介绍

视频地址：www.bilibili.com/video/av78062009/

相关源码：github.com/anonymousGiga/Rust-link...

详细内容

本节我们实现可持久化链表的Drop。我们在之前实现可变链表的Drop的时候，我们是使用循环来进行回收的。这里我们如果采用统一的方法，是行不通的，因为我们此处通过Rc进行共享，必须等待引用计数为0 才能将数据从Rc中移除。

Drop的实现

Drop实现如下：

//实现Drop
impl<T> Drop for List<T> {
    fn drop(&mut self) {let mut head = self.head.take();    
        while let Some(node) = head {if let Ok(mut node) = Rc::try_unwrap(node) {
                head = node.next.take();} else {break;}}}
}

完整代码如下：

use std::rc::Rc;
pub struct List<T> {
    head: Link<T>,
}

type Link<T> = Option<Rc<Node<T>>>;

struct Node<T> {
    elem: T,
    next: Link<T>,
}

impl<T> List<T> {
    pub fn new() -> Self {
        List { head: None }}

    pub fn append(&mut self, elem: T) -> List<T> {
        List { head: Some(Rc::new(Node {
            elem: elem,
            next: self.head.clone(),}))}    
    }

    pub fn tail(&self) -> List<T> {
        List { head: self.head.as_ref().and_then(|node| {
            node.next.clone()})}}

    pub fn head(&self) -> Option<&T> {
        self.head.as_ref().map(|node| &node.elem)}
}

//实现Iter
pub struct Iter<'a, T> {
    next: Option<&'a Node<T>>,
}

impl<T> List<T> {
    pub fn iter(&self) -> Iter<T> {
        Iter { next: self.head.as_deref() }}
}

impl<'a, T> Iterator for Iter<'a, T> {
    type Item = &'a T;

    fn next(&mut self) -> Option<Self::Item> {
        self.next.map(|node| {
            self.next = node.next.as_deref();&node.elem
        })}
}

//实现Drop
impl<T> Drop for List<T> {
    fn drop(&mut self) {let mut head = self.head.take();    
        while let Some(node) = head {if let Ok(mut node) = Rc::try_unwrap(node) {
                head = node.next.take();} else {break;}}}
}

#[cfg(test)]
mod test {
    use super::List;
    #[test] 
    fn basics() {let mut list = List::new();
        assert_eq!(list.head(), None);

        let list = list.append(1).append(2).append(3);
        assert_eq!(list.head(), Some(&3));

        let list = list.tail();
        assert_eq!(list.head(), Some(&2));

        let list = list.tail();
        assert_eq!(list.head(), Some(&1));

        let list = list.tail();
        assert_eq!(list.head(), None);}

    #[test] 
    fn iter() {let list = List::new().append(1).append(2).append(3);let mut iter = list.iter();

        assert_eq!(iter.next(), Some(&3));
        assert_eq!(iter.next(), Some(&2));
        assert_eq!(iter.next(), Some(&1));}
}