尝试运算符

尝试运算符

Runtime errors like connection-refused or file-not-found are handled with the Result type, but matching this type on every call can be cumbersome. The try-operator ? is used to return errors to the caller. It lets you turn the common

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::io::Read;
use std::{fs, io};
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:?}");
}

This slide should take about 5 minutes.

简化 read_username 函数以使用 ?。

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。
Note that main can return a Result<(), E> as long as it implements std::process::Termination. In practice, this means that E implements Debug. The executable will print the Err variant and return a nonzero exit status on error.

29.2. 尝试运算符

尝试运算符