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<main>
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<h1 id="数据布局2-再裸一些吧"><a class="header" href="#数据布局2-再裸一些吧">数据布局2: 再裸一些吧</a></h1>
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<blockquote>
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<p>TL;DR 在之前部分中，将安全的指针 <code>&</code> 、<code>&mut</code> 和 <code>Box</code> 跟不安全的裸指针 <code>*mut</code> 和 <code>*const</code> 混用是 UB 的根源之一，原因是安全指针会引入额外的约束，但是裸指针并不会遵守这些约束。</p>
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</blockquote>
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<p>一个好消息，一个坏消息。坏消息是我们又要开始写链表了，悲剧 = , = 好消息呢是之前我们已经讨论过该如何设计了，之前做的工作基本都是正确的，除了混用安全指针和不安全指针的部分。</p>
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<h2 id="布局"><a class="header" href="#布局">布局</a></h2>
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<p>在新的布局中我们将只使用裸指针，然后大家就等着好消息吧！</p>
|
|
|
<p>下面是之前的“破代码“ ：</p>
|
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|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
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|
</span><span class="boring">fn main() {
|
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|
</span>pub struct List<T> {
|
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|
head: Link<T>,
|
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|
tail: *mut Node<T>, // 好人一枚
|
|
|
}
|
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|
type Link<T> = Option<Box<Node<T>>>; // 恶魔一只
|
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|
struct Node<T> {
|
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|
elem: T,
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next: Link<T>,
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}
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<span class="boring">}</span></code></pre>
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<p>现在删除恶魔:</p>
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|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>pub struct List<T> {
|
|
|
head: Link<T>,
|
|
|
tail: *mut Node<T>,
|
|
|
}
|
|
|
|
|
|
type Link<T> = *mut Node<T>; // 嘀，新的好人卡，请查收
|
|
|
|
|
|
struct Node<T> {
|
|
|
elem: T,
|
|
|
next: Link<T>,
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>细心的同学可能会发现，此时 <code>Link<T></code> 和 <code>tail</code> 的类型 <code>*mut Node<T></code> 一致了。我们可以把 <code>tail</code> 的类型也改为 <code>Link<T></code> ，让代码更一致。</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>pub struct List<T> {
|
|
|
head: LinkT>,
|
|
|
// tail: *mut Node<T>,
|
|
|
tail: Link<T>, // 新老好人卡此时一致了
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>请大家牢记：当使用裸指针时，<code>Option</code> 对我们是相当不友好的，所以这里不再使用。在后面还将引入 <code>NonNull</code> 类型，但是现在还无需操心。</p>
|
|
|
<h2 id="基本操作"><a class="header" href="#基本操作">基本操作</a></h2>
|
|
|
<p><code>List::new</code> 与之前几乎没有区别：</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>use ptr;
|
|
|
|
|
|
impl<T> List<T> {
|
|
|
pub fn new() -> Self {
|
|
|
List { head: ptr::null_mut(), tail: ptr::null_mut() }
|
|
|
}
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p><code>Push</code> 也几乎没区…</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>pub fn push(&mut self, elem: T) {
|
|
|
let mut new_tail = Box::new(
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>等等，我们不再使用 <code>Box</code> 了，既然如此，该怎么分配内存呢？</p>
|
|
|
<p>也许我们可以使用 <code>std::alloc::alloc</code>，但是大家想象一下拿着武士刀进厨房切菜的场景，所以，还是算了吧。</p>
|
|
|
<p>我们想要 <code>Box</code> 又不想要，这里有一个也许很野但是管用的方法:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>struct Node<T> {
|
|
|
elem: T,
|
|
|
real_next: Option<Box<Node<T>>>,
|
|
|
next: *mut Node<T>,
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>先创建一个 <code>Box</code> ，并使用一个裸指针指向 <code>Box</code> 中的 <code>Node</code>，然后就一直使用该裸指针直到我们处理完 <code>Node</code> 且可以销毁它之时。最后，可以将 <code>Box</code> 从 <code>real_next</code> 中 <code>take</code> 出来，并 <code>drop</code> 掉。</p>
|
|
|
<p>从上面来看，这个非常符合我们之前的简化版借用栈模型？借用 <code>Box</code>，再借用一个裸指针，然后先弹出该裸指针，再弹出 <code>Box</code>，嗯，果然很符合。</p>
|
|
|
<p>但是问题来了，这样做看上去有趣，但是你能保证这个简化版借用栈顺利的工作吗？所以，我们还是使用 <a href="https://doc.rust-lang.org/std/boxed/struct.Box.html#method.into_raw">Box::into_raw</a> 函数吧！</p>
|
|
|
<blockquote>
|
|
|
<p><code>pub fn into_raw(b: Box<T>) -> *mut T</code></p>
|
|
|
<p>消费掉 <code>Box</code> (拿走所有权)，返回一个裸指针。该指针会被正确的对齐且不为 null</p>
|
|
|
<p>在调用该函数后，调用者需要对之前被 Box 所管理的内存负责，特别地，调用者需要正确的清理 <code>T</code> 并释放相应的内存。最简单的方式是通过 <code>Box::from_raw</code> 函数将裸指针再转回到 <code>Box</code>，然后 <code>Box</code> 的析构器就可以自动执行清理了。</p>
|
|
|
<p>注意：这是一个关联函数，因此 <code>b.into_raw()</code> 是不正确的，我们得使用 <code>Box::into_raw(b)</code>。因此该函数不会跟内部类型的同名方法冲突。</p>
|
|
|
<h3 id="示例"><a class="header" href="#示例">示例</a></h3>
|
|
|
<p>将裸指针转换成 <code>Box</code> 以实现自动的清理:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>let x = Box::new(String::from("Hello"));
|
|
|
let ptr = Box::into_raw(x);
|
|
|
let x = unsafe { Box::from_raw(ptr) };
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
</blockquote>
|
|
|
<p>太棒了，简直为我们量身定制。而且它还很符合我们试图遵循的规则： 从安全的东东开始，将其转换成裸指针，最后再将裸指针转回安全的东东以实现安全的 drop。</p>
|
|
|
<p>现在，我们就可以到处使用裸指针，只不过要注意 unsafe 的范围要尽可能小。大家可以再看下<a href="https://course.rs/advance/unsafe/intro.html#控制-unsafe-的使用边界">之前的章节</a> 巩固巩固。</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>pub fn push(&mut self, elem: T) {
|
|
|
// 一开始就将 Box 转换成裸指针
|
|
|
let new_tail = Box::into_raw(Box::new(Node {
|
|
|
elem: elem,
|
|
|
next: ptr::null_mut(),
|
|
|
}));
|
|
|
|
|
|
if !self.tail.is_null() {
|
|
|
unsafe {
|
|
|
(*self.tail).next = new_tail;
|
|
|
}
|
|
|
} else {
|
|
|
self.head = new_tail;
|
|
|
}
|
|
|
|
|
|
self.tail = new_tail;
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>嘿，都说 unsafe 不应该使用，但没想到 unsafe 真的是好！现在代码整体看起来简洁多了。</p>
|
|
|
<p>继续实现 <code>pop</code>，它跟之前区别不大，但是我们不要忘了使用 <code>Box::from_raw</code> 来清理内存:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>pub fn pop(&mut self) -> Option<T> {
|
|
|
unsafe {
|
|
|
if self.head.is_null() {
|
|
|
None
|
|
|
} else {
|
|
|
let head;
|
|
|
unsafe {
|
|
|
head = Box::from_raw(self.head);
|
|
|
}
|
|
|
self.head = head.next;
|
|
|
|
|
|
if self.head.is_null() {
|
|
|
self.tail = ptr::null_mut();
|
|
|
}
|
|
|
|
|
|
Some(head.elem)
|
|
|
}
|
|
|
}
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>纪念下死去的 <code>take</code> 和 <code>map</code>，现在我们得手动检查和设置 <code>null</code> 了。</p>
|
|
|
<p>然后再实现下析构器，直接循环 <code>pop</code> 即可，怎么说，简单可爱，谁不爱呢?</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>impl<T> Drop for List<T> {
|
|
|
fn drop(&mut self) {
|
|
|
while let Some(_) = self.pop() { }
|
|
|
}
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>现在到了检验正确性的时候:</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>#[cfg(test)]
|
|
|
mod test {
|
|
|
use super::List;
|
|
|
#[test]
|
|
|
fn basics() {
|
|
|
let mut list = List::new();
|
|
|
|
|
|
// Check empty list behaves right
|
|
|
assert_eq!(list.pop(), None);
|
|
|
|
|
|
// Populate list
|
|
|
list.push(1);
|
|
|
list.push(2);
|
|
|
list.push(3);
|
|
|
|
|
|
// Check normal removal
|
|
|
assert_eq!(list.pop(), Some(1));
|
|
|
assert_eq!(list.pop(), Some(2));
|
|
|
|
|
|
// Push some more just to make sure nothing's corrupted
|
|
|
list.push(4);
|
|
|
list.push(5);
|
|
|
|
|
|
// Check normal removal
|
|
|
assert_eq!(list.pop(), Some(3));
|
|
|
assert_eq!(list.pop(), Some(4));
|
|
|
|
|
|
// Check exhaustion
|
|
|
assert_eq!(list.pop(), Some(5));
|
|
|
assert_eq!(list.pop(), None);
|
|
|
|
|
|
// Check the exhaustion case fixed the pointer right
|
|
|
list.push(6);
|
|
|
list.push(7);
|
|
|
|
|
|
// Check normal removal
|
|
|
assert_eq!(list.pop(), Some(6));
|
|
|
assert_eq!(list.pop(), Some(7));
|
|
|
assert_eq!(list.pop(), None);
|
|
|
}
|
|
|
}
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<pre><code class="language-shell">$ cargo test
|
|
|
|
|
|
running 12 tests
|
|
|
test fifth::test::basics ... ok
|
|
|
test first::test::basics ... ok
|
|
|
test fourth::test::basics ... ok
|
|
|
test fourth::test::peek ... ok
|
|
|
test second::test::basics ... ok
|
|
|
test fourth::test::into_iter ... 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. 12 passed; 0 failed; 0 ignored; 0 measured
|
|
|
</code></pre>
|
|
|
<p>测试没问题，还有一个拦路虎 <code>miri</code> 呢。</p>
|
|
|
<pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>MIRIFLAGS="-Zmiri-tag-raw-pointers" cargo +nightly-2022-01-21 miri test
|
|
|
|
|
|
running 12 tests
|
|
|
test fifth::test::basics ... ok
|
|
|
test first::test::basics ... ok
|
|
|
test fourth::test::basics ... ok
|
|
|
test fourth::test::peek ... ok
|
|
|
test second::test::basics ... ok
|
|
|
test fourth::test::into_iter ... 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. 12 passed; 0 failed; 0 ignored; 0 measured
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>苦尽甘来，苦尽甘来啊！我们这些章节的努力没有白费，它终于成功的工作了。</p>
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