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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&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" class="active"><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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<h1 class="menu-title">Rust 程序设计语言 简体中文版</h1>
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<h2 id="面向对象语言的特征"><a class="header" href="#面向对象语言的特征">面向对象语言的特征</a></h2>
<blockquote>
<p><a href="https://github.com/rust-lang/book/blob/main/src/ch18-01-what-is-oo.md">ch18-01-what-is-oo.md</a>
<br>
commit 398d6f48d2e6b7b15efd51c4541d446e89de3892</p>
</blockquote>
<p>关于一门语言必须具备哪些特征才能被视为面向对象，目前在编程社区中并没有共识。Rust 受到了许多编程范式的影响，包括面向对象编程（OOP）；例如，在第 13 章中，我们探讨了来自函数式编程的特性。可以说，面向对象的语言共有一些共同的特征，即对象、封装和继承。我们将会讨论这些特征分别是什么，以及 Rust 是否支持它们。</p>
<h3 id="对象包含数据和行为"><a class="header" href="#对象包含数据和行为">对象包含数据和行为</a></h3>
<p>由 Erich Gamma、Richard Helm、Ralph Johnson 和 John Vlissides（Addison-Wesley Professional, 1994）编写的书 <em>Design Patterns: Elements of Reusable Object-Oriented Software</em> ，通称 <em>The Gang of Four</em> (“四人帮”)，是一本面向对象设计模式的目录。它这样定义面向对象编程：</p>
<blockquote>
<p>Object-oriented programs are made up of objects. An <em>object</em> packages both
data and the procedures that operate on that data. The procedures are
typically called <em>methods</em> or <em>operations</em>.</p>
<p>面向对象的程序由对象组成。一个 <strong>对象</strong> 包含数据和操作这些数据的过程。这些过程通常被称为 <strong>方法</strong> 或 <strong>操作</strong>。</p>
</blockquote>
<p>在这个定义下，Rust 是面向对象的：结构体和枚举包含数据而 <code>impl</code> 块提供了在结构体和枚举之上的方法。虽然带有方法的结构体和枚举并不被 <strong>称为</strong> 对象，但是它们提供了与对象相同的功能，参考 <em>The Gang of Four</em> 中对象的定义。</p>
<h3 id="封装隐藏了实现细节"><a class="header" href="#封装隐藏了实现细节">封装隐藏了实现细节</a></h3>
<p>另一个通常与面向对象编程关联的概念是 <strong>封装</strong>（<em>encapsulation</em>）：一个对象的实现细节对使用该对象的代码不可访问。因此，对象交互的唯一方式是通过其公共 API；使用对象的代码不应能直接触及对象的内部并改变数据或行为。这使得程序员能够更改和重构一个对象的内部实现，而无需改变使用该对象的代码。</p>
<p>我们在第 7 章讨论了如何控制封装：我们可以使用 <code>pub</code> 关键字来决定代码中的哪些模块、类型、函数和方法是公有的，而默认情况下其他所有内容都是私有的。例如，我们可以定义一个 <code>AveragedCollection</code> 结构体，其中有一个存有 <code>Vec&lt;i32&gt;</code> 的字段。该结构体还可以有一个字段存储其平均值，以便需要时取用。示例 17-1 给出了 <code>AveragedCollection</code> 结构体的定义：</p>
<p><span class="filename">文件名：src/lib.rs</span></p>
<pre><code class="language-rust noplayground">pub struct AveragedCollection {
list: Vec&lt;i32&gt;,
average: f64,
}</code></pre>
<p><span class="caption">示例 17-1: <code>AveragedCollection</code> 结构体维护了一个整型列表及其所有元素的平均值。</span></p>
<p>该结构体被标记为 <code>pub</code>，这样其他代码就可以使用它，但结构体内的字段保持私有。这在这种情况下很重要，因为我们想确保每当列表中添加或删除值时，平均值也会更新。我们通过实现结构体上的 <code>add</code>、<code>remove</code> 和 <code>average</code> 方法来做到这一点，如示例 17-2 所示：</p>
<p><span class="filename">文件名：src/lib.rs</span></p>
<pre><code class="language-rust noplayground"><span class="boring">pub struct AveragedCollection {
</span><span class="boring"> list: Vec&lt;i32&gt;,
</span><span class="boring"> average: f64,
</span><span class="boring">}
</span><span class="boring">
</span>impl AveragedCollection {
pub fn add(&amp;mut self, value: i32) {
self.list.push(value);
self.update_average();
}
pub fn remove(&amp;mut self) -&gt; Option&lt;i32&gt; {
let result = self.list.pop();
match result {
Some(value) =&gt; {
self.update_average();
Some(value)
}
None =&gt; None,
}
}
pub fn average(&amp;self) -&gt; f64 {
self.average
}
fn update_average(&amp;mut self) {
let total: i32 = self.list.iter().sum();
self.average = total as f64 / self.list.len() as f64;
}
}</code></pre>
<p><span class="caption">示例 17-2: 在 <code>AveragedCollection</code> 结构体上实现了 <code>add</code>、<code>remove</code> 和 <code>average</code> 公有方法</span></p>
<p>公有方法 <code>add</code>、<code>remove</code> 和 <code>average</code> 是修改 <code>AveragedCollection</code> 实例的唯一方式。当使用 <code>add</code> 方法把一个元素加入到 <code>list</code> 或者使用 <code>remove</code> 方法来删除时，这些方法的实现同时会调用私有的 <code>update_average</code> 方法来更新 <code>average</code> 字段。</p>
<p><code>list</code> 和 <code>average</code> 是私有的，所以没有其他方式来使得外部的代码直接向 <code>list</code> 增加或者删除元素，否则 <code>list</code> 改变时可能会导致 <code>average</code> 字段不同步。<code>average</code> 方法返回 <code>average</code> 字段的值，这使得外部的代码只能读取 <code>average</code> 而不能修改它。</p>
<p>因为我们已经封装了 <code>AveragedCollection</code> 的实现细节，改动数据结构等内部实现非常简单。例如，可以使用 <code>HashSet&lt;i32&gt;</code> 代替 <code>Vec&lt;i32&gt;</code> 作为 <code>list</code> 字段的类型。只要 <code>add</code>、<code>remove</code> 和 <code>average</code> 这些公有方法的签名保持不变，使用 <code>AveragedCollection</code> 的代码就无需改变。如果我们将 <code>list</code> 设为公有，情况就未必如此： <code>HashSet&lt;i32&gt;</code> 和 <code>Vec&lt;i32&gt;</code> 使用不同的方法增加或移除项，所以如果外部代码直接修改 <code>list</code> ，很可能需要进行更改。</p>
<p>如果封装被认为是面向对象语言所必要的特征，那么 Rust 满足这个要求。在代码中不同的部分控制 <code>pub</code> 的使用来封装实现细节。</p>
<h2 id="继承作为类型系统与代码共享"><a class="header" href="#继承作为类型系统与代码共享">继承，作为类型系统与代码共享</a></h2>
<p><strong>继承</strong>（<em>Inheritance</em>）是一种机制：一个对象可以从另一个对象的定义中继承元素，从而获得父对象的数据和行为，无需再次定义。</p>
<p>如果一种语言必须具有继承才能被认为是面向对象语言，那么 Rust 不是其中之一。Rust 不支持定义一个结构体时继承父结构体的字段和方法，除非使用宏。</p>
<p>然而，如果您习惯于在编程过程中使用继承，那么根据运用继承的原因，Rust 提供了其他解决方案。</p>
<p>选择继承有两个主要的原因。其一是代码复用：您可以为一种类型实现特定的行为，继承可将其复用到不同的类型上。在 Rust 代码中可以使用默认 trait 方法实现来进行有限的代码复用，就像示例 10-14 中在 <code>Summary</code> trait 上增加的 <code>summarize</code> 方法的默认实现。任何实现了 <code>Summary</code> trait 的类型都可以使用 <code>summarize</code> 方法而无须进一步实现。这类似于父类有一个方法的实现，继承的子类也拥有这个方法的实现。当实现 <code>Summary</code> trait 时也可以选择覆盖 <code>summarize</code> 的默认实现，这类似于子类覆盖从父类继承方法的实现。</p>
<p>其二与类型系统有关：子类型可以用于父类型被使用的地方。这也被称为 <strong>多态</strong>（<em>polymorphism</em>）：如果多个对象共享某些特征，可以在运行时将它们互相替代。</p>
<blockquote>
<p>多态（Polymorphism）</p>
<p>对很多人来说，多态性与继承同义。但它实际上是一个更广义的概念，指的是可以处理多种类型数据的代码。对继承而言，这些类型通常是子类。
Rust 使用泛型来抽象不同可能的类型，并通过 trait bounds 来约束这些类型所必须提供的内容。这有时被称为 <em>bounded parametric polymorphism</em>。</p>
</blockquote>
<p>作为一种语言设计的解决方案，继承在许多新的编程语言中逐渐不被青睐，因为它经常有分享过多代码的风险。子类不应总是共享父类的所有特征，但是继承始终如此。它还引入了在子类上调用方法的可能性，这些方法可能没有意义，或因为方法不适用于子类而导致错误。此外，一些语言只允许单一继承（意味着子类只能从一个类继承），进一步限制了程序设计的灵活性。</p>
<p>出于这些原因，Rust 使用 trait 对象而非继承。接下来我们会讨论 Rust 如何使用 trait 对象实现多态性。</p>
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