rust - 返回对可选结构成员的可变引用

Question

我想延迟连接到 Redis 数据库。我有一个 Db 结构，它包含 Redis Client。默认情况下，它是 None。这是 Python 中的示例代码:

import redis


class Db:

    def __init__(self):
        self.client = None

    def get_client(self):
        if self.client is None:
            self.client = redis.StrictRedis(host='127.0.0.1')
        return self.client

我试过了

extern crate redis;

use redis::Client;

struct Db {
    client: Option<Client>,
}

impl Db {
    fn new() -> Db {
        Db { client: None }
    }

    fn get_client(&mut self) -> Result<&Client, &'static str> {
        if let Some(ref client) = self.client {
            Ok(client)
        } else {
            let connection_string = "redis://127.0.0.1";
            match Client::open(connection_string) {
                Ok(client) => {
                    self.client = Some(client);
                    Ok(&self.client.unwrap())
                }
                Err(err) => Err("Error!"),
            }
        }
    }
}

fn main() {
    let mut db = Db::new();
    db.get_client();
}

而且我有编译错误。我几乎明白编译器说的是什么，但我不知道如何解决这个问题。

    error: borrowed value does not live long enough
  --> src/main.rs:28:29
   |
28 |                         Ok(&self.client.unwrap())
   |                             ^^^^^^^^^^^^^^^^^^^^ does not live long enough
29 |                     },
   |                     - temporary value only lives until here
   |
note: borrowed value must be valid for the anonymous lifetime #1 defined on the body at 19:66...
  --> src/main.rs:19:67
   |
19 |         fn get_client(&mut self) -> Result<&Client, &'static str> {
   |                                                                   ^

error[E0507]: cannot move out of borrowed content
  --> src/main.rs:28:29
   |
28 |                         Ok(&self.client.unwrap())
   |                             ^^^^ cannot move out of borrowed content

Answer 1

如果您调用 unwrap() 你动T来自 Option .因为你只借了self这导致了 cannot move out of borrowed content错误。

如果你想借一个Option<T>里面的值你可以使用 as_ref 方法:

extern crate redis;

use redis::Client;

struct Db {
    client: Option<Client>,
}

impl Db {
    fn new() -> Db {
        Db { client: None }
    }

    fn get_client(&mut self) -> Result<&Client, &'static str> {
        if let Some(ref client) = self.client {
            Ok(client)
        } else {
            let connection_string = "redis://127.0.0.1";
            match Client::open(connection_string) {
                Ok(client) => {
                    self.client = Some(client);
                    Ok(self.client.as_ref().unwrap())
                }
                Err(_) => Err("Error!"),
            }
        }
    }
}

fn main() {
    let mut db = Db::new();
    db.get_client().expect("get_client failed");
}