使用 ? 展开 Option

你可以使用 match 语句来展开 Option，但通常使用 ? 操作符会更简单。如果 x 是一个 Option，那么计算 x? 如果 x 是 Some 将返回底层的值，否则它将终止正在执行的任何函数并返回 None。

fn next_birthday(current_age: Option<u8>) -> Option<String> {
    // If `current_age` is `None`, this returns `None`.
    // If `current_age` is `Some`, the inner `u8` value + 1
    // gets assigned to `next_age`
    let next_age: u8 = current_age? + 1;
    Some(format!("Next year I will be {}", next_age))
}

你可以将多个 ? 链接在一起，使你的代码更具可读性。

struct Person {
    job: Option<Job>,
}

#[derive(Clone, Copy)]
struct Job {
    phone_number: Option<PhoneNumber>,
}

#[derive(Clone, Copy)]
struct PhoneNumber {
    area_code: Option<u8>,
    number: u32,
}

impl Person {

    // Gets the area code of the phone number of the person's job, if it exists.
    fn work_phone_area_code(&self) -> Option<u8> {
        // This would need many nested `match` statements without the `?` operator.
        // It would take a lot more code - try writing it yourself and see which
        // is easier.
        self.job?.phone_number?.area_code
    }
}

fn main() {
    let p = Person {
        job: Some(Job {
            phone_number: Some(PhoneNumber {
                area_code: Some(61),
                number: 439222222,
            }),
        }),
    };

    assert_eq!(p.work_phone_area_code(), Some(61));
}