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添加 Rust 语言圣经（镜像）
2 years ago
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<h1 id="基本操作"><a class="header" href="#基本操作">基本操作</a></h1>
<blockquote>
<p>本章节的代码中有一个隐藏的 bug，因为它藏身于 unsafe 中，因此不会导致报错，我们会在后续章节解决这个问题，所以，请不要在生产环境使用此处的代码</p>
</blockquote>
<p>在开始之前，大家需要先了解 unsafe 的<a href="https://course.rs/advance/unsafe/intro.html">相关知识</a>。那么，言归正传，该如何构建一个链表？在之前我们是这么做的：</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; List&lt;T&gt; {
pub fn new() -&gt; Self {
List { head: None, tail: None }
}
}
<span class="boring">}
</span></code></pre></pre>
<p>但是我们不再在 <code>tail</code> 中使用 <code>Option</code>:</p>
<pre><code class="language-shell">$ cargo build
error[E0308]: mismatched types
--&gt; src/fifth.rs:15:34
|
15 | List { head: None, tail: None }
| ^^^^ expected *-ptr, found
| enum `std::option::Option`
|
= note: expected type `*mut fifth::Node&lt;T&gt;`
found type `std::option::Option&lt;_&gt;`
</code></pre>
<p>我们是可以使用 <code>Option</code> 包裹一层，但是 <code>*mut</code> 裸指针之所以裸，是因为它狂，它可以是 <code>null</code> ! 因此 <code>Option</code> 就变得没有意义:</p>
<pre><pre class="playground"><code class="language-rust edition2021">
<span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>use std::ptr;
// defns...
impl&lt;T&gt; List&lt;T&gt; {
pub fn new() -&gt; Self {
List { head: None, tail: ptr::null_mut() }
}
}
<span class="boring">}
</span></code></pre></pre>
<p>如上所示，通过 <code>std::ptr::null_mut</code> 函数可以获取一个 <code>null</code>，当然，还可以使用 <code>0 as *mut _</code>，但是...已经这么不安全了，好歹我们要留一点代码可读性上的尊严吧 = , =</p>
<p>好了，现在是时候去重新实现 <code>push</code> ，之前获取的是 <code>Option&lt;&amp;mut Node&lt;T&gt;&gt;</code> 成为我们的拦路虎，这次来看看如果是获取 <code>*mut Node&lt;T&gt;</code> 还会不会有类似的问题。</p>
<p>首先，该如何将一个普通的引用变成裸指针？答案是：强制转换 Coercions。</p>
<pre><pre class="playground"><code class="language-rust edition2021">
<span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>let raw_tail: *mut _ = &amp;mut *new_tail;
<span class="boring">}
</span></code></pre></pre>
<p>来看看 <code>push</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 push(&amp;mut self, elem: T) {
let mut new_tail = Box::new(Node {
elem: elem,
next: None,
});
let raw_tail: *mut _ = &amp;mut *new_tail;
// .is_null 会检查是否为 null, 在功能上等价于 `None` 的检查
if !self.tail.is_null() {
// 如果 old tail 存在，那将其指向新的 tail
self.tail.next = Some(new_tail);
} else {
// 否则让 head 指向新的 tail
self.head = Some(new_tail);
}
self.tail = raw_tail;
}
<span class="boring">}
</span></code></pre></pre>
<pre><code class="language-shell">$ cargo build
error[E0609]: no field `next` on type `*mut fifth::Node&lt;T&gt;`
--&gt; src/fifth.rs:31:23
|
31 | self.tail.next = Some(new_tail);
| ----------^^^^
| |
| help: `self.tail` is a raw pointer;
| try dereferencing it: `(*self.tail).next`
</code></pre>
<p>当使用裸指针时，一些 Rust 提供的便利条件也将不复存在，例如由于不安全性的存在，裸指针需要我们手动去解引用( deref ):</p>
<pre><pre class="playground"><code class="language-rust edition2021">
<span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>*self.tail.next = Some(new_tail);
<span class="boring">}
</span></code></pre></pre>
<pre><code class="language-shell">$ cargo build
error[E0609]: no field `next` on type `*mut fifth::Node&lt;T&gt;`
--&gt; src/fifth.rs:31:23
|
31 | *self.tail.next = Some(new_tail);
| -----------^^^^
| |
| help: `self.tail` is a raw pointer;
| try dereferencing it: `(*self.tail).next`
</code></pre>
<p>哦哦，运算符的优先级问题:</p>
<pre><pre class="playground"><code class="language-rust edition2021">
<span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>(*self.tail).next = Some(new_tail);
<span class="boring">}
</span></code></pre></pre>
<pre><code class="language-shell">$ cargo build
error[E0133]: dereference of raw pointer is unsafe and requires
unsafe function or block
--&gt; src/fifth.rs:31:13
|
31 | (*self.tail).next = Some(new_tail);
| ^^^^^^^^^^^^^^^^^ dereference of raw pointer
|
= note: raw pointers may be NULL, dangling or unaligned;
they can violate aliasing rules and cause data races:
all of these are undefined behavior
</code></pre>
<p>哎...太难了，错误一个连一个，好在编译器给出了提示：由于我们在进行不安全的操作，因此需要使用 <code>unsafe</code> 语句块。那么问题来了，是将某几行代码包在 <code>unsafe</code> 中还是将整个函数包在 <code>unsafe</code> 中呢？如果大家不知道哪个是正确答案的话，证明<a href="https://course.rs/advance/unsafe/intro.html#%E6%8E%A7%E5%88%B6-unsafe-%E7%9A%84%E4%BD%BF%E7%94%A8%E8%BE%B9%E7%95%8C">之前的章节</a>还是没有仔细学，请回去再看一下，巩固巩固:) </p>
<pre><pre class="playground"><code class="language-rust edition2021">
<span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>pub fn push(&amp;mut self, elem: T) {
let mut new_tail = Box::new(Node {
elem: elem,
next: None,
});
let raw_tail: *mut _ = &amp;mut *new_tail;
if !self.tail.is_null() {
// 你好编译器，我知道我在做危险的事情，我向你保证：就算犯错了，也和你没有关系，都是我这个不优秀的程序员的责任
unsafe {
(*self.tail).next = Some(new_tail);
}
} else {
self.head = Some(new_tail);
}
self.tail = raw_tail;
}
<span class="boring">}
</span></code></pre></pre>
<pre><code class="language-shell">$ cargo build
warning: field is never used: `elem`
--&gt; src/fifth.rs:11:5
|
11 | elem: T,
| ^^^^^^^
|
= note: #[warn(dead_code)] on by default
</code></pre>
<p>细心的同学可能会发现:不是所有的裸指针代码都有 unsafe 的身影。原因在于：<strong>创建原生指针是安全的行为，而解引用原生指针才是不安全的行为</strong></p>
<p>呼，长出了一口气，终于成功实现了 <code>push</code> ，下面来看看 <code>pop</code>:</p>
<pre><pre class="playground"><code class="language-rust edition2021">
<span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>pub fn pop(&amp;mut self) -&gt; Option&lt;T&gt; {
self.head.take().map(|head| {
let head = *head;
self.head = head.next;
if self.head.is_none() {
self.tail = ptr::null_mut();
}
head.elem
})
}
<span class="boring">}
</span></code></pre></pre>
<p>测试下:</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;
#[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>
<p>摊牌了，我们偷懒了，这些测试就是从之前的栈链表赋值过来的，但是依然做了些改变，例如在末尾增加了几个步骤以确保在 <code>pop</code> 中不会发生尾指针损坏( tail-pointer corruption )的情况。</p>
<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>
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