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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" class="active"><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&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="使用-use-关键字将路径引入作用域"><a class="header" href="#使用-use-关键字将路径引入作用域">使用 <code>use</code> 关键字将路径引入作用域</a></h2>
<blockquote>
<p><a href="https://github.com/rust-lang/book/blob/main/src/ch07-04-bringing-paths-into-scope-with-the-use-keyword.md">ch07-04-bringing-paths-into-scope-with-the-use-keyword.md</a>
<br>
commit c77d7a1279dbc7a9d76e80c5ac9d742dd529538c</p>
</blockquote>
<p>不得不编写路径来调用函数显得不便且重复。在示例 7-7 中，无论我们选择 <code>add_to_waitlist</code> 函数的绝对路径还是相对路径，每次我们想要调用 <code>add_to_waitlist</code> 时，都必须指定<code>front_of_house</code> 和 <code>hosting</code>。幸运的是，有一种方法可以简化这个过程。我们可以使用 <code>use</code> 关键字创建一个短路径，然后就可以在作用域中的任何地方使用这个更短的名字。</p>
<p>在示例 7-11 中，我们将 <code>crate::front_of_house::hosting</code> 模块引入了 <code>eat_at_restaurant</code> 函数的作用域，而我们只需要指定 <code>hosting::add_to_waitlist</code> 即可在 <code>eat_at_restaurant</code> 中调用 <code>add_to_waitlist</code> 函数。</p>
<p><span class="filename">文件名：src/lib.rs</span></p>
<pre><code class="language-rust noplayground test_harness">mod front_of_house {
pub mod hosting {
pub fn add_to_waitlist() {}
}
}
use crate::front_of_house::hosting;
pub fn eat_at_restaurant() {
hosting::add_to_waitlist();
}</code></pre>
<p><span class="caption">示例 7-11: 使用 <code>use</code> 将模块引入作用域</span></p>
<p>在作用域中增加 <code>use</code> 和路径类似于在文件系统中创建软连接（符号连接，symbolic link）。通过在 crate 根增加 <code>use crate::front_of_house::hosting</code>，现在 <code>hosting</code> 在作用域中就是有效的名称了，如同 <code>hosting</code> 模块被定义于 crate 根一样。通过 <code>use</code> 引入作用域的路径也会检查私有性，同其它路径一样。</p>
<p>注意 <code>use</code> 只能创建 <code>use</code> 所在的特定作用域内的短路径。示例 7-12 将 <code>eat_at_restaurant</code> 函数移动到了一个叫 <code>customer</code> 的子模块，这又是一个不同于 <code>use</code> 语句的作用域，所以函数体不能编译。</p>
<p><span class="filename">文件名：src/lib.rs</span></p>
<pre><code class="language-rust noplayground test_harness does_not_compile ignore">mod front_of_house {
pub mod hosting {
pub fn add_to_waitlist() {}
}
}
use crate::front_of_house::hosting;
mod customer {
pub fn eat_at_restaurant() {
hosting::add_to_waitlist();
}
}</code></pre>
<p><span class="caption">示例 7-12: <code>use</code> 语句只适用于其所在的作用域</span></p>
<p>编译器错误显示短路径不再适用于 <code>customer</code> 模块中：</p>
<pre><code class="language-console">$ cargo build
Compiling restaurant v0.1.0 (file:///projects/restaurant)
error[E0433]: failed to resolve: use of undeclared crate or module `hosting`
--&gt; src/lib.rs:11:9
|
11 | hosting::add_to_waitlist();
| ^^^^^^^ use of undeclared crate or module `hosting`
|
help: consider importing this module through its public re-export
|
10 + use crate::hosting;
|
warning: unused import: `crate::front_of_house::hosting`
--&gt; src/lib.rs:7:5
|
7 | use crate::front_of_house::hosting;
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: `#[warn(unused_imports)]` on by default
For more information about this error, try `rustc --explain E0433`.
warning: `restaurant` (lib) generated 1 warning
error: could not compile `restaurant` (lib) due to 1 previous error; 1 warning emitted
</code></pre>
<p>注意这里还有一个警告说 <code>use</code> 在其作用域内不再被使用！为了修复这个问题，可以将 <code>use</code> 移动到 <code>customer</code> 模块内，或者在子模块 <code>customer</code> 内通过 <code>super::hosting</code> 引用父模块中的这个短路径。</p>
<h3 id="创建惯用的-use-路径"><a class="header" href="#创建惯用的-use-路径">创建惯用的 <code>use</code> 路径</a></h3>
<p>在示例 7-11 中，你可能会比较疑惑，为什么我们是指定 <code>use crate::front_of_house::hosting</code> ，然后在 <code>eat_at_restaurant</code> 中调用 <code>hosting::add_to_waitlist</code> ，而不是通过指定一直到 <code>add_to_waitlist</code> 函数的 <code>use</code> 路径来得到相同的结果，如示例 7-13 所示。</p>
<p><span class="filename">文件名：src/lib.rs</span></p>
<pre><code class="language-rust noplayground test_harness">mod front_of_house {
pub mod hosting {
pub fn add_to_waitlist() {}
}
}
use crate::front_of_house::hosting::add_to_waitlist;
pub fn eat_at_restaurant() {
add_to_waitlist();
}</code></pre>
<p><span class="caption">示例 7-13: 使用 <code>use</code> 将 <code>add_to_waitlist</code> 函数引入作用域，这并不符合习惯</span></p>
<p>虽然示例 7-11 和 7-13 都完成了相同的任务，但示例 7-11 是使用 <code>use</code> 将函数引入作用域的习惯用法。要想使用 <code>use</code> 将函数的父模块引入作用域，我们必须在调用函数时指定父模块，这样可以清晰地表明函数不是在本地定义的，同时使完整路径的重复度最小化。示例 7-13 中的代码不清楚 <code>add_to_waitlist</code> 是在哪里被定义的。</p>
<p>另一方面，使用 <code>use</code> 引入结构体、枚举和其他项时，习惯是指定它们的完整路径。示例 7-14 展示了将 <code>HashMap</code> 结构体引入二进制 crate 作用域的习惯用法。</p>
<p><span class="filename">文件名：src/main.rs</span></p>
<pre><pre class="playground"><code class="language-rust edition2021">use std::collections::HashMap;
fn main() {
let mut map = HashMap::new();
map.insert(1, 2);
}</code></pre></pre>
<p><span class="caption">示例 7-14: 将 <code>HashMap</code> 引入作用域的习惯用法</span></p>
<p>这种习惯用法背后没有什么硬性要求：它只是一种惯例，人们已经习惯了以这种方式阅读和编写 Rust 代码。</p>
<p>这个习惯用法有一个例外，那就是我们想使用 <code>use</code> 语句将两个具有相同名称的项带入作用域，因为 Rust 不允许这样做。示例 7-15 展示了如何将两个具有相同名称但不同父模块的 <code>Result</code> 类型引入作用域，以及如何引用它们。</p>
<p><span class="filename">文件名：src/lib.rs</span></p>
<pre><code class="language-rust noplayground">use std::fmt;
use std::io;
fn function1() -&gt; fmt::Result {
// --snip--
<span class="boring"> Ok(())
</span>}
fn function2() -&gt; io::Result&lt;()&gt; {
// --snip--
<span class="boring"> Ok(())
</span>}</code></pre>
<p><span class="caption">示例 7-15: 使用父模块将两个具有相同名称的类型引入同一作用域</span></p>
<p>如你所见，使用父模块可以区分这两个 <code>Result</code> 类型。如果我们是指定 <code>use std::fmt::Result</code> 和 <code>use std::io::Result</code>，我们将在同一作用域拥有了两个 <code>Result</code> 类型，当我们使用 <code>Result</code> 时，Rust 则不知道我们要用的是哪个。</p>
<h3 id="使用-as-关键字提供新的名称"><a class="header" href="#使用-as-关键字提供新的名称">使用 <code>as</code> 关键字提供新的名称</a></h3>
<p>使用 <code>use</code> 将两个同名类型引入同一作用域这个问题还有另一个解决办法：在这个类型的路径后面，我们使用 <code>as</code> 指定一个新的本地名称或者别名。示例 7-16 展示了另一个编写示例 7-15 中代码的方法，通过 <code>as</code> 重命名其中一个 <code>Result</code> 类型。</p>
<p><span class="filename">文件名：src/lib.rs</span></p>
<pre><code class="language-rust noplayground">use std::fmt::Result;
use std::io::Result as IoResult;
fn function1() -&gt; Result {
// --snip--
<span class="boring"> Ok(())
</span>}
fn function2() -&gt; IoResult&lt;()&gt; {
// --snip--
<span class="boring"> Ok(())
</span>}</code></pre>
<p><span class="caption">示例 7-16: 使用 <code>as</code> 关键字重命名引入作用域的类型</span></p>
<p>在第二个 <code>use</code> 语句中，我们选择 <code>IoResult</code> 作为 <code>std::io::Result</code> 的新名称，它与从 <code>std::fmt</code> 引入作用域的 <code>Result</code> 并不冲突。示例 7-15 和示例 7-16 都是惯用的，如何选择都取决于你！</p>
<h3 id="使用-pub-use-重导出名称"><a class="header" href="#使用-pub-use-重导出名称">使用 <code>pub use</code> 重导出名称</a></h3>
<p>使用 <code>use</code> 关键字，将某个名称导入当前作用域后，这个名称在此作用域中就可以使用了，但它对此作用域之外还是私有的。如果想让其他人调用我们的代码时，也能够正常使用这个名称，就好像它本来就在当前作用域一样，那我们可以将 <code>pub</code> 和 <code>use</code> 合起来使用。这种技术被称为 “<em>重导出</em>（<em>re-exporting</em>）”：我们不仅将一个名称导入了当前作用域，还允许别人把它导入他们自己的作用域。</p>
<p>示例 7-17 将示例 7-11 根模块中的 <code>use</code> 改为 <code>pub use</code> 。</p>
<p><span class="filename">文件名：src/lib.rs</span></p>
<pre><code class="language-rust noplayground test_harness">mod front_of_house {
pub mod hosting {
pub fn add_to_waitlist() {}
}
}
pub use crate::front_of_house::hosting;
pub fn eat_at_restaurant() {
hosting::add_to_waitlist();
}</code></pre>
<p><span class="caption">示例 7-17: 通过 <code>pub use</code> 使名称可从新作用域中被导入至任何代码</span></p>
<p>在这个修改之前，外部代码需要使用路径 <code>restaurant::front_of_house::hosting::add_to_waitlist()</code> 来调用 <code>add_to_waitlist</code> 函数。现在这个 <code>pub use</code> 从根模块重导出了 <code>hosting</code> 模块，外部代码现在可以使用路径 <code>restaurant::hosting::add_to_waitlist</code>。</p>
<p>当你代码的内部结构与调用你代码的程序员所想象的结构不同时，重导出会很有用。例如，在这个餐馆的比喻中，经营餐馆的人会想到“前台”和“后台”。但顾客在光顾一家餐馆时，可能不会以这些术语来考虑餐馆的各个部分。使用 <code>pub use</code>，我们可以使用一种结构编写代码，却将不同的结构形式暴露出来。这样做使我们的库井井有条，也使开发这个库的程序员和调用这个库的程序员都更加方便。在<a href="ch14-02-publishing-to-crates-io.html#%E4%BD%BF%E7%94%A8-pub-use-%E5%AF%BC%E5%87%BA%E5%90%88%E9%80%82%E7%9A%84%E5%85%AC%E6%9C%89-api">“使用 <code>pub use</code> 导出合适的公有 API”</a>部分让我们再看另一个 <code>pub use</code> 的例子来了解这如何影响 crate 的文档。</p>
<h3 id="使用外部包"><a class="header" href="#使用外部包">使用外部包</a></h3>
<p>在第二章中我们编写了一个猜猜看游戏。那个项目使用了一个外部包，<code>rand</code>，来生成随机数。为了在项目中使用 <code>rand</code>，在 <em>Cargo.toml</em> 中加入了如下行：</p>
<p><span class="filename">文件名：Cargo.toml</span></p>
<pre><code class="language-toml">rand = "0.8.5"
</code></pre>
<p>在 <em>Cargo.toml</em> 中加入 <code>rand</code> 依赖告诉了 Cargo 要从 <a href="https://crates.io">crates.io</a> 下载 <code>rand</code> 和其依赖，并使其可在项目代码中使用。</p>
<p>接着，为了将 <code>rand</code> 定义引入项目包的作用域，我们加入一行 <code>use</code> 起始的包名，它以 <code>rand</code> 包名开头并列出了需要引入作用域的项。回忆一下第二章的 “生成一个随机数” 部分，我们曾将 <code>Rng</code> trait 引入作用域并调用了 <code>rand::thread_rng</code> 函数：</p>
<pre><code class="language-rust ignore"><span class="boring">use std::io;
</span>use rand::Rng;
fn main() {
<span class="boring"> println!("Guess the number!");
</span><span class="boring">
</span> let secret_number = rand::thread_rng().gen_range(1..=100);
<span class="boring">
</span><span class="boring"> println!("The secret number is: {secret_number}");
</span><span class="boring">
</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"> io::stdin()
</span><span class="boring"> .read_line(&amp;mut guess)
</span><span class="boring"> .expect("Failed to read line");
</span><span class="boring">
</span><span class="boring"> println!("You guessed: {guess}");
</span>}</code></pre>
<p><a href="https://crates.io">crates.io</a> 上有很多 Rust 社区成员发布的包，将其引入你自己的项目都需要一道相同的步骤：在 <em>Cargo.toml</em> 列出它们并通过 <code>use</code> 将其中定义的项引入项目包的作用域中。</p>
<p>注意 <code>std</code> 标准库对于你的包来说也是外部 crate。因为标准库随 Rust 语言一同分发，无需修改 <em>Cargo.toml</em> 来引入 <code>std</code>，不过需要通过 <code>use</code> 将标准库中定义的项引入项目包的作用域中来引用它们，比如我们使用的 <code>HashMap</code>：</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>use std::collections::HashMap;
<span class="boring">}</span></code></pre></pre>
<p>这是一个以标准库 crate 名 <code>std</code> 开头的绝对路径。</p>
<h3 id="嵌套路径来消除大量的-use-行"><a class="header" href="#嵌套路径来消除大量的-use-行">嵌套路径来消除大量的 <code>use</code> 行</a></h3>
<p>当需要引入很多定义于相同包或相同模块的项时，为每一项单独列出一行会占用源码很大的空间。例如猜猜看章节示例 2-4 中有两行 <code>use</code> 语句都从 <code>std</code> 引入项到作用域：</p>
<p><span class="filename">文件名：src/main.rs</span></p>
<pre><code class="language-rust ignore"><span class="boring">use rand::Rng;
</span>// --snip--
use std::cmp::Ordering;
use std::io;
// --snip--
<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"> 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"> io::stdin()
</span><span class="boring"> .read_line(&amp;mut guess)
</span><span class="boring"> .expect("Failed to read line");
</span><span class="boring">
</span><span class="boring"> println!("You guessed: {guess}");
</span><span class="boring">
</span><span class="boring"> match guess.cmp(&amp;secret_number) {
</span><span class="boring"> Ordering::Less =&gt; println!("Too small!"),
</span><span class="boring"> Ordering::Greater =&gt; println!("Too big!"),
</span><span class="boring"> Ordering::Equal =&gt; println!("You win!"),
</span><span class="boring"> }
</span><span class="boring">}</span></code></pre>
<p>相反，我们可以使用嵌套路径将相同的项在一行中引入作用域。这么做需要指定路径的相同部分，接着是两个冒号，接着是大括号中的各自不同的路径部分，如示例 7-18 所示。</p>
<p><span class="filename">文件名：src/main.rs</span></p>
<pre><code class="language-rust ignore"><span class="boring">use rand::Rng;
</span>// --snip--
use std::{cmp::Ordering, io};
// --snip--
<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"> 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"> io::stdin()
</span><span class="boring"> .read_line(&amp;mut guess)
</span><span class="boring"> .expect("Failed to read line");
</span><span class="boring">
</span><span class="boring"> let guess: u32 = guess.trim().parse().expect("Please type a number!");
</span><span class="boring">
</span><span class="boring"> println!("You guessed: {guess}");
</span><span class="boring">
</span><span class="boring"> match guess.cmp(&amp;secret_number) {
</span><span class="boring"> Ordering::Less =&gt; println!("Too small!"),
</span><span class="boring"> Ordering::Greater =&gt; println!("Too big!"),
</span><span class="boring"> Ordering::Equal =&gt; println!("You win!"),
</span><span class="boring"> }
</span><span class="boring">}</span></code></pre>
<p><span class="caption">示例 7-18: 指定嵌套的路径在一行中将多个带有相同前缀的项引入作用域</span></p>
<p>在较大的程序中，使用嵌套路径从相同包或模块中引入很多项，可以显著减少所需的独立 <code>use</code> 语句的数量！</p>
<p>我们可以在路径的任何层级使用嵌套路径，这在组合两个共享子路径的 <code>use</code> 语句时非常有用。例如，示例 7-19 中展示了两个 <code>use</code> 语句：一个将 <code>std::io</code> 引入作用域，另一个将 <code>std::io::Write</code> 引入作用域：</p>
<p><span class="filename">文件名：src/lib.rs</span></p>
<pre><code class="language-rust noplayground">use std::io;
use std::io::Write;</code></pre>
<p><span class="caption">示例 7-19: 通过两行 <code>use</code> 语句引入两个路径，其中一个是另一个的子路径</span></p>
<p>两个路径的相同部分是 <code>std::io</code>，这正是第一个路径。为了在一行 <code>use</code> 语句中引入这两个路径，可以在嵌套路径中使用 <code>self</code>，如示例 7-20 所示。</p>
<p><span class="filename">文件名：src/lib.rs</span></p>
<pre><code class="language-rust noplayground">use std::io::{self, Write};</code></pre>
<p><span class="caption">示例 7-20: 将示例 7-19 中部分重复的路径合并为一个 <code>use</code> 语句</span></p>
<p>这一行便将 <code>std::io</code> 和 <code>std::io::Write</code> 同时引入作用域。</p>
<h3 id="通过-glob-运算符将所有的公有定义引入作用域"><a class="header" href="#通过-glob-运算符将所有的公有定义引入作用域">通过 glob 运算符将所有的公有定义引入作用域</a></h3>
<p>如果希望将一个路径下 <strong>所有</strong> 公有项引入作用域，可以指定路径后跟 <code>*</code>，glob 运算符：</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>use std::collections::*;
<span class="boring">}</span></code></pre></pre>
<p>这个 <code>use</code> 语句将 <code>std::collections</code> 中定义的所有公有项引入当前作用域。使用 glob 运算符时请多加小心！Glob 会使得我们难以推导作用域中有什么名称和它们是在何处定义的。</p>
<p>glob 运算符经常用于测试模块 <code>tests</code> 中，这时会将所有内容引入作用域；我们将在第十一章 “如何编写测试” 部分讲解。glob 运算符有时也用于 prelude 模式；查看 <a href="https://doc.rust-lang.org/std/prelude/index.html#other-preludes">标准库中的文档</a> 了解这个模式的更多细节。</p>
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