如何防止值被移动？

Question

我在尝试解决 the robot simulator Exercism exercise 时遇到了很多乐趣，但我面临着一个价值转移问题，我似乎无法想出一个优雅的解决方案：

impl Robot {
    pub fn new(x: isize, y: isize, d: Direction) -> Self {
        Robot { position: Coordinate { x: x, y: y }, direction: d }
    }

    pub fn turn_right(mut self) -> Self {
        match self.direction {
           // ...
        };
        self
    }

    pub fn turn_left(mut self) -> Self {
        match self.direction {
           // ...
        };
        self
    }

    pub fn advance(mut self) -> Self {
        match self.direction {
           // ...
        };
        self
    }

    pub fn instructions(self, instructions: &str) -> Self {
        for instruction in instructions.chars() {
            match instruction {
                'A' => { self.advance(); },
                'R' => { self.turn_right(); },
                'L' => { self.turn_left(); },
                _   => {
                    println!("{} is not a valid instruction", instruction);
                },
            };
        }
        self
    }

我收到此错误：

enter code hereerror[E0382]: use of moved value: `self`
  --> src/lib.rs:60:26
   |
60 |                 'A' => { self.advance(); },
   |                          ^^^^ value moved here in previous iteration of loop
   |
   = note: move occurs because `self` has type `Robot`, which does not implement the `Copy` trait

error[E0382]: use of moved value: `self`
  --> src/lib.rs:61:26
   |
60 |                 'A' => { self.advance(); },
   |                          ---- value moved here
61 |                 'R' => { self.turn_right(); },
   |                          ^^^^ value used here after move
   |
   = note: move occurs because `self` has type `Robot`, which does not implement the `Copy` trait

我想我收到错误是因为 advance() returns self，但我不明白为什么值在块内使用时仍然移动。我真的必须实施 Copy 还是我错过了一个终生用例？

Answer 1

I think I get the error because advance() returns self ?

不，您收到该错误是因为 advance 消耗 self（您的其他方法也会消耗）。

您的问题的惯用解决方案几乎肯定是让您的方法对 self 采用可变引用 (&mut)，而不是按值采用 self。例如。签名 pub fn turn_right(mut self) -> Self 将变为 pub fn turn_right(&mut self)（请注意，后者不会 return 任何东西）。您可以通过引用操纵机器人的状态，您的 instructions 函数应该可以正常工作。

如果出于某种原因您希望继续让方法按值获取 self，您可以重写 instructions 如下：

pub fn instructions(self, instructions: &str) -> Self {
    let mut robot = self;
    for instruction in instructions.chars() {
        robot = match instruction {
            'A' => { robot.advance() },
            'R' => { robot.turn_right() },
            'L' => { robot.turn_left() },
            _   => {
                println!("{} is not a valid instruction", instruction);
                robot
            },
        };
    }
    robot
}

即继续按值传递机器人的状态，但要确保新状态在每次循环迭代时都绑定到一个变量。（我没试过编译这段代码，但原理应该是合理的。）

Answer 2

看看其他用户的回答，其实你可以用 fold 来做：

pub fn instructions(self, instructions: &str) -> Self {
    instructions.chars().fold(self, |robot, c| {
        match c {
            'L' => robot.turn_left(),
            'R' => robot.turn_right(),
            'A' => robot.advance(),
            _ => panic!("unexpected char")
        }
    })
}

似乎一直在向后移动机器人进入范围。

如何防止值被移动？

How to prevent a value from being moved?

move

lifetime

rust