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<title>使用迭代器处理元素序列</title>
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</a></div> <!----></nav> <ul class="sidebar-links"><li><section class="sidebar-group depth-0"><a href="/title-page" class="sidebar-heading clickable open"><span>Rust 程序设计语言</span> <!----></a> <ul class="sidebar-links sidebar-group-items"><li><a href="/foreword.html" class="sidebar-link">前言</a></li><li><a href="/ch00-00-introduction.html" class="sidebar-link">介绍</a></li><li><section class="sidebar-group is-sub-group depth-1"><a href="/ch01-00-getting-started.html" class="sidebar-heading clickable"><span>1. 入门指南</span> <!----></a> <ul class="sidebar-links sidebar-group-items"><li><a href="/ch01-01-installation.html" class="sidebar-link">1.1. 安装</a></li><li><a href="/ch01-02-hello-world.html" class="sidebar-link">1.2. Hello, World!</a></li><li><a href="/ch01-03-hello-cargo.html" class="sidebar-link">1.3. Hello, Cargo!</a></li></ul></section></li><li><a href="/ch02-00-guessing-game-tutorial.html" class="sidebar-link">2. 猜猜看游戏教程</a></li><li><section class="sidebar-group is-sub-group depth-1"><a href="/ch03-00-common-programming-concepts.html" class="sidebar-heading clickable"><span>3. 常见编程概念</span> <!----></a> <ul class="sidebar-links sidebar-group-items"><li><a href="/ch03-01-variables-and-mutability.html" class="sidebar-link">3.1. 变量与可变性</a></li><li><a href="/ch03-02-data-types.html" class="sidebar-link">3.2. 数据类型</a></li><li><a href="/ch03-03-how-functions-work.html" class="sidebar-link">3.3. 函数如何工作</a></li><li><a href="/ch03-04-comments.html" class="sidebar-link">3.4. 注释</a></li><li><a href="/ch03-05-control-flow.html" class="sidebar-link">3.5. 控制流</a></li></ul></section></li><li><section class="sidebar-group is-sub-group depth-1"><a href="/ch04-00-understanding-ownership.html" class="sidebar-heading clickable"><span>4. 认识所有权</span> <!----></a> <ul class="sidebar-links sidebar-group-items"><li><a href="/ch04-01-what-is-ownership.html" class="sidebar-link">4.1. 什么是所有权？</a></li><li><a href="/ch04-02-references-and-borrowing.html" class="sidebar-link">4.2. 引用与借用</a></li><li><a href="/ch04-03-slices.html" class="sidebar-link">4.3. Slices</a></li></ul></section></li><li><section class="sidebar-group is-sub-group depth-1"><a href="/ch05-00-structs.html" class="sidebar-heading clickable"><span>5. 使用结构体来组织相关联的数据</span> <!----></a> <ul class="sidebar-links sidebar-group-items"><li><a href="/ch05-01-defining-structs.html" class="sidebar-link">5.1. 定义并实例化结构体</a></li><li><a href="/ch05-02-example-structs.html" class="sidebar-link">5.2. 一个使用结构体的示例程序</a></li><li><a href="/ch05-03-method-syntax.html" class="sidebar-link">5.3. 方法语法</a></li></ul></section></li><li><section class="sidebar-group is-sub-group depth-1"><a href="/ch06-00-enums.html" class="sidebar-heading clickable"><span>6. 枚举与模式匹配</span> <!----></a> <ul class="sidebar-links sidebar-group-items"><li><a href="/ch06-01-defining-an-enum.html" class="sidebar-link">6.1. 定义枚举</a></li><li><a href="/ch06-02-match.html" class="sidebar-link">6.2. match控制流运算符</a></li><li><a href="/ch06-03-if-let.html" class="sidebar-link">6.3. if let简洁控制流</a></li></ul></section></li><li><section class="sidebar-group is-sub-group depth-1"><a href="/ch07-00-managing-growing-projects-with-packages-crates-and-modules.html" class="sidebar-heading clickable"><span>7. 使用包、Crate 和模块管理不断增长的项目</span> <!----></a> <ul class="sidebar-links sidebar-group-items"><li><a href="/ch07-01-packages-and-crates.html" class="sidebar-link">7.1. 包和 crate</a></li><li><a href="/ch07-02-defining-modules-to-control-scope-and-privacy.html" class="sidebar-link">7.2. 定义模块来控制作用域与私有性</a></li><li><a href="/ch07-03-paths-for-referring-to-an-item-in-the-module-tree.html" class="sidebar-link">7.3. 路径用于引用模块树中的项</a></li><li><a href="/ch07-04-bringing-paths-into-scope-with-the-use-keyword.html" class="sidebar-link">7.4. 使用use关键字将名称引入作用域</a></li><li><a href="/ch07-05-separating-modules-into-different-files.html" class="sidebar-link">7.5. 将模块分割进不同文件</a></li></ul></section></li><li><section class="sidebar-group is-sub-group depth-1"><a href="/ch08-00-common-collections.html" class="sidebar-heading clickable"><span>8. 常见集合</span> <!----></a> <ul class="sidebar-links sidebar-group-items"><li><a href="/ch08-01-vectors.html" class="sidebar-link">8.1. vector</a></li><li><a href="/ch08-02-strings.html" class="sidebar-link">8.2. 字符串</a></li><li><a href="/ch08-03-hash-maps.html" class="sidebar-link">8.3. 哈希 map</a></li></ul></section></li><li><section class="sidebar-group is-sub-group depth-1"><a href="/ch09-00-error-handling.html" class="sidebar-heading clickable"><span>9. 错误处理</span> <!----></a> <ul class="sidebar-links sidebar-group-items"><li><a href="/ch09-01-unrecoverable-errors-with-panic.html" class="sidebar-link">9.1. panic!与不可恢复的错误</a></li><li><a href="/ch09-02-recoverable-errors-with-result.html" class="sidebar-link">9.2. Result与可恢复的错误</a></li><li><a href="/ch09-03-to-panic-or-not-to-panic.html" class="sidebar-link">9.3. panic!还是不panic!</a></li></ul></section></li><li><section class="sidebar-group is-sub-group depth-1"><a href="/ch10-00-generics.html" class="sidebar-heading clickable"><span>10. 泛型、trait 与生命周期</span> <!----></a> <ul class="sidebar-links sidebar-group-items"><li><a href="/ch10-01-syntax.html" class="sidebar-link">10.1. 泛型数据类型</a></li><li><a href="/ch10-02-traits.html" class="sidebar-link">10.2. trait：定义共享的行为</a></li><li><a href="/ch10-03-lifetime-syntax.html" class="sidebar-link">10.3. 生命周期与引用有效性</a></li></ul></section></li><li><section class="sidebar-group is-sub-group depth-1"><a href="/ch11-00-testing.html" class="sidebar-heading clickable"><span>11. 测试</span> <!----></a> <ul class="sidebar-links sidebar-group-items"><li><a href="/ch11-01-writing-tests.html" class="sidebar-link">11.1. 编写测试</a></li><li><a href="/ch11-02-running-tests.html" class="sidebar-link">11.2. 运行测试</a></li><li><a href="/ch11-03-test-organization.html" class="sidebar-link">11.3. 测试的组织结构</a></li></ul></section></li><li><section class="sidebar-group is-sub-group depth-1"><a href="/ch12-00-an-io-project.html" class="sidebar-heading clickable"><span>12. 一个 I/O 项目：构建命令行程序</span> <!----></a> <ul class="sidebar-links sidebar-group-items"><li><a href="/ch12-01-accepting-command-line-arguments.html" class="sidebar-link">12.1. 接受命令行参数</a></li><li><a href="/ch12-02-reading-a-file.html" class="sidebar-link">12.2. 读取文件</a></li><li><a href="/ch12-03-improving-error-handling-and-modularity.html" class="sidebar-link">12.3. 重构以改进模块化与错误处理</a></li><li><a href="/ch12-04-testing-the-librarys-functionality.html" class="sidebar-link">12.4. 采用测试驱动开发完善库的功能</a></li><li><a href="/ch12-05-working-with-environment-variables.html" class="sidebar-link">12.5. 处理环境变量</a></li><li><a href="/ch12-06-writing-to-stderr-instead-of-stdout.html" class="sidebar-link">12.6. 将错误信息输出到标准错误而不是标准输出</a></li></ul></section></li><li><section class="sidebar-group is-sub-group depth-1"><a href="/ch13-00-functional-features.html" class="sidebar-heading clickable open"><span>13. Rust 中的函数式语言功能：迭代器与闭包</span> <!----></a> <ul class="sidebar-links sidebar-group-items"><li><a href="/ch13-01-closures.html" class="sidebar-link">13.1. 闭包：可以捕获其环境的匿名函数</a></li><li><a href="/ch13-02-iterators.html" aria-current="page" class="active sidebar-link">13.2. 使用迭代器处理元素序列</a></li><li><a href="/ch13-03-improving-our-io-project.html" class="sidebar-link">13.3. 改进之前的 I/O 项目</a></li><li><a href="/ch13-04-performance.html" class="sidebar-link">13.4. 性能比较：循环对迭代器</a></li></ul></section></li><li><section class="sidebar-group is-sub-group depth-1"><a href="/ch14-00-more-about-cargo.html" class="sidebar-heading clickable"><span>14. 更多关于 Cargo 和 Crates.io 的内容</span> <!----></a> <ul class="sidebar-links sidebar-group-items"><li><a href="/ch14-01-release-profiles.html" class="sidebar-link">14.1. 采用发布配置自定义构建</a></li><li><a href="/ch14-02-publishing-to-crates-io.html" class="sidebar-link">14.2. 将 crate 发布到 Crates.io</a></li><li><a href="/ch14-03-cargo-workspaces.html" class="sidebar-link">14.3. Cargo 工作空间</a></li><li><a href="/ch14-04-installing-binaries.html" class="sidebar-link">14.4. 使用cargo install从 Crates.io 安装二进制文件</a></li><li><a href="/ch14-05-extending-cargo.html" class="sidebar-link">14.5. Cargo 自定义扩展命令</a></li></ul></section></li><li><section class="sidebar-group is-sub-group depth-1"><a href="/ch15-00-smart-pointers.html" class="sidebar-heading clickable"><span>15. 智能指针</span> <!----></a> <ul class="sidebar-links sidebar-group-items"><li><a href="/ch15-01-box.html" class="sidebar-link">15.1. Box<T>指向堆上数据，并且可确定大小</a></li><li><a href="/ch15-02-deref.html" class="sidebar-link">15.2. 通过Dereftrait 将智能指针当作常规引用处理</a></li><li><a href="/ch15-03-drop.html" class="sidebar-link">15.3. DropTrait 运行清理代码</a></li><li><a href="/ch15-04-rc.html" class="sidebar-link">15.4. Rc<T>引用计数智能指针</a></li><li><a href="/ch15-05-interior-mutability.html" class="sidebar-link">15.5. RefCell<T>与内部可变性模式</a></li><li><a href="/ch15-06-reference-cycles.html" class="sidebar-link">15.6. 引用循环与内存泄漏是安全的</a></li></ul></section></li><li><section class="sidebar-group is-sub-group depth-1"><a href="/ch16-00-concurrency.html" class="sidebar-heading clickable"><span>16. 无畏并发</span> <!----></a> <ul class="sidebar-links sidebar-group-items"><li><a href="/ch16-01-threads.html" class="sidebar-link">16.1. 线程</a></li><li><a href="/ch16-02-message-passing.html" class="sidebar-link">16.2. 消息传递</a></li><li><a href="/ch16-03-shared-state.html" class="sidebar-link">16.3. 共享状态</a></li><li><a href="/ch16-04-extensible-concurrency-sync-and-send.html" class="sidebar-link">16.4. 可扩展的并发：Sync与Send</a></li></ul></section></li><li><section class="sidebar-group is-sub-group depth-1"><a href="/ch17-00-oop.html" class="sidebar-heading clickable"><span>17. Rust 的面向对象编程特性</span> <!----></a> <ul class="sidebar-links sidebar-group-items"><li><a href="/ch17-01-what-is-oo.html" class="sidebar-link">17.1. 面向对象语言的特点</a></li><li><a href="/ch17-02-trait-objects.html" class="sidebar-link">17.2. 为使用不同类型的值而设计的 trait 对象</a></li><li><a href="/ch17-03-oo-design-patterns.html" class="sidebar-link">17.3. 面向对象设计模式的实现</a></li></ul></section></li><li><section class="sidebar-group is-sub-group depth-1"><a href="/ch18-00-patterns.html" class="sidebar-heading clickable"><span>18. 模式用来匹配值的结构</span> <!----></a> <ul class="sidebar-links sidebar-group-items"><li><a href="/ch18-01-all-the-places-for-patterns.html" class="sidebar-link">18.1. 所有可能会用到模式的位置</a></li><li><a href="/ch18-02-refutability.html" class="sidebar-link">18.2. Refutability：何时模式可能会匹配失败</a></li><li><a href="/ch18-03-pattern-syntax.html" class="sidebar-link">18.3. 模式的全部语法</a></li></ul></section></li><li><section class="sidebar-group is-sub-group depth-1"><a href="/ch19-00-advanced-features.html" class="sidebar-heading clickable"><span>19. 高级特征</span> <!----></a> <ul class="sidebar-links sidebar-group-items"><li><a href="/ch19-01-unsafe-rust.html" class="sidebar-link">19.1. 不安全的 Rust</a></li><li><a href="/ch19-03-advanced-traits.html" class="sidebar-link">19.2. 高级 trait</a></li><li><a href="/ch19-04-advanced-types.html" class="sidebar-link">19.3. 高级类型</a></li><li><a href="/ch19-05-advanced-functions-and-closures.html" class="sidebar-link">19.4. 高级函数与闭包</a></li><li><a href="/ch19-06-macros.html" class="sidebar-link">19.5. 宏</a></li></ul></section></li><li><section class="sidebar-group is-sub-group depth-1"><a href="/ch20-00-final-project-a-web-server.html" class="sidebar-heading clickable"><span>20. 最后的项目: 构建多线程 web server</span> <!----></a> <ul class="sidebar-links sidebar-group-items"><li><a href="/ch20-01-single-threaded.html" class="sidebar-link">20.1. 单线程 web server</a></li><li><a href="/ch20-02-multithreaded.html" class="sidebar-link">20.2. 将单线程 server 变为多线程 server</a></li><li><a href="/ch20-03-graceful-shutdown-and-cleanup.html" class="sidebar-link">20.3. 优雅停机与清理</a></li></ul></section></li><li><section class="sidebar-group is-sub-group depth-1"><a href="/appendix-00.html" class="sidebar-heading clickable"><span>21. 附录</span> <!----></a> <ul class="sidebar-links sidebar-group-items"><li><a href="/appendix-01-keywords.html" class="sidebar-link">21.1. 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2020
</a></span></div></div> <div data-v-dad8a512><main class="page"><div class="page-title" style="display:none;"><h1>使用迭代器处理元素序列</h1> <div data-v-3b7f5bdf><!----> <!----> <!----> <!----></div></div> <div class="theme-reco-content content__default" style="display:none;"><h2 id="使用迭代器处理元素序列"><a href="#使用迭代器处理元素序列" class="header-anchor">#</a> 使用迭代器处理元素序列</h2> <blockquote><p><a href="https://github.com/rust-lang/book/blob/master/src/ch13-02-iterators.md" target="_blank" rel="noopener noreferrer">ch13-02-iterators.md<svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg></a> <br>
commit 8edf0457ab571b375b87357e1353ae0dd2127abe</p></blockquote> <p>迭代器模式允许你对一个项的序列进行某些处理。<strong>迭代器</strong>（<em>iterator</em>）负责遍历序列中的每一项和决定序列何时结束的逻辑。当使用迭代器时，我们无需重新实现这些逻辑。</p> <p>在 Rust 中，迭代器是 <strong>惰性的</strong>（<em>lazy</em>），这意味着在调用方法使用迭代器之前它都不会有效果。例如，示例 13-13 中的代码通过调用定义于 <code>Vec</code> 上的 <code>iter</code> 方法在一个 vector <code>v1</code> 上创建了一个迭代器。这段代码本身没有任何用处：</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">let</span> v1 <span class="token operator">=</span> <span class="token macro property">vec!</span><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> v1_iter <span class="token operator">=</span> v1<span class="token punctuation">.</span><span class="token function">iter</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre></div><p><span class="caption">示例 13-13：创建一个迭代器</span></p> <p>一旦创建迭代器之后，可以选择用多种方式利用它。在第三章的示例 3-5 中，我们使用迭代器和 <code>for</code> 循环在每一个项上执行了一些代码，虽然直到现在为止我们一直没有具体讨论调用 <code>iter</code> 到底具体做了什么。</p> <p>示例 13-14 中的例子将迭代器的创建和 <code>for</code> 循环中的使用分开。迭代器被储存在 <code>v1_iter</code> 变量中，而这时没有进行迭代。一旦 <code>for</code> 循环开始使用 <code>v1_iter</code>，接着迭代器中的每一个元素被用于循环的一次迭代，这会打印出其每一个值：</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">let</span> v1 <span class="token operator">=</span> <span class="token macro property">vec!</span><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> v1_iter <span class="token operator">=</span> v1<span class="token punctuation">.</span><span class="token function">iter</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">for</span> val <span class="token keyword">in</span> v1_iter <span class="token punctuation">{</span>
<span class="token macro property">println!</span><span class="token punctuation">(</span><span class="token string">"Got: {}"</span><span class="token punctuation">,</span> val<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre></div><p><span class="caption">示例 13-14：在一个 <code>for</code> 循环中使用迭代器</span></p> <p>在标准库中没有提供迭代器的语言中，我们可能会使用一个从 0 开始的索引变量，使用这个变量索引 vector 中的值，并循环增加其值直到达到 vector 的元素数量。</p> <p>迭代器为我们处理了所有这些逻辑，这减少了重复代码并消除了潜在的混乱。另外，迭代器的实现方式提供了对多种不同的序列使用相同逻辑的灵活性，而不仅仅是像 vector 这样可索引的数据结构.让我们看看迭代器是如何做到这些的。</p> <h3 id="iterator-trait-和-next-方法"><a href="#iterator-trait-和-next-方法" class="header-anchor">#</a> <code>Iterator</code> trait 和 <code>next</code> 方法</h3> <p>迭代器都实现了一个叫做 <code>Iterator</code> 的定义于标准库的 trait。这个 trait 的定义看起来像这样：</p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">pub</span> <span class="token keyword">trait</span> <span class="token class-name">Iterator</span> <span class="token punctuation">{</span>
<span class="token keyword">type</span> <span class="token class-name">Item</span><span class="token punctuation">;</span>
<span class="token keyword">fn</span> <span class="token function-definition function">next</span><span class="token punctuation">(</span><span class="token operator">&</span><span class="token keyword">mut</span> <span class="token keyword">self</span><span class="token punctuation">)</span> <span class="token punctuation">-></span> <span class="token class-name">Option</span><span class="token operator"><</span><span class="token keyword">Self</span><span class="token punctuation">::</span><span class="token class-name">Item</span><span class="token operator">></span><span class="token punctuation">;</span>
<span class="token comment">// 此处省略了方法的默认实现</span>
<span class="token punctuation">}</span>
</code></pre></div><p>注意这里有一下我们还未讲到的新语法：<code>type Item</code> 和 <code>Self::Item</code>，他们定义了 trait 的 <strong>关联类型</strong>（<em>associated type</em>）。第十九章会深入讲解关联类型，不过现在只需知道这段代码表明实现 <code>Iterator</code> trait 要求同时定义一个 <code>Item</code> 类型，这个 <code>Item</code> 类型被用作 <code>next</code> 方法的返回值类型。换句话说，<code>Item</code> 类型将是迭代器返回元素的类型。</p> <p><code>next</code> 是 <code>Iterator</code> 实现者被要求定义的唯一方法。<code>next</code> 一次返回迭代器中的一个项，封装在 <code>Some</code> 中，当迭代器结束时，它返回 <code>None</code>。</p> <p>可以直接调用迭代器的 <code>next</code> 方法；示例 13-15 有一个测试展示了重复调用由 vector 创建的迭代器的 <code>next</code> 方法所得到的值：</p> <p><span class="filename">文件名: src/lib.rs</span></p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token attribute attr-name">#[test]</span>
<span class="token keyword">fn</span> <span class="token function-definition function">iterator_demonstration</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">let</span> v1 <span class="token operator">=</span> <span class="token macro property">vec!</span><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> <span class="token keyword">mut</span> v1_iter <span class="token operator">=</span> v1<span class="token punctuation">.</span><span class="token function">iter</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token macro property">assert_eq!</span><span class="token punctuation">(</span>v1_iter<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token class-name">Some</span><span class="token punctuation">(</span><span class="token operator">&</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token macro property">assert_eq!</span><span class="token punctuation">(</span>v1_iter<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token class-name">Some</span><span class="token punctuation">(</span><span class="token operator">&</span><span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token macro property">assert_eq!</span><span class="token punctuation">(</span>v1_iter<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token class-name">Some</span><span class="token punctuation">(</span><span class="token operator">&</span><span class="token number">3</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token macro property">assert_eq!</span><span class="token punctuation">(</span>v1_iter<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token class-name">None</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre></div><p><span class="caption">示例 13-15：在迭代器上（直接）调用 <code>next</code> 方法</span></p> <p>注意 <code>v1_iter</code> 需要是可变的：在迭代器上调用 <code>next</code> 方法改变了迭代器中用来记录序列位置的状态。换句话说，代码 <strong>消费</strong>（consume）了，或使用了迭代器。每一个 <code>next</code> 调用都会从迭代器中消费一个项。使用 <code>for</code> 循环时无需使 <code>v1_iter</code> 可变因为 <code>for</code> 循环会获取 <code>v1_iter</code> 的所有权并在后台使 <code>v1_iter</code> 可变。</p> <p>另外需要注意到从 <code>next</code> 调用中得到的值是 vector 的不可变引用。<code>iter</code> 方法生成一个不可变引用的迭代器。如果我们需要一个获取 <code>v1</code> 所有权并返回拥有所有权的迭代器，则可以调用 <code>into_iter</code> 而不是 <code>iter</code>。类似的，如果我们希望迭代可变引用，则可以调用 <code>iter_mut</code> 而不是 <code>iter</code>。</p> <h3 id="消费迭代器的方法"><a href="#消费迭代器的方法" class="header-anchor">#</a> 消费迭代器的方法</h3> <p><code>Iterator</code> trait 有一系列不同的由标准库提供默认实现的方法；你可以在 <code>Iterator</code> trait 的标准库 API 文档中找到所有这些方法。一些方法在其定义中调用了 <code>next</code> 方法，这也就是为什么在实现 <code>Iterator</code> trait 时要求实现 <code>next</code> 方法的原因。</p> <p>这些调用 <code>next</code> 方法的方法被称为 <strong>消费适配器</strong>（<em>consuming adaptors</em>），因为调用他们会消耗迭代器。一个消费适配器的例子是 <code>sum</code> 方法。这个方法获取迭代器的所有权并反复调用 <code>next</code> 来遍历迭代器，因而会消费迭代器。当其遍历每一个项时，它将每一个项加总到一个总和并在迭代完成时返回总和。示例 13-16 有一个展示 <code>sum</code> 方法使用的测试：</p> <p><span class="filename">文件名: src/lib.rs</span></p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token attribute attr-name">#[test]</span>
<span class="token keyword">fn</span> <span class="token function-definition function">iterator_sum</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">let</span> v1 <span class="token operator">=</span> <span class="token macro property">vec!</span><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> v1_iter <span class="token operator">=</span> v1<span class="token punctuation">.</span><span class="token function">iter</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> total<span class="token punctuation">:</span> <span class="token keyword">i32</span> <span class="token operator">=</span> v1_iter<span class="token punctuation">.</span><span class="token function">sum</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token macro property">assert_eq!</span><span class="token punctuation">(</span>total<span class="token punctuation">,</span> <span class="token number">6</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre></div><p><span class="caption">示例 13-16：调用 <code>sum</code> 方法获取迭代器所有项的总和</span></p> <p>调用 <code>sum</code> 之后不再允许使用 <code>v1_iter</code> 因为调用 <code>sum</code> 时它会获取迭代器的所有权。</p> <h3 id="产生其他迭代器的方法"><a href="#产生其他迭代器的方法" class="header-anchor">#</a> 产生其他迭代器的方法</h3> <p><code>Iterator</code> trait 中定义了另一类方法，被称为 <strong>迭代器适配器</strong>（<em>iterator adaptors</em>），他们允许我们将当前迭代器变为不同类型的迭代器。可以链式调用多个迭代器适配器。不过因为所有的迭代器都是惰性的，必须调用一个消费适配器方法以便获取迭代器适配器调用的结果。</p> <p>示例 13-17 展示了一个调用迭代器适配器方法 <code>map</code> 的例子，该 <code>map</code> 方法使用闭包来调用每个元素以生成新的迭代器。 这里的闭包创建了一个新的迭代器，对其中 vector 中的每个元素都被加 1。不过这些代码会产生一个警告：</p> <p><span class="filename">文件名: src/main.rs</span></p> <div class="language-rust,not_desired_behavior extra-class"><pre class="language-text"><code>let v1: Vec<i32> = vec![1, 2, 3];
v1.iter().map(|x| x + 1);
</code></pre></div><p><span class="caption">示例 13-17：调用迭代器适配器 <code>map</code> 来创建一个新迭代器</span></p> <p>得到的警告是：</p> <div class="language-text extra-class"><pre class="language-text"><code>warning: unused `std::iter::Map` which must be used: iterator adaptors are lazy
and do nothing unless consumed
--> src/main.rs:4:5
|
4 | v1.iter().map(|x| x + 1);
| ^^^^^^^^^^^^^^^^^^^^^^^^^
|
= note: #[warn(unused_must_use)] on by default
</code></pre></div><p>示例 13-17 中的代码实际上并没有做任何事；所指定的闭包从未被调用过。警告提醒了我们为什么：迭代器适配器是惰性的，而这里我们需要消费迭代器。</p> <p>为了修复这个警告并消费迭代器获取有用的结果，我们将使用第十二章示例 12-1 结合 <code>env::args</code> 使用的 <code>collect</code> 方法。这个方法消费迭代器并将结果收集到一个数据结构中。</p> <p>在示例 13-18 中，我们将遍历由 <code>map</code> 调用生成的迭代器的结果收集到一个 vector 中，它将会含有原始 vector 中每个元素加 1 的结果：</p> <p><span class="filename">文件名: src/main.rs</span></p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">let</span> v1<span class="token punctuation">:</span> <span class="token class-name">Vec</span><span class="token operator"><</span><span class="token keyword">i32</span><span class="token operator">></span> <span class="token operator">=</span> <span class="token macro property">vec!</span><span class="token punctuation">[</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> v2<span class="token punctuation">:</span> <span class="token class-name">Vec</span><span class="token operator"><</span>_<span class="token operator">></span> <span class="token operator">=</span> v1<span class="token punctuation">.</span><span class="token function">iter</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span><span class="token closure-params"><span class="token closure-punctuation punctuation">|</span>x<span class="token closure-punctuation punctuation">|</span></span> x <span class="token operator">+</span> <span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">collect</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token macro property">assert_eq!</span><span class="token punctuation">(</span>v2<span class="token punctuation">,</span> <span class="token macro property">vec!</span><span class="token punctuation">[</span><span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">]</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
</code></pre></div><p><span class="caption">示例 13-18：调用 <code>map</code> 方法创建一个新迭代器，接着调用 <code>collect</code> 方法消费新迭代器并创建一个 vector</span></p> <p>因为 <code>map</code> 获取一个闭包，可以指定任何希望在遍历的每个元素上执行的操作。这是一个展示如何使用闭包来自定义行为同时又复用 <code>Iterator</code> trait 提供的迭代行为的绝佳例子。</p> <h3 id="使用闭包获取环境"><a href="#使用闭包获取环境" class="header-anchor">#</a> 使用闭包获取环境</h3> <p>现在我们介绍了迭代器，让我们展示一个通过使用 <code>filter</code> 迭代器适配器和捕获环境的闭包的常规用例。迭代器的 <code>filter</code> 方法获取一个使用迭代器的每一个项并返回布尔值的闭包。如果闭包返回 <code>true</code>，其值将会包含在 <code>filter</code> 提供的新迭代器中。如果闭包返回 <code>false</code>，其值不会包含在结果迭代器中。</p> <p>示例 13-19 展示了使用 <code>filter</code> 和一个捕获环境中变量 <code>shoe_size</code> 的闭包，这样闭包就可以遍历一个 <code>Shoe</code> 结构体集合以便只返回指定大小的鞋子：</p> <p><span class="filename">文件名: src/lib.rs</span></p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token attribute attr-name">#[derive(PartialEq, Debug)]</span>
<span class="token keyword">struct</span> <span class="token type-definition class-name">Shoe</span> <span class="token punctuation">{</span>
size<span class="token punctuation">:</span> <span class="token keyword">u32</span><span class="token punctuation">,</span>
style<span class="token punctuation">:</span> <span class="token class-name">String</span><span class="token punctuation">,</span>
<span class="token punctuation">}</span>
<span class="token keyword">fn</span> <span class="token function-definition function">shoes_in_my_size</span><span class="token punctuation">(</span>shoes<span class="token punctuation">:</span> <span class="token class-name">Vec</span><span class="token operator"><</span><span class="token class-name">Shoe</span><span class="token operator">></span><span class="token punctuation">,</span> shoe_size<span class="token punctuation">:</span> <span class="token keyword">u32</span><span class="token punctuation">)</span> <span class="token punctuation">-></span> <span class="token class-name">Vec</span><span class="token operator"><</span><span class="token class-name">Shoe</span><span class="token operator">></span> <span class="token punctuation">{</span>
shoes<span class="token punctuation">.</span><span class="token function">into_iter</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">.</span><span class="token function">filter</span><span class="token punctuation">(</span><span class="token closure-params"><span class="token closure-punctuation punctuation">|</span>s<span class="token closure-punctuation punctuation">|</span></span> s<span class="token punctuation">.</span>size <span class="token operator">==</span> shoe_size<span class="token punctuation">)</span>
<span class="token punctuation">.</span><span class="token function">collect</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
<span class="token punctuation">}</span>
<span class="token attribute attr-name">#[test]</span>
<span class="token keyword">fn</span> <span class="token function-definition function">filters_by_size</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">let</span> shoes <span class="token operator">=</span> <span class="token macro property">vec!</span><span class="token punctuation">[</span>
<span class="token class-name">Shoe</span> <span class="token punctuation">{</span> size<span class="token punctuation">:</span> <span class="token number">10</span><span class="token punctuation">,</span> style<span class="token punctuation">:</span> <span class="token class-name">String</span><span class="token punctuation">::</span><span class="token function">from</span><span class="token punctuation">(</span><span class="token string">"sneaker"</span><span class="token punctuation">)</span> <span class="token punctuation">}</span><span class="token punctuation">,</span>
<span class="token class-name">Shoe</span> <span class="token punctuation">{</span> size<span class="token punctuation">:</span> <span class="token number">13</span><span class="token punctuation">,</span> style<span class="token punctuation">:</span> <span class="token class-name">String</span><span class="token punctuation">::</span><span class="token function">from</span><span class="token punctuation">(</span><span class="token string">"sandal"</span><span class="token punctuation">)</span> <span class="token punctuation">}</span><span class="token punctuation">,</span>
<span class="token class-name">Shoe</span> <span class="token punctuation">{</span> size<span class="token punctuation">:</span> <span class="token number">10</span><span class="token punctuation">,</span> style<span class="token punctuation">:</span> <span class="token class-name">String</span><span class="token punctuation">::</span><span class="token function">from</span><span class="token punctuation">(</span><span class="token string">"boot"</span><span class="token punctuation">)</span> <span class="token punctuation">}</span><span class="token punctuation">,</span>
<span class="token punctuation">]</span><span class="token punctuation">;</span>
<span class="token keyword">let</span> in_my_size <span class="token operator">=</span> <span class="token function">shoes_in_my_size</span><span class="token punctuation">(</span>shoes<span class="token punctuation">,</span> <span class="token number">10</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token macro property">assert_eq!</span><span class="token punctuation">(</span>
in_my_size<span class="token punctuation">,</span>
<span class="token macro property">vec!</span><span class="token punctuation">[</span>
<span class="token class-name">Shoe</span> <span class="token punctuation">{</span> size<span class="token punctuation">:</span> <span class="token number">10</span><span class="token punctuation">,</span> style<span class="token punctuation">:</span> <span class="token class-name">String</span><span class="token punctuation">::</span><span class="token function">from</span><span class="token punctuation">(</span><span class="token string">"sneaker"</span><span class="token punctuation">)</span> <span class="token punctuation">}</span><span class="token punctuation">,</span>
<span class="token class-name">Shoe</span> <span class="token punctuation">{</span> size<span class="token punctuation">:</span> <span class="token number">10</span><span class="token punctuation">,</span> style<span class="token punctuation">:</span> <span class="token class-name">String</span><span class="token punctuation">::</span><span class="token function">from</span><span class="token punctuation">(</span><span class="token string">"boot"</span><span class="token punctuation">)</span> <span class="token punctuation">}</span><span class="token punctuation">,</span>
<span class="token punctuation">]</span>
<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre></div><p><span class="caption">示例 13-19：使用 <code>filter</code> 方法和一个捕获 <code>shoe_size</code> 的闭包</span></p> <p><code>shoes_in_my_size</code> 函数获取一个鞋子 vector 的所有权和一个鞋子大小作为参数。它返回一个只包含指定大小鞋子的 vector。</p> <p><code>shoes_in_my_size</code> 函数体中调用了 <code>into_iter</code> 来创建一个获取 vector 所有权的迭代器。接着调用 <code>filter</code> 将这个迭代器适配成一个只含有那些闭包返回 <code>true</code> 的元素的新迭代器。</p> <p>闭包从环境中捕获了 <code>shoe_size</code> 变量并使用其值与每一只鞋的大小作比较，只保留指定大小的鞋子。最终，调用 <code>collect</code> 将迭代器适配器返回的值收集进一个 vector 并返回。</p> <p>这个测试展示当调用 <code>shoes_in_my_size</code> 时，我们只会得到与指定值相同大小的鞋子。</p> <h3 id="实现-iterator-trait-来创建自定义迭代器"><a href="#实现-iterator-trait-来创建自定义迭代器" class="header-anchor">#</a> 实现 <code>Iterator</code> trait 来创建自定义迭代器</h3> <p>我们已经展示了可以通过在 vector 上调用 <code>iter</code>、<code>into_iter</code> 或 <code>iter_mut</code> 来创建一个迭代器。也可以用标准库中其他的集合类型创建迭代器，比如哈希 map。另外，可以实现 <code>Iterator</code> trait 来创建任何我们希望的迭代器。正如之前提到的，定义中唯一要求提供的方法就是 <code>next</code> 方法。一旦定义了它，就可以使用所有其他由 <code>Iterator</code> trait 提供的拥有默认实现的方法来创建自定义迭代器了！</p> <p>作为展示，让我们创建一个只会从 1 数到 5 的迭代器。首先，创建一个结构体来存放一些值，接着实现 <code>Iterator</code> trait 将这个结构体放入迭代器中并在此实现中使用其值。</p> <p>示例 13-20 有一个 <code>Counter</code> 结构体定义和一个创建 <code>Counter</code> 实例的关联函数 <code>new</code>：</p> <p><span class="filename">文件名: src/lib.rs</span></p> <div class="language-rust extra-class"><pre class="language-rust"><code><span class="token keyword">struct</span> <span class="token type-definition class-name">Counter</span> <span class="token punctuation">{</span>
count<span class="token punctuation">:</span> <span class="token keyword">u32</span><span class="token punctuation">,</span>
<span class="token punctuation">}</span>
<span class="token keyword">impl</span> <span class="token class-name">Counter</span> <span class="token punctuation">{</span>
<span class="token keyword">fn</span> <span class="token function-definition function">new</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">-></span> <span class="token class-name">Counter</span> <span class="token punctuation">{</span>
<span class="token class-name">Counter</span> <span class="token punctuation">{</span> count<span class="token punctuation">:</span> <span class="token number">0</span> <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p><span class="caption">示例 13-20：定义 <code>Counter</code> 结构体和一个创建 <code>count</code> 初值为 0 的 <code>Counter</code> 实例的 <code>new</code> 函数</span></p> <p><code>Counter</code> 结构体有一个字段 <code>count</code>。这个字段存放一个 <code>u32</code> 值，它会记录处理 1 到 5 的迭代过程中的位置。<code>count</code> 是私有的因为我们希望 <code>Counter</code> 的实现来管理这个值。<code>new</code> 函数通过总是从为 0 的 <code>count</code> 字段开始新实例来确保我们需要的行为。</p> <p>接下来将为 <code>Counter</code> 类型实现 <code>Iterator</code> trait，通过定义 <code>next</code> 方法来指定使用迭代器时的行为，如示例 13-21 所示：</p> <p><span class="filename">文件名: src/lib.rs</span></p> <div class="language-rust extra-class"><pre class="language-rust"><code># <span class="token keyword">struct</span> <span class="token type-definition class-name">Counter</span> <span class="token punctuation">{</span>
# count<span class="token punctuation">:</span> <span class="token keyword">u32</span><span class="token punctuation">,</span>
# <span class="token punctuation">}</span>
#
<span class="token keyword">impl</span> <span class="token class-name">Iterator</span> <span class="token keyword">for</span> <span class="token class-name">Counter</span> <span class="token punctuation">{</span>
<span class="token keyword">type</span> <span class="token class-name">Item</span> <span class="token operator">=</span> <span class="token keyword">u32</span><span class="token punctuation">;</span>
<span class="token keyword">fn</span> <span class="token function-definition function">next</span><span class="token punctuation">(</span><span class="token operator">&</span><span class="token keyword">mut</span> <span class="token keyword">self</span><span class="token punctuation">)</span> <span class="token punctuation">-></span> <span class="token class-name">Option</span><span class="token operator"><</span><span class="token keyword">Self</span><span class="token punctuation">::</span><span class="token class-name">Item</span><span class="token operator">></span> <span class="token punctuation">{</span>
<span class="token keyword">self</span><span class="token punctuation">.</span>count <span class="token operator">+=</span> <span class="token number">1</span><span class="token punctuation">;</span>
<span class="token keyword">if</span> <span class="token keyword">self</span><span class="token punctuation">.</span>count <span class="token operator"><</span> <span class="token number">6</span> <span class="token punctuation">{</span>
<span class="token class-name">Some</span><span class="token punctuation">(</span><span class="token keyword">self</span><span class="token punctuation">.</span>count<span class="token punctuation">)</span>
<span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
<span class="token class-name">None</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p><span class="caption">示例 13-21：在 <code>Counter</code> 结构体上实现 <code>Iterator</code> trait</span></p> <p>这里将迭代器的关联类型 <code>Item</code> 设置为 <code>u32</code>，意味着迭代器会返回 <code>u32</code> 值集合。再一次，这里仍无需担心关联类型，第十九章会讲到。</p> <p>我们希望迭代器对其内部状态加一，这也就是为何将 <code>count</code> 初始化为 0：我们希望迭代器首先返回 1。如果 <code>count</code> 值小于 6，<code>next</code> 会返回封装在 <code>Some</code> 中的当前值，不过如果 <code>count</code> 大于或等于 6，迭代器会返回 <code>None</code>。</p> <h4 id="使用-counter-迭代器的-next-方法"><a href="#使用-counter-迭代器的-next-方法" class="header-anchor">#</a> 使用 <code>Counter</code> 迭代器的 <code>next</code> 方法</h4> <p>一旦实现了 <code>Iterator</code> trait，我们就有了一个迭代器！示例 13-22 展示了一个测试用来演示使用 <code>Counter</code> 结构体的迭代器功能，通过直接调用 <code>next</code> 方法，正如示例 13-15 中从 vector 创建的迭代器那样：</p> <p><span class="filename">文件名: src/lib.rs</span></p> <div class="language-rust extra-class"><pre class="language-rust"><code># <span class="token keyword">struct</span> <span class="token type-definition class-name">Counter</span> <span class="token punctuation">{</span>
# count<span class="token punctuation">:</span> <span class="token keyword">u32</span><span class="token punctuation">,</span>
# <span class="token punctuation">}</span>
#
# <span class="token keyword">impl</span> <span class="token class-name">Iterator</span> <span class="token keyword">for</span> <span class="token class-name">Counter</span> <span class="token punctuation">{</span>
# <span class="token keyword">type</span> <span class="token class-name">Item</span> <span class="token operator">=</span> <span class="token keyword">u32</span><span class="token punctuation">;</span>
#
# <span class="token keyword">fn</span> <span class="token function-definition function">next</span><span class="token punctuation">(</span><span class="token operator">&</span><span class="token keyword">mut</span> <span class="token keyword">self</span><span class="token punctuation">)</span> <span class="token punctuation">-></span> <span class="token class-name">Option</span><span class="token operator"><</span><span class="token keyword">Self</span><span class="token punctuation">::</span><span class="token class-name">Item</span><span class="token operator">></span> <span class="token punctuation">{</span>
# <span class="token keyword">self</span><span class="token punctuation">.</span>count <span class="token operator">+=</span> <span class="token number">1</span><span class="token punctuation">;</span>
#
# <span class="token keyword">if</span> <span class="token keyword">self</span><span class="token punctuation">.</span>count <span class="token operator"><</span> <span class="token number">6</span> <span class="token punctuation">{</span>
# <span class="token class-name">Some</span><span class="token punctuation">(</span><span class="token keyword">self</span><span class="token punctuation">.</span>count<span class="token punctuation">)</span>
# <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
# <span class="token class-name">None</span>
# <span class="token punctuation">}</span>
# <span class="token punctuation">}</span>
# <span class="token punctuation">}</span>
#
<span class="token attribute attr-name">#[test]</span>
<span class="token keyword">fn</span> <span class="token function-definition function">calling_next_directly</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">let</span> <span class="token keyword">mut</span> counter <span class="token operator">=</span> <span class="token class-name">Counter</span><span class="token punctuation">::</span><span class="token function">new</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token macro property">assert_eq!</span><span class="token punctuation">(</span>counter<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token class-name">Some</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token macro property">assert_eq!</span><span class="token punctuation">(</span>counter<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token class-name">Some</span><span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token macro property">assert_eq!</span><span class="token punctuation">(</span>counter<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token class-name">Some</span><span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token macro property">assert_eq!</span><span class="token punctuation">(</span>counter<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token class-name">Some</span><span class="token punctuation">(</span><span class="token number">4</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token macro property">assert_eq!</span><span class="token punctuation">(</span>counter<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token class-name">Some</span><span class="token punctuation">(</span><span class="token number">5</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token macro property">assert_eq!</span><span class="token punctuation">(</span>counter<span class="token punctuation">.</span><span class="token function">next</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token class-name">None</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre></div><p><span class="caption">示例 13-22：测试 <code>next</code> 方法实现的功能</span></p> <p>这个测试在 <code>counter</code> 变量中新建了一个 <code>Counter</code> 实例并接着反复调用 <code>next</code> 方法，来验证我们实现的行为符合这个迭代器返回从 1 到 5 的值的预期。</p> <h4 id="使用自定义迭代器中其他-iterator-trait-方法"><a href="#使用自定义迭代器中其他-iterator-trait-方法" class="header-anchor">#</a> 使用自定义迭代器中其他 <code>Iterator</code> trait 方法</h4> <p>通过定义 <code>next</code> 方法实现 <code>Iterator</code> trait，我们现在就可以使用任何标准库定义的拥有默认实现的 <code>Iterator</code> trait 方法了，因为他们都使用了 <code>next</code> 方法的功能。</p> <p>例如，出于某种原因我们希望获取 <code>Counter</code> 实例产生的值，将这些值与另一个 <code>Counter</code> 实例在省略了第一个值之后产生的值配对，将每一对值相乘，只保留那些可以被三整除的结果，然后将所有保留的结果相加，这可以如示例 13-23 中的测试这样做：</p> <p><span class="filename">文件名: src/lib.rs</span></p> <div class="language-rust extra-class"><pre class="language-rust"><code># <span class="token keyword">struct</span> <span class="token type-definition class-name">Counter</span> <span class="token punctuation">{</span>
# count<span class="token punctuation">:</span> <span class="token keyword">u32</span><span class="token punctuation">,</span>
# <span class="token punctuation">}</span>
#
# <span class="token keyword">impl</span> <span class="token class-name">Counter</span> <span class="token punctuation">{</span>
# <span class="token keyword">fn</span> <span class="token function-definition function">new</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">-></span> <span class="token class-name">Counter</span> <span class="token punctuation">{</span>
# <span class="token class-name">Counter</span> <span class="token punctuation">{</span> count<span class="token punctuation">:</span> <span class="token number">0</span> <span class="token punctuation">}</span>
# <span class="token punctuation">}</span>
# <span class="token punctuation">}</span>
#
# <span class="token keyword">impl</span> <span class="token class-name">Iterator</span> <span class="token keyword">for</span> <span class="token class-name">Counter</span> <span class="token punctuation">{</span>
# <span class="token comment">// 迭代器会产生 u32s</span>
# <span class="token keyword">type</span> <span class="token class-name">Item</span> <span class="token operator">=</span> <span class="token keyword">u32</span><span class="token punctuation">;</span>
#
# <span class="token keyword">fn</span> <span class="token function-definition function">next</span><span class="token punctuation">(</span><span class="token operator">&</span><span class="token keyword">mut</span> <span class="token keyword">self</span><span class="token punctuation">)</span> <span class="token punctuation">-></span> <span class="token class-name">Option</span><span class="token operator"><</span><span class="token keyword">Self</span><span class="token punctuation">::</span><span class="token class-name">Item</span><span class="token operator">></span> <span class="token punctuation">{</span>
# <span class="token comment">// count 自增 1。也就是为什么从 0 开始。</span>
# <span class="token keyword">self</span><span class="token punctuation">.</span>count <span class="token operator">+=</span> <span class="token number">1</span><span class="token punctuation">;</span>
#
# <span class="token comment">// 检测是否结束结束计数。</span>
# <span class="token keyword">if</span> <span class="token keyword">self</span><span class="token punctuation">.</span>count <span class="token operator"><</span> <span class="token number">6</span> <span class="token punctuation">{</span>
# <span class="token class-name">Some</span><span class="token punctuation">(</span><span class="token keyword">self</span><span class="token punctuation">.</span>count<span class="token punctuation">)</span>
# <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
# <span class="token class-name">None</span>
# <span class="token punctuation">}</span>
# <span class="token punctuation">}</span>
# <span class="token punctuation">}</span>
#
<span class="token attribute attr-name">#[test]</span>
<span class="token keyword">fn</span> <span class="token function-definition function">using_other_iterator_trait_methods</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
<span class="token keyword">let</span> sum<span class="token punctuation">:</span> <span class="token keyword">u32</span> <span class="token operator">=</span> <span class="token class-name">Counter</span><span class="token punctuation">::</span><span class="token function">new</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">zip</span><span class="token punctuation">(</span><span class="token class-name">Counter</span><span class="token punctuation">::</span><span class="token function">new</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">skip</span><span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">)</span>
<span class="token punctuation">.</span><span class="token function">map</span><span class="token punctuation">(</span><span class="token closure-params"><span class="token closure-punctuation punctuation">|</span><span class="token punctuation">(</span>a<span class="token punctuation">,</span> b<span class="token punctuation">)</span><span class="token closure-punctuation punctuation">|</span></span> a <span class="token operator">*</span> b<span class="token punctuation">)</span>
<span class="token punctuation">.</span><span class="token function">filter</span><span class="token punctuation">(</span><span class="token closure-params"><span class="token closure-punctuation punctuation">|</span>x<span class="token closure-punctuation punctuation">|</span></span> x <span class="token operator">%</span> <span class="token number">3</span> <span class="token operator">==</span> <span class="token number">0</span><span class="token punctuation">)</span>
<span class="token punctuation">.</span><span class="token function">sum</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token macro property">assert_eq!</span><span class="token punctuation">(</span><span class="token number">18</span><span class="token punctuation">,</span> sum<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre></div><p><span class="caption">示例 13-23：使用自定义的 <code>Counter</code> 迭代器的多种方法</span></p> <p>注意 <code>zip</code> 只产生四对值；理论上第五对值 <code>(5, None)</code> 从未被产生，因为 <code>zip</code> 在任一输入迭代器返回 <code>None</code> 时也返回 <code>None</code>。</p> <p>所有这些方法调用都是可能的，因为我们指定了 <code>next</code> 方法如何工作，而标准库则提供了其它调用 <code>next</code> 的方法的默认实现。</p></div> <footer class="page-edit" style="display:none;"><!----> <!----></footer> <!----> <!----> <!----></main> <!----></div></div></div></div><div class="global-ui"><div class="back-to-ceiling" style="right:1rem;bottom:6rem;width:2.5rem;height:2.5rem;border-radius:.25rem;line-height:2.5rem;display:none;" data-v-c6073ba8 data-v-c6073ba8><svg t="1574745035067" viewBox="0 0 1024 1024" version="1.1" xmlns="http://www.w3.org/2000/svg" p-id="5404" class="icon" data-v-c6073ba8><path d="M526.60727968 10.90185116a27.675 27.675 0 0 0-29.21455937 0c-131.36607665 82.28402758-218.69155461 228.01873535-218.69155402 394.07834331a462.20625001 462.20625001 0 0 0 5.36959153 69.94390903c1.00431239 6.55289093-0.34802892 13.13561351-3.76865779 18.80351572-32.63518765 54.11355614-51.75690182 118.55860487-51.7569018 187.94566865a371.06718723 371.06718723 0 0 0 11.50484808 91.98906777c6.53300375 25.50556257 41.68394495 28.14064038 52.69160883 4.22606766 17.37162448-37.73630017 42.14135425-72.50938081 72.80769204-103.21549295 2.18761121 3.04276886 4.15646224 6.24463696 6.40373557 9.22774369a1871.4375 1871.4375 0 0 0 140.04691725 5.34970492 1866.36093723 1866.36093723 0 0 0 140.04691723-5.34970492c2.24727335-2.98310674 4.21612437-6.18497483 6.3937923-9.2178004 30.66633723 30.70611158 55.4360664 65.4791928 72.80769147 103.21549355 11.00766384 23.91457269 46.15860503 21.27949489 52.69160879-4.22606768a371.15156223 371.15156223 0 0 0 11.514792-91.99901164c0-69.36717486-19.13165746-133.82216804-51.75690182-187.92578088-3.42062944-5.66790279-4.76302748-12.26056868-3.76865837-18.80351632a462.20625001 462.20625001 0 0 0 5.36959269-69.943909c-0.00994388-166.08943902-87.32547796-311.81420293-218.6915546-394.09823051zM605.93803103 357.87693858a93.93749974 93.93749974 0 1 1-187.89594924 6.1e-7 93.93749974 93.93749974 0 0 1 187.89594924-6.1e-7z" p-id="5405" data-v-c6073ba8></path><path d="M429.50777625 765.63860547C429.50777625 803.39355007 466.44236686 1000.39046097 512.00932183 1000.39046097c45.56695499 0 82.4922232-197.00623328 82.5015456-234.7518555 0-37.75494459-36.9345906-68.35043303-82.4922232-68.34111062-45.57627738-0.00932239-82.52019037 30.59548842-82.51086798 68.34111062z" p-id="5406" data-v-c6073ba8></path></svg></div></div></div>
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