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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 h
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<main>
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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>
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<blockquote>
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<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>
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<br>
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commit 398d6f48d2e6b7b15efd51c4541d446e89de3892</p>
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</blockquote>
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<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>
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<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>
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<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>
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<blockquote>
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<p>Object-oriented programs are made up of objects. An <em>object</em> packages both
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data and the procedures that operate on that data. The procedures are
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typically called <em>methods</em> or <em>operations</em>.</p>
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<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>
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</blockquote>
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<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>
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<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>
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<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>
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<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<i32></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>
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<p><span class="filename">文件名：src/lib.rs</span></p>
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<pre><code class="language-rust noplayground">pub struct AveragedCollection {
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list: Vec<i32>,
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average: f64,
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}</code></pre>
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<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>
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<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>
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<p><span class="filename">文件名：src/lib.rs</span></p>
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<pre><code class="language-rust noplayground"><span class="boring">pub struct AveragedCollection {
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</span><span class="boring"> list: Vec<i32>,
|
|||
|
|
</span><span class="boring"> average: f64,
|
|||
|
|
</span><span class="boring">}
|
|||
|
|
</span><span class="boring">
|
|||
|
|
</span>impl AveragedCollection {
|
|||
|
|
pub fn add(&mut self, value: i32) {
|
|||
|
|
self.list.push(value);
|
|||
|
|
self.update_average();
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
pub fn remove(&mut self) -> Option<i32> {
|
|||
|
|
let result = self.list.pop();
|
|||
|
|
match result {
|
|||
|
|
Some(value) => {
|
|||
|
|
self.update_average();
|
|||
|
|
Some(value)
|
|||
|
|
}
|
|||
|
|
None => None,
|
|||
|
|
}
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
pub fn average(&self) -> f64 {
|
|||
|
|
self.average
|
|||
|
|
}
|
|||
|
|
|
|||
|
|
fn update_average(&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<i32></code> 代替 <code>Vec<i32></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<i32></code> 和 <code>Vec<i32></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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