使用 ? 传播错误

try 操作符 ? 用于将错误返回给调用方。它能把常用命令

match some_expression {
    Ok(value) => value,
    Err(err) => return Err(err),
}

转换成更简单的命令

some_expression?

We can use this to simplify our error handling code:

use std::{fs, io};
use std::io::Read;

fn read_username(path: &str) -> Result<String, io::Error> {
    let username_file_result = fs::File::open(path);
    let mut username_file = match username_file_result {
        Ok(file) => file,
        Err(err) => return Err(err),
    };

    let mut username = String::new();
    match username_file.read_to_string(&mut username) {
        Ok(_) => Ok(username),
        Err(err) => Err(err),
    }
}

fn main() {
    //fs::write("config.dat", "alice").unwrap();
    let username = read_username("config.dat");
    println!("username or error: {username:?}");
}

关键点：

  • username 变量可以是 Ok(string) 或 Err(error)。
  • 可以使用 fs::write 调用来测试不同的场景：没有文件、空文件、包含用户名的文件。
  • The return type of the function has to be compatible with the nested functions it calls. For instance, a function returning a Result<T, Err> can only apply the ? operator on a function returning a Result<AnyT, Err>. It cannot apply the ? operator on a function returning an Option<AnyT> or Result<T, OtherErr> unless OtherErr implements From<Err>. Reciprocally, a function returning an Option<T> can only apply the ? operator on a function returning an Option<AnyT>.
    • 您可以使用其他“Option”和“Result”方法（例如“Option::ok_or”“Result::ok”“Result::err”）将不兼容的类型转换为另一种类型。