rust - 为什么我们需要为 Option 变量调用 take()

Question

在这段代码中:

pub struct Post {
    state: Option<Box<dyn State>>,
    content: String,
}

impl Post {
    pub fn new() -> Post {
        Post {
            state: Some(Box::new(Draft {})),
            content: String::new(),
        }
    }
    
    pub fn add_text(&mut self, text: &str) {
        self.content.push_str(text);
    }
    
    pub fn content(&self) -> &str {
        ""
    }

    pub fn request_review(&mut self) {
        if let Some(s) = self.state.take() {
            self.state = Some(s.request_review())
        }
    }
}

trait State {
    fn request_review(self: Box<Self>) -> Box<dyn State>; 
}

struct Draft {}

impl State for Draft {
    fn request_review(self: Box<Self>) -> Box<dyn State> {
        Box::new(PendingReview {})
    }
}

struct PendingReview {
    fn request_review(self: Box<Self>) -> Box<dyn State> {
        self
    }
}

调用take()；书上说:

To consume the old state, the request_review method needs to take ownership of the state value. This is where the Option in the state field of Post comes in: we call the take method to take the Some value out of the state field and leave a None in its place.

We need to set state to None temporarily rather than setting it directly with code like self.state = self.state.request_review(); to get ownership of the state value. This ensures Post can’t use the old state value after we’ve transformed it into a new state.

如果我们直接设置 Post 怎么可能使用它的旧状态？

Answer 1

如果你这样编码:

pub struct Post {
    state: Box<dyn State>,
    content: String,
}

trait State {
    fn request_review(self: Box<Self>) -> Box<dyn State>; 
}

impl Post {
    // ... 
    pub fn request_review(&mut self) {
        self.state = self.state.request_review();
    }
    // ... 
}

你会得到一个编译器错误:

self.state = self.state.request_review();
             ^^^^^^ move occurs because `self.state` has type `std::boxed::Box<dyn State>`, which does not implement the `Copy` trait'.

这是因为调用 State::request_review会搬家Box<self> ，这是在堆上分配的，除非你实现 Copy，否则 Rust 不允许你将值从堆中移走。 ，否则那里还剩下什么？本书使用Option::take()将所有权移出并离开None在这个地方。