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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 >
< 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 >
< 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 >
< 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 >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > enum Result< T, E> {
Ok(T),
Err(E),
}
< span class = "boring" > }< / span > < / code > < / pre >
< 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 >
< pre class = "playground" > < code class = "language-rust edition2021" > use std::fs::File;
fn main() {
let f = File::open("hello.txt");
}< / code > < / pre >
< 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， < / 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， < 文件句柄,io::Error> `
let f = File::open("hello.txt");
let mut f = match f {
// 打开文件成功，
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，
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，
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，
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 )， < 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 >
< h2 id = "课后练习" > < a class = "header" href = "#课后练习" > 课后练习< / a > < / h2 >
< blockquote >
< 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 >
< / blockquote >
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