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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" class="active"><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&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="方法语法"><a class="header" href="#方法语法">方法语法</a></h2>
<blockquote>
<p><a href="https://github.com/rust-lang/book/blob/main/src/ch05-03-method-syntax.md">ch05-03-method-syntax.md</a>
<br>
commit d339373a838fd312a8a9bcc9487e1ffbc9e1582f</p>
</blockquote>
<p><strong>方法</strong>（method）与函数类似：它们使用 <code>fn</code> 关键字和名称声明，可以拥有参数和返回值，同时包含在某处调用该方法时会执行的代码。不过方法与函数是不同的，因为它们在结构体的上下文中被定义（或者是枚举或 trait 对象的上下文，将分别在<a href="ch06-00-enums.html">第六章</a>和<a href="ch18-02-trait-objects.html">第十八章</a>讲解），并且它们第一个参数总是 <code>self</code>，它代表调用该方法的结构体实例。</p>
<h3 id="定义方法"><a class="header" href="#定义方法">定义方法</a></h3>
<p>让我们把前面实现的获取一个 <code>Rectangle</code> 实例作为参数的 <code>area</code> 函数，改写成一个定义于 <code>Rectangle</code> 结构体上的 <code>area</code> 方法，如示例 5-13 所示：</p>
<p><span class="filename">文件名：src/main.rs</span></p>
<pre><pre class="playground"><code class="language-rust edition2021">#[derive(Debug)]
struct Rectangle {
width: u32,
height: u32,
}
impl Rectangle {
fn area(&amp;self) -&gt; u32 {
self.width * self.height
}
}
fn main() {
let rect1 = Rectangle {
width: 30,
height: 50,
};
println!(
"The area of the rectangle is {} square pixels.",
rect1.area()
);
}</code></pre></pre>
<p><span class="caption">示例 5-13：在 <code>Rectangle</code> 结构体上定义 <code>area</code> 方法</span></p>
<p>为了使函数定义于 <code>Rectangle</code> 的上下文中，我们开始了一个 <code>impl</code> 块（<code>impl</code> 是 <em>implementation</em> 的缩写），这个 <code>impl</code> 块中的所有内容都将与 <code>Rectangle</code> 类型相关联。接着将 <code>area</code> 函数移动到 <code>impl</code> 大括号中，并将签名中的第一个（在这里也是唯一一个）参数和函数体中其他地方的对应参数改成 <code>self</code>。然后在 <code>main</code> 中将我们先前调用 <code>area</code> 方法并传递 <code>rect1</code> 作为参数的地方，改成使用 <strong>方法语法</strong>（<em>method syntax</em>）在 <code>Rectangle</code> 实例上调用 <code>area</code> 方法。方法语法获取一个实例并加上一个点号，后跟方法名、圆括号以及任何参数。</p>
<p>在 <code>area</code> 的签名中，使用 <code>&amp;self</code> 来替代 <code>rectangle: &amp;Rectangle</code>，<code>&amp;self</code> 实际上是 <code>self: &amp;Self</code> 的缩写。在一个 <code>impl</code> 块中，<code>Self</code> 类型是 <code>impl</code> 块的类型的别名。方法的第一个参数必须有一个名为 <code>self</code> 的<code>Self</code> 类型的参数，所以 Rust 让你在第一个参数位置上只用 <code>self</code> 这个名字来简化。注意，我们仍然需要在 <code>self</code> 前面使用 <code>&amp;</code> 来表示这个方法借用了 <code>Self</code> 实例，就像我们在 <code>rectangle: &amp;Rectangle</code> 中做的那样。方法可以选择获得 <code>self</code> 的所有权，或者像我们这里一样不可变地借用 <code>self</code>，或者可变地借用 <code>self</code>，就跟其他参数一样。</p>
<p>这里选择 <code>&amp;self</code> 的理由跟在函数版本中使用 <code>&amp;Rectangle</code> 是相同的：我们并不想获取所有权，只希望能够读取结构体中的数据，而不是写入。如果想要在方法中改变调用方法的实例，需要将第一个参数改为 <code>&amp;mut self</code>。通过仅仅使用 <code>self</code> 作为第一个参数来使方法获取实例的所有权是很少见的；这种技术通常用在当方法将 <code>self</code> 转换成别的实例的时候，这时我们想要防止调用者在转换之后使用原始的实例。</p>
<p>使用方法替代函数，除了可使用方法语法和不需要在每个函数签名中重复 <code>self</code> 的类型之外，其主要好处在于组织性。我们将某个类型实例能做的所有事情都一起放入 <code>impl</code> 块中，而不是让将来的用户在我们的库中到处寻找 <code>Rectangle</code> 的功能。</p>
<p>请注意，我们可以选择将方法的名称与结构中的一个字段相同。例如，我们可以在 <code>Rectangle</code> 上定义一个方法，并命名为 <code>width</code>：</p>
<p><span class="filename">文件名：src/main.rs</span></p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#[derive(Debug)]
</span><span class="boring">struct Rectangle {
</span><span class="boring"> width: u32,
</span><span class="boring"> height: u32,
</span><span class="boring">}
</span><span class="boring">
</span>impl Rectangle {
fn width(&amp;self) -&gt; bool {
self.width &gt; 0
}
}
fn main() {
let rect1 = Rectangle {
width: 30,
height: 50,
};
if rect1.width() {
println!("The rectangle has a nonzero width; it is {}", rect1.width);
}
}</code></pre></pre>
<p>在这里，我们选择让 <code>width</code> 方法在实例的 <code>width</code> 字段的值大于 <code>0</code> 时返回 <code>true</code>，等于 <code>0</code> 时则返回 <code>false</code>：我们可以出于任何目的，在同名的方法中使用同名的字段。在 <code>main</code> 中，当我们在 <code>rect1.width</code> 后面加上括号时。Rust 知道我们指的是方法 <code>width</code>。当我们不使用圆括号时，Rust 知道我们指的是字段 <code>width</code>。</p>
<p>通常，但并不总是如此，与字段同名的方法将被定义为只返回字段中的值，而不做其他事情。这样的方法被称为 <em>getters</em>，Rust 并不像其他一些语言那样为结构字段自动实现它们。Getters 很有用，因为你可以把字段变成私有的，但方法是公共的，这样就可以把对字段的只读访问作为该类型公共 API 的一部分。我们将在<a href="ch07-03-paths-for-referring-to-an-item-in-the-module-tree.html#%E4%BD%BF%E7%94%A8-pub-%E5%85%B3%E9%94%AE%E5%AD%97%E6%9A%B4%E9%9C%B2%E8%B7%AF%E5%BE%84">第七章</a>中讨论什么是公有和私有，以及如何将一个字段或方法指定为公有或私有。</p>
<blockquote>
<h3 id="--运算符到哪去了"><a class="header" href="#--运算符到哪去了"><code>-&gt;</code> 运算符到哪去了？</a></h3>
<p>在 C/C++ 语言中，有两个不同的运算符来调用方法：<code>.</code> 直接在对象上调用方法，而 <code>-&gt;</code> 在一个对象的指针上调用方法，这时需要先解引用（dereference）指针。换句话说，如果 <code>object</code> 是一个指针，那么 <code>object-&gt;something()</code> 就像 <code>(*object).something()</code> 一样。</p>
<p>Rust 并没有一个与 <code>-&gt;</code> 等效的运算符；相反，Rust 有一个叫 <strong>自动引用和解引用</strong>（<em>automatic referencing and dereferencing</em>）的功能。方法调用是 Rust 中少数几个拥有这种行为的地方。</p>
<p>它是这样工作的：当使用 <code>object.something()</code> 调用方法时，Rust 会自动为 <code>object</code> 添加 <code>&amp;</code>、<code>&amp;mut</code> 或 <code>*</code> 以便使 <code>object</code> 与方法签名匹配。也就是说，这些代码是等价的：</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span><span class="boring">#[derive(Debug,Copy,Clone)]
</span><span class="boring">struct Point {
</span><span class="boring"> x: f64,
</span><span class="boring"> y: f64,
</span><span class="boring">}
</span><span class="boring">
</span><span class="boring">impl Point {
</span><span class="boring"> fn distance(&amp;self, other: &amp;Point) -&gt; f64 {
</span><span class="boring"> let x_squared = f64::powi(other.x - self.x, 2);
</span><span class="boring"> let y_squared = f64::powi(other.y - self.y, 2);
</span><span class="boring">
</span><span class="boring"> f64::sqrt(x_squared + y_squared)
</span><span class="boring"> }
</span><span class="boring">}
</span><span class="boring">let p1 = Point { x: 0.0, y: 0.0 };
</span><span class="boring">let p2 = Point { x: 5.0, y: 6.5 };
</span>p1.distance(&amp;p2);
(&amp;p1).distance(&amp;p2);
<span class="boring">}</span></code></pre></pre>
<p>第一行看起来简洁的多。这种自动引用的行为之所以有效，是因为方法有一个明确的接收者———— <code>self</code> 的类型。在给出接收者和方法名的前提下，Rust 可以明确地计算出方法是仅仅读取（<code>&amp;self</code>），做出修改（<code>&amp;mut self</code>）或者是获取所有权（<code>self</code>）。事实上，Rust 对方法接收者的隐式借用让所有权在实践中更友好。</p>
</blockquote>
<h3 id="带有更多参数的方法"><a class="header" href="#带有更多参数的方法">带有更多参数的方法</a></h3>
<p>让我们通过实现 <code>Rectangle</code> 结构体上的另一方法来练习使用方法。这回，我们让一个 <code>Rectangle</code> 的实例获取另一个 <code>Rectangle</code> 实例，如果 <code>self</code> （第一个 <code>Rectangle</code>）能完全包含第二个长方形则返回 <code>true</code>；否则返回 <code>false</code>。一旦我们定义了 <code>can_hold</code> 方法，就可以编写示例 5-14 中的代码。</p>
<p><span class="filename">文件名：src/main.rs</span></p>
<pre><code class="language-rust ignore">fn main() {
let rect1 = Rectangle {
width: 30,
height: 50,
};
let rect2 = Rectangle {
width: 10,
height: 40,
};
let rect3 = Rectangle {
width: 60,
height: 45,
};
println!("Can rect1 hold rect2? {}", rect1.can_hold(&amp;rect2));
println!("Can rect1 hold rect3? {}", rect1.can_hold(&amp;rect3));
}</code></pre>
<p><span class="caption">示例 5-14：使用还未实现的 <code>can_hold</code> 方法</span></p>
<p>同时我们希望看到如下输出，因为 <code>rect2</code> 的两个维度都小于 <code>rect1</code>，而 <code>rect3</code> 比 <code>rect1</code> 要宽：</p>
<pre><code class="language-text">Can rect1 hold rect2? true
Can rect1 hold rect3? false
</code></pre>
<p>因为我们想定义一个方法，所以它应该位于 <code>impl Rectangle</code> 块中。方法名是 <code>can_hold</code>，并且它会获取另一个 <code>Rectangle</code> 的不可变借用作为参数。通过观察调用方法的代码可以看出参数是什么类型的：<code>rect1.can_hold(&amp;rect2)</code> 传入了 <code>&amp;rect2</code>，它是一个 <code>Rectangle</code> 的实例 <code>rect2</code> 的不可变借用。这是可以理解的，因为我们只需要读取 <code>rect2</code>（而不是写入，这意味着我们需要一个不可变借用），而且希望 <code>main</code> 保持 <code>rect2</code> 的所有权，这样就可以在调用这个方法后继续使用它。<code>can_hold</code> 的返回值是一个布尔值，其实现会分别检查 <code>self</code> 的宽高是否都大于另一个 <code>Rectangle</code>。让我们在示例 5-13 的 <code>impl</code> 块中增加这个新的 <code>can_hold</code> 方法，如示例 5-15 所示：</p>
<p><span class="filename">文件名：src/main.rs</span></p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#[derive(Debug)]
</span><span class="boring">struct Rectangle {
</span><span class="boring"> width: u32,
</span><span class="boring"> height: u32,
</span><span class="boring">}
</span><span class="boring">
</span>impl Rectangle {
fn area(&amp;self) -&gt; u32 {
self.width * self.height
}
fn can_hold(&amp;self, other: &amp;Rectangle) -&gt; bool {
self.width &gt; other.width &amp;&amp; self.height &gt; other.height
}
}
<span class="boring">
</span><span class="boring">fn main() {
</span><span class="boring"> let rect1 = Rectangle {
</span><span class="boring"> width: 30,
</span><span class="boring"> height: 50,
</span><span class="boring"> };
</span><span class="boring"> let rect2 = Rectangle {
</span><span class="boring"> width: 10,
</span><span class="boring"> height: 40,
</span><span class="boring"> };
</span><span class="boring"> let rect3 = Rectangle {
</span><span class="boring"> width: 60,
</span><span class="boring"> height: 45,
</span><span class="boring"> };
</span><span class="boring">
</span><span class="boring"> println!("Can rect1 hold rect2? {}", rect1.can_hold(&amp;rect2));
</span><span class="boring"> println!("Can rect1 hold rect3? {}", rect1.can_hold(&amp;rect3));
</span><span class="boring">}</span></code></pre></pre>
<p><span class="caption">示例 5-15：在 <code>Rectangle</code> 上实现 <code>can_hold</code> 方法，它获取另一个 <code>Rectangle</code> 实例作为参数</span></p>
<p>如果结合示例 5-14 的 <code>main</code> 函数来运行，就会看到期望的输出。在方法签名中，可以在 <code>self</code> 后增加多个参数，而且这些参数就像函数中的参数一样工作。</p>
<h3 id="关联函数"><a class="header" href="#关联函数">关联函数</a></h3>
<p>所有在 <code>impl</code> 块中定义的函数被称为 <strong>关联函数</strong>（<em>associated functions</em>），因为它们与 <code>impl</code> 后面命名的类型相关。我们可以定义不以 <code>self</code> 为第一参数的关联函数（因此不是方法），因为它们并不作用于一个结构体的实例。我们已经使用了一个这样的函数：在 <code>String</code> 类型上定义的 <code>String::from</code> 函数。</p>
<p>不是方法的关联函数经常被用作返回一个结构体新实例的构造函数。这些函数的名称通常为 <code>new</code> ，但 <code>new</code> 并不是一个关键字。例如我们可以提供一个叫做 <code>square</code> 关联函数，它接受一个维度参数并且同时作为宽和高，这样可以更轻松的创建一个正方形 <code>Rectangle</code> 而不必指定两次同样的值：</p>
<p><span class="filename">文件名：src/main.rs</span></p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#[derive(Debug)]
</span><span class="boring">struct Rectangle {
</span><span class="boring"> width: u32,
</span><span class="boring"> height: u32,
</span><span class="boring">}
</span><span class="boring">
</span>impl Rectangle {
fn square(size: u32) -&gt; Self {
Self {
width: size,
height: size,
}
}
}
<span class="boring">
</span><span class="boring">fn main() {
</span><span class="boring"> let sq = Rectangle::square(3);
</span><span class="boring">}</span></code></pre></pre>
<p>关键字 <code>Self</code> 在函数的返回类型中代指在 <code>impl</code> 关键字后出现的类型，在这里是 <code>Rectangle</code></p>
<p>使用结构体名和 <code>::</code> 语法来调用这个关联函数：比如 <code>let sq = Rectangle::square(3);</code>。这个函数位于结构体的命名空间中：<code>::</code> 语法用于关联函数和模块创建的命名空间。<a href="ch07-02-defining-modules-to-control-scope-and-privacy.html">第七章</a>会讲到模块。</p>
<h3 id="多个-impl-块"><a class="header" href="#多个-impl-块">多个 <code>impl</code> 块</a></h3>
<p>每个结构体都允许拥有多个 <code>impl</code> 块。例如，示例 5-16 中的代码等同于示例 5-15，但每个方法有其自己的 <code>impl</code> 块。</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#[derive(Debug)]
</span><span class="boring">struct Rectangle {
</span><span class="boring"> width: u32,
</span><span class="boring"> height: u32,
</span><span class="boring">}
</span><span class="boring">
</span>impl Rectangle {
fn area(&amp;self) -&gt; u32 {
self.width * self.height
}
}
impl Rectangle {
fn can_hold(&amp;self, other: &amp;Rectangle) -&gt; bool {
self.width &gt; other.width &amp;&amp; self.height &gt; other.height
}
}
<span class="boring">
</span><span class="boring">fn main() {
</span><span class="boring"> let rect1 = Rectangle {
</span><span class="boring"> width: 30,
</span><span class="boring"> height: 50,
</span><span class="boring"> };
</span><span class="boring"> let rect2 = Rectangle {
</span><span class="boring"> width: 10,
</span><span class="boring"> height: 40,
</span><span class="boring"> };
</span><span class="boring"> let rect3 = Rectangle {
</span><span class="boring"> width: 60,
</span><span class="boring"> height: 45,
</span><span class="boring"> };
</span><span class="boring">
</span><span class="boring"> println!("Can rect1 hold rect2? {}", rect1.can_hold(&amp;rect2));
</span><span class="boring"> println!("Can rect1 hold rect3? {}", rect1.can_hold(&amp;rect3));
</span><span class="boring">}</span></code></pre></pre>
<p><span class="caption">示例 5-16：使用多个 <code>impl</code> 块重写示例 5-15</span></p>
<p>这里没有理由将这些方法分散在多个 <code>impl</code> 块中，不过这是有效的语法。第十章讨论泛型和 trait 时会看到实用的多 <code>impl</code> 块的用例。</p>
<h2 id="总结"><a class="header" href="#总结">总结</a></h2>
<p>结构体让你可以创建出在你的领域中有意义的自定义类型。通过结构体，我们可以将相关联的数据片段联系起来并命名它们，这样可以使得代码更加清晰。在 <code>impl</code> 块中，你可以定义与你的类型相关联的函数，而方法是一种相关联的函数，让你指定结构体的实例所具有的行为。</p>
<p>但结构体并不是创建自定义类型的唯一方法：让我们转向 Rust 的枚举功能，为你的工具箱再添一个工具。</p>
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