005 透過連結串列學Rust之實現Drop

linghuyichong發表於2021-06-20
Rust

影片地址：www.bilibili.com/video/av78062009/
相關原始碼：github.com/anonymousGiga/Rust-link...

本節我們為連結串列實現Drop函式。按照我們常規的方式，我們可能的實現方式如下：

impl Drop for List {
    fn drop(&mut self) {
        // NOTE: you can't actually explicitly call `drop` in real Rust code;
        // we're pretending to be the compiler!
        self.head.drop(); // tail recursive - good!
    }
}

impl Drop for Link {
    fn drop(&mut self) {
        match *self {
            Link::Empty => {} // Done!
            Link::More(ref mut boxed_node) => {
                boxed_node.drop(); // tail recursive - good!
            }
        }
    }
}

impl Drop for Box<Node> {
    fn drop(&mut self) {
        self.ptr.drop(); // uh oh, not tail recursive!
        deallocate(self.ptr);
    }
}

impl Drop for Node {
    fn drop(&mut self) {
        self.next.drop();
    }
}

但是此種實現方式在編譯時候會報錯，因為Box中包含Node無法以遞迴的方式刪除，因此，正確的實現方式如下：

impl Drop for List {
    fn drop(&mut self) {
        let mut cur_link = mem::replace(&mut self.head, Link::Empty);
        while let Link::More(mut boxed_node) = cur_link {
            cur_link = mem::replace(&mut boxed_node.next, Link::Empty);
        }
    }
}

完整的程式碼如下：

use std::mem;

#[derive(Debug)]
pub struct List{
    head: Link,
}

#[derive(Debug)]
enum Link {
    Empty,
    More(Box<Node>),
}

#[derive(Debug)]
struct Node {
    elem: i32,
    next: Link,
}

impl List {
    pub fn new() -> Self {
        List { head: Link::Empty }
    }

    pub fn push(&mut self, elem: i32) {
        let node = Box::new(Node {
            elem: elem,
            next: mem::replace(&mut self.head, Link::Empty),
        });

        self.head = Link::More(node);
    }

    pub fn pop(&mut self) -> Option<i32> {
        match mem::replace(&mut self.head, Link::Empty) {
            Link::Empty => None,
            Link::More(node) => {
                self.head = node.next;
                Some(node.elem)    
            }
        }
    }
}

//impl Drop for List {
//    fn drop(&mut self) {
//        self.head.drop();
//    }
//}
//
//impl Drop for Link {
//    fn drop(&mut self) {
//        match *self {
//            Link::Empty => {} // Done!
//            Link::More(ref mut boxed_node) => {
//                boxed_node.drop(); // tail recursive - good!
//            }
//        }
//    }
//}
//
//impl Drop for Box<Node> {
//    fn drop(&mut self) {
//        self.ptr.drop(); // uh oh, not tail recursive!
//        deallocate(self.ptr);
//    }
//}
//
//impl Drop for Node {
//    fn drop(&mut self) {
//        self.next.drop();
//    }
//}

impl Drop for List {
    fn drop(&mut self) {
        let mut cur_link = mem::replace(&mut self.head, Link::Empty);
        while let Link::More(mut boxed_node) = cur_link {
            cur_link = mem::replace(&mut boxed_node.next, Link::Empty);
        }
    }
}

#[cfg(test)]
mod tests {
    use super::List;

    #[test]
    fn basics() {
        let mut list = List::new();

        assert_eq!(list.pop(), None);

        list.push(1);
        list.push(2);
        list.push(3);

        assert_eq!(list.pop(), Some(3));
        assert_eq!(list.pop(), Some(2));

        list.push(4);
        list.push(5);

        assert_eq!(list.pop(), Some(5));
        assert_eq!(list.pop(), Some(4));

        assert_eq!(list.pop(), Some(1));
        assert_eq!(list.pop(), None);
    }
}
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