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<title>定义模块来控制作用域与私有性 - Rust 程序设计语言 简体中文版</title>
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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" class="active"><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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<h2 id="定义模块来控制作用域与私有性"><a class="header" href="#定义模块来控制作用域与私有性">定义模块来控制作用域与私有性</a></h2>
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<blockquote>
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<p><a href="https://github.com/rust-lang/book/blob/main/src/ch07-02-defining-modules-to-control-scope-and-privacy.md">ch07-02-defining-modules-to-control-scope-and-privacy.md</a>
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<br>
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commit 310ea6cb0dd855eaf510c9ba05648bc5836ead0c</p>
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</blockquote>
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<p>在本节，我们将讨论模块和其它一些关于模块系统的部分，如允许你命名项的 <em>路径</em>（<em>paths</em>）；用来将路径引入作用域的 <code>use</code> 关键字；以及使项变为公有的 <code>pub</code> 关键字。我们还将讨论 <code>as</code> 关键字、外部包和 glob 运算符。现在，让我们把注意力放在模块上！</p>
|
||
<p>首先，我们将从一系列的规则开始，在你未来组织代码的时候，这些规则可被用作简单的参考。接下来我们将会详细的解释每条规则。</p>
|
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<h2 id="模块小抄"><a class="header" href="#模块小抄">模块小抄</a></h2>
|
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<p>这里我们提供一个简单的参考，用来解释模块、路径、<code>use</code>关键词和<code>pub</code>关键词如何在编译器中工作，以及大部分开发者如何组织他们的代码。我们将在本章节中举例说明每条规则，不过这是一个解释模块工作方式的良好参考。</p>
|
||
<ul>
|
||
<li><strong>从 crate 根节点开始</strong>: 当编译一个 crate, 编译器首先在 crate 根文件（通常，对于一个库 crate 而言是<em>src/lib.rs</em>，对于一个二进制 crate 而言是<em>src/main.rs</em>）中寻找需要被编译的代码。</li>
|
||
<li><strong>声明模块</strong>: 在 crate 根文件中，你可以声明一个新模块；比如，你用<code>mod garden;</code>声明了一个叫做<code>garden</code>的模块。编译器会在下列路径中寻找模块代码：
|
||
<ul>
|
||
<li>内联，在大括号中，当<code>mod garden</code>后方不是一个分号而是一个大括号</li>
|
||
<li>在文件 <em>src/garden.rs</em></li>
|
||
<li>在文件 <em>src/garden/mod.rs</em></li>
|
||
</ul>
|
||
</li>
|
||
<li><strong>声明子模块</strong>: 在除了 crate 根节点以外的其他文件中，你可以定义子模块。比如，你可能在<em>src/garden.rs</em>中定义了<code>mod vegetables;</code>。编译器会在以父模块命名的目录中寻找子模块代码：
|
||
<ul>
|
||
<li>内联，在大括号中，当<code>mod vegetables</code>后方不是一个分号而是一个大括号</li>
|
||
<li>在文件 <em>src/garden/vegetables.rs</em></li>
|
||
<li>在文件 <em>src/garden/vegetables/mod.rs</em></li>
|
||
</ul>
|
||
</li>
|
||
<li><strong>模块中的代码路径</strong>: 一旦一个模块是你 crate 的一部分，你可以在隐私规则允许的前提下，从同一个 crate 内的任意地方，通过代码路径引用该模块的代码。举例而言，一个 garden vegetables 模块下的<code>Asparagus</code>类型可以在<code>crate::garden::vegetables::Asparagus</code>被找到。</li>
|
||
<li><strong>私有 vs 公用</strong>: 一个模块里的代码默认对其父模块私有。为了使一个模块公用，应当在声明时使用<code>pub mod</code>替代<code>mod</code>。为了使一个公用模块内部的成员公用，应当在声明前使用<code>pub</code>。</li>
|
||
<li><strong><code>use</code> 关键字</strong>: 在一个作用域内，<code>use</code>关键字创建了一个成员的快捷方式，用来减少长路径的重复。在任何可以引用<code>crate::garden::vegetables::Asparagus</code>的作用域，你可以通过 <code>use crate::garden::vegetables::Asparagus;</code>创建一个快捷方式，然后你就可以在作用域中只写<code>Asparagus</code>来使用该类型。</li>
|
||
</ul>
|
||
<p>这里我们创建一个名为<code>backyard</code>的二进制 crate 来说明这些规则。该 crate 的路径同样命名为<code>backyard</code>，该路径包含了这些文件和目录：</p>
|
||
<pre><code class="language-text">backyard
|
||
├── Cargo.lock
|
||
├── Cargo.toml
|
||
└── src
|
||
├── garden
|
||
│ └── vegetables.rs
|
||
├── garden.rs
|
||
└── main.rs
|
||
</code></pre>
|
||
<p>这个例子中的 crate 根文件是<em>src/main.rs</em>，该文件包括了：</p>
|
||
<p><span class="filename">文件名：src/main.rs</span></p>
|
||
<pre><code class="language-rust noplayground ignore">use crate::garden::vegetables::Asparagus;
|
||
|
||
pub mod garden;
|
||
|
||
fn main() {
|
||
let plant = Asparagus {};
|
||
println!("I'm growing {plant:?}!");
|
||
}</code></pre>
|
||
<p><code>pub mod garden;</code>行告诉编译器应该包含在<em>src/garden.rs</em>文件中发现的代码：</p>
|
||
<p><span class="filename">文件名：src/garden.rs</span></p>
|
||
<pre><code class="language-rust noplayground ignore">pub mod vegetables;</code></pre>
|
||
<p>在此处， <code>pub mod vegetables;</code>意味着在<em>src/garden/vegetables.rs</em>中的代码也应该被包括。这些代码是：</p>
|
||
<pre><code class="language-rust noplayground ignore">#[derive(Debug)]
|
||
pub struct Asparagus {}</code></pre>
|
||
<p>现在让我们深入了解这些规则的细节并在实际中演示它们！</p>
|
||
<h3 id="在模块中对相关代码进行分组"><a class="header" href="#在模块中对相关代码进行分组">在模块中对相关代码进行分组</a></h3>
|
||
<p><em>模块</em> 让我们可以将一个 crate 中的代码进行分组，以提高可读性与重用性。因为一个模块中的代码默认是私有的，所以还可以利用模块控制项的 <em>私有性</em>。私有项是不可为外部使用的内在详细实现。我们也可以将模块和它其中的项标记为公开的，这样，外部代码就可以使用并依赖与它们。</p>
|
||
<p>在餐饮业，餐馆中会有一些地方被称之为 <em>前台</em>（<em>front of house</em>），还有另外一些地方被称之为 <em>后台</em>（<em>back of house</em>）。前台是招待顾客的地方，在这里，店主可以为顾客安排座位，服务员接受顾客下单和付款，调酒师会制作饮品。后台则是由厨师工作的厨房，洗碗工的工作地点，以及经理做行政工作的地方组成。</p>
|
||
<p>我们可以将函数放置到嵌套的模块中，来使我们的 crate 结构与实际的餐厅结构相同。通过执行 <code>cargo new --lib restaurant</code>，来创建一个新的名为 <code>restaurant</code> 的库。然后将示例 7-1 中所罗列出来的代码放入 <em>src/lib.rs</em> 中，来定义一些模块和函数。</p>
|
||
<p><span class="filename">文件名：src/lib.rs</span></p>
|
||
<pre><code class="language-rust noplayground">mod front_of_house {
|
||
mod hosting {
|
||
fn add_to_waitlist() {}
|
||
|
||
fn seat_at_table() {}
|
||
}
|
||
|
||
mod serving {
|
||
fn take_order() {}
|
||
|
||
fn serve_order() {}
|
||
|
||
fn take_payment() {}
|
||
}
|
||
}</code></pre>
|
||
<p><span class="caption">示例 7-1：一个包含了其他内置了函数的模块的 <code>front_of_house</code> 模块</span></p>
|
||
<p>我们定义一个模块，是以 <code>mod</code> 关键字为起始，然后指定模块的名字（本例中叫做 <code>front_of_house</code>），并且用花括号包围模块的主体。在模块内，我们还可以定义其他的模块，就像本例中的 <code>hosting</code> 和 <code>serving</code> 模块。模块还可以保存一些定义的其他项，比如结构体、枚举、常量、特性、或者函数。</p>
|
||
<p>通过使用模块，我们可以将相关的定义分组到一起，并指出它们为什么相关。程序员可以通过使用这段代码，更加容易地找到他们想要的定义，因为他们可以基于分组来对代码进行导航，而不需要阅读所有的定义。程序员向这段代码中添加一个新的功能时，他们也会知道代码应该放置在何处，可以保持程序的组织性。</p>
|
||
<p>在前面我们提到了，<code>src/main.rs</code> 和 <code>src/lib.rs</code> 叫做 crate 根。之所以这样叫它们是因为这两个文件的内容都分别在 crate 模块结构的根组成了一个名为 <code>crate</code> 的模块，该结构被称为 <em>模块树</em>（<em>module tree</em>）。</p>
|
||
<p>示例 7-2 展示了示例 7-1 中的模块树的结构。</p>
|
||
<pre><code class="language-text">crate
|
||
└── front_of_house
|
||
├── hosting
|
||
│ ├── add_to_waitlist
|
||
│ └── seat_at_table
|
||
└── serving
|
||
├── take_order
|
||
├── serve_order
|
||
└── take_payment
|
||
</code></pre>
|
||
<p><span class="caption">示例 7-2: 示例 7-1 中代码的模块树</span></p>
|
||
<p>这个树展示了一些模块是如何被嵌入到另一个模块的（例如，<code>hosting</code> 嵌套在 <code>front_of_house</code> 中）。这个树还展示了一些模块是互为 <em>兄弟</em>（<em>siblings</em>）的，这意味着它们定义在同一模块中（<code>hosting</code> 和 <code>serving</code> 被一起定义在 <code>front_of_house</code> 中）。继续沿用家庭关系的比喻，如果一个模块 A 被包含在模块 B 中，我们将模块 A 称为模块 B 的 <em>子</em>（<em>child</em>），模块 B 则是模块 A 的 <em>父</em>（<em>parent</em>）。注意，整个模块树都植根于名为 <code>crate</code> 的隐式模块下。</p>
|
||
<p>这个模块树可能会令你想起电脑上文件系统的目录树；这是一个非常恰当的类比！就像文件系统的目录，你可以使用模块来组织你的代码。并且，就像目录中的文件，我们需要一种方法来找到模块。</p>
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