【Rust】问号的使用

环境

• Rust 1.56.1
• VSCode 1.61.2

概念

参考：https://doc.rust-lang.org/rust-by-example/error/multiple_error_types/reenter_question_mark.html

示例

main.rs

use std::error;
use std::fmt;

type Result<T> = std::result::Result<T, Box<dyn error::Error>>;

#[derive(Debug)]
struct EmptyVec;

impl fmt::Display for EmptyVec {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "invalid first item to double")
    }
}

impl error::Error for EmptyVec {}

fn double_first(vec: Vec<&str>) -> Result<i32> {
    let first = vec.first().ok_or(EmptyVec)?;
    let parsed = first.parse::<i32>()?;
    Ok(2 * parsed)
}

fn print(result: Result<i32>) {
    match result {
        Ok(n) => println!("The first doubled is {}", n),
        Err(e) => println!("Error: {}", e),
    }
}

fn main() {
    let numbers = vec!["42", "93", "18"];
    let empty = vec![];
    let strings = vec!["tofu", "93", "18"];

    print(double_first(numbers));
    print(double_first(empty));
    print(double_first(strings));
}

总结

了解了 Rust 中问号的使用，使用它来展开对于 Option 的处理。

附录