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<h1 id="droparc-及完整代码"><a class="header" href="#droparc-及完整代码">Drop、Arc 及完整代码</a></h1>
<h2 id="drop"><a class="header" href="#drop">Drop</a></h2>
<p>与之前链表存在的问题相似，新的链表也有递归的问题。下面是之前的解决方法:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>impl&lt;T&gt; Drop for List&lt;T&gt; {
fn drop(&amp;mut self) {
let mut cur_link = self.head.take();
while let Some(mut boxed_node) = cur_link {
cur_link = boxed_node.next.take();
}
}
}
<span class="boring">}</span></code></pre></pre>
<p>但是 <code>boxed_node.next.take()</code> 的方式在新的链表中无法使用，因为我们没办法去修改 <code>Rc</code> 持有的值。</p>
<p>考虑一下相关的逻辑，可以发现，如果当前的节点仅被当前链表所引用(Rc 的引用计数为 1)，那该节点是可以安全 <code>drop</code> 的: </p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>impl&lt;T&gt; Drop for List&lt;T&gt; {
fn drop(&amp;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;
}
}
}
}
<span class="boring">}</span></code></pre></pre>
<p>这里有一个没见过的方法 <code>Rc::Try_unwrap</code> ，该方法会判断当前的 <code>Rc</code> 是否只有一个强引用，若是，则返回 <code>Rc</code> 持有的值，否则返回一个错误。</p>
<p>可以看出，我们会一直 drop 到第一个被其它链表所引用的节点：</p>
<pre><code class="language-shell">list1 -&gt; A ---+
|
v
list2 ------&gt; B -&gt; C -&gt; D
^
|
list3 -&gt; X ---+
</code></pre>
<p>例如如果要 drop <code>List2</code>，那会从头节点开始一直 drop 到 <code>B</code> 节点时停止，剩余的 <code>B -&gt; C -&gt; D</code> 三个节点由于引用计数不为 1 (同时被多个链表引用) ，因此不会被 drop。</p>
<p>测试下新的代码:</p>
<pre><code class="language-shell">$ cargo test
Compiling lists v0.1.0 (/Users/ABeingessner/dev/too-many-lists/lists)
Finished dev [unoptimized + debuginfo] target(s) in 1.10s
Running /Users/ABeingessner/dev/too-many-lists/lists/target/debug/deps/lists-86544f1d97438f1f
running 8 tests
test first::test::basics ... ok
test second::test::basics ... ok
test second::test::into_iter ... ok
test second::test::iter ... ok
test second::test::iter_mut ... ok
test second::test::peek ... ok
test third::test::basics ... ok
test third::test::iter ... ok
test result: ok. 8 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
</code></pre>
<p>完美通过，下面再来考虑一个问题，如果我们的链表要在多线程环境使用该怎么办？</p>
<h2 id="arc"><a class="header" href="#arc">Arc</a></h2>
<p>不可变链表的一个很大的好处就在于多线程访问时自带安全性，毕竟共享可变性是多线程危险的源泉，最好也是最简单的解决办法就是直接干掉可变性。</p>
<p>但是 <code>Rc&lt;T&gt;</code> 本身并不是线程安全的，原因在之前的章节也有讲：它内部的引用计数器并不是线程安全的，通俗来讲，计数器没有加锁也没有实现原子性。</p>
<p>再结合之前章节学过的内容，绝大部分同学应该都能想到, <code>Arc&lt;T&gt;</code> 就是我们的最终答案。</p>
<p>那么还有一个问题，我们怎么知道一个类型是不是类型安全？会不会在多线程误用了非线程安全的类型呢？这就是 Rust 安全性的另一个强大之处：Rust 通过提供 <code>Send</code> 和 <code>Sync</code> 两个特征来保证线程安全。</p>
<blockquote>
<p>关于 <code>Send</code> 和 <code>Sync</code> 的详细介绍，请参见<a href="https://course.rs/advance/concurrency-with-threads/send-sync.html">此章节</a></p>
</blockquote>
<h2 id="完整代码"><a class="header" href="#完整代码">完整代码</a></h2>
<p>又到了喜闻乐见的环节，新链表的代码相比之前反而还更简单了，不可变就是香！</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>use std::rc::Rc;
pub struct List&lt;T&gt; {
head: Link&lt;T&gt;,
}
type Link&lt;T&gt; = Option&lt;Rc&lt;Node&lt;T&gt;&gt;&gt;;
struct Node&lt;T&gt; {
elem: T,
next: Link&lt;T&gt;,
}
impl&lt;T&gt; List&lt;T&gt; {
pub fn new() -&gt; Self {
List { head: None }
}
pub fn prepend(&amp;self, elem: T) -&gt; List&lt;T&gt; {
List { head: Some(Rc::new(Node {
elem: elem,
next: self.head.clone(),
}))}
}
pub fn tail(&amp;self) -&gt; List&lt;T&gt; {
List { head: self.head.as_ref().and_then(|node| node.next.clone()) }
}
pub fn head(&amp;self) -&gt; Option&lt;&amp;T&gt; {
self.head.as_ref().map(|node| &amp;node.elem)
}
pub fn iter(&amp;self) -&gt; Iter&lt;'_, T&gt; {
Iter { next: self.head.as_deref() }
}
}
impl&lt;T&gt; Drop for List&lt;T&gt; {
fn drop(&amp;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;
}
}
}
}
pub struct Iter&lt;'a, T&gt; {
next: Option&lt;&amp;'a Node&lt;T&gt;&gt;,
}
impl&lt;'a, T&gt; Iterator for Iter&lt;'a, T&gt; {
type Item = &amp;'a T;
fn next(&amp;mut self) -&gt; Option&lt;Self::Item&gt; {
self.next.map(|node| {
self.next = node.next.as_deref();
&amp;node.elem
})
}
}
#[cfg(test)]
mod test {
use super::List;
#[test]
fn basics() {
let list = List::new();
assert_eq!(list.head(), None);
let list = list.prepend(1).prepend(2).prepend(3);
assert_eq!(list.head(), Some(&amp;3));
let list = list.tail();
assert_eq!(list.head(), Some(&amp;2));
let list = list.tail();
assert_eq!(list.head(), Some(&amp;1));
let list = list.tail();
assert_eq!(list.head(), None);
// Make sure empty tail works
let list = list.tail();
assert_eq!(list.head(), None);
}
#[test]
fn iter() {
let list = List::new().prepend(1).prepend(2).prepend(3);
let mut iter = list.iter();
assert_eq!(iter.next(), Some(&amp;3));
assert_eq!(iter.next(), Some(&amp;2));
assert_eq!(iter.next(), Some(&amp;1));
}
}
<span class="boring">}</span></code></pre></pre>
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