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<h1 id="泛型trait-和生命周期"><a class="header" href="#泛型trait-和生命周期">泛型、Trait 和生命周期</a></h1>
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<p><a href="https://github.com/rust-lang/book/blob/3986f214fd82427b4401adf4d7dc0911c917e1e8/src/ch10-00-generics.md">ch10-00-generics.md</a></p>
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<p>每种编程语言都有一些工具，用来高效处理概念上的重复。在 Rust 中，这样的工具之一就是<strong>泛型</strong>（<em>generics</em>）：它是具体类型或其他属性的抽象占位符。我们可以描述泛型的行为，或者它们与其他泛型之间的关系，而不需要在编译和运行代码时就提前知道它们具体会被什么替换。</p>
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<p>函数可以像接收未知值那样，接收某种泛型类型的参数，而不是像 <code>i32</code> 或 <code>String</code> 这样的具体类型，从而让同一段代码可以作用于多种具体值。事实上，我们已经在第六章见过 <code>Option<T></code>，在第八章见过 <code>Vec<T></code> 和 <code>HashMap<K, V></code>，在第九章见过 <code>Result<T, E></code>。本章将继续探索如何使用泛型来定义我们自己的类型、函数和方法！</p>
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<p>首先，我们会回顾如何通过提取函数来减少代码重复。然后，我们会使用同样的技巧，把两个只在参数类型上不同的函数变成一个泛型函数。我们也会解释如何在结构体和枚举定义中使用泛型类型。</p>
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<p>然后，你会学到如何使用 <strong>trait</strong> 以泛型的方式定义行为。你可以把 trait 和泛型类型组合起来，把某个泛型类型限制为只接受具有特定行为的类型，而不是任意类型。</p>
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<p>最后，我们会讨论 <strong>生命周期</strong>（<em>lifetimes</em>）：它是一类向编译器提供“引用之间如何相互关联”信息的泛型。生命周期让我们能够向编译器提供足够的信息，使它在更多场景下也能确认引用是有效的。</p>
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<h2 id="提取函数来减少重复"><a class="header" href="#提取函数来减少重复">提取函数来减少重复</a></h2>
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<p>泛型允许我们使用一个可以代表多种类型的占位符来替换特定类型，以此来减少代码冗余。在深入了解泛型的语法之前，我们首先来看一种没有使用泛型的减少冗余的方法，即提取一个函数。在这个函数中，我们用一个可以代表多种值的占位符来替换具体的值。接着我们使用相同的技术来提取一个泛型函数！通过学习如何识别并提取可以整合进一个函数的重复代码，你也会开始识别出可以使用泛型的重复代码。</p>
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<p>让我们从下面这个寻找列表中最大值的小程序开始，如示例 10-1 所示：</p>
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<p><span class="filename">文件名：src/main.rs</span></p>
|
|
|
<pre class="playground"><code class="language-rust edition2024">fn main() {
|
|
|
let number_list = vec![34, 50, 25, 100, 65];
|
|
|
|
|
|
let mut largest = &number_list[0];
|
|
|
|
|
|
for number in &number_list {
|
|
|
if number > largest {
|
|
|
largest = number;
|
|
|
}
|
|
|
}
|
|
|
|
|
|
println!("The largest number is {largest}");
|
|
|
<span class="boring"> assert_eq!(*largest, 100);
|
|
|
</span>}</code></pre>
|
|
|
<p><span class="caption">示例 10-1：在一个数字列表中寻找最大值的函数</span></p>
|
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|
<p>我们把一个整数列表存进变量 <code>number_list</code>，再把列表中第一个数字的引用放进名为 <code>largest</code> 的变量。然后遍历列表中的所有数字：如果当前数字比 <code>largest</code> 中存储的数字更大，就更新这个变量中的引用。反之，如果当前数字小于或等于目前为止见到的最大值，这个变量就保持不变，代码继续处理列表中的下一个数字。当列表中所有数字都比较完后，<code>largest</code> 就会指向最大值；在这个例子里，它是 100。</p>
|
|
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<p>现在我们又接到一个任务：要在两个不同的数字列表中找出最大值。为此，我们可以选择把示例 10-1 中的代码复制一份，在程序的两个不同位置使用同样的逻辑，如示例 10-2 所示：</p>
|
|
|
<p><span class="filename">文件名：src/main.rs</span></p>
|
|
|
<pre class="playground"><code class="language-rust edition2024">fn main() {
|
|
|
let number_list = vec![34, 50, 25, 100, 65];
|
|
|
|
|
|
let mut largest = &number_list[0];
|
|
|
|
|
|
for number in &number_list {
|
|
|
if number > largest {
|
|
|
largest = number;
|
|
|
}
|
|
|
}
|
|
|
|
|
|
println!("The largest number is {largest}");
|
|
|
|
|
|
let number_list = vec![102, 34, 6000, 89, 54, 2, 43, 8];
|
|
|
|
|
|
let mut largest = &number_list[0];
|
|
|
|
|
|
for number in &number_list {
|
|
|
if number > largest {
|
|
|
largest = number;
|
|
|
}
|
|
|
}
|
|
|
|
|
|
println!("The largest number is {largest}");
|
|
|
}</code></pre>
|
|
|
<p><span class="caption">示例 10-2：寻找<strong>两个</strong>数组最大值的代码</span></p>
|
|
|
<p>虽然这段代码能工作，但复制代码既繁琐又容易出错。而且当我们想修改逻辑时，还得记得去更新多处地方。</p>
|
|
|
<p>为了消除这种重复，我们将通过定义一个以任意整数列表为参数的函数来创建抽象。这个方案让代码更清晰，也能让我们以更抽象的方式表达“找出一个列表中最大数字”这个概念。</p>
|
|
|
<p>在示例 10-3 中，我们把寻找最大值的代码提取到了一个名为 <code>largest</code> 的函数中。然后调用这个函数，来找出示例 10-2 中两个列表里的最大值。将来如果我们还有别的 <code>i32</code> 值列表，也同样可以复用这个函数。</p>
|
|
|
<p><span class="filename">文件名：src/main.rs</span></p>
|
|
|
<pre class="playground"><code class="language-rust edition2024">fn largest(list: &[i32]) -> &i32 {
|
|
|
let mut largest = &list[0];
|
|
|
|
|
|
for item in list {
|
|
|
if item > largest {
|
|
|
largest = item;
|
|
|
}
|
|
|
}
|
|
|
|
|
|
largest
|
|
|
}
|
|
|
|
|
|
fn main() {
|
|
|
let number_list = vec![34, 50, 25, 100, 65];
|
|
|
|
|
|
let result = largest(&number_list);
|
|
|
println!("The largest number is {result}");
|
|
|
<span class="boring"> assert_eq!(*result, 100);
|
|
|
</span>
|
|
|
let number_list = vec![102, 34, 6000, 89, 54, 2, 43, 8];
|
|
|
|
|
|
let result = largest(&number_list);
|
|
|
println!("The largest number is {result}");
|
|
|
<span class="boring"> assert_eq!(*result, 6000);
|
|
|
</span>}</code></pre>
|
|
|
<p><span class="caption">示例 10-3：抽象后的寻找两个数字列表最大值的代码</span></p>
|
|
|
<p><code>largest</code> 函数有一个名为 <code>list</code> 的参数，它表示任何传给该函数的 <code>i32</code> 值切片。因此，当我们调用这个函数时，代码会运行在我们传入的具体值上。</p>
|
|
|
<p>总的来说，我们把代码从示例 10-2 改成示例 10-3 时，经历了下面这些步骤：</p>
|
|
|
<ol>
|
|
|
<li>找出重复代码。</li>
|
|
|
<li>将重复代码提取到了一个函数中，并在函数签名中指定了代码中的输入和返回值。</li>
|
|
|
<li>将重复代码的两个实例，改为调用函数。</li>
|
|
|
</ol>
|
|
|
<p>接下来，我们会用同样的步骤借助泛型来减少重复。就像函数体可以处理抽象的 <code>list</code>、而不是特定的值一样，泛型也允许代码处理抽象类型。</p>
|
|
|
<p>例如，假设我们有两个函数：一个用来找出 <code>i32</code> 切片中的最大项，另一个用来找出 <code>char</code> 切片中的最大项。我们该如何消除这种重复呢？继续往下看。</p>
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