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添加 Rust 语言圣经（镜像）
2 years ago
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<h1 id="一些收尾工作以及最终代码"><a class="header" href="#一些收尾工作以及最终代码">一些收尾工作以及最终代码</a></h1>
<p>在之前的章节中，我们完成了 Bad 单链表栈的数据定义和基本操作，下面一起来写一些测试代码。</p>
<h2 id="单元测试"><a class="header" href="#单元测试">单元测试</a></h2>
<blockquote>
<p>关于如何编写测试，请参见<a href="https://course.rs/test/write-tests.html">自动化测试章节</a></p>
</blockquote>
<p>首先，单元测试代码要放在待测试的目标代码旁边，也就是同一个文件中:</p>
<pre><pre class="playground"><code class="language-rust edition2021">
<span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>// in first.rs
#[cfg(test)]
mod test {
#[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(3));
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(5));
assert_eq!(list.pop(), Some(4));
// Check exhaustion
assert_eq!(list.pop(), Some(1));
assert_eq!(list.pop(), None);
}
}
<span class="boring">}
</span></code></pre></pre>
<p>在 <code>src/first.rs</code> 中添加以上测试模块，然后使用 <code>cargo test</code> 运行相关的测试用例：</p>
<pre><code class="language-shell">$ cargo test
error[E0433]: failed to resolve: use of undeclared type or module `List`
--&gt; src/first.rs:43:24
|
43 | let mut list = List::new();
| ^^^^ use of undeclared type or module `List`
</code></pre>
<p>Ooops! 报错了，从错误内容来看，是因为我们在一个不同的模块 <code>test</code> 中，引入了 <code>first</code> 模块中的代码，由于前者是后者的子模块，因此可以使用以下方式引入 <code>first</code> 模块中的 <code>List</code> 定义:</p>
<pre><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;
// 其它代码保持不变
}
<span class="boring">}
</span></code></pre></pre>
<p>大家可以再次尝试使用 <code>cargo test</code> 运行测试用例，具体的结果就不再展开，关于结果的解读，请参看文章开头的链接。</p>
<h2 id="drop"><a class="header" href="#drop">Drop</a></h2>
<p>现在还有一个问题，我们是否需要手动来清理释放我们的链表？答案是 No，因为 Rust 为我们提供了 <code>Drop</code> 特征，若变量实现了该特征，则在它离开作用域时将自动调用解构函数以实现资源清理释放工作，最妙的是，这一切都发生在编译期，因此没有多余的性能开销。</p>
<blockquote>
<p>关于 Drop 特征的详细介绍，请参见<a href="https://course.rs/advance/smart-pointer/drop.html">智能指针 - Drop</a></p>
</blockquote>
<p>事实上，我们无需手动为自定义类型实现 <code>Drop</code> 特征，原因是 Rust 自动为几乎所有类型都实现了 <code>Drop</code>，例如我们自定义的结构体，只要结构体的所有字段都实现了 <code>Drop</code>，那结构体也会自动实现 <code>Drop</code> !</p>
<p>但是，有的时候这种自动实现可能不够优秀，例如考虑以下链表:</p>
<pre><code class="language-shell">list -&gt; A -&gt; B -&gt; C
</code></pre>
<p>当 <code>List</code> 被自动 <code>drop</code> 后，接着会去尝试 <code>Drop</code> A，然后是 <code>B</code>，最后是 <code>C</code>。这个时候，其中一部分读者可能会紧张起来，因此这其实是一段递归代码，可能会直接撑爆我们的 stack 栈。</p>
<p>例如以下的测试代码会试图创建一个很长的链表，然后会导致栈溢出错误:</p>
<pre><pre class="playground"><code class="language-rust edition2021">
<span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>#[test]
fn long_list() {
let mut list = List::new();
for i in 0..100000 {
list.push(i);
}
drop(list);
}
<span class="boring">}
</span></code></pre></pre>
<pre><code class="language-shell">thread 'first::test::long_list' has overflowed its stack
</code></pre>
<p>可能另一部分同学会想 &quot;这显然是<a href="https://zh.wikipedia.org/wiki/%E5%B0%BE%E8%B0%83%E7%94%A8">尾递归</a>，一个靠谱的编程语言是不会让尾递归撑爆我们的 stack&quot;。然后，这个想法并不正确，下面让我们尝试模拟编译器来看看 <code>Drop</code> 会如何实现:</p>
<pre><pre class="playground"><code class="language-rust edition2021">
<span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>impl Drop for List {
fn drop(&amp;mut self) {
// NOTE: 在 Rust 代码中，我们不能显式的调用 `drop` 方法，只能调用 std::mem::drop 函数
// 这里只是在模拟编译器!
self.head.drop(); // 尾递归 - good!
}
}
impl Drop for Link {
fn drop(&amp;mut self) {
match *self {
Link::Empty =&gt; {} // Done!
Link::More(ref mut boxed_node) =&gt; {
boxed_node.drop(); // 尾递归 - good!
}
}
}
}
impl Drop for Box&lt;Node&gt; {
fn drop(&amp;mut self) {
self.ptr.drop(); // 糟糕，这里不是尾递归!
deallocate(self.ptr); // 不是尾递归的原因是在 `drop` 后，还有额外的操作
}
}
impl Drop for Node {
fn drop(&amp;mut self) {
self.next.drop();
}
}
<span class="boring">}
</span></code></pre></pre>
<p>从上面的代码和注释可以看出为 <code>Box&lt;Node&gt;</code> 实现的 <code>drop</code> 方法中，在 <code>self.ptr.drop</code> 后调用的 <code>deallocate</code> 会导致非尾递归的情况发生。</p>
<p>因此我们需要手动为 <code>List</code> 实现 <code>Drop</code> 特征:</p>
<pre><pre class="playground"><code class="language-rust edition2021">
<span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>impl Drop for List {
fn drop(&amp;mut self) {
let mut cur_link = mem::replace(&amp;mut self.head, Link::Empty);
while let Link::More(mut boxed_node) = cur_link {
cur_link = mem::replace(&amp;mut boxed_node.next, Link::Empty);
// boxed_node 在这里超出作用域并被 drop,
// 由于它的 `next` 字段拥有的 `Node` 被设置为 Link::Empty,
// 因此这里并不会有无边界的递归发生
}
}
}
<span class="boring">}
</span></code></pre></pre>
<p>测试下上面的实现以及之前的长链表例子:</p>
<pre><code class="language-shell">$ cargo test
Running target/debug/lists-5c71138492ad4b4a
running 2 tests
test first::test::basics ... ok
test first::test::long_list ... ok
test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured
</code></pre>
<p>完美！</p>
<p><span style="float:left"><img src="https://rust-unofficial.github.io/too-many-lists/img/profbee.gif" /></span></p>
<h4 id="为什么要提前优化"><a class="header" href="#为什么要提前优化">为什么要提前优化？</a></h4>
<p>事实上，我们在这里做了提前优化，否则可以使用 <code>while let Some(_) = self.pop() { }</code>, 这种实现显然更加简单. 那么问题来了：它们的区别是什么，有哪些性能上的好处？特别是在链表不仅仅支持 <code>i32</code> 时。</p>
<details>
<summary>点击这里展开答案</summary>
<p><code>self.pop()</code> 的会返回 <code>Option&lt;i32&gt;</code>, 而我们之前的实现仅仅对智能指针 <code>Box&lt;Node&gt;</code> 进行操作。前者会对值进行拷贝，而后者仅仅使用的是指针类型。</p>
<p>当链表中包含的值是其他较大的类型时，那这个拷贝的开销将变得非常高昂。</p>
</details>
<h2 id="最终代码"><a class="header" href="#最终代码">最终代码</a></h2>
<pre><pre class="playground"><code class="language-rust edition2021">
<span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>use std::mem;
pub struct List {
head: Link,
}
enum Link {
Empty,
More(Box&lt;Node&gt;),
}
struct Node {
elem: i32,
next: Link,
}
impl List {
pub fn new() -&gt; Self {
List { head: Link::Empty }
}
pub fn push(&amp;mut self, elem: i32) {
let new_node = Box::new(Node {
elem: elem,
next: mem::replace(&amp;mut self.head, Link::Empty),
});
self.head = Link::More(new_node);
}
pub fn pop(&amp;mut self) -&gt; Option&lt;i32&gt; {
match mem::replace(&amp;mut self.head, Link::Empty) {
Link::Empty =&gt; None,
Link::More(node) =&gt; {
self.head = node.next;
Some(node.elem)
}
}
}
}
impl Drop for List {
fn drop(&amp;mut self) {
let mut cur_link = mem::replace(&amp;mut self.head, Link::Empty);
while let Link::More(mut boxed_node) = cur_link {
cur_link = mem::replace(&amp;mut boxed_node.next, Link::Empty);
}
}
}
#[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(3));
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(5));
assert_eq!(list.pop(), Some(4));
// Check exhaustion
assert_eq!(list.pop(), Some(1));
assert_eq!(list.pop(), None);
}
}
<span class="boring">}
</span></code></pre></pre>
<p>从代码行数也可以看出，我们实现的肯定不是一个精致的链表：总共只有 80 行代码，其中一半还是测试！</p>
<p>但是万事开头难，既然开了一个好头，那接下来我们一鼓作气，继续看看更精致的链表长什么样。</p>
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