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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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<h2 id="改进-io-项目"><a class="header" href="#改进-io-项目">改进 I/O 项目</a></h2>
<blockquote>
<p><a href="https://github.com/rust-lang/book/blob/main/src/ch13-03-improving-our-io-project.md">ch13-03-improving-our-io-project.md</a>
<br>
commit 2cd1b5593d26dc6a03c20f8619187ad4b2485552</p>
</blockquote>
<p>掌握了这些关于迭代器的新知识后，我们可以使用迭代器来改进第十二章中 I/O 项目的实现来使得代码更简洁明了。接下来，让我们看看迭代器如何改进 <code>Config::build</code> 函数和 <code>search</code> 函数的实现。</p>
<h3 id="使用迭代器去除-clone"><a class="header" href="#使用迭代器去除-clone">使用迭代器去除 <code>clone</code></a></h3>
<p>在示例 12-6 中，我们增加了一些代码获取一个 <code>String</code> 类型的 slice 并创建一个 <code>Config</code> 结构体的实例，它们索引 slice 中的值并克隆这些值以便 <code>Config</code> 结构体可以拥有这些值。在示例 13-17 中重现了第十二章结尾示例 12-23 中 <code>Config::build</code> 函数的实现：</p>
<p><span class="filename">文件名：src/lib.rs</span></p>
<pre><code class="language-rust ignore"><span class="boring">use std::env;
</span><span class="boring">use std::error::Error;
</span><span class="boring">use std::fs;
</span><span class="boring">
</span><span class="boring">pub struct Config {
</span><span class="boring"> pub query: String,
</span><span class="boring"> pub file_path: String,
</span><span class="boring"> pub ignore_case: bool,
</span><span class="boring">}
</span><span class="boring">
</span>impl Config {
pub fn build(args: &amp;[String]) -&gt; Result&lt;Config, &amp;'static str&gt; {
if args.len() &lt; 3 {
return Err("not enough arguments");
}
let query = args[1].clone();
let file_path = args[2].clone();
let ignore_case = env::var("IGNORE_CASE").is_ok();
Ok(Config {
query,
file_path,
ignore_case,
})
}
}
<span class="boring">
</span><span class="boring">pub fn run(config: Config) -&gt; Result&lt;(), Box&lt;dyn Error&gt;&gt; {
</span><span class="boring"> let contents = fs::read_to_string(config.file_path)?;
</span><span class="boring">
</span><span class="boring"> let results = if config.ignore_case {
</span><span class="boring"> search_case_insensitive(&amp;config.query, &amp;contents)
</span><span class="boring"> } else {
</span><span class="boring"> search(&amp;config.query, &amp;contents)
</span><span class="boring"> };
</span><span class="boring">
</span><span class="boring"> for line in results {
</span><span class="boring"> println!("{line}");
</span><span class="boring"> }
</span><span class="boring">
</span><span class="boring"> Ok(())
</span><span class="boring">}
</span><span class="boring">
</span><span class="boring">pub fn search&lt;'a&gt;(query: &amp;str, contents: &amp;'a str) -&gt; Vec&lt;&amp;'a str&gt; {
</span><span class="boring"> let mut results = Vec::new();
</span><span class="boring">
</span><span class="boring"> for line in contents.lines() {
</span><span class="boring"> if line.contains(query) {
</span><span class="boring"> results.push(line);
</span><span class="boring"> }
</span><span class="boring"> }
</span><span class="boring">
</span><span class="boring"> results
</span><span class="boring">}
</span><span class="boring">
</span><span class="boring">pub fn search_case_insensitive&lt;'a&gt;(
</span><span class="boring"> query: &amp;str,
</span><span class="boring"> contents: &amp;'a str,
</span><span class="boring">) -&gt; Vec&lt;&amp;'a str&gt; {
</span><span class="boring"> let query = query.to_lowercase();
</span><span class="boring"> let mut results = Vec::new();
</span><span class="boring">
</span><span class="boring"> for line in contents.lines() {
</span><span class="boring"> if line.to_lowercase().contains(&amp;query) {
</span><span class="boring"> results.push(line);
</span><span class="boring"> }
</span><span class="boring"> }
</span><span class="boring">
</span><span class="boring"> results
</span><span class="boring">}
</span><span class="boring">
</span><span class="boring">#[cfg(test)]
</span><span class="boring">mod tests {
</span><span class="boring"> use super::*;
</span><span class="boring">
</span><span class="boring"> #[test]
</span><span class="boring"> fn case_sensitive() {
</span><span class="boring"> let query = "duct";
</span><span class="boring"> let contents = "\
</span><span class="boring">Rust:
</span><span class="boring">safe, fast, productive.
</span><span class="boring">Pick three.
</span><span class="boring">Duct tape.";
</span><span class="boring">
</span><span class="boring"> assert_eq!(vec!["safe, fast, productive."], search(query, contents));
</span><span class="boring"> }
</span><span class="boring">
</span><span class="boring"> #[test]
</span><span class="boring"> fn case_insensitive() {
</span><span class="boring"> let query = "rUsT";
</span><span class="boring"> let contents = "\
</span><span class="boring">Rust:
</span><span class="boring">safe, fast, productive.
</span><span class="boring">Pick three.
</span><span class="boring">Trust me.";
</span><span class="boring">
</span><span class="boring"> assert_eq!(
</span><span class="boring"> vec!["Rust:", "Trust me."],
</span><span class="boring"> search_case_insensitive(query, contents)
</span><span class="boring"> );
</span><span class="boring"> }
</span><span class="boring">}</span></code></pre>
<p><span class="caption">示例 13-17：重现示例 12-23 的 <code>Config::build</code> 函数</span></p>
<p>当时我们说过不必担心低效的 <code>clone</code> 调用，因为我们以后会将其移除。好吧，就是现在！</p>
<p>起初这里需要 <code>clone</code> 的原因是参数 <code>args</code> 中有一个 <code>String</code> 元素的 slice，而 <code>build</code> 函数并不拥有 <code>args</code>。为了能够返回 <code>Config</code> 实例的所有权，我们需要克隆 <code>Config</code> 中字段 <code>query</code> 和 <code>file_path</code> 的值，这样 <code>Config</code> 实例就能拥有这些值。</p>
<p>在学习了迭代器之后，我们可以将 <code>build</code> 函数改为获取一个有所有权的迭代器作为参数，而不是借用 slice。我们将使用迭代器功能代替之前检查 slice 长度和索引特定位置的代码。这样可以更清晰地表达 <code>Config::build</code> 函数的操作，因为迭代器会负责访问这些值。</p>
<p>一旦 <code>Config::build</code> 获取了迭代器的所有权并不再使用借用的索引操作，就可以将迭代器中的 <code>String</code> 值移动到 <code>Config</code> 中，而不是调用 <code>clone</code> 分配新的空间。</p>
<h4 id="直接使用返回的迭代器"><a class="header" href="#直接使用返回的迭代器">直接使用返回的迭代器</a></h4>
<p>打开 I/O 项目的 <em>src/main.rs</em> 文件，它看起来应该像这样：</p>
<p><span class="filename">文件名：src/main.rs</span></p>
<pre><code class="language-rust ignore"><span class="boring">use std::env;
</span><span class="boring">use std::process;
</span><span class="boring">
</span><span class="boring">use minigrep::Config;
</span><span class="boring">
</span>fn main() {
let args: Vec&lt;String&gt; = env::args().collect();
let config = Config::build(&amp;args).unwrap_or_else(|err| {
eprintln!("Problem parsing arguments: {err}");
process::exit(1);
});
// --snip--
<span class="boring">
</span><span class="boring"> if let Err(e) = minigrep::run(config) {
</span><span class="boring"> eprintln!("Application error: {e}");
</span><span class="boring"> process::exit(1);
</span><span class="boring"> }
</span>}</code></pre>
<p>首先我们修改第十二章结尾示例 12-24 中的 <code>main</code> 函数的开头为示例 13-18 中的代码。在更新 <code>Config::build</code> 之前这些代码还不能编译：</p>
<p><span class="filename">文件名：src/main.rs</span></p>
<pre><code class="language-rust ignore does_not_compile"><span class="boring">use std::env;
</span><span class="boring">use std::process;
</span><span class="boring">
</span><span class="boring">use minigrep::Config;
</span><span class="boring">
</span>fn main() {
let config = Config::build(env::args()).unwrap_or_else(|err| {
eprintln!("Problem parsing arguments: {err}");
process::exit(1);
});
// --snip--
<span class="boring">
</span><span class="boring"> if let Err(e) = minigrep::run(config) {
</span><span class="boring"> eprintln!("Application error: {e}");
</span><span class="boring"> process::exit(1);
</span><span class="boring"> }
</span>}</code></pre>
<p><span class="caption">示例 13-25：将 <code>env::args</code> 的返回值传递给 <code>Config::build</code></span></p>
<p><code>env::args</code> 函数返回一个迭代器！不同于将迭代器的值收集到一个 vector 中接着传递一个 slice 给 <code>Config::build</code>，现在我们直接将 <code>env::args</code> 返回的迭代器的所有权传递给 <code>Config::build</code>。</p>
<p>接下来需要更新 <code>Config::build</code> 的定义。在 I/O 项目的 <em>src/lib.rs</em> 中，将 <code>Config::build</code> 的签名改为如示例 13-19 所示。这仍然不能编译因为我们还需更新函数体。</p>
<p><span class="filename">文件名：src/lib.rs</span></p>
<pre><code class="language-rust ignore does_not_compile"><span class="boring">use std::env;
</span><span class="boring">use std::error::Error;
</span><span class="boring">use std::fs;
</span><span class="boring">
</span><span class="boring">pub struct Config {
</span><span class="boring"> pub query: String,
</span><span class="boring"> pub file_path: String,
</span><span class="boring"> pub ignore_case: bool,
</span><span class="boring">}
</span><span class="boring">
</span>impl Config {
pub fn build(
mut args: impl Iterator&lt;Item = String&gt;,
) -&gt; Result&lt;Config, &amp;'static str&gt; {
// --snip--
<span class="boring"> if args.len() &lt; 3 {
</span><span class="boring"> return Err("not enough arguments");
</span><span class="boring"> }
</span><span class="boring">
</span><span class="boring"> let query = args[1].clone();
</span><span class="boring"> let file_path = args[2].clone();
</span><span class="boring">
</span><span class="boring"> let ignore_case = env::var("IGNORE_CASE").is_ok();
</span><span class="boring">
</span><span class="boring"> Ok(Config {
</span><span class="boring"> query,
</span><span class="boring"> file_path,
</span><span class="boring"> ignore_case,
</span><span class="boring"> })
</span><span class="boring"> }
</span><span class="boring">}
</span><span class="boring">
</span><span class="boring">pub fn run(config: Config) -&gt; Result&lt;(), Box&lt;dyn Error&gt;&gt; {
</span><span class="boring"> let contents = fs::read_to_string(config.file_path)?;
</span><span class="boring">
</span><span class="boring"> let results = if config.ignore_case {
</span><span class="boring"> search_case_insensitive(&amp;config.query, &amp;contents)
</span><span class="boring"> } else {
</span><span class="boring"> search(&amp;config.query, &amp;contents)
</span><span class="boring"> };
</span><span class="boring">
</span><span class="boring"> for line in results {
</span><span class="boring"> println!("{line}");
</span><span class="boring"> }
</span><span class="boring">
</span><span class="boring"> Ok(())
</span><span class="boring">}
</span><span class="boring">
</span><span class="boring">pub fn search&lt;'a&gt;(query: &amp;str, contents: &amp;'a str) -&gt; Vec&lt;&amp;'a str&gt; {
</span><span class="boring"> let mut results = Vec::new();
</span><span class="boring">
</span><span class="boring"> for line in contents.lines() {
</span><span class="boring"> if line.contains(query) {
</span><span class="boring"> results.push(line);
</span><span class="boring"> }
</span><span class="boring"> }
</span><span class="boring">
</span><span class="boring"> results
</span><span class="boring">}
</span><span class="boring">
</span><span class="boring">pub fn search_case_insensitive&lt;'a&gt;(
</span><span class="boring"> query: &amp;str,
</span><span class="boring"> contents: &amp;'a str,
</span><span class="boring">) -&gt; Vec&lt;&amp;'a str&gt; {
</span><span class="boring"> let query = query.to_lowercase();
</span><span class="boring"> let mut results = Vec::new();
</span><span class="boring">
</span><span class="boring"> for line in contents.lines() {
</span><span class="boring"> if line.to_lowercase().contains(&amp;query) {
</span><span class="boring"> results.push(line);
</span><span class="boring"> }
</span><span class="boring"> }
</span><span class="boring">
</span><span class="boring"> results
</span><span class="boring">}
</span><span class="boring">
</span><span class="boring">#[cfg(test)]
</span><span class="boring">mod tests {
</span><span class="boring"> use super::*;
</span><span class="boring">
</span><span class="boring"> #[test]
</span><span class="boring"> fn case_sensitive() {
</span><span class="boring"> let query = "duct";
</span><span class="boring"> let contents = "\
</span><span class="boring">Rust:
</span><span class="boring">safe, fast, productive.
</span><span class="boring">Pick three.
</span><span class="boring">Duct tape.";
</span><span class="boring">
</span><span class="boring"> assert_eq!(vec!["safe, fast, productive."], search(query, contents));
</span><span class="boring"> }
</span><span class="boring">
</span><span class="boring"> #[test]
</span><span class="boring"> fn case_insensitive() {
</span><span class="boring"> let query = "rUsT";
</span><span class="boring"> let contents = "\
</span><span class="boring">Rust:
</span><span class="boring">safe, fast, productive.
</span><span class="boring">Pick three.
</span><span class="boring">Trust me.";
</span><span class="boring">
</span><span class="boring"> assert_eq!(
</span><span class="boring"> vec!["Rust:", "Trust me."],
</span><span class="boring"> search_case_insensitive(query, contents)
</span><span class="boring"> );
</span><span class="boring"> }
</span><span class="boring">}</span></code></pre>
<p><span class="caption">示例 13-19：以迭代器作为参数更新 <code>Config::build</code> 的签名</span></p>
<p><code>env::args</code> 函数的标准库文档显示，它返回的迭代器的类型为 <code>std::env::Args</code>，并且这个类型实现了 <code>Iterator</code> trait 并返回 <code>String</code> 值。</p>
<p>我们已经更新了 <code>Config::build</code> 函数的签名，因此参数 <code>args</code> 有一个带有 trait bounds <code>impl Iterator&lt;Item = String&gt;</code> 的泛型类型，而不是 <code>&amp;[String]</code>。这里用到了第十章 <a href="ch10-02-traits.html#trait-%E4%BD%9C%E4%B8%BA%E5%8F%82%E6%95%B0">“trait 作为参数”</a> 部分讨论过的 <code>impl Trait</code> 语法，这意味着 <code>args</code> 可以是任何实现了 <code>Iterator</code> trait 并返回 <code>String</code> 项（item）的类型。</p>
<p>由于我们获取了 <code>args</code> 的所有权，并且将通过迭代来修改 <code>args</code>，因此我们可以在 <code>args</code> 参数的声明中添加 <code>mut</code> 关键字，使其可变。</p>
<h4 id="使用-iterator-trait-代替索引"><a class="header" href="#使用-iterator-trait-代替索引">使用 <code>Iterator</code> trait 代替索引</a></h4>
<p>接下来，我们将修改 <code>Config::build</code> 的函数体。因为 <code>args</code> 实现了 <code>Iterator</code> trait，因此我们知道可以对其调用 <code>next</code> 方法！示例 13-20 更新了示例 12-23 中的代码，以使用 <code>next</code> 方法：</p>
<p><span class="filename">文件名：src/lib.rs</span></p>
<pre><code class="language-rust noplayground"><span class="boring">use std::env;
</span><span class="boring">use std::error::Error;
</span><span class="boring">use std::fs;
</span><span class="boring">
</span><span class="boring">pub struct Config {
</span><span class="boring"> pub query: String,
</span><span class="boring"> pub file_path: String,
</span><span class="boring"> pub ignore_case: bool,
</span><span class="boring">}
</span><span class="boring">
</span>impl Config {
pub fn build(
mut args: impl Iterator&lt;Item = String&gt;,
) -&gt; Result&lt;Config, &amp;'static str&gt; {
args.next();
let query = match args.next() {
Some(arg) =&gt; arg,
None =&gt; return Err("Didn't get a query string"),
};
let file_path = match args.next() {
Some(arg) =&gt; arg,
None =&gt; return Err("Didn't get a file path"),
};
let ignore_case = env::var("IGNORE_CASE").is_ok();
Ok(Config {
query,
file_path,
ignore_case,
})
}
}
<span class="boring">
</span><span class="boring">pub fn run(config: Config) -&gt; Result&lt;(), Box&lt;dyn Error&gt;&gt; {
</span><span class="boring"> let contents = fs::read_to_string(config.file_path)?;
</span><span class="boring">
</span><span class="boring"> let results = if config.ignore_case {
</span><span class="boring"> search_case_insensitive(&amp;config.query, &amp;contents)
</span><span class="boring"> } else {
</span><span class="boring"> search(&amp;config.query, &amp;contents)
</span><span class="boring"> };
</span><span class="boring">
</span><span class="boring"> for line in results {
</span><span class="boring"> println!("{line}");
</span><span class="boring"> }
</span><span class="boring">
</span><span class="boring"> Ok(())
</span><span class="boring">}
</span><span class="boring">
</span><span class="boring">pub fn search&lt;'a&gt;(query: &amp;str, contents: &amp;'a str) -&gt; Vec&lt;&amp;'a str&gt; {
</span><span class="boring"> let mut results = Vec::new();
</span><span class="boring">
</span><span class="boring"> for line in contents.lines() {
</span><span class="boring"> if line.contains(query) {
</span><span class="boring"> results.push(line);
</span><span class="boring"> }
</span><span class="boring"> }
</span><span class="boring">
</span><span class="boring"> results
</span><span class="boring">}
</span><span class="boring">
</span><span class="boring">pub fn search_case_insensitive&lt;'a&gt;(
</span><span class="boring"> query: &amp;str,
</span><span class="boring"> contents: &amp;'a str,
</span><span class="boring">) -&gt; Vec&lt;&amp;'a str&gt; {
</span><span class="boring"> let query = query.to_lowercase();
</span><span class="boring"> let mut results = Vec::new();
</span><span class="boring">
</span><span class="boring"> for line in contents.lines() {
</span><span class="boring"> if line.to_lowercase().contains(&amp;query) {
</span><span class="boring"> results.push(line);
</span><span class="boring"> }
</span><span class="boring"> }
</span><span class="boring">
</span><span class="boring"> results
</span><span class="boring">}
</span><span class="boring">
</span><span class="boring">#[cfg(test)]
</span><span class="boring">mod tests {
</span><span class="boring"> use super::*;
</span><span class="boring">
</span><span class="boring"> #[test]
</span><span class="boring"> fn case_sensitive() {
</span><span class="boring"> let query = "duct";
</span><span class="boring"> let contents = "\
</span><span class="boring">Rust:
</span><span class="boring">safe, fast, productive.
</span><span class="boring">Pick three.
</span><span class="boring">Duct tape.";
</span><span class="boring">
</span><span class="boring"> assert_eq!(vec!["safe, fast, productive."], search(query, contents));
</span><span class="boring"> }
</span><span class="boring">
</span><span class="boring"> #[test]
</span><span class="boring"> fn case_insensitive() {
</span><span class="boring"> let query = "rUsT";
</span><span class="boring"> let contents = "\
</span><span class="boring">Rust:
</span><span class="boring">safe, fast, productive.
</span><span class="boring">Pick three.
</span><span class="boring">Trust me.";
</span><span class="boring">
</span><span class="boring"> assert_eq!(
</span><span class="boring"> vec!["Rust:", "Trust me."],
</span><span class="boring"> search_case_insensitive(query, contents)
</span><span class="boring"> );
</span><span class="boring"> }
</span><span class="boring">}</span></code></pre>
<p><span class="caption">示例 13-20：修改 <code>Config::build</code> 的函数体来使用迭代器方法</span></p>
<p>请记住 <code>env::args</code> 返回值的第一个值是程序的名称。我们希望忽略它并获取下一个值，所以首先调用 <code>next</code> 且不对其返回值做任何操作。然后，我们再次调用 <code>next</code> 来获取要放入 <code>Config</code> 结构体的 <code>query</code> 字段的值。如果 <code>next</code> 返回 <code>Some</code>，使用 <code>match</code> 来提取其值。如果它返回 <code>None</code>，则意味着没有提供足够的参数并通过 <code>Err</code> 值提早返回。我们对对 <code>file_path</code> 的值也进行同样的操作。</p>
<h3 id="使用迭代器适配器来使代码更简明"><a class="header" href="#使用迭代器适配器来使代码更简明">使用迭代器适配器来使代码更简明</a></h3>
<p>I/O 项目中其他可以利用迭代器的地方是 <code>search</code> 函数，示例 13-21 中重现了第十二章结尾示例 12-19 中此函数的定义：</p>
<p><span class="filename">文件名：src/lib.rs</span></p>
<pre><code class="language-rust ignore"><span class="boring">use std::error::Error;
</span><span class="boring">use std::fs;
</span><span class="boring">
</span><span class="boring">pub struct Config {
</span><span class="boring"> pub query: String,
</span><span class="boring"> pub file_path: String,
</span><span class="boring">}
</span><span class="boring">
</span><span class="boring">impl Config {
</span><span class="boring"> pub fn build(args: &amp;[String]) -&gt; Result&lt;Config, &amp;'static str&gt; {
</span><span class="boring"> if args.len() &lt; 3 {
</span><span class="boring"> return Err("not enough arguments");
</span><span class="boring"> }
</span><span class="boring">
</span><span class="boring"> let query = args[1].clone();
</span><span class="boring"> let file_path = args[2].clone();
</span><span class="boring">
</span><span class="boring"> Ok(Config { query, file_path })
</span><span class="boring"> }
</span><span class="boring">}
</span><span class="boring">
</span><span class="boring">pub fn run(config: Config) -&gt; Result&lt;(), Box&lt;dyn Error&gt;&gt; {
</span><span class="boring"> let contents = fs::read_to_string(config.file_path)?;
</span><span class="boring">
</span><span class="boring"> Ok(())
</span><span class="boring">}
</span><span class="boring">
</span>pub fn search&lt;'a&gt;(query: &amp;str, contents: &amp;'a str) -&gt; Vec&lt;&amp;'a str&gt; {
let mut results = Vec::new();
for line in contents.lines() {
if line.contains(query) {
results.push(line);
}
}
results
}
<span class="boring">
</span><span class="boring">#[cfg(test)]
</span><span class="boring">mod tests {
</span><span class="boring"> use super::*;
</span><span class="boring">
</span><span class="boring"> #[test]
</span><span class="boring"> fn one_result() {
</span><span class="boring"> let query = "duct";
</span><span class="boring"> let contents = "\
</span><span class="boring">Rust:
</span><span class="boring">safe, fast, productive.
</span><span class="boring">Pick three.";
</span><span class="boring">
</span><span class="boring"> assert_eq!(vec!["safe, fast, productive."], search(query, contents));
</span><span class="boring"> }
</span><span class="boring">}</span></code></pre>
<p><span class="caption">示例 13-21：示例 12-19 中 <code>search</code> 函数的定义</span></p>
<p>可以通过使用迭代器适配器方法来编写更简明的代码。这样做还可以避免使用一个可变的中间 <code>results</code> vector。函数式编程风格倾向于最小化可变状态的数量来使代码更简洁。去除可变状态可能会使未来的并行搜索优化变得更容易，因为我们不必管理对 <code>results</code> vector 的并发访问。示例 13-22 展示了这一变化：</p>
<p><span class="filename">文件名：src/lib.rs</span></p>
<pre><code class="language-rust ignore"><span class="boring">use std::env;
</span><span class="boring">use std::error::Error;
</span><span class="boring">use std::fs;
</span><span class="boring">
</span><span class="boring">pub struct Config {
</span><span class="boring"> pub query: String,
</span><span class="boring"> pub file_path: String,
</span><span class="boring"> pub ignore_case: bool,
</span><span class="boring">}
</span><span class="boring">
</span><span class="boring">impl Config {
</span><span class="boring"> pub fn build(
</span><span class="boring"> mut args: impl Iterator&lt;Item = String&gt;,
</span><span class="boring"> ) -&gt; Result&lt;Config, &amp;'static str&gt; {
</span><span class="boring"> args.next();
</span><span class="boring">
</span><span class="boring"> let query = match args.next() {
</span><span class="boring"> Some(arg) =&gt; arg,
</span><span class="boring"> None =&gt; return Err("Didn't get a query string"),
</span><span class="boring"> };
</span><span class="boring">
</span><span class="boring"> let file_path = match args.next() {
</span><span class="boring"> Some(arg) =&gt; arg,
</span><span class="boring"> None =&gt; return Err("Didn't get a file path"),
</span><span class="boring"> };
</span><span class="boring">
</span><span class="boring"> let ignore_case = env::var("IGNORE_CASE").is_ok();
</span><span class="boring">
</span><span class="boring"> Ok(Config {
</span><span class="boring"> query,
</span><span class="boring"> file_path,
</span><span class="boring"> ignore_case,
</span><span class="boring"> })
</span><span class="boring"> }
</span><span class="boring">}
</span><span class="boring">
</span><span class="boring">pub fn run(config: Config) -&gt; Result&lt;(), Box&lt;dyn Error&gt;&gt; {
</span><span class="boring"> let contents = fs::read_to_string(config.file_path)?;
</span><span class="boring">
</span><span class="boring"> let results = if config.ignore_case {
</span><span class="boring"> search_case_insensitive(&amp;config.query, &amp;contents)
</span><span class="boring"> } else {
</span><span class="boring"> search(&amp;config.query, &amp;contents)
</span><span class="boring"> };
</span><span class="boring">
</span><span class="boring"> for line in results {
</span><span class="boring"> println!("{line}");
</span><span class="boring"> }
</span><span class="boring">
</span><span class="boring"> Ok(())
</span><span class="boring">}
</span><span class="boring">
</span>pub fn search&lt;'a&gt;(query: &amp;str, contents: &amp;'a str) -&gt; Vec&lt;&amp;'a str&gt; {
contents
.lines()
.filter(|line| line.contains(query))
.collect()
}
<span class="boring">
</span><span class="boring">pub fn search_case_insensitive&lt;'a&gt;(
</span><span class="boring"> query: &amp;str,
</span><span class="boring"> contents: &amp;'a str,
</span><span class="boring">) -&gt; Vec&lt;&amp;'a str&gt; {
</span><span class="boring"> let query = query.to_lowercase();
</span><span class="boring"> let mut results = Vec::new();
</span><span class="boring">
</span><span class="boring"> for line in contents.lines() {
</span><span class="boring"> if line.to_lowercase().contains(&amp;query) {
</span><span class="boring"> results.push(line);
</span><span class="boring"> }
</span><span class="boring"> }
</span><span class="boring">
</span><span class="boring"> results
</span><span class="boring">}
</span><span class="boring">
</span><span class="boring">#[cfg(test)]
</span><span class="boring">mod tests {
</span><span class="boring"> use super::*;
</span><span class="boring">
</span><span class="boring"> #[test]
</span><span class="boring"> fn case_sensitive() {
</span><span class="boring"> let query = "duct";
</span><span class="boring"> let contents = "\
</span><span class="boring">Rust:
</span><span class="boring">safe, fast, productive.
</span><span class="boring">Pick three.
</span><span class="boring">Duct tape.";
</span><span class="boring">
</span><span class="boring"> assert_eq!(vec!["safe, fast, productive."], search(query, contents));
</span><span class="boring"> }
</span><span class="boring">
</span><span class="boring"> #[test]
</span><span class="boring"> fn case_insensitive() {
</span><span class="boring"> let query = "rUsT";
</span><span class="boring"> let contents = "\
</span><span class="boring">Rust:
</span><span class="boring">safe, fast, productive.
</span><span class="boring">Pick three.
</span><span class="boring">Trust me.";
</span><span class="boring">
</span><span class="boring"> assert_eq!(
</span><span class="boring"> vec!["Rust:", "Trust me."],
</span><span class="boring"> search_case_insensitive(query, contents)
</span><span class="boring"> );
</span><span class="boring"> }
</span><span class="boring">}</span></code></pre>
<p><span class="caption">示例 13-22：在 <code>search</code> 函数实现中使用迭代器适配器</span></p>
<p>回忆一下，<code>search</code> 函数的目的是返回所有 <code>contents</code> 中包含 <code>query</code> 的行。类似于示例 13-16 中的 <code>filter</code> 例子，这段代码使用 <code>filter</code> 适配器来保留 <code>line.contains(query)</code> 返回 <code>true</code> 的行。接着使用 <code>collect</code> 将匹配行收集到另一个 vector 中。这样就容易多了！尝试对 <code>search_case_insensitive</code> 函数做出同样的使用迭代器方法的修改吧。</p>
<h3 id="选择循环或迭代器"><a class="header" href="#选择循环或迭代器">选择循环或迭代器</a></h3>
<p>接下来的逻辑问题就是在代码中应该选择哪种风格，以及原因：是使用示例 13-21 中的原始实现还是使用示例 13-22 中使用迭代器的版本？大部分 Rust 程序员倾向于使用迭代器风格。开始这有点难以掌握，不过一旦你对不同迭代器的工作方式有了感觉之后，迭代器反而更容易理解。相比摆弄不同的循环并创建新 vector，（迭代器）代码则更关注循环的高层次目的。这抽象掉那些老生常谈的代码，这样就更容易看清代码所特有的概念，比如迭代器中每个元素必须满足的过滤条件。</p>
<p>不过这两种实现真的完全等价吗？直觉上的假设是更底层的循环会更快一些。让我们聊聊性能吧。</p>
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