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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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<p><a href="https://github.com/rust-lang/book/blob/eb54c18184c0555acd9f636f5ef7875236b0ff53/src/ch18-01-what-is-oo.md">ch18-01-what-is-oo.md</a></p>
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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>，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>同时封装了数据以及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，但是参考 <em>The Gang of Four</em> 中对象的定义，它们提供了与对象相同的功能。</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见。因此，对象交互的唯一方式是通过其公有 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>我们在第七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向量中值的平均值，从而无需在每次需要时重新计算。换句话说，<code>AveragedCollection</code> 会为我们缓存已计算的平均值。示例 18-1 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>
|
|
|
<pre><code class="language-rust noplayground">pub struct AveragedCollection {
|
|
|
list: Vec<i32>,
|
|
|
average: f64,
|
|
|
}</code></pre>
|
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|
<p><span class="caption">示例 18-1: <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，但结构体内的字段仍保持私有。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 18-2 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<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 18-2: 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> 是访问或修改 <code>AveragedCollection</code> 实例中数据的唯一途径。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>
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<h3 id="作为类型系统与代码共享的继承"><a class="header" href="#作为类型系统与代码共享的继承">作为类型系统与代码共享的继承</a></h3>
|
|
|
<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>如果一种语言必须拥有继承才能算作面向对象，那么 Rust 就不是这样的语言。Rust 没有办法在不借助宏的情况下，定义一个结构体去继承父结构体的字段和方法实现。</p>
|
|
|
<p>不过，如果你已经习惯了把继承作为编程工具箱的一部分，那么 Rust 也会根据你最初想借助继承解决的问题，提供其他方案。</p>
|
|
|
<p>选择继承通常有两个主要原因。其一是复用代码：你可以先为某种类型实现特定行为，然后借助继承把这份实现复用到另一种类型上。在 Rust 中，可以通过 trait 方法的默认实现，在一定程度上做到这一点。你在示例 10-14 中已经见过：我们为 <code>Summary</code> trait 的 <code>summarize</code> 方法提供了默认实现。这样，任何实现 <code>Summary</code> trait 的类型，都会自动拥有 <code>summarize</code> 方法，而无需额外编写代码。这很像父类已经实现了某个方法，而继承它的子类也随之拥有这份实现。同样地，在实现 <code>Summary</code> trait 时，我们也可以覆盖 <code>summarize</code> 的默认实现，这又类似于子类去重写从父类继承而来的方法实现。</p>
|
|
|
<p>另一个使用继承的原因和类型系统有关：它可以让子类型出现在父类型能出现的地方。这也被称为<strong>多态</strong>（<em>polymorphism</em>），意思是如果多个对象共享某些共同特征，那么在运行时就可以把它们彼此替换使用。</p>
|
|
|
<blockquote>
|
|
|
<h3 id="多态polymorphism"><a class="header" href="#多态polymorphism">多态（Polymorphism）</a></h3>
|
|
|
<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。</p>
|
|
|
<p>Rust 使用泛型来抽象不同可能的类型，并通过 trait bound 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>
|
|
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</blockquote>
|
|
|
<p>Rust 通过不提供继承，选择了另一组不同的权衡。继承常常有“共享了超出需要的代码”的风险。子类并不总是应该继承父类的全部特征，但使用继承时却往往会这样发生。这会让程序设计变得不够灵活。它还会引入这样一种可能：在子类上调用一些其实并不适用的方法，结果这些方法要么根本说不通，要么会导致错误。另外，一些语言只支持<strong>单继承</strong>（即一个子类只能继承一个父类），这也进一步限制了程序设计的灵活性。</p>
|
|
|
<p>基于这些原因，Rust 采取了不同的做法：它使用 trait object，而不是继承，来实现运行时多态。接下来我们就来看看 trait object 是如何工作的。</p>
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