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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" class="active"><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"><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<T> 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<T> 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<T> 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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<main>
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<h2 id="match-控制流结构"><a class="header" href="#match-控制流结构"><code>match</code> 控制流结构</a></h2>
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<blockquote>
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<p><a href="https://github.com/rust-lang/book/blob/main/src/ch06-02-match.md">ch06-02-match.md</a>
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<br>
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commit 3962c0224b274e2358e0acf06443af64df115359</p>
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</blockquote>
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<p>Rust 有一个叫做 <code>match</code> 的极为强大的控制流运算符，它允许我们将一个值与一系列的模式相比较，并根据相匹配的模式执行相应代码。模式可由字面值、变量、通配符和许多其他内容构成；<a href="ch19-00-patterns.html">第十九章</a>会涉及到所有不同种类的模式以及它们的作用。<code>match</code> 的力量来源于模式的表现力以及编译器检查，它确保了所有可能的情况都得到处理。</p>
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|
<p>可以把 <code>match</code> 表达式想象成某种硬币分类器：硬币滑入有着不同大小孔洞的轨道，每一个硬币都会掉入符合它大小的孔洞。同样地，值也会通过 <code>match</code> 的每一个模式，并且在遇到第一个 “符合” 的模式时，值会进入相关联的代码块并在执行中被使用。</p>
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<p>因为刚刚提到了硬币，让我们用它们来作为一个使用 <code>match</code> 的例子！我们可以编写一个函数来获取一个未知的硬币，并以一种类似验钞机的方式，确定它是何种硬币并返回它的美分值，如示例 6-3 中所示。</p>
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">enum Coin {
|
|
|
Penny,
|
|
|
Nickel,
|
|
|
Dime,
|
|
|
Quarter,
|
|
|
}
|
|
|
|
|
|
fn value_in_cents(coin: Coin) -> u8 {
|
|
|
match coin {
|
|
|
Coin::Penny => 1,
|
|
|
Coin::Nickel => 5,
|
|
|
Coin::Dime => 10,
|
|
|
Coin::Quarter => 25,
|
|
|
}
|
|
|
}
|
|
|
<span class="boring">
|
|
|
</span><span class="boring">fn main() {}</span></code></pre></pre>
|
|
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<p><span class="caption">示例 6-3：一个枚举和一个以枚举成员作为模式的 <code>match</code> 表达式</span></p>
|
|
|
<p>拆开 <code>value_in_cents</code> 函数中的 <code>match</code> 来看。首先，我们列出 <code>match</code> 关键字后跟一个表达式，在这个例子中是 <code>coin</code> 的值。这看起来非常像 <code>if</code> 所使用的条件表达式，不过这里有一个非常大的区别：对于 <code>if</code>，表达式必须返回一个布尔值，而这里它可以是任何类型的。例子中的 <code>coin</code> 的类型是示例 6-3 中定义的 <code>Coin</code> 枚举。</p>
|
|
|
<p>接下来是 <code>match</code> 的分支。一个分支有两个部分：一个模式和一些代码。第一个分支的模式是值 <code>Coin::Penny</code> 而之后的 <code>=></code> 运算符将模式和将要运行的代码分开。这里的代码就仅仅是值 <code>1</code>。每一个分支之间使用逗号分隔。</p>
|
|
|
<p>当 <code>match</code> 表达式执行时，它将结果值按顺序与每一个分支的模式相比较。如果模式匹配了这个值，这个模式相关联的代码将被执行。如果模式并不匹配这个值，将继续执行下一个分支，非常类似一个硬币分类器。可以拥有任意多的分支：示例 6-3 中的 <code>match</code> 有四个分支。</p>
|
|
|
<p>每个分支相关联的代码是一个表达式，而表达式的结果值将作为整个 <code>match</code> 表达式的返回值。</p>
|
|
|
<p>如果分支代码较短的话通常不使用大括号，正如示例 6-3 中的每个分支都只是返回一个值。如果想要在分支中运行多行代码，可以使用大括号，而分支后的逗号是可选的。例如，如下代码在每次使用<code>Coin::Penny</code> 调用时都会打印出 “Lucky penny!”，同时仍然返回代码块最后的值，<code>1</code>：</p>
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">enum Coin {
|
|
|
</span><span class="boring"> Penny,
|
|
|
</span><span class="boring"> Nickel,
|
|
|
</span><span class="boring"> Dime,
|
|
|
</span><span class="boring"> Quarter,
|
|
|
</span><span class="boring">}
|
|
|
</span><span class="boring">
|
|
|
</span>fn value_in_cents(coin: Coin) -> u8 {
|
|
|
match coin {
|
|
|
Coin::Penny => {
|
|
|
println!("Lucky penny!");
|
|
|
1
|
|
|
}
|
|
|
Coin::Nickel => 5,
|
|
|
Coin::Dime => 10,
|
|
|
Coin::Quarter => 25,
|
|
|
}
|
|
|
}
|
|
|
<span class="boring">
|
|
|
</span><span class="boring">fn main() {}</span></code></pre></pre>
|
|
|
<h3 id="绑定值的模式"><a class="header" href="#绑定值的模式">绑定值的模式</a></h3>
|
|
|
<p>匹配分支的另一个有用的功能是可以绑定匹配的模式的部分值。这也就是如何从枚举成员中提取值的。</p>
|
|
|
<p>作为一个例子，让我们修改枚举的一个成员来存放数据。1999 年到 2008 年间，美国在 25 美分的硬币的一侧为 50 个州的每一个都印刷了不同的设计。其他的硬币都没有这种区分州的设计，所以只有这些 25 美分硬币有特殊的价值。可以将这些信息加入我们的 <code>enum</code>，通过改变 <code>Quarter</code> 成员来包含一个 <code>State</code> 值，示例 6-4 中完成了这些修改：</p>
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021">#[derive(Debug)] // 这样可以立刻看到州的名称
|
|
|
enum UsState {
|
|
|
Alabama,
|
|
|
Alaska,
|
|
|
// --snip--
|
|
|
}
|
|
|
|
|
|
enum Coin {
|
|
|
Penny,
|
|
|
Nickel,
|
|
|
Dime,
|
|
|
Quarter(UsState),
|
|
|
}
|
|
|
<span class="boring">
|
|
|
</span><span class="boring">fn main() {}</span></code></pre></pre>
|
|
|
<p><span class="caption">示例 6-4：<code>Quarter</code> 成员也存放了一个 <code>UsState</code> 值的 <code>Coin</code> 枚举</span></p>
|
|
|
<p>想象一下我们的一个朋友尝试收集所有 50 个州的 25 美分硬币。在根据硬币类型分类零钱的同时，也可以报告出每个 25 美分硬币所对应的州名称，这样如果我们的朋友没有的话，他可以将其加入收藏。</p>
|
|
|
<p>在这些代码的匹配表达式中，我们在匹配 <code>Coin::Quarter</code> 成员的分支的模式中增加了一个叫做 <code>state</code> 的变量。当匹配到 <code>Coin::Quarter</code> 时，变量 <code>state</code> 将会绑定 25 美分硬币所对应州的值。接着在那个分支的代码中使用 <code>state</code>，如下：</p>
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#[derive(Debug)]
|
|
|
</span><span class="boring">enum UsState {
|
|
|
</span><span class="boring"> Alabama,
|
|
|
</span><span class="boring"> Alaska,
|
|
|
</span><span class="boring"> // --snip--
|
|
|
</span><span class="boring">}
|
|
|
</span><span class="boring">
|
|
|
</span><span class="boring">enum Coin {
|
|
|
</span><span class="boring"> Penny,
|
|
|
</span><span class="boring"> Nickel,
|
|
|
</span><span class="boring"> Dime,
|
|
|
</span><span class="boring"> Quarter(UsState),
|
|
|
</span><span class="boring">}
|
|
|
</span><span class="boring">
|
|
|
</span>fn value_in_cents(coin: Coin) -> u8 {
|
|
|
match coin {
|
|
|
Coin::Penny => 1,
|
|
|
Coin::Nickel => 5,
|
|
|
Coin::Dime => 10,
|
|
|
Coin::Quarter(state) => {
|
|
|
println!("State quarter from {state:?}!");
|
|
|
25
|
|
|
}
|
|
|
}
|
|
|
}
|
|
|
<span class="boring">
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span><span class="boring"> value_in_cents(Coin::Quarter(UsState::Alaska));
|
|
|
</span><span class="boring">}</span></code></pre></pre>
|
|
|
<p>如果调用 <code>value_in_cents(Coin::Quarter(UsState::Alaska))</code>，<code>coin</code> 将是 <code>Coin::Quarter(UsState::Alaska)</code>。当将值与每个分支相比较时，没有分支会匹配，直到遇到 <code>Coin::Quarter(state)</code>。这时，<code>state</code> 绑定的将会是值 <code>UsState::Alaska</code>。接着就可以在 <code>println!</code> 表达式中使用这个绑定了，像这样就可以获取 <code>Coin</code> 枚举的 <code>Quarter</code> 成员中内部的州的值。</p>
|
|
|
<h3 id="匹配-optiont"><a class="header" href="#匹配-optiont">匹配 <code>Option<T></code></a></h3>
|
|
|
<p>我们在之前的部分中使用 <code>Option<T></code> 时，是为了从 <code>Some</code> 中取出其内部的 <code>T</code> 值；我们还可以像处理 <code>Coin</code> 枚举那样使用 <code>match</code> 处理 <code>Option<T></code>！只不过这回比较的不再是硬币，而是 <code>Option<T></code> 的成员，但 <code>match</code> 表达式的工作方式保持不变。</p>
|
|
|
<p>比如我们想要编写一个函数，它获取一个 <code>Option<i32></code> ，如果其中含有一个值，将其加一。如果其中没有值，函数应该返回 <code>None</code> 值，而不尝试执行任何操作。</p>
|
|
|
<p>得益于 <code>match</code>，编写这个函数非常简单，它将看起来像示例 6-5 中这样：</p>
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">fn main() {
|
|
|
</span> fn plus_one(x: Option<i32>) -> Option<i32> {
|
|
|
match x {
|
|
|
None => None,
|
|
|
Some(i) => Some(i + 1),
|
|
|
}
|
|
|
}
|
|
|
|
|
|
let five = Some(5);
|
|
|
let six = plus_one(five);
|
|
|
let none = plus_one(None);
|
|
|
<span class="boring">}</span></code></pre></pre>
|
|
|
<p><span class="caption">示例 6-5：一个在 <code>Option<i32></code> 上使用 <code>match</code> 表达式的函数</span></p>
|
|
|
<h4 id="匹配-somet"><a class="header" href="#匹配-somet">匹配 <code>Some(T)</code></a></h4>
|
|
|
<p>让我们更仔细地检查 <code>plus_one</code> 的第一行操作。当调用 <code>plus_one(five)</code> 时，<code>plus_one</code> 函数体中的 <code>x</code> 将会是值 <code>Some(5)</code>。接着将其与每个分支比较。</p>
|
|
|
<pre><code class="language-rust ignore"><span class="boring">fn main() {
|
|
|
</span><span class="boring"> fn plus_one(x: Option<i32>) -> Option<i32> {
|
|
|
</span><span class="boring"> match x {
|
|
|
</span> None => None,
|
|
|
<span class="boring"> Some(i) => Some(i + 1),
|
|
|
</span><span class="boring"> }
|
|
|
</span><span class="boring"> }
|
|
|
</span><span class="boring">
|
|
|
</span><span class="boring"> let five = Some(5);
|
|
|
</span><span class="boring"> let six = plus_one(five);
|
|
|
</span><span class="boring"> let none = plus_one(None);
|
|
|
</span><span class="boring">}</span></code></pre>
|
|
|
<p>值 <code>Some(5)</code> 并不匹配模式 <code>None</code>，所以继续进行下一个分支。</p>
|
|
|
<pre><code class="language-rust ignore"><span class="boring">fn main() {
|
|
|
</span><span class="boring"> fn plus_one(x: Option<i32>) -> Option<i32> {
|
|
|
</span><span class="boring"> match x {
|
|
|
</span><span class="boring"> None => None,
|
|
|
</span> Some(i) => Some(i + 1),
|
|
|
<span class="boring"> }
|
|
|
</span><span class="boring"> }
|
|
|
</span><span class="boring">
|
|
|
</span><span class="boring"> let five = Some(5);
|
|
|
</span><span class="boring"> let six = plus_one(five);
|
|
|
</span><span class="boring"> let none = plus_one(None);
|
|
|
</span><span class="boring">}</span></code></pre>
|
|
|
<p><code>Some(5)</code> 与 <code>Some(i)</code> 匹配吗？当然匹配！它们是相同的成员。<code>i</code> 绑定了 <code>Some</code> 中包含的值，所以 <code>i</code> 的值是 <code>5</code>。接着匹配分支的代码被执行，所以我们将 <code>i</code> 的值加一并返回一个含有值 <code>6</code> 的新 <code>Some</code>。</p>
|
|
|
<p>接着考虑下示例 6-5 中 <code>plus_one</code> 的第二个调用，这里 <code>x</code> 是 <code>None</code>。我们进入 <code>match</code> 并与第一个分支相比较。</p>
|
|
|
<pre><code class="language-rust ignore"><span class="boring">fn main() {
|
|
|
</span><span class="boring"> fn plus_one(x: Option<i32>) -> Option<i32> {
|
|
|
</span><span class="boring"> match x {
|
|
|
</span> None => None,
|
|
|
<span class="boring"> Some(i) => Some(i + 1),
|
|
|
</span><span class="boring"> }
|
|
|
</span><span class="boring"> }
|
|
|
</span><span class="boring">
|
|
|
</span><span class="boring"> let five = Some(5);
|
|
|
</span><span class="boring"> let six = plus_one(five);
|
|
|
</span><span class="boring"> let none = plus_one(None);
|
|
|
</span><span class="boring">}</span></code></pre>
|
|
|
<p>匹配上了！这里没有值来加一，所以程序结束并返回 <code>=></code> 右侧的值 <code>None</code>，因为第一个分支就匹配到了，其他的分支将不再比较。</p>
|
|
|
<p>将 <code>match</code> 与枚举相结合在很多场景中都是有用的。你会在 Rust 代码中看到很多这样的模式：<code>match</code> 一个枚举，绑定其中的值到一个变量，接着根据其值执行代码。这在一开始有点复杂，不过一旦习惯了，你会希望所有语言都拥有它！这一直是用户的最爱。</p>
|
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|
<h3 id="匹配是穷尽的"><a class="header" href="#匹配是穷尽的">匹配是穷尽的</a></h3>
|
|
|
<p><code>match</code> 还有另一方面需要讨论：这些分支必须覆盖了所有的可能性。考虑一下 <code>plus_one</code> 函数的这个版本，它有一个 bug 并不能编译：</p>
|
|
|
<pre><code class="language-rust ignore does_not_compile"><span class="boring">fn main() {
|
|
|
</span> fn plus_one(x: Option<i32>) -> Option<i32> {
|
|
|
match x {
|
|
|
Some(i) => Some(i + 1),
|
|
|
}
|
|
|
}
|
|
|
<span class="boring">
|
|
|
</span><span class="boring"> let five = Some(5);
|
|
|
</span><span class="boring"> let six = plus_one(five);
|
|
|
</span><span class="boring"> let none = plus_one(None);
|
|
|
</span><span class="boring">}</span></code></pre>
|
|
|
<p>我们没有处理 <code>None</code> 的情况，所以这些代码会造成一个 bug。幸运的是，这是一个 Rust 知道如何处理的 bug。如果尝试编译这段代码，会得到这个错误：</p>
|
|
|
<pre><code class="language-console">$ cargo run
|
|
|
Compiling enums v0.1.0 (file:///projects/enums)
|
|
|
error[E0004]: non-exhaustive patterns: `None` not covered
|
|
|
--> src/main.rs:3:15
|
|
|
|
|
|
|
3 | match x {
|
|
|
| ^ pattern `None` not covered
|
|
|
|
|
|
|
note: `Option<i32>` defined here
|
|
|
--> /rustc/eeb90cda1969383f56a2637cbd3037bdf598841c/library/core/src/option.rs:574:1
|
|
|
::: /rustc/eeb90cda1969383f56a2637cbd3037bdf598841c/library/core/src/option.rs:578:5
|
|
|
|
|
|
|
= note: not covered
|
|
|
= note: the matched value is of type `Option<i32>`
|
|
|
help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
|
|
|
|
|
|
|
4 ~ Some(i) => Some(i + 1),
|
|
|
5 ~ None => todo!(),
|
|
|
|
|
|
|
|
|
|
For more information about this error, try `rustc --explain E0004`.
|
|
|
error: could not compile `enums` (bin "enums") due to 1 previous error
|
|
|
</code></pre>
|
|
|
<p>Rust 知道我们没有覆盖所有可能的情况甚至知道哪些模式被忘记了！Rust 中的匹配是 <strong>穷尽的</strong>（<em>exhaustive</em>）：必须穷举到最后的可能性来使代码有效。特别的在这个 <code>Option<T></code> 的例子中，Rust 防止我们忘记明确的处理 <code>None</code> 的情况，这让我们免于假设拥有一个实际上为空的值，从而使之前提到的价值亿万的错误不可能发生。</p>
|
|
|
<h3 id="通配模式和-_-占位符"><a class="header" href="#通配模式和-_-占位符">通配模式和 <code>_</code> 占位符</a></h3>
|
|
|
<p>让我们看一个例子，我们希望对一些特定的值采取特殊操作，而对其他的值采取默认操作。想象我们正在玩一个游戏，如果你掷出骰子的值为 3，角色不会移动，而是会得到一顶新奇的帽子。如果你掷出了 7，你的角色将失去新奇的帽子。对于其他的数值，你的角色会在棋盘上移动相应的格子。这是一个实现了上述逻辑的 <code>match</code>，骰子的结果是硬编码而不是一个随机值，其他的逻辑部分使用了没有函数体的函数来表示，实现它们超出了本例的范围：</p>
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">fn main() {
|
|
|
</span> let dice_roll = 9;
|
|
|
match dice_roll {
|
|
|
3 => add_fancy_hat(),
|
|
|
7 => remove_fancy_hat(),
|
|
|
other => move_player(other),
|
|
|
}
|
|
|
|
|
|
fn add_fancy_hat() {}
|
|
|
fn remove_fancy_hat() {}
|
|
|
fn move_player(num_spaces: u8) {}
|
|
|
<span class="boring">}</span></code></pre></pre>
|
|
|
<p>对于前两个分支，匹配模式是字面值 <code>3</code> 和 <code>7</code>，最后一个分支则涵盖了所有其他可能的值，模式是我们命名为 <code>other</code> 的一个变量。<code>other</code> 分支的代码通过将其传递给 <code>move_player</code> 函数来使用这个变量。</p>
|
|
|
<p>即使我们没有列出 <code>u8</code> 所有可能的值，这段代码依然能够编译，因为最后一个模式将匹配所有未被特殊列出的值。这种通配模式满足了 <code>match</code> 必须被穷尽的要求。请注意，我们必须将通配分支放在最后，因为模式是按顺序匹配的。如果我们在通配分支后添加其他分支，Rust 将会警告我们，因为此后的分支永远不会被匹配到。</p>
|
|
|
<p>Rust 还提供了一个模式，当我们不想使用通配模式获取的值时，请使用 <code>_</code> ，这是一个特殊的模式，可以匹配任意值而不绑定到该值。这告诉 Rust 我们不会使用这个值，所以 Rust 也不会警告我们存在未使用的变量。</p>
|
|
|
<p>让我们改变游戏规则：现在，当你掷出的值不是 3 或 7 的时候，你必须再次掷出。这种情况下我们不需要使用这个值，所以我们改动代码使用 <code>_</code> 来替代变量 <code>other</code> ：</p>
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">fn main() {
|
|
|
</span> let dice_roll = 9;
|
|
|
match dice_roll {
|
|
|
3 => add_fancy_hat(),
|
|
|
7 => remove_fancy_hat(),
|
|
|
_ => reroll(),
|
|
|
}
|
|
|
|
|
|
fn add_fancy_hat() {}
|
|
|
fn remove_fancy_hat() {}
|
|
|
fn reroll() {}
|
|
|
<span class="boring">}</span></code></pre></pre>
|
|
|
<p>这个例子也满足穷举性要求，因为我们在最后一个分支中明确地忽略了其他的值。我们没有忘记处理任何东西。</p>
|
|
|
<p>最后，让我们再次改变游戏规则，如果你掷出 3 或 7 以外的值，你的回合将无事发生。我们可以使用单元值（在<a href="ch03-02-data-types.html#%E5%85%83%E7%BB%84%E7%B1%BB%E5%9E%8B">“元组类型”</a><!-- ignore -->一节中提到的空元组）作为 <code>_</code> 分支的代码：</p>
|
|
|
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">fn main() {
|
|
|
</span> let dice_roll = 9;
|
|
|
match dice_roll {
|
|
|
3 => add_fancy_hat(),
|
|
|
7 => remove_fancy_hat(),
|
|
|
_ => (),
|
|
|
}
|
|
|
|
|
|
fn add_fancy_hat() {}
|
|
|
fn remove_fancy_hat() {}
|
|
|
<span class="boring">}</span></code></pre></pre>
|
|
|
<p>在这里，我们明确告诉 Rust 我们不会使用与前面模式不匹配的值，并且这种情况下我们不想运行任何代码。</p>
|
|
|
<p>我们将在<a href="ch19-00-patterns.html">第十九章</a><!-- ignore -->中介绍更多关于模式和匹配的内容。现在，让我们继续讨论 <code>if let</code> 语法，这在 <code>match</code> 表达式有点啰嗦的情况下很有用。</p>
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