="true">20.</strong> 高级特征</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch20-01-unsafe-rust.html"><strong aria-hidden="true">20.1.</strong> 不安全的 Rust</a></li><li class="chapter-item expanded "><a href="ch20-03-advanced-traits.html"><strong aria-hidden="true">20.2.</strong> 高级 trait</a></li><li class="chapter-item expanded "><a href="ch20-04-advanced-types.html"><strong aria-hidden="true">20.3.</strong> 高级类型</a></li><li class="chapter-item expanded "><a href="ch20-05-advanced-functions-and-closures.html"><strong aria-hidden="true">20.4.</strong> 高级函数与闭包</a></li><li class="chapter-item expanded "><a href="ch20-06-macros.html"><strong aria-hidden="true">20.5.</strong> 宏</a></li></ol></li><li class="chapter-item expanded "><a href="ch21-00-final-project-a-web-server.html"><strong aria-hidden="true">21.</strong> 最后的项目：构建多线程 web server</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch21-01-single-threaded.html"><strong aria-hidden="true">21.1.</strong> 建立单线程 web server</a></li><li class="chapter-item expanded "><a href="ch21-02-multithreaded.html"><strong aria-hidden="true">21.2.</strong> 将单线程 server 变为多线程 server</a></li><li class="chapter-item expanded "><a href="ch21-03-graceful-shutdown-and-cleanup.html"><strong aria-hidden="true">21.3.</strong> 优雅停机与清理</a></li></ol></li><li class="chapter-item expanded "><a href="appendix-00.html"><strong aria-hidden="true">22.</strong> 附录</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="appendix-01-keywords.html"><strong aria-hidden="true">22.1.</strong> A - 关键字</a></li><li class="chapter-item expanded "><a href="appendix-02-operators.html"><strong aria-hidden="true">22.2.</strong> B - 运算符与符号</a></li><li class="chapter-item expanded "><a href="appendix-03-derivable-traits.html"><strong aria-hidden="true">22.3.</strong> C - 可派生的 trait</a></li><li class="chapter-item expanded "><a href="appendix-04-useful-development-tools.html"><strong aria-hidden="true">22.4.</strong> D - 实用开发工具</a></li><li class="chapter-item expanded "><a href="appendix-05-editions.html"><strong aria-hidden="true">22.5.</strong> E - 版本</a></li><li class="chapter-item expanded "><a href="appendix-06-translation.html"><strong aria-hidden="true">22.6.</strong> F - 本书译本</a></li><li class="chapter-item expanded "><a href="appendix-07-nightly-rust.html"><strong aria-hidden="true">22.7.</strong> G - Rust 是如何开发的与 “Nightly Rust”</a></li></ol></li></ol>
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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>
<blockquote>
<p><a href="https://github.com/rust-lang/book/blob/main/src/ch10-02-traits.md">ch10-02-traits.md</a>
<br>
commit 92bfbfacf88ee9a814cea0a58e9c019c529ef4ae</p>
</blockquote>
<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>
<blockquote>
<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>
</blockquote>
<h3 id="定义-trait"><a class="header" href="#定义-trait">定义 trait</a></h3>
<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>
<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>Tweet</code> 最多只能存放 280 个字符的内容，以及像是否转推或是否是对推友的回复这样的元数据。</p>
<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>Tweet</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>
<p><span class="filename">文件名：src/lib.rs</span></p>
<pre><code class="language-rust noplayground">pub trait Summary {
fn summarize(&amp;self) -&gt; String;
}</code></pre>
<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>
<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 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(&amp;self) -&gt; String</code>。</p>
<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>Tweet</code> 结构体，我们选择将 <code>summarize</code> 定义为用户名后跟推文的全部文本作为返回值，并假设推文内容已经被限制为 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(&amp;self) -&gt; 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(&amp;self) -&gt; String {
format!("{}, by {} ({})", self.headline, self.author, self.location)
}
}
pub struct Tweet {
pub username: String,
pub content: String,
pub reply: bool,
pub retweet: bool,
}
impl Summary for Tweet {
fn summarize(&amp;self) -&gt; String {
format!("{}: {}", self.username, self.content)
}
}</code></pre>
<p><span class="caption">示例 10-13：在 <code>NewsArticle</code> 和 <code>Tweet</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>Tweet</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>Tweet</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::{Summary, Tweet};
fn main() {
let tweet = Tweet {
username: String::from("horse_ebooks"),
content: String::from(
"of course, as you probably already know, people",
),
reply: false,
retweet: false,
};
println!("1 new tweet: {}", tweet.summarize());
}</code></pre>
<p>这会打印出 <code>1 new tweet: 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>Tweet</code> 实现如标准库中的 <code>Display</code> trait，这是因为 <code>Tweet</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&lt;T&gt;</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&lt;T&gt;</code> 实现 <code>Display</code> trait。这是因为 <code>Display</code> 和 <code>Vec&lt;T&gt;</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(&amp;self) -&gt; 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 Tweet {
</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 retweet: bool,
</span><span class="boring">}
</span><span class="boring">
</span><span class="boring">impl Summary for Tweet {
</span><span class="boring"> fn summarize(&amp;self) -&gt; 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>Tweet</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(&amp;self) -&gt; String;
fn summarize(&amp;self) -&gt; String {
format!("(Read more from {}...)", self.summarize_author())
}
}
<span class="boring">
</span><span class="boring">pub struct Tweet {
</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 retweet: bool,
</span><span class="boring">}
</span><span class="boring">
</span><span class="boring">impl Summary for Tweet {
</span><span class="boring"> fn summarize_author(&amp;self) -&gt; 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(&amp;self) -&gt; String;
</span><span class="boring">
</span><span class="boring"> fn summarize(&amp;self) -&gt; 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 Tweet {
</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 retweet: bool,
</span><span class="boring">}
</span><span class="boring">
</span>impl Summary for Tweet {
fn summarize_author(&amp;self) -&gt; String {
format!("@{}", self.username)
}
}</code></pre>
<p>一旦定义了 <code>summarize_author</code>，我们就可以对 <code>Tweet</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, Summary, Tweet};
</span><span class="boring">
</span><span class="boring">fn main() {
</span> let tweet = Tweet {
username: String::from("horse_ebooks"),
content: String::from(
"of course, as you probably already know, people",
),
reply: false,
retweet: false,
};
println!("1 new tweet: {}", tweet.summarize());
<span class="boring">}</span></code></pre>
<p>这会打印出 <code>1 new tweet: (Read more from @horse_ebooks...)</code>。</p>
<p>注意无法从相同方法的重载实现中调用默认方法。</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>Tweet</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(&amp;self) -&gt; 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(&amp;self) -&gt; 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 Tweet {
</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 retweet: bool,
</span><span class="boring">}
</span><span class="boring">
</span><span class="boring">impl Summary for Tweet {
</span><span class="boring"> fn summarize(&amp;self) -&gt; 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: &amp;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>Tweet</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&lt;T: Summary&gt;(item: &amp;T) {
println!("Breaking news! {}", item.summarize());
}</code></pre>
<p>这与之前的例子相同，不过稍微冗长了一些。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: &amp;impl Summary, item2: &amp;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？这只有在使用 trait bound 时才有可能：</p>
<pre><code class="language-rust ignore">pub fn notify&lt;T: Summary&gt;(item1: &amp;T, item2: &amp;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>如果 <code>notify</code> 需要显示 <code>item</code> 的格式化形式，同时也要使用 <code>summarize</code> 方法，那么 <code>item</code> 就需要同时实现两个不同的 trait：<code>Display</code> 和 <code>Summary</code>。这可以通过 <code>+</code> 语法实现：</p>
<pre><code class="language-rust ignore">pub fn notify(item: &amp;(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&lt;T: Summary + Display&gt;(item: &amp;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&lt;T: Display + Clone, U: Clone + Debug&gt;(t: &amp;T, u: &amp;U) -&gt; 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&lt;T, U&gt;(t: &amp;T, u: &amp;U) -&gt; 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(&amp;self) -&gt; 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(&amp;self) -&gt; 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 Tweet {
</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 retweet: bool,
</span><span class="boring">}
</span><span class="boring">
</span><span class="boring">impl Summary for Tweet {
</span><span class="boring"> fn summarize(&amp;self) -&gt; 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() -&gt; impl Summary {
Tweet {
username: String::from("horse_ebooks"),
content: String::from(
"of course, as you probably already know, people",
),
reply: false,
retweet: 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>Tweet</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>Tweet</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(&amp;self) -&gt; 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(&amp;self) -&gt; 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 Tweet {
</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 retweet: bool,
</span><span class="boring">}
</span><span class="boring">
</span><span class="boring">impl Summary for Tweet {
</span><span class="boring"> fn summarize(&amp;self) -&gt; 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) -&gt; 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 {
Tweet {
username: String::from("horse_ebooks"),
content: String::from(
"of course, as you probably already know, people",
),
reply: false,
retweet: false,
}
}
}</code></pre>
<p>这里尝试返回 <code>NewsArticle</code> 或 <code>Tweet</code>。这不能编译，因为 <code>impl Trait</code> 工作方式的限制。第十八章的 <a href="ch18-02-trait-objects.html#%E9%A1%BE%E5%8F%8A%E4%B8%8D%E5%90%8C%E7%B1%BB%E5%9E%8B%E5%80%BC%E7%9A%84-trait-%E5%AF%B9%E8%B1%A1">“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&lt;T&gt;</code> 总是实现了 <code>new</code> 方法并返回一个 <code>Pair&lt;T&gt;</code> 的实例（回忆一下第五章的 <a href="ch05-03-method-syntax.html#%E5%AE%9A%E4%B9%89%E6%96%B9%E6%B3%95">“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&lt;T&gt;</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&lt;T&gt;</code> 才会实现 <code>cmp_display</code> 方法：</p>
<pre><code class="language-rust noplayground">use std::fmt::Display;
struct Pair&lt;T&gt; {
x: T,
y: T,
}
impl&lt;T&gt; Pair&lt;T&gt; {
fn new(x: T, y: T) -&gt; Self {
Self { x, y }
}
}
impl&lt;T: Display + PartialOrd&gt; Pair&lt;T&gt; {
fn cmp_display(&amp;self) {
if self.x &gt;= 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&lt;T: Display&gt; 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><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></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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