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<main>
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<h1 id="可恢复的错误-result"><a class="header" href="#可恢复的错误-result">可恢复的错误 Result</a></h1>
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<p>还记得上一节中，提到的关于文件读取的思考题吧？当时我们解决了读取文件时遇到不可恢复错误该怎么处理的问题，现在来看看，读取过程中，正常返回和遇到可以恢复的错误时该如何处理。</p>
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<p>假设，我们有一台消息服务器，每个用户都通过 websocket 连接到该服务器来接收和发送消息，该过程就涉及到 socket 文件的读写，那么此时，如果一个用户的读写发生了错误，显然不能直接 <code>panic</code>，否则服务器会直接崩溃，所有用户都会断开连接，因此我们需要一种更温和的错误处理方式：<code>Result<T, E></code>。</p>
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<p>之前章节有提到过，<code>Result<T, E></code> 是一个枚举类型，定义如下：</p>
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<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
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</span><span class="boring">fn main() {
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</span>enum Result<T, E> {
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Ok(T),
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Err(E),
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}
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<span class="boring">}</span></code></pre>
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<p>泛型参数 <code>T</code> 代表成功时存入的正确值的类型，存放方式是 <code>Ok(T)</code>，<code>E</code> 代表错误时存入的错误值，存放方式是 <code>Err(E)</code>，枯燥的讲解永远不及代码生动准确，因此先来看下打开文件的例子：</p>
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<pre class="playground"><code class="language-rust edition2021">use std::fs::File;
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fn main() {
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let f = File::open("hello.txt");
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}</code></pre>
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|
|
<p>以上 <code>File::open</code> 返回一个 <code>Result</code> 类型，那么问题来了：</p>
|
|
|
<blockquote>
|
|
|
<h4 id="如何获知变量类型或者函数的返回类型"><a class="header" href="#如何获知变量类型或者函数的返回类型">如何获知变量类型或者函数的返回类型</a></h4>
|
|
|
<p>有几种常用的方式，此处更推荐第二种方法：</p>
|
|
|
<ul>
|
|
|
<li>第一种是查询标准库或者三方库文档，搜索 <code>File</code>，然后找到它的 <code>open</code> 方法</li>
|
|
|
<li>在 <a href="https://course.rs/first-try/editor.html">Rust IDE</a> 章节，我们推荐了 <code>VSCode</code> IDE 和 <code>rust-analyzer</code> 插件，如果你成功安装的话，那么就可以在 <code>VSCode</code> 中很方便的通过代码跳转的方式查看代码，同时 <code>rust-analyzer</code> 插件还会对代码中的类型进行标注，非常方便好用！</li>
|
|
|
<li>你还可以尝试故意标记一个错误的类型，然后让编译器告诉你：</li>
|
|
|
</ul>
|
|
|
</blockquote>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>let f: u32 = File::open("hello.txt");
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>错误提示如下：</p>
|
|
|
<pre><code class="language-console">error[E0308]: mismatched types
|
|
|
--> src/main.rs:4:18
|
|
|
|
|
|
|
4 | let f: u32 = File::open("hello.txt");
|
|
|
| ^^^^^^^^^^^^^^^^^^^^^^^ expected u32, found enum
|
|
|
`std::result::Result`
|
|
|
|
|
|
|
= note: expected type `u32`
|
|
|
found type `std::result::Result<std::fs::File, std::io::Error>`
|
|
|
</code></pre>
|
|
|
<p>上面代码，故意将 <code>f</code> 类型标记成整形，编译器立刻不乐意了，你是在忽悠我吗？打开文件操作返回一个整形？来，大哥来告诉你返回什么：<code>std::result::Result<std::fs::File, std::io::Error></code>，我的天呐，怎么这么长的类型！</p>
|
|
|
<p>别慌，其实很简单，首先 <code>Result</code> 本身是定义在 <code>std::result</code> 中的，但是因为 <code>Result</code> 很常用，所以就被包含在了 <a href="https://course.rs/appendix/prelude.html"><code>prelude</code></a> 中（将常用的东东提前引入到当前作用域内），因此无需手动引入 <code>std::result::Result</code>，那么返回类型可以简化为 <code>Result<std::fs::File,std::io::Error></code>，你看看是不是很像标准的 <code>Result<T, E></code> 枚举定义？只不过 <code>T</code> 被替换成了具体的类型 <code>std::fs::File</code>，是一个文件句柄类型，<code>E</code> 被替换成 <code>std::io::Error</code>，是一个 IO 错误类型。</p>
|
|
|
<p>这个返回值类型说明 <code>File::open</code> 调用如果成功则返回一个可以进行读写的文件句柄，如果失败，则返回一个 IO 错误：文件不存在或者没有访问文件的权限等。总之 <code>File::open</code> 需要一个方式告知调用者是成功还是失败，并同时返回具体的文件句柄（成功）或错误信息（失败），万幸的是，这些信息可以通过 <code>Result</code> 枚举提供：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">use std::fs::File;
|
|
|
|
|
|
fn main() {
|
|
|
let f = File::open("hello.txt");
|
|
|
|
|
|
let f = match f {
|
|
|
Ok(file) => file,
|
|
|
Err(error) => {
|
|
|
panic!("Problem opening the file: {:?}", error)
|
|
|
},
|
|
|
};
|
|
|
}</code></pre>
|
|
|
<p>代码很清晰，对打开文件后的 <code>Result<T, E></code> 类型进行匹配取值，如果是成功，则将 <code>Ok(file)</code> 中存放的的文件句柄 <code>file</code> 赋值给 <code>f</code>，如果失败，则将 <code>Err(error)</code> 中存放的错误信息 <code>error</code> 使用 <code>panic</code> 抛出来，进而结束程序，这非常符合上文提到过的 <code>panic</code> 使用场景。</p>
|
|
|
<p>好吧，也没有那么合理 :)</p>
|
|
|
<h2 id="对返回的错误进行处理"><a class="header" href="#对返回的错误进行处理">对返回的错误进行处理</a></h2>
|
|
|
<p>直接 <code>panic</code> 还是过于粗暴，因为实际上 IO 的错误有很多种，我们需要对部分错误进行特殊处理，而不是所有错误都直接崩溃：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">use std::fs::File;
|
|
|
use std::io::ErrorKind;
|
|
|
|
|
|
fn main() {
|
|
|
let f = File::open("hello.txt");
|
|
|
|
|
|
let f = match f {
|
|
|
Ok(file) => file,
|
|
|
Err(error) => match error.kind() {
|
|
|
ErrorKind::NotFound => match File::create("hello.txt") {
|
|
|
Ok(fc) => fc,
|
|
|
Err(e) => panic!("Problem creating the file: {:?}", e),
|
|
|
},
|
|
|
other_error => panic!("Problem opening the file: {:?}", other_error),
|
|
|
},
|
|
|
};
|
|
|
}</code></pre>
|
|
|
<p>上面代码在匹配出 <code>error</code> 后，又对 <code>error</code> 进行了详细的匹配解析，最终结果：</p>
|
|
|
<ul>
|
|
|
<li>如果是文件不存在错误 <code>ErrorKind::NotFound</code>，就创建文件，这里创建文件<code>File::create</code> 也是返回 <code>Result</code>，因此继续用 <code>match</code> 对其结果进行处理：创建成功，将新的文件句柄赋值给 <code>f</code>，如果失败，则 <code>panic</code></li>
|
|
|
<li>剩下的错误，一律 <code>panic</code></li>
|
|
|
</ul>
|
|
|
<p>虽然很清晰，但是代码还是有些啰嗦，我们会在<a href="https://course.rs/advance/errors.html">简化错误处理</a>一章重点讲述如何写出更优雅的错误。</p>
|
|
|
<h2 id="失败就-panic-unwrap-和-expect"><a class="header" href="#失败就-panic-unwrap-和-expect">失败就 panic: unwrap 和 expect</a></h2>
|
|
|
<p>上一节中，已经看到过这两兄弟的简单介绍，这里再来回顾下。</p>
|
|
|
<p>在不需要处理错误的场景，例如写原型、示例时，我们不想使用 <code>match</code> 去匹配 <code>Result<T, E> </code> 以获取其中的 <code>T</code> 值，因为 <code>match</code> 的穷尽匹配特性，你总要去处理下 <code>Err</code> 分支。那么有没有办法简化这个过程？有，答案就是 <code>unwrap</code> 和 <code>expect</code>。</p>
|
|
|
<p>它们的作用就是，如果返回成功，就将 <code>Ok(T)</code> 中的值取出来，如果失败，就直接 <code>panic</code>，真的勇士绝不多 BB，直接崩溃。</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">use std::fs::File;
|
|
|
|
|
|
fn main() {
|
|
|
let f = File::open("hello.txt").unwrap();
|
|
|
}</code></pre>
|
|
|
<p>如果调用这段代码时 <em>hello.txt</em> 文件不存在，那么 <code>unwrap</code> 就将直接 <code>panic</code>：</p>
|
|
|
<pre><code class="language-console">thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Os { code: 2, kind: NotFound, message: "No such file or directory" }', src/main.rs:4:37
|
|
|
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
|
|
|
</code></pre>
|
|
|
<p><code>expect</code> 跟 <code>unwrap</code> 很像，也是遇到错误直接 <code>panic</code>, 但是会带上自定义的错误提示信息，相当于重载了错误打印的函数：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">use std::fs::File;
|
|
|
|
|
|
fn main() {
|
|
|
let f = File::open("hello.txt").expect("Failed to open hello.txt");
|
|
|
}</code></pre>
|
|
|
<p>报错如下：</p>
|
|
|
<pre><code class="language-console">thread 'main' panicked at 'Failed to open hello.txt: Os { code: 2, kind: NotFound, message: "No such file or directory" }', src/main.rs:4:37
|
|
|
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
|
|
|
</code></pre>
|
|
|
<p>可以看出，<code>expect</code> 相比 <code>unwrap</code> 能提供更精确的错误信息，在有些场景也会更加实用。</p>
|
|
|
<h2 id="传播错误"><a class="header" href="#传播错误">传播错误</a></h2>
|
|
|
<p>咱们的程序几乎不太可能只有 <code>A->B</code> 形式的函数调用，一个设计良好的程序，一个功能涉及十几层的函数调用都有可能。而错误处理也往往不是哪里调用出错，就在哪里处理，实际应用中，大概率会把错误层层上传然后交给调用链的上游函数进行处理，错误传播将极为常见。</p>
|
|
|
<p>例如以下函数从文件中读取用户名，然后将结果进行返回：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>use std::fs::File;
|
|
|
use std::io::{self, Read};
|
|
|
|
|
|
fn read_username_from_file() -> Result<String, io::Error> {
|
|
|
// 打开文件，f是`Result<文件句柄,io::Error>`
|
|
|
let f = File::open("hello.txt");
|
|
|
|
|
|
let mut f = match f {
|
|
|
// 打开文件成功，将file句柄赋值给f
|
|
|
Ok(file) => file,
|
|
|
// 打开文件失败，将错误返回(向上传播)
|
|
|
Err(e) => return Err(e),
|
|
|
};
|
|
|
// 创建动态字符串s
|
|
|
let mut s = String::new();
|
|
|
// 从f文件句柄读取数据并写入s中
|
|
|
match f.read_to_string(&mut s) {
|
|
|
// 读取成功，返回Ok封装的字符串
|
|
|
Ok(_) => Ok(s),
|
|
|
// 将错误向上传播
|
|
|
Err(e) => Err(e),
|
|
|
}
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>有几点值得注意：</p>
|
|
|
<ul>
|
|
|
<li>该函数返回一个 <code>Result<String, io::Error></code> 类型，当读取用户名成功时，返回 <code>Ok(String)</code>，失败时，返回 <code>Err(io:Error)</code></li>
|
|
|
<li><code>File::open</code> 和 <code>f.read_to_string</code> 返回的 <code>Result<T, E></code> 中的 <code>E</code> 就是 <code>io::Error</code></li>
|
|
|
</ul>
|
|
|
<p>由此可见，该函数将 <code>io::Error</code> 的错误往上进行传播，该函数的调用者最终会对 <code>Result<String,io::Error></code> 进行再处理，至于怎么处理就是调用者的事，如果是错误，它可以选择继续向上传播错误，也可以直接 <code>panic</code>，亦或将具体的错误原因包装后写入 socket 中呈现给终端用户。</p>
|
|
|
<p>但是上面的代码也有自己的问题，那就是太长了（优秀的程序员身上的优点极多，其中最大的优点就是<em>懒</em>），我自认为也有那么一点点优秀，因此见不得这么啰嗦的代码，下面咱们来讲讲如何简化它。</p>
|
|
|
<h3 id="传播界的大明星-"><a class="header" href="#传播界的大明星-">传播界的大明星: ?</a></h3>
|
|
|
<p>大明星出场，必须得有排面，来看看 <code>?</code> 的排面：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>use std::fs::File;
|
|
|
use std::io;
|
|
|
use std::io::Read;
|
|
|
|
|
|
fn read_username_from_file() -> Result<String, io::Error> {
|
|
|
let mut f = File::open("hello.txt")?;
|
|
|
let mut s = String::new();
|
|
|
f.read_to_string(&mut s)?;
|
|
|
Ok(s)
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>看到没，这就是排面，相比前面的 <code>match</code> 处理错误的函数，代码直接减少了一半不止，但是，一山更比一山难，看不懂啊！</p>
|
|
|
<p>其实 <code>?</code> 就是一个宏，它的作用跟上面的 <code>match</code> 几乎一模一样：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>let mut f = match f {
|
|
|
// 打开文件成功，将file句柄赋值给f
|
|
|
Ok(file) => file,
|
|
|
// 打开文件失败，将错误返回(向上传播)
|
|
|
Err(e) => return Err(e),
|
|
|
};
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>如果结果是 <code>Ok(T)</code>，则把 <code>T</code> 赋值给 <code>f</code>，如果结果是 <code>Err(E)</code>，则返回该错误，所以 <code>?</code> 特别适合用来传播错误。</p>
|
|
|
<p>虽然 <code>?</code> 和 <code>match</code> 功能一致，但是事实上 <code>?</code> 会更胜一筹。何解？</p>
|
|
|
<p>想象一下，一个设计良好的系统中，肯定有自定义的错误特征，错误之间很可能会存在上下级关系，例如标准库中的 <code>std::io::Error </code>和 <code>std::error::Error</code>，前者是 IO 相关的错误结构体，后者是一个最最通用的标准错误特征，同时前者实现了后者，因此 <code>std::io::Error</code> 可以转换为 <code>std:error::Error</code>。</p>
|
|
|
<p>明白了以上的错误转换，<code>?</code> 的更胜一筹就很好理解了，它可以自动进行类型提升（转换）：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>fn open_file() -> Result<File, Box<dyn std::error::Error>> {
|
|
|
let mut f = File::open("hello.txt")?;
|
|
|
Ok(f)
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>上面代码中 <code>File::open</code> 报错时返回的错误是 <code>std::io::Error</code> 类型，但是 <code>open_file</code> 函数返回的错误类型是 <code>std::error::Error</code> 的特征对象，可以看到一个错误类型通过 <code>?</code> 返回后，变成了另一个错误类型，这就是 <code>?</code> 的神奇之处。</p>
|
|
|
<p>根本原因是在于标准库中定义的 <code>From</code> 特征，该特征有一个方法 <code>from</code>，用于把一个类型转成另外一个类型，<code>?</code> 可以自动调用该方法，然后进行隐式类型转换。因此只要函数返回的错误 <code>ReturnError</code> 实现了 <code>From<OtherError></code> 特征，那么 <code>?</code> 就会自动把 <code>OtherError</code> 转换为 <code>ReturnError</code>。</p>
|
|
|
<p>这种转换非常好用，意味着你可以用一个大而全的 <code>ReturnError</code> 来覆盖所有错误类型，只需要为各种子错误类型实现这种转换即可。</p>
|
|
|
<p>强中自有强中手，一码更比一码短：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>use std::fs::File;
|
|
|
use std::io;
|
|
|
use std::io::Read;
|
|
|
|
|
|
fn read_username_from_file() -> Result<String, io::Error> {
|
|
|
let mut s = String::new();
|
|
|
|
|
|
File::open("hello.txt")?.read_to_string(&mut s)?;
|
|
|
|
|
|
Ok(s)
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>瞧见没？ <code>?</code> 还能实现链式调用，<code>File::open</code> 遇到错误就返回，没有错误就将 <code>Ok</code> 中的值取出来用于下一个方法调用，简直太精妙了，从 Go 语言过来的我，内心狂喜（其实学 Rust 的苦和痛我才不会告诉你们）。</p>
|
|
|
<p>不仅有更强，还要有最强，我不信还有人比我更短(不要误解)：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>use std::fs;
|
|
|
use std::io;
|
|
|
|
|
|
fn read_username_from_file() -> Result<String, io::Error> {
|
|
|
// read_to_string是定义在std::io中的方法，因此需要在上面进行引用
|
|
|
fs::read_to_string("hello.txt")
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>从文件读取数据到字符串中，是比较常见的操作，因此 Rust 标准库为我们提供了 <code>fs::read_to_string</code> 函数，该函数内部会打开一个文件、创建 <code>String</code>、读取文件内容最后写入字符串并返回，因为该函数其实与本章讲的内容关系不大，因此放在最后来讲，其实只是我想震你们一下 :)</p>
|
|
|
<h4 id="-用于-option-的返回"><a class="header" href="#-用于-option-的返回">? 用于 Option 的返回</a></h4>
|
|
|
<p><code>?</code> 不仅仅可以用于 <code>Result</code> 的传播，还能用于 <code>Option</code> 的传播，再来回忆下 <code>Option</code> 的定义：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>pub enum Option<T> {
|
|
|
Some(T),
|
|
|
None
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p><code>Result</code> 通过 <code>?</code> 返回错误，那么 <code>Option</code> 就通过 <code>?</code> 返回 <code>None</code>：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>fn first(arr: &[i32]) -> Option<&i32> {
|
|
|
let v = arr.get(0)?;
|
|
|
Some(v)
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>上面的函数中，<code>arr.get</code> 返回一个 <code>Option<&i32></code> 类型，因为 <code>?</code> 的使用，如果 <code>get</code> 的结果是 <code>None</code>，则直接返回 <code>None</code>，如果是 <code>Some(&i32)</code>，则把里面的值赋给 <code>v</code>。</p>
|
|
|
<p>其实这个函数有些画蛇添足，我们完全可以写出更简单的版本：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>fn first(arr: &[i32]) -> Option<&i32> {
|
|
|
arr.get(0)
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>有一句话怎么说？没有需求，制造需求也要上……大家别跟我学习，这是软件开发大忌。只能用代码洗洗眼了：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>fn last_char_of_first_line(text: &str) -> Option<char> {
|
|
|
text.lines().next()?.chars().last()
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>上面代码展示了在链式调用中使用 <code>?</code> 提前返回 <code>None</code> 的用法， <code>.next</code> 方法返回的是 <code>Option</code> 类型：如果返回 <code>Some(&str)</code>，那么继续调用 <code>chars</code> 方法，如果返回 <code>None</code>，则直接从整个函数中返回 <code>None</code>，不再继续进行链式调用。</p>
|
|
|
<h4 id="新手用--常会犯的错误"><a class="header" href="#新手用--常会犯的错误">新手用 ? 常会犯的错误</a></h4>
|
|
|
<p>初学者在用 <code>?</code> 时，老是会犯错，例如写出这样的代码：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>fn first(arr: &[i32]) -> Option<&i32> {
|
|
|
arr.get(0)?
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>这段代码无法通过编译，切记：<code>?</code> 操作符需要一个变量来承载正确的值，这个函数只会返回 <code>Some(&i32)</code> 或者 <code>None</code>，只有错误值能直接返回，正确的值不行，所以如果数组中存在 0 号元素，那么函数第二行使用 <code>?</code> 后的返回类型为 <code>&i32</code> 而不是 <code>Some(&i32)</code>。因此 <code>?</code> 只能用于以下形式：</p>
|
|
|
<ul>
|
|
|
<li><code>let v = xxx()?;</code></li>
|
|
|
<li><code>xxx()?.yyy()?;</code></li>
|
|
|
</ul>
|
|
|
<h4 id="带返回值的-main-函数"><a class="header" href="#带返回值的-main-函数">带返回值的 main 函数</a></h4>
|
|
|
<p>在了解了 <code>?</code> 的使用限制后，这段代码你很容易看出它无法编译：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">use std::fs::File;
|
|
|
|
|
|
fn main() {
|
|
|
let f = File::open("hello.txt")?;
|
|
|
}</code></pre>
|
|
|
<p>运行后会报错:</p>
|
|
|
<pre><code class="language-shell">$ cargo run
|
|
|
...
|
|
|
the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `FromResidual`)
|
|
|
--> src/main.rs:4:48
|
|
|
|
|
|
|
3 | fn main() {
|
|
|
| --------- this function should return `Result` or `Option` to accept `?`
|
|
|
4 | let greeting_file = File::open("hello.txt")?;
|
|
|
| ^ cannot use the `?` operator in a function that returns `()`
|
|
|
|
|
|
|
= help: the trait `FromResidual<Result<Infallible, std::io::Error>>` is not implemented for `()`
|
|
|
</code></pre>
|
|
|
<p>因为 <code>?</code> 要求 <code>Result<T, E></code> 形式的返回值，而 <code>main</code> 函数的返回是 <code>()</code>，因此无法满足，那是不是就无解了呢？</p>
|
|
|
<p>实际上 Rust 还支持另外一种形式的 <code>main</code> 函数：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021">use std::error::Error;
|
|
|
use std::fs::File;
|
|
|
|
|
|
fn main() -> Result<(), Box<dyn Error>> {
|
|
|
let f = File::open("hello.txt")?;
|
|
|
|
|
|
Ok(())
|
|
|
}</code></pre>
|
|
|
<p>这样就能使用 <code>?</code> 提前返回了，同时我们又一次看到了<code>Box<dyn Error></code> 特征对象，因为 <code>std::error:Error</code> 是 Rust 中抽象层次最高的错误，其它标准库中的错误都实现了该特征，因此我们可以用该特征对象代表一切错误，就算 <code>main</code> 函数中调用任何标准库函数发生错误，都可以通过 <code>Box<dyn Error></code> 这个特征对象进行返回。</p>
|
|
|
<p>至于 <code>main</code> 函数可以有多种返回值，那是因为实现了 <a href="https://doc.rust-lang.org/std/process/trait.Termination.html">std::process::Termination</a> 特征，目前为止该特征还没进入稳定版 Rust 中，也许未来你可以为自己的类型实现该特征！</p>
|
|
|
<h4 id="try"><a class="header" href="#try">try!</a></h4>
|
|
|
<p>在 <code>?</code> 横空出世之前( Rust 1.13 )，Rust 开发者还可以使用 <code>try!</code> 来处理错误，该宏的大致定义如下：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>macro_rules! try {
|
|
|
($e:expr) => (match $e {
|
|
|
Ok(val) => val,
|
|
|
Err(err) => return Err(::std::convert::From::from(err)),
|
|
|
});
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>简单看一下与 <code>?</code> 的对比:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>// `?`
|
|
|
let x = function_with_error()?; // 若返回 Err, 则立刻返回；若返回 Ok(255)，则将 x 的值设置为 255
|
|
|
|
|
|
// `try!()`
|
|
|
let x = try!(function_with_error());
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>可以看出 <code>?</code> 的优势非常明显，何况 <code>?</code> 还能做链式调用。</p>
|
|
|
<p>总之，<code>try!</code> 作为前浪已经死在了沙滩上，<strong>在当前版本中，我们要尽量避免使用 try!</strong>。</p>
|
|
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<h2 id="课后练习"><a class="header" href="#课后练习">课后练习</a></h2>
|
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<blockquote>
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<p><a href="https://practice-zh.course.rs/result-panic/result.html">Rust By Practice</a>，支持代码在线编辑和运行，并提供详细的<a href="https://github.com/sunface/rust-by-practice/blob/master/solutions/result-panic/result.md">习题解答</a>。</p>
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