2017-02-25 23:47:33 +08:00
|
|
|
|
<!DOCTYPE HTML>
|
|
|
|
|
|
<html lang="en">
|
|
|
|
|
|
<head>
|
|
|
|
|
|
<meta charset="UTF-8">
|
2017-02-26 15:11:54 +08:00
|
|
|
|
<title>泛型、trait 和生命周期 - Rust 程序设计语言 简体中文版</title>
|
2017-02-25 23:47:33 +08:00
|
|
|
|
<meta content="text/html; charset=utf-8" http-equiv="Content-Type">
|
2017-02-26 15:11:54 +08:00
|
|
|
|
<meta name="description" content="Rust 程序设计语言 简体中文版">
|
2017-02-25 23:47:33 +08:00
|
|
|
|
<meta name="viewport" content="width=device-width, initial-scale=1">
|
|
|
|
|
|
|
|
|
|
|
|
<base href="">
|
|
|
|
|
|
|
|
|
|
|
|
<link rel="stylesheet" href="book.css">
|
|
|
|
|
|
<link href='https://fonts.googleapis.com/css?family=Open+Sans:300italic,400italic,600italic,700italic,800italic,400,300,600,700,800' rel='stylesheet' type='text/css'>
|
|
|
|
|
|
|
|
|
|
|
|
<link rel="shortcut icon" href="favicon.png">
|
|
|
|
|
|
|
|
|
|
|
|
<!-- Font Awesome -->
|
|
|
|
|
|
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/font-awesome/4.3.0/css/font-awesome.min.css">
|
|
|
|
|
|
|
|
|
|
|
|
<link rel="stylesheet" href="highlight.css">
|
|
|
|
|
|
<link rel="stylesheet" href="tomorrow-night.css">
|
|
|
|
|
|
|
|
|
|
|
|
<!-- MathJax -->
|
|
|
|
|
|
<script type="text/javascript" src="https://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML"></script>
|
|
|
|
|
|
|
|
|
|
|
|
<!-- Fetch JQuery from CDN but have a local fallback -->
|
|
|
|
|
|
<script src="https://code.jquery.com/jquery-2.1.4.min.js"></script>
|
|
|
|
|
|
<script>
|
|
|
|
|
|
if (typeof jQuery == 'undefined') {
|
|
|
|
|
|
document.write(unescape("%3Cscript src='jquery.js'%3E%3C/script%3E"));
|
|
|
|
|
|
}
|
|
|
|
|
|
</script>
|
|
|
|
|
|
</head>
|
|
|
|
|
|
<body class="light">
|
|
|
|
|
|
<!-- Set the theme before any content is loaded, prevents flash -->
|
|
|
|
|
|
<script type="text/javascript">
|
|
|
|
|
|
var theme = localStorage.getItem('theme');
|
|
|
|
|
|
if (theme == null) { theme = 'light'; }
|
|
|
|
|
|
$('body').removeClass().addClass(theme);
|
|
|
|
|
|
</script>
|
|
|
|
|
|
|
|
|
|
|
|
<!-- Hide / unhide sidebar before it is displayed -->
|
|
|
|
|
|
<script type="text/javascript">
|
|
|
|
|
|
var sidebar = localStorage.getItem('sidebar');
|
|
|
|
|
|
if (sidebar === "hidden") { $("html").addClass("sidebar-hidden") }
|
|
|
|
|
|
else if (sidebar === "visible") { $("html").addClass("sidebar-visible") }
|
|
|
|
|
|
</script>
|
|
|
|
|
|
|
|
|
|
|
|
<div id="sidebar" class="sidebar">
|
2017-03-18 21:39:27 +08:00
|
|
|
|
<ul class="chapter"><li><a href="ch01-00-introduction.html"><strong>1.</strong> 介绍</a></li><li><ul class="section"><li><a href="ch01-01-installation.html"><strong>1.1.</strong> 安装</a></li><li><a href="ch01-02-hello-world.html"><strong>1.2.</strong> Hello, World!</a></li></ul></li><li><a href="ch02-00-guessing-game-tutorial.html"><strong>2.</strong> 猜猜看教程</a></li><li><a href="ch03-00-common-programming-concepts.html"><strong>3.</strong> 通用编程概念</a></li><li><ul class="section"><li><a href="ch03-01-variables-and-mutability.html"><strong>3.1.</strong> 变量和可变性</a></li><li><a href="ch03-02-data-types.html"><strong>3.2.</strong> 数据类型</a></li><li><a href="ch03-03-how-functions-work.html"><strong>3.3.</strong> 函数如何工作</a></li><li><a href="ch03-04-comments.html"><strong>3.4.</strong> 注释</a></li><li><a href="ch03-05-control-flow.html"><strong>3.5.</strong> 控制流</a></li></ul></li><li><a href="ch04-00-understanding-ownership.html"><strong>4.</strong> 认识所有权</a></li><li><ul class="section"><li><a href="ch04-01-what-is-ownership.html"><strong>4.1.</strong> 什么是所有权</a></li><li><a href="ch04-02-references-and-borrowing.html"><strong>4.2.</strong> 引用 & 借用</a></li><li><a href="ch04-03-slices.html"><strong>4.3.</strong> Slices</a></li></ul></li><li><a href="ch05-00-structs.html"><strong>5.</strong> 结构体</a></li><li><ul class="section"><li><a href="ch05-01-method-syntax.html"><strong>5.1.</strong> 方法语法</a></li></ul></li><li><a href="ch06-00-enums.html"><strong>6.</strong> 枚举和模式匹配</a></li><li><ul class="section"><li><a href="ch06-01-defining-an-enum.html"><strong>6.1.</strong> 定义枚举</a></li><li><a href="ch06-02-match.html"><strong>6.2.</strong> <code>match</code>控制流运算符</a></li><li><a href="ch06-03-if-let.html"><strong>6.3.</strong> <code>if let</code>简单控制流</a></li></ul></li><li><a href="ch07-00-modules.html"><strong>7.</strong> 模块</a></li><li><ul class="section"><li><a href="ch07-01-mod-and-the-filesystem.html"><strong>7.1.</strong> <code>mod</code>和文件系统</a></li><li><a href="ch07-02-controlling-visibility-with-pub.html"><strong>7.2.</strong> 使用<code>pub</code>控制可见性</a></li><li><a href="ch07-03-importing-names-with-use.html"><strong>7.3.</strong> 使用<code>use</code>导入命名</a></li></ul></li><li><a href="ch08-00-common-collections.html"><strong>8.</strong> 通用集合类型</a></li><li><ul class="section"><li><a href="ch08-01-vectors.html"><strong>8.1.</strong> vector</a></li><li><a href="ch08-02-strings.html"><strong>8.2.</strong> 字符串</a></li><li><a href="ch08-03-hash-maps.html"><strong>8.3.</strong> 哈希 map</a></li></ul></li><li><a href="ch09-00-error-handling.html"><strong>9.</strong> 错误处理</a></li><li><ul class="section"><li><a href="ch09-01-unrecoverable-errors-with-panic.html"><strong>9.1.</strong> <code>panic!</code>与不可恢复的错误</a></li><li><a href="ch09-02-recoverable-errors-with-result.html"><strong>9.2.</strong> <code>Result</code>与可恢复的错误</a></li><li><a href="ch09-03-to-panic-or-not-to-panic.html"><strong>9.3.</strong> <code>panic!</code>还是不<code>panic!</code></a></li></ul></li><li><a href="ch10-00-generics.html" class="active"><strong>10.</strong> 泛型、trait 和生命周期</a></li><li><ul class="section"><li><a href="ch10-01-syntax.html"><strong>10.1.</strong> 泛型数据类型</a></li><li><a href="ch10-02-traits.html"><strong>10.2.</strong> trait：定义共享的行为</a></li><li><a href="ch10-03-lifetime-syntax.html"><strong>10.3.</strong> 生命周期与引用有效性</a></li></ul></li><li><a href="ch11-00-testing.html"><strong>11.</strong> 测试</a></li><li><ul class="section"><li><a href="ch11-01-writing-tests.html"><strong>11.1.</strong> 编写测试</a></li><li><a href="ch11-02-running-tests.html"><strong>11.2.</strong> 运行测试</a></li><li><a href="ch11-03-test-organization.html"><strong>11.3.</strong> 测试的组织结构</a></li></ul></li><li><a href="ch12-00-
|
2017-02-25 23:47:33 +08:00
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
|
|
<div id="page-wrapper" class="page-wrapper">
|
|
|
|
|
|
|
|
|
|
|
|
<div class="page">
|
|
|
|
|
|
<div id="menu-bar" class="menu-bar">
|
|
|
|
|
|
<div class="left-buttons">
|
|
|
|
|
|
<i id="sidebar-toggle" class="fa fa-bars"></i>
|
|
|
|
|
|
<i id="theme-toggle" class="fa fa-paint-brush"></i>
|
|
|
|
|
|
</div>
|
|
|
|
|
|
|
2017-02-26 15:11:54 +08:00
|
|
|
|
<h1 class="menu-title">Rust 程序设计语言 简体中文版</h1>
|
2017-02-25 23:47:33 +08:00
|
|
|
|
|
|
|
|
|
|
<div class="right-buttons">
|
|
|
|
|
|
<i id="print-button" class="fa fa-print" title="Print this book"></i>
|
|
|
|
|
|
</div>
|
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
|
|
<div id="content" class="content">
|
2017-02-26 15:11:54 +08:00
|
|
|
|
<a class="header" href="#泛型trait-和生命周期" name="泛型trait-和生命周期"><h1>泛型、trait 和生命周期</h1></a>
|
2017-02-25 23:47:33 +08:00
|
|
|
|
<blockquote>
|
2017-03-23 22:37:22 +08:00
|
|
|
|
<p><a href="https://github.com/rust-lang/book/blob/master/second-edition/src/ch10-00-generics.md">ch10-00-generics.md</a>
|
2017-02-25 23:47:33 +08:00
|
|
|
|
<br>
|
2017-03-23 22:37:22 +08:00
|
|
|
|
commit 3f2a1bd8dbb19cc48b210fc4fb35c305c8d81b56</p>
|
2017-02-25 23:47:33 +08:00
|
|
|
|
</blockquote>
|
|
|
|
|
|
<p>每一个编程语言都有高效的处理重复概念的工具；在 Rust 中工具之一就是<strong>泛型</strong>（<em>generics</em>）。泛型是具体类型或其他属性的抽象替代。我们可以表达泛型的属性，比如他们的行为或如何与其他泛型相关联，而不需要在编写和编译代码时知道他们在这里实际上代表什么。</p>
|
2017-03-23 22:37:22 +08:00
|
|
|
|
<p>同理为了编写一份可以用于多种具体值的代码，函数并不知道其参数为何值，这时就可以让函数获取泛型而不是像<code>i32</code>或<code>String</code>这样的具体值。我们已经使用过第六章的<code>Option<T></code>，第八章的<code>Vec<T></code>和<code>HashMap<K, V></code>，以及第九章的<code>Result<T, E></code>这些泛型了。本章会探索如何使用泛型定义我们自己自己的类型、函数和方法！</p>
|
2017-02-25 23:47:33 +08:00
|
|
|
|
<p>首先，我们将回顾一下提取函数以减少代码重复的机制。接着使用一个只在参数类型上不同的泛型函数来实现相同的功能。我们也会讲到结构体和枚举定义中的泛型。</p>
|
|
|
|
|
|
<p>之后，我们讨论 <em>traits</em>，这是一个定义泛型行为的方法。trait 可以与泛型结合来将泛型限制为拥有特定行为的类型，而不是任意类型。</p>
|
|
|
|
|
|
<p>最后介绍<strong>生命周期</strong>（<em>lifetimes</em>），它是一类允许我们向编译器提供引用如何相互关联的泛型。Rust 的生命周期功能允许在很多场景下借用值同时仍然使编译器能够检查这些引用的有效性。</p>
|
2017-02-26 15:11:54 +08:00
|
|
|
|
<a class="header" href="#提取函数来减少重复" name="提取函数来减少重复"><h2>提取函数来减少重复</h2></a>
|
2017-02-25 23:47:33 +08:00
|
|
|
|
<p>在介绍泛型语法之前，首先来回顾一个不使用泛型的处理重复的技术：提取一个函数。当熟悉了这个技术以后，我们将使用相同的机制来提取一个泛型函数！如同你识别出可以提取到函数中重复代码那样，你也会开始识别出能够使用泛型的重复代码。</p>
|
|
|
|
|
|
<p>考虑一下这个寻找列表中最大值的小程序，如列表 10-1 所示：</p>
|
2017-03-23 22:37:22 +08:00
|
|
|
|
<p><span class="filename">Filename: src/main.rs</span></p>
|
2017-02-25 23:47:33 +08:00
|
|
|
|
<pre><code class="language-rust">fn main() {
|
|
|
|
|
|
let numbers = vec![34, 50, 25, 100, 65];
|
|
|
|
|
|
|
|
|
|
|
|
let mut largest = numbers[0];
|
|
|
|
|
|
|
|
|
|
|
|
for number in numbers {
|
|
|
|
|
|
if number > largest {
|
|
|
|
|
|
largest = number;
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
println!("The largest number is {}", largest);
|
|
|
|
|
|
# assert_eq!(largest, 100);
|
|
|
|
|
|
}
|
|
|
|
|
|
</code></pre>
|
2017-03-23 22:37:22 +08:00
|
|
|
|
<p><span class="caption">Listing 10-1: Code to find the largest number in a list
|
|
|
|
|
|
of numbers</span></p>
|
2017-02-25 23:47:33 +08:00
|
|
|
|
<p>这段代码获取一个整型列表，存放在变量<code>numbers</code>中。它将列表的第一项放入了变量<code>largest</code>中。接着遍历了列表中的所有数字，如果当前值大于<code>largest</code>中储存的值，将<code>largest</code>替换为这个值。如果当前值小于目前为止的最大值，<code>largest</code>保持不变。当列表中所有值都被考虑到之后，<code>largest</code>将会是最大值，在这里也就是 100。</p>
|
|
|
|
|
|
<p>如果需要在两个不同的列表中寻找最大值，我们可以重复列表 10-1 中的代码这样程序中就会存在两段相同逻辑的代码，如列表 10-2 所示：</p>
|
2017-03-23 22:37:22 +08:00
|
|
|
|
<p><span class="filename">Filename: src/main.rs</span></p>
|
2017-02-25 23:47:33 +08:00
|
|
|
|
<pre><code class="language-rust">fn main() {
|
|
|
|
|
|
let numbers = vec![34, 50, 25, 100, 65];
|
|
|
|
|
|
|
|
|
|
|
|
let mut largest = numbers[0];
|
|
|
|
|
|
|
|
|
|
|
|
for number in numbers {
|
|
|
|
|
|
if number > largest {
|
|
|
|
|
|
largest = number;
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
println!("The largest number is {}", largest);
|
|
|
|
|
|
|
|
|
|
|
|
let numbers = vec![102, 34, 6000, 89, 54, 2, 43, 8];
|
|
|
|
|
|
|
|
|
|
|
|
let mut largest = numbers[0];
|
|
|
|
|
|
|
|
|
|
|
|
for number in numbers {
|
|
|
|
|
|
if number > largest {
|
|
|
|
|
|
largest = number;
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
println!("The largest number is {}", largest);
|
|
|
|
|
|
}
|
|
|
|
|
|
</code></pre>
|
2017-03-23 22:37:22 +08:00
|
|
|
|
<p><span class="caption">Listing 10-2: Code to find the largest number in <em>two</em>
|
|
|
|
|
|
lists of numbers</span></p>
|
2017-02-25 23:47:33 +08:00
|
|
|
|
<p>虽然代码能够执行，但是重复的代码是冗余且已于出错的，并且意味着当更新逻辑时需要修改多处地方的代码。</p>
|
|
|
|
|
|
<!-- Are we safe assuming the reader will be familiar with the term
|
|
|
|
|
|
"abstraction" in this context, or do we want to give a brief definition? -->
|
|
|
|
|
|
<!-- Yes, our audience will be familiar with this term. /Carol -->
|
|
|
|
|
|
<p>为了消除重复，我们可以创建一层抽象，在这个例子中将表现为一个获取任意整型列表作为参数并对其进行处理的函数。这将增加代码的简洁性并让我们将表达和推导寻找列表中最大值的这个概念与使用这个概念的特定位置相互独。
|
|
|
|
|
|
立。</p>
|
|
|
|
|
|
<p>在列表 10-3 的程序中将寻找最大值的代码提取到了一个叫做<code>largest</code>的函数中。这个程序可以找出两个不同数字列表的最大值，不过列表 10-1 中的代码只存在于一个位置：</p>
|
2017-03-23 22:37:22 +08:00
|
|
|
|
<p><span class="filename">Filename: src/main.rs</span></p>
|
2017-02-25 23:47:33 +08:00
|
|
|
|
<pre><code class="language-rust">fn largest(list: &[i32]) -> i32 {
|
|
|
|
|
|
let mut largest = list[0];
|
|
|
|
|
|
|
|
|
|
|
|
for &item in list.iter() {
|
|
|
|
|
|
if item > largest {
|
|
|
|
|
|
largest = item;
|
|
|
|
|
|
}
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
largest
|
|
|
|
|
|
}
|
|
|
|
|
|
|
|
|
|
|
|
fn main() {
|
|
|
|
|
|
let numbers = vec![34, 50, 25, 100, 65];
|
|
|
|
|
|
|
|
|
|
|
|
let result = largest(&numbers);
|
|
|
|
|
|
println!("The largest number is {}", result);
|
|
|
|
|
|
# assert_eq!(result, 100);
|
|
|
|
|
|
|
|
|
|
|
|
let numbers = vec![102, 34, 6000, 89, 54, 2, 43, 8];
|
|
|
|
|
|
|
|
|
|
|
|
let result = largest(&numbers);
|
|
|
|
|
|
println!("The largest number is {}", result);
|
|
|
|
|
|
# assert_eq!(result, 6000);
|
|
|
|
|
|
}
|
|
|
|
|
|
</code></pre>
|
2017-03-23 22:37:22 +08:00
|
|
|
|
<p><span class="caption">Listing 10-3: Abstracted code to find the largest number
|
|
|
|
|
|
in two lists</span></p>
|
2017-02-25 23:47:33 +08:00
|
|
|
|
<p>这个函数有一个参数<code>list</code>，它代表会传递给函数的任何具体<code>i32</code>值的 slice。函数定义中的<code>list</code>代表任何<code>&[i32]</code>。当调用<code>largest</code>函数时，其代码实际上运行于我们传递的特定值上。</p>
|
|
|
|
|
|
<p>从列表 10-2 到列表 10-3 中涉及的机制经历了如下几步：</p>
|
|
|
|
|
|
<ol>
|
|
|
|
|
|
<li>我们注意到了重复代码。</li>
|
|
|
|
|
|
<li>我们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>我们将两个具体的存在重复代码的位置替换为了函数调用。</li>
|
|
|
|
|
|
</ol>
|
|
|
|
|
|
<p>在不同的场景使用不同的方式泛型也可以利用相同的步骤来减少重复代码。与函数体中现在作用于一个抽象的<code>list</code>而不是具体值一样，使用泛型的代码也作用于抽象类型。支持泛型背后的概念与你已经了解的支持函数的概念是一样的，不过是实现方式不同。</p>
|
|
|
|
|
|
<p>如果我们有两个函数，一个寻找一个<code>i32</code>值的 slice 中的最大项而另一个寻找<code>char</code>值的 slice 中的最大项该怎么办？该如何消除重复呢？让我们拭目以待！</p>
|
|
|
|
|
|
|
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
|
|
<!-- Mobile navigation buttons -->
|
|
|
|
|
|
|
|
|
|
|
|
<a href="ch09-03-to-panic-or-not-to-panic.html" class="mobile-nav-chapters previous">
|
|
|
|
|
|
<i class="fa fa-angle-left"></i>
|
|
|
|
|
|
</a>
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
<a href="ch10-01-syntax.html" class="mobile-nav-chapters next">
|
|
|
|
|
|
<i class="fa fa-angle-right"></i>
|
|
|
|
|
|
</a>
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
<a href="ch09-03-to-panic-or-not-to-panic.html" class="nav-chapters previous" title="You can navigate through the chapters using the arrow keys">
|
|
|
|
|
|
<i class="fa fa-angle-left"></i>
|
|
|
|
|
|
</a>
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
<a href="ch10-01-syntax.html" class="nav-chapters next" title="You can navigate through the chapters using the arrow keys">
|
|
|
|
|
|
<i class="fa fa-angle-right"></i>
|
|
|
|
|
|
</a>
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
<!-- Local fallback for Font Awesome -->
|
|
|
|
|
|
<script>
|
|
|
|
|
|
if ($(".fa").css("font-family") !== "FontAwesome") {
|
|
|
|
|
|
$('<link rel="stylesheet" type="text/css" href="_FontAwesome/css/font-awesome.css">').prependTo('head');
|
|
|
|
|
|
}
|
|
|
|
|
|
</script>
|
|
|
|
|
|
|
|
|
|
|
|
<!-- Livereload script (if served using the cli tool) -->
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
<script src="highlight.js"></script>
|
|
|
|
|
|
<script src="book.js"></script>
|
|
|
|
|
|
</body>
|
|
|
|
|
|
</html>
|
