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<h1 id="额外的操作"><a class="header" href="#额外的操作">额外的操作</a></h1>
<p>在搞定 <code>push</code>、<code>pop</code> 后，剩下的基本跟栈链表的实现没有啥区别。只有会改变链表长度的操作才会使用<ruby>尾<rt>tail</rt></ruby>指针。</p>
<p>当然，现在一切都是裸指针，因此我们要重写代码来使用它们，在此过程中必须要确保没有遗漏地修改所有地方。</p>
<p>首先，先从栈链表实现中拷贝以下代码:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>// ...
pub struct IntoIter&lt;T&gt;(List&lt;T&gt;);
pub struct Iter&lt;'a, T&gt; {
next: Option&lt;&amp;'a Node&lt;T&gt;&gt;,
}
pub struct IterMut&lt;'a, T&gt; {
next: Option&lt;&amp;'a mut Node&lt;T&gt;&gt;,
}
<span class="boring">}</span></code></pre></pre>
<p>这里的 <code>Iter</code> 和 <code>IterMut</code> 并没有实现裸指针，先来修改下：</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>pub struct IntoIter&lt;T&gt;(List&lt;T&gt;);
pub struct Iter&lt;'a, T&gt; {
next: *mut Node&lt;T&gt;,
}
pub struct IterMut&lt;'a, T&gt; {
next: *mut Node&lt;T&gt;,
}
impl&lt;T&gt; List&lt;T&gt; {
pub fn into_iter(self) -&gt; IntoIter&lt;T&gt; {
IntoIter(self)
}
pub fn iter(&amp;self) -&gt; Iter&lt;'_, T&gt; {
Iter { next: self.head }
}
pub fn iter_mut(&amp;mut self) -&gt; IterMut&lt;'_, T&gt; {
IterMut { next: self.head }
}
}
<span class="boring">}</span></code></pre></pre>
<p>看起来不错!</p>
<pre><code class="language-text">error[E0392]: parameter `'a` is never used
--&gt; src\fifth.rs:17:17
|
17 | pub struct Iter&lt;'a, T&gt; {
| ^^ unused parameter
|
= help: consider removing `'a`, referring to it in a field,
or using a marker such as `PhantomData`
error[E0392]: parameter `'a` is never used
--&gt; src\fifth.rs:21:20
|
21 | pub struct IterMut&lt;'a, T&gt; {
| ^^ unused parameter
|
= help: consider removing `'a`, referring to it in a field,
or using a marker such as `PhantomData`
</code></pre>
<p>咦？这里的 <a href="https://doc.rust-lang.org/std/marker/struct.PhantomData.html">PhantomData</a> 是什么?</p>
<blockquote>
<p>PhantomData 是<ruby>零大小<rt>zero sized</rt></ruby>的类型</p>
<p>在你的类型中添加一个 <code>PhantomData&lt;T&gt;</code> 字段，可以告诉编译器你的类型对 <code>T</code> 进行了使用，虽然并没有。说白了，就是让编译器不再给出 <code>T</code> 未被使用的警告或者错误。</p>
<p>如果想要更深入的了解，可以看下 <a href="https://doc.rust-lang.org/nightly/nomicon/">Nomicon</a></p>
</blockquote>
<p>大概最适用于 PhantomData 的场景就是一个结构体拥有未使用的生命周期，典型的就是在 unsafe 中使用。</p>
<p>总之，之前的错误是可以通过 PhantomData 来解决的，但是我想将这个秘密武器留到下一章中的双向链表，它才是真正的需要。</p>
<p>那现在只能破坏我们之前的豪言壮语了，灰溜溜的继续使用引用貌似也是不错的选择。能使用引用的原因是：我们可以创建一个迭代器，在其中使用安全引用，然后再丢弃迭代器。一旦迭代器被丢弃后，就可以继续使用 <code>push</code> 和 <code>pop</code> 了。</p>
<p>事实上，在迭代期间，我们还是需要解引用大量的裸指针，但是可以把引用看作裸指针的再借用。</p>
<p>偷偷的说一句：对于这个方法，我不敢保证一定能成功，先来试试吧..</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>pub struct IntoIter&lt;T&gt;(List&lt;T&gt;);
pub struct Iter&lt;'a, T&gt; {
next: Option&lt;&amp;'a Node&lt;T&gt;&gt;,
}
pub struct IterMut&lt;'a, T&gt; {
next: Option&lt;&amp;'a mut Node&lt;T&gt;&gt;,
}
impl&lt;T&gt; List&lt;T&gt; {
pub fn into_iter(self) -&gt; IntoIter&lt;T&gt; {
IntoIter(self)
}
pub fn iter(&amp;self) -&gt; Iter&lt;'_, T&gt; {
unsafe {
Iter { next: self.head.as_ref() }
}
}
pub fn iter_mut(&amp;mut self) -&gt; IterMut&lt;'_, T&gt; {
unsafe {
IterMut { next: self.head.as_mut() }
}
}
}
<span class="boring">}</span></code></pre></pre>
<p>为了存储引用，这里使用 <code>Option</code> 来包裹，并通过 <a href="https://doc.rust-lang.org/std/primitive.pointer.html#method.as_ref-1"><code>ptr::as_ref</code></a> 和 <a href="https://doc.rust-lang.org/std/primitive.pointer.html#method.as_mut"><code>ptr::as_mut</code></a> 来将裸指针转换成引用。</p>
<p>通常，我会尽量避免使用 <code>as_ref</code> 这类方法，因为它们在做一些不可思议的转换！但是上面却是极少数可以使用的场景之一。</p>
<p>这两个方法的使用往往会伴随很多警告，其中最有趣的是：</p>
<blockquote>
<p>你必须要遵循混叠(Aliasing)的规则，原因是返回的生命周期 <code>'a</code> 只是任意选择的，并不能代表数据真实的生命周期。特别的，在这段生命周期的过程中，指针指向的内存区域绝不能被其它指针所访问。</p>
</blockquote>
<p>好消息是，我们貌似不存在这个问题，因为混叠是我们一直在讨论和避免的问题。除此之外，还有一个恶魔：</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>pub unsafe fn as_mut&lt;'a&gt;(self) -&gt; Option&lt;&amp;'a mut T&gt;
<span class="boring">}</span></code></pre></pre>
<p>大家注意到这个凭空出现的 <code>'a</code> 吗？这里 <code>self</code> 是一个值类型，按照生命周期的规则，<code>'a</code> 无根之木，它就是<a href="https://course.rs/advance/lifetime/advance.html#%E6%97%A0%E7%95%8C%E7%94%9F%E5%91%BD%E5%91%A8%E6%9C%9F">无界生命周期</a>。</p>
<p>兄弟们，我很紧张，但是该继续的还是得继续，让我们从栈链表中再复制一些代码过来：</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; Iterator for IntoIter&lt;T&gt; {
type Item = T;
fn next(&amp;mut self) -&gt; Option&lt;Self::Item&gt; {
self.0.pop()
}
}
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; {
unsafe {
self.next.map(|node| {
self.next = node.next.as_ref();
&amp;node.elem
})
}
}
}
impl&lt;'a, T&gt; Iterator for IterMut&lt;'a, T&gt; {
type Item = &amp;'a mut T;
fn next(&amp;mut self) -&gt; Option&lt;Self::Item&gt; {
unsafe {
self.next.take().map(|node| {
self.next = node.next.as_mut();
&amp;mut node.elem
})
}
}
}
<span class="boring">}</span></code></pre></pre>
<p>验证下测试用例：</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>cargo test
running 15 tests
test fifth::test::basics ... ok
test fifth::test::into_iter ... ok
test fifth::test::iter ... ok
test fifth::test::iter_mut ... ok
test first::test::basics ... ok
test fourth::test::basics ... ok
test fourth::test::into_iter ... ok
test fourth::test::peek ... 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::iter ... ok
test third::test::basics ... ok
test result: ok. 15 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out;
<span class="boring">}</span></code></pre></pre>
<p>还有 miri:</p>
<pre><code class="language-text">MIRIFLAGS=&quot;-Zmiri-tag-raw-pointers&quot; cargo +nightly-2022-01-21 miri test
running 15 tests
test fifth::test::basics ... ok
test fifth::test::into_iter ... ok
test fifth::test::iter ... ok
test fifth::test::iter_mut ... ok
test first::test::basics ... ok
test fourth::test::basics ... ok
test fourth::test::into_iter ... ok
test fourth::test::peek ... 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. 15 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
</code></pre>
<p>嗯，还有 <code>peek</code> 和 <code>peek_mut</code> 的实现：</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>pub fn peek(&amp;self) -&gt; Option&lt;&amp;T&gt; {
unsafe {
self.head.as_ref()
}
}
pub fn peek_mut(&amp;mut self) -&gt; Option&lt;&amp;mut T&gt; {
unsafe {
self.head.as_mut()
}
}
<span class="boring">}</span></code></pre></pre>
<p>实现这么简单，运行起来肯定没问题：</p>
<pre><code class="language-text">$ cargo build
error[E0308]: mismatched types
--&gt; src\fifth.rs:66:13
|
25 | impl&lt;T&gt; List&lt;T&gt; {
| - this type parameter
...
64 | pub fn peek(&amp;self) -&gt; Option&lt;&amp;T&gt; {
| ---------- expected `Option&lt;&amp;T&gt;`
| because of return type
65 | unsafe {
66 | self.head.as_ref()
| ^^^^^^^^^^^^^^^^^^ expected type parameter `T`,
| found struct `fifth::Node`
|
= note: expected enum `Option&lt;&amp;T&gt;`
found enum `Option&lt;&amp;fifth::Node&lt;T&gt;&gt;`
</code></pre>
<p>哦，这个简单，map 以下就可以了:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>pub fn peek(&amp;self) -&gt; Option&lt;&amp;T&gt; {
unsafe {
self.head.as_ref().map(|node| &amp;node.elem)
}
}
pub fn peek_mut(&amp;mut self) -&gt; Option&lt;&amp;mut T&gt; {
unsafe {
self.head.as_mut().map(|node| &amp;mut node.elem)
}
}
<span class="boring">}</span></code></pre></pre>
<p>我感觉有很多错误正在赶来的路上，因此大家需要提高警惕，要么先写一个测试吧：把我们的 API 都混合在一起，让 miri 来享用 - miri food!</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>#[test]
fn miri_food() {
let mut list = List::new();
list.push(1);
list.push(2);
list.push(3);
assert!(list.pop() == Some(1));
list.push(4);
assert!(list.pop() == Some(2));
list.push(5);
assert!(list.peek() == Some(&amp;3));
list.push(6);
list.peek_mut().map(|x| *x *= 10);
assert!(list.peek() == Some(&amp;30));
assert!(list.pop() == Some(30));
for elem in list.iter_mut() {
*elem *= 100;
}
let mut iter = list.iter();
assert_eq!(iter.next(), Some(&amp;400));
assert_eq!(iter.next(), Some(&amp;500));
assert_eq!(iter.next(), Some(&amp;600));
assert_eq!(iter.next(), None);
assert_eq!(iter.next(), None);
assert!(list.pop() == Some(400));
list.peek_mut().map(|x| *x *= 10);
assert!(list.peek() == Some(&amp;5000));
list.push(7);
// Drop it on the ground and let the dtor exercise itself
}
<span class="boring">}</span></code></pre></pre>
<pre><code class="language-text">cargo test
running 16 tests
test fifth::test::basics ... ok
test fifth::test::into_iter ... ok
test fifth::test::iter ... ok
test fifth::test::iter_mut ... ok
test fifth::test::miri_food ... ok
test first::test::basics ... ok
test fourth::test::basics ... ok
test fourth::test::into_iter ... ok
test fourth::test::peek ... ok
test second::test::into_iter ... ok
test second::test::basics ... ok
test second::test::iter_mut ... ok
test second::test::peek ... ok
test third::test::iter ... ok
test second::test::iter ... ok
test third::test::basics ... ok
test result: ok. 16 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
MIRIFLAGS=&quot;-Zmiri-tag-raw-pointers&quot; cargo +nightly-2022-01-21 miri test
running 16 tests
test fifth::test::basics ... ok
test fifth::test::into_iter ... ok
test fifth::test::iter ... ok
test fifth::test::iter_mut ... ok
test fifth::test::miri_food ... ok
test first::test::basics ... ok
test fourth::test::basics ... ok
test fourth::test::into_iter ... ok
test fourth::test::peek ... ok
test second::test::into_iter ... ok
test second::test::basics ... ok
test second::test::iter_mut ... ok
test second::test::peek ... ok
test third::test::iter ... ok
test second::test::iter ... ok
test third::test::basics ... ok
test result: ok. 16 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
</code></pre>
<p>完美。</p>
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