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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"><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" class="active"><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="高级类型"><a class="header" href="#高级类型">高级类型</a></h2>
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<blockquote>
|
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<p><a href="https://github.com/rust-lang/book/blob/main/src/ch20-04-advanced-types.md">ch20-04-advanced-types.md</a>
|
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<br>
|
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commit 95e931170404cb98d476b19017cbbdbc00d0834d</p>
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</blockquote>
|
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<p>Rust 的类型系统有一些我们曾经提到但没有讨论过的功能。首先我们从一个关于为什么 newtype 与类型一样有用的更宽泛的讨论开始。接着会转向类型别名（type aliases），一个类似于 newtype 但有着稍微不同的语义的功能。我们还会讨论 <code>!</code> 类型和动态大小类型。</p>
|
||
<blockquote>
|
||
<p>这一部分假设你已经阅读了之前的 <a href="ch20-03-advanced-traits.html#newtype-%E6%A8%A1%E5%BC%8F%E7%94%A8%E4%BB%A5%E5%9C%A8%E5%A4%96%E9%83%A8%E7%B1%BB%E5%9E%8B%E4%B8%8A%E5%AE%9E%E7%8E%B0%E5%A4%96%E9%83%A8-trait">“newtype 模式用于在外部类型上实现外部 trait”</a> 部分。</p>
|
||
</blockquote>
|
||
<h3 id="为了类型安全和抽象而使用-newtype-模式"><a class="header" href="#为了类型安全和抽象而使用-newtype-模式">为了类型安全和抽象而使用 newtype 模式</a></h3>
|
||
<p>newtype 模式也可以用于一些其他我们还未讨论的功能，包括静态的确保某值不被混淆，和用来表示一个值的单位。实际上示例 19-15 中已经有一个这样的例子：<code>Millimeters</code> 和 <code>Meters</code> 结构体都在 newtype 中封装了 <code>u32</code> 值。如果编写了一个有 <code>Millimeters</code> 类型参数的函数，不小心使用 <code>Meters</code> 或普通的 <code>u32</code> 值来调用该函数的程序是不能编译的。</p>
|
||
<p>newtype 模式也可以用于抽象掉一些类型的实现细节：例如，封装类型可以暴露出与直接使用其内部私有类型时所不同的公有 API。</p>
|
||
<p>newtype 也可以隐藏其内部的泛型类型。例如，可以提供一个封装了 <code>HashMap<i32, String></code> 的 <code>People</code> 类型，用来储存人名以及相应的 ID。使用 <code>People</code> 的代码只需与提供的公有 API 交互即可，比如向 <code>People</code> 集合增加名字字符串的方法，这样这些代码就无需知道在内部我们将一个 <code>i32</code> ID 赋予了这个名字了。newtype 模式是一种实现第十八章 <a href="ch18-01-what-is-oo.html#%E5%B0%81%E8%A3%85%E9%9A%90%E8%97%8F%E4%BA%86%E5%AE%9E%E7%8E%B0%E7%BB%86%E8%8A%82">“封装隐藏了实现细节”</a> 部分所讨论的隐藏实现细节的封装的轻量级方法。</p>
|
||
<h3 id="类型别名用来创建类型同义词"><a class="header" href="#类型别名用来创建类型同义词">类型别名用来创建类型同义词</a></h3>
|
||
<p>Rust 提供了声明 <strong>类型别名</strong>（<em>type alias</em>）的能力，使用 <code>type</code> 关键字来给予现有类型另一个名字。例如，可以像这样创建 <code>i32</code> 的别名 <code>Kilometers</code>：</p>
|
||
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">fn main() {
|
||
</span> type Kilometers = i32;
|
||
<span class="boring">
|
||
</span><span class="boring"> let x: i32 = 5;
|
||
</span><span class="boring"> let y: Kilometers = 5;
|
||
</span><span class="boring">
|
||
</span><span class="boring"> println!("x + y = {}", x + y);
|
||
</span><span class="boring">}</span></code></pre></pre>
|
||
<p>这意味着 <code>Kilometers</code> 是 <code>i32</code> 的 <strong>同义词</strong>（<em>synonym</em>）；不同于示例 19-15 中创建的 <code>Millimeters</code> 和 <code>Meters</code> 类型。<code>Kilometers</code> 不是一个新的、单独的类型。<code>Kilometers</code> 类型的值将被完全当作 <code>i32</code> 类型值来对待：</p>
|
||
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">fn main() {
|
||
</span> type Kilometers = i32;
|
||
|
||
let x: i32 = 5;
|
||
let y: Kilometers = 5;
|
||
|
||
println!("x + y = {}", x + y);
|
||
<span class="boring">}</span></code></pre></pre>
|
||
<p>因为 <code>Kilometers</code> 是 <code>i32</code> 的别名，它们是同一类型，可以将 <code>i32</code> 与 <code>Kilometers</code> 相加，也可以将 <code>Kilometers</code> 传递给获取 <code>i32</code> 参数的函数。但通过这种手段无法获得上一部分讨论的 newtype 模式所提供的类型检查的好处。换句话说，如果在哪里混用 <code>Kilometers</code> 和 <code>i32</code> 的值，编译器也不会给出一个错误。</p>
|
||
<p>类型别名的主要用途是减少重复。例如，可能会有这样很长的类型：</p>
|
||
<pre><code class="language-rust ignore">Box<dyn Fn() + Send + 'static></code></pre>
|
||
<p>在函数签名或类型注解中每次都书写这个类型将是枯燥且易于出错的。想象一下如示例 19-24 这样全是如此代码的项目：</p>
|
||
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">fn main() {
|
||
</span> let f: Box<dyn Fn() + Send + 'static> = Box::new(|| println!("hi"));
|
||
|
||
fn takes_long_type(f: Box<dyn Fn() + Send + 'static>) {
|
||
// --snip--
|
||
}
|
||
|
||
fn returns_long_type() -> Box<dyn Fn() + Send + 'static> {
|
||
// --snip--
|
||
<span class="boring"> Box::new(|| ())
|
||
</span> }
|
||
<span class="boring">}</span></code></pre></pre>
|
||
<p><span class="caption">示例 19-24: 在很多地方使用名称很长的类型</span></p>
|
||
<p>类型别名通过减少项目中重复代码的数量来使其更加易于控制。这里我们为这个冗长的类型引入了一个叫做 <code>Thunk</code> 的别名，这样就可以如示例 19-25 所示将所有使用这个类型的地方替换为更短的 <code>Thunk</code>：</p>
|
||
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">fn main() {
|
||
</span> type Thunk = Box<dyn Fn() + Send + 'static>;
|
||
|
||
let f: Thunk = Box::new(|| println!("hi"));
|
||
|
||
fn takes_long_type(f: Thunk) {
|
||
// --snip--
|
||
}
|
||
|
||
fn returns_long_type() -> Thunk {
|
||
// --snip--
|
||
<span class="boring"> Box::new(|| ())
|
||
</span> }
|
||
<span class="boring">}</span></code></pre></pre>
|
||
<p><span class="caption">示例 19-25: 引入类型别名 <code>Thunk</code> 来减少重复</span></p>
|
||
<p>这样读写起来就容易多了！为类型别名选择一个好名字也可以帮助你表达意图（单词 <em>thunk</em> 表示会在之后被计算的代码，所以这是一个存放闭包的合适的名字）。</p>
|
||
<p>类型别名也经常与 <code>Result<T, E></code> 结合使用来减少重复。考虑一下标准库中的 <code>std::io</code> 模块。I/O 操作通常会返回一个 <code>Result<T, E></code>，因为这些操作可能会失败。标准库中的 <code>std::io::Error</code> 结构体代表了所有可能的 I/O 错误。<code>std::io</code> 中大部分函数会返回 <code>Result<T, E></code>，其中 <code>E</code> 是 <code>std::io::Error</code>，比如 <code>Write</code> trait 中的这些函数：</p>
|
||
<pre><code class="language-rust noplayground">use std::fmt;
|
||
use std::io::Error;
|
||
|
||
pub trait Write {
|
||
fn write(&mut self, buf: &[u8]) -> Result<usize, Error>;
|
||
fn flush(&mut self) -> Result<(), Error>;
|
||
|
||
fn write_all(&mut self, buf: &[u8]) -> Result<(), Error>;
|
||
fn write_fmt(&mut self, fmt: fmt::Arguments) -> Result<(), Error>;
|
||
}</code></pre>
|
||
<p>这里出现了很多的 <code>Result<..., Error></code>。为此，<code>std::io</code> 有这个类型别名声明：</p>
|
||
<pre><code class="language-rust noplayground"><span class="boring">use std::fmt;
|
||
</span><span class="boring">
|
||
</span>type Result<T> = std::result::Result<T, std::io::Error>;
|
||
<span class="boring">
|
||
</span><span class="boring">pub trait Write {
|
||
</span><span class="boring"> fn write(&mut self, buf: &[u8]) -> Result<usize>;
|
||
</span><span class="boring"> fn flush(&mut self) -> Result<()>;
|
||
</span><span class="boring">
|
||
</span><span class="boring"> fn write_all(&mut self, buf: &[u8]) -> Result<()>;
|
||
</span><span class="boring"> fn write_fmt(&mut self, fmt: fmt::Arguments) -> Result<()>;
|
||
</span><span class="boring">}</span></code></pre>
|
||
<p>因为这位于 <code>std::io</code> 中，可用的完全限定的别名是 <code>std::io::Result<T></code>；也就是说，<code>Result<T, E></code> 中 <code>E</code> 放入了 <code>std::io::Error</code>。<code>Write</code> trait 中的函数最终看起来像这样：</p>
|
||
<pre><code class="language-rust noplayground"><span class="boring">use std::fmt;
|
||
</span><span class="boring">
|
||
</span><span class="boring">type Result<T> = std::result::Result<T, std::io::Error>;
|
||
</span><span class="boring">
|
||
</span>pub trait Write {
|
||
fn write(&mut self, buf: &[u8]) -> Result<usize>;
|
||
fn flush(&mut self) -> Result<()>;
|
||
|
||
fn write_all(&mut self, buf: &[u8]) -> Result<()>;
|
||
fn write_fmt(&mut self, fmt: fmt::Arguments) -> Result<()>;
|
||
}</code></pre>
|
||
<p>类型别名在两个方面有帮助：易于编写 <strong>并</strong> 在整个 <code>std::io</code> 中提供了一致的接口。因为这是一个别名，它只是另一个 <code>Result<T, E></code>，这意味着可以在其上使用 <code>Result<T, E></code> 的任何方法，以及像 <code>?</code> 这样的特殊语法。</p>
|
||
<h3 id="从不返回的-never-type"><a class="header" href="#从不返回的-never-type">从不返回的 never type</a></h3>
|
||
<p>Rust 有一个叫做 <code>!</code> 的特殊类型。在类型理论术语中，它被称为 <em>empty type</em>，因为它没有值。我们更倾向于称之为 <em>never type</em>。这个名字描述了它的作用：在函数从不返回的时候充当返回值。例如：</p>
|
||
<pre><code class="language-rust noplayground">fn bar() -> ! {
|
||
// --snip--
|
||
<span class="boring"> panic!();
|
||
</span>}</code></pre>
|
||
<p>这读 “函数 <code>bar</code> 从不返回”，而从不返回的函数被称为 <strong>发散函数</strong>（<em>diverging functions</em>）。不能创建 <code>!</code> 类型的值，所以 <code>bar</code> 也不可能返回值。</p>
|
||
<p>不过一个不能创建值的类型有什么用呢？如果你回想一下示例 2-5 中的代码，曾经有一些看起来像这样的代码，如示例 19-26 所重现的：</p>
|
||
<pre><code class="language-rust ignore"><span class="boring">use rand::Rng;
|
||
</span><span class="boring">use std::cmp::Ordering;
|
||
</span><span class="boring">use std::io;
|
||
</span><span class="boring">
|
||
</span><span class="boring">fn main() {
|
||
</span><span class="boring"> println!("Guess the number!");
|
||
</span><span class="boring">
|
||
</span><span class="boring"> let secret_number = rand::thread_rng().gen_range(1..=100);
|
||
</span><span class="boring">
|
||
</span><span class="boring"> println!("The secret number is: {secret_number}");
|
||
</span><span class="boring">
|
||
</span><span class="boring"> loop {
|
||
</span><span class="boring"> println!("Please input your guess.");
|
||
</span><span class="boring">
|
||
</span><span class="boring"> let mut guess = String::new();
|
||
</span><span class="boring">
|
||
</span><span class="boring"> // --snip--
|
||
</span><span class="boring">
|
||
</span><span class="boring"> io::stdin()
|
||
</span><span class="boring"> .read_line(&mut guess)
|
||
</span><span class="boring"> .expect("Failed to read line");
|
||
</span><span class="boring">
|
||
</span> let guess: u32 = match guess.trim().parse() {
|
||
Ok(num) => num,
|
||
Err(_) => continue,
|
||
};
|
||
<span class="boring">
|
||
</span><span class="boring"> println!("You guessed: {guess}");
|
||
</span><span class="boring">
|
||
</span><span class="boring"> // --snip--
|
||
</span><span class="boring">
|
||
</span><span class="boring"> match guess.cmp(&secret_number) {
|
||
</span><span class="boring"> Ordering::Less => println!("Too small!"),
|
||
</span><span class="boring"> Ordering::Greater => println!("Too big!"),
|
||
</span><span class="boring"> Ordering::Equal => {
|
||
</span><span class="boring"> println!("You win!");
|
||
</span><span class="boring"> break;
|
||
</span><span class="boring"> }
|
||
</span><span class="boring"> }
|
||
</span><span class="boring"> }
|
||
</span><span class="boring">}</span></code></pre>
|
||
<p><span class="caption">示例 19-26: <code>match</code> 语句和一个以 <code>continue</code> 结束的分支</span></p>
|
||
<p>当时我们忽略了代码中的一些细节。在第六章 <a href="ch06-02-match.html#match-%E6%8E%A7%E5%88%B6%E6%B5%81%E7%BB%93%E6%9E%84">“<code>match</code> 控制流运算符”</a> 部分，我们学习了 <code>match</code> 的分支必须返回相同的类型。如下代码不能工作：</p>
|
||
<pre><code class="language-rust ignore does_not_compile"><span class="boring">fn main() {
|
||
</span><span class="boring"> let guess = "3";
|
||
</span> let guess = match guess.trim().parse() {
|
||
Ok(_) => 5,
|
||
Err(_) => "hello",
|
||
};
|
||
<span class="boring">}</span></code></pre>
|
||
<p>这里的 <code>guess</code> 必须既是整型 <strong>也是</strong> 字符串，而 Rust 要求 <code>guess</code> 只能是一个类型。那么 <code>continue</code> 返回了什么呢？为什么示例 19-26 中会允许一个分支返回 <code>u32</code> 而另一个分支却以 <code>continue</code> 结束呢？</p>
|
||
<p>正如你可能猜到的，<code>continue</code> 的值是 <code>!</code>。也就是说，当 Rust 要计算 <code>guess</code> 的类型时，它查看这两个分支。前者是 <code>u32</code> 值，而后者是 <code>!</code> 值。因为 <code>!</code> 并没有一个值，Rust 决定 <code>guess</code> 的类型是 <code>u32</code>。</p>
|
||
<p>描述 <code>!</code> 的行为的正式方式是 never type 可以强转为任何其他类型。允许 <code>match</code> 的分支以 <code>continue</code> 结束是因为 <code>continue</code> 并不真正返回一个值；相反它把控制权交回上层循环，所以在 <code>Err</code> 的情况，事实上并未对 <code>guess</code> 赋值。</p>
|
||
<p>never type 的另一个用途是 <code>panic!</code>。还记得 <code>Option<T></code> 上的 <code>unwrap</code> 函数吗？它产生一个值或 panic。这里是它的定义：</p>
|
||
<pre><code class="language-rust ignore"><span class="boring">enum Option<T> {
|
||
</span><span class="boring"> Some(T),
|
||
</span><span class="boring"> None,
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span><span class="boring">use crate::Option::*;
|
||
</span><span class="boring">
|
||
</span>impl<T> Option<T> {
|
||
pub fn unwrap(self) -> T {
|
||
match self {
|
||
Some(val) => val,
|
||
None => panic!("called `Option::unwrap()` on a `None` value"),
|
||
}
|
||
}
|
||
}</code></pre>
|
||
<p>这里与示例 19-34 中的 <code>match</code> 发生了相同的情况：Rust 知道 <code>val</code> 是 <code>T</code> 类型，<code>panic!</code> 是 <code>!</code> 类型，所以整个 <code>match</code> 表达式的结果是 <code>T</code> 类型。这能工作是因为 <code>panic!</code> 并不产生一个值；它会终止程序。对于 <code>None</code> 的情况，<code>unwrap</code> 并不返回一个值，所以这些代码是有效的。</p>
|
||
<p>最后一个有着 <code>!</code> 类型的表达式是 <code>loop</code>：</p>
|
||
<pre><code class="language-rust ignore"><span class="boring">fn main() {
|
||
</span> print!("forever ");
|
||
|
||
loop {
|
||
print!("and ever ");
|
||
}
|
||
<span class="boring">}</span></code></pre>
|
||
<p>这里，循环永远也不结束，所以此表达式的值是 <code>!</code>。但是如果引入 <code>break</code> 这就不为真了，因为循环在执行到 <code>break</code> 后就会终止。</p>
|
||
<h3 id="动态大小类型和-sized-trait"><a class="header" href="#动态大小类型和-sized-trait">动态大小类型和 <code>Sized</code> trait</a></h3>
|
||
<p>Rust 需要知道有关类型的某些细节，例如为特定类型的值需要分配多少空间。这便是起初留下的一个类型系统中令人迷惑的角落：即 <strong>动态大小类型</strong>（<em>dynamically sized types</em>）。这有时被称为 “DST” 或 “unsized types”，这些类型允许我们处理只有在运行时才知道大小的类型。</p>
|
||
<p>让我们深入研究一个贯穿本书都在使用的动态大小类型的细节：<code>str</code>。没错，不是 <code>&str</code>，而是 <code>str</code> 本身。<code>str</code> 是一个 DST；直到运行时我们都不知道字符串有多长。因为直到运行时都不能知道其大小，也就意味着不能创建 <code>str</code> 类型的变量，也不能获取 <code>str</code> 类型的参数。考虑一下这些代码，它们不能工作：</p>
|
||
<pre><code class="language-rust ignore does_not_compile"><span class="boring">fn main() {
|
||
</span> let s1: str = "Hello there!";
|
||
let s2: str = "How's it going?";
|
||
<span class="boring">}</span></code></pre>
|
||
<p>Rust 需要知道应该为特定类型的值分配多少内存，同时所有同一类型的值必须使用相同数量的内存。如果允许编写这样的代码，也就意味着这两个 <code>str</code> 需要占用完全相同大小的空间，不过它们有着不同的长度。这也就是为什么不可能创建一个存放动态大小类型的变量的原因。</p>
|
||
<p>那么该怎么办呢？你已经知道了这种问题的答案：<code>s1</code> 和 <code>s2</code> 的类型是 <code>&str</code> 而不是 <code>str</code>。如果你回想第四章 <a href="ch04-03-slices.html#%E5%AD%97%E7%AC%A6%E4%B8%B2-slice">“字符串 slice”</a> 部分，slice 数据结构仅仅储存了开始位置和 slice 的长度。所以虽然 <code>&T</code> 是一个储存了 <code>T</code> 所在的内存位置的单个值，<code>&str</code> 则是 <strong>两个</strong> 值：<code>str</code> 的地址和其长度。这样，<code>&str</code> 就有了一个在编译时可以知道的大小：它是 <code>usize</code> 长度的两倍。也就是说，我们总是知道 <code>&str</code> 的大小，而无论其引用的字符串是多长。这里是 Rust 中动态大小类型的常规用法：它们有一些额外的元信息来储存动态信息的大小。这引出了动态大小类型的黄金规则：必须将动态大小类型的值置于某种指针之后。</p>
|
||
<p>可以将 <code>str</code> 与所有类型的指针结合：比如 <code>Box<str></code> 或 <code>Rc<str></code>。事实上，之前我们已经见过了，不过是另一个动态大小类型：trait。每一个 trait 都是一个可以通过 trait 名称来引用的动态大小类型。在第十八章 <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，我们提到了为了将 trait 用于 trait 对象，必须将它们放入指针之后，比如 <code>&dyn Trait</code> 或 <code>Box<dyn Trait></code>（<code>Rc<dyn Trait></code> 也可以）。</p>
|
||
<p>为了处理 DST，Rust 提供了 <code>Sized</code> trait 来决定一个类型的大小是否在编译时可知。这个 trait 自动为编译器在编译时就知道大小的类型实现。另外，Rust 隐式的为每一个泛型函数增加了 <code>Sized</code> bound。也就是说，对于如下泛型函数定义：</p>
|
||
<pre><code class="language-rust ignore">fn generic<T>(t: T) {
|
||
// --snip--
|
||
}</code></pre>
|
||
<p>实际上被当作如下处理：</p>
|
||
<pre><code class="language-rust ignore">fn generic<T: Sized>(t: T) {
|
||
// --snip--
|
||
}</code></pre>
|
||
<p>泛型函数默认只能用于在编译时已知大小的类型。然而可以使用如下特殊语法来放宽这个限制：</p>
|
||
<pre><code class="language-rust ignore">fn generic<T: ?Sized>(t: &T) {
|
||
// --snip--
|
||
}</code></pre>
|
||
<p><code>?Sized</code> 上的 trait bound 意味着 “<code>T</code> 可能是也可能不是 <code>Sized</code>” 同时这个注解会覆盖泛型类型必须在编译时拥有固定大小的默认规则。这种意义的 <code>?Trait</code> 语法只能用于 <code>Sized</code> ，而不能用于任何其他 trait。</p>
|
||
<p>另外注意我们将 <code>t</code> 参数的类型从 <code>T</code> 变为了 <code>&T</code>：因为其类型可能不是 <code>Sized</code> 的，所以需要将其置于某种指针之后。在这个例子中选择了引用。</p>
|
||
<p>接下来，让我们讨论一下函数和闭包！</p>
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