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<main>
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<h1 id="基本操作"><a class="header" href="#基本操作">基本操作</a></h1>
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<blockquote>
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<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>
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</blockquote>
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<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>
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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>impl<T> List<T> {
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pub fn new() -> Self {
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List { head: None, tail: None }
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}
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}
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<span class="boring">}</span></code></pre>
|
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<p>但是我们不再在 <code>tail</code> 中使用 <code>Option</code>:</p>
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<pre><code class="language-shell">$ cargo build
|
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error[E0308]: mismatched types
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|
--> src/fifth.rs:15:34
|
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15 | List { head: None, tail: None }
|
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| ^^^^ expected *-ptr, found
|
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| enum `std::option::Option`
|
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|
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= note: expected type `*mut fifth::Node<T>`
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found type `std::option::Option<_>`
|
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</code></pre>
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<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>
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|
|
<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<T> List<T> {
|
|
|
pub fn new() -> Self {
|
|
|
List { head: None, tail: ptr::null_mut() }
|
|
|
}
|
|
|
}
|
|
|
<span class="boring">}</span></code></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<&mut Node<T>></code> 成为我们的拦路虎，这次来看看如果是获取 <code>*mut Node<T></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 class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
|
|
|
</span><span class="boring">fn main() {
|
|
|
</span>let raw_tail: *mut _ = &mut *new_tail;
|
|
|
<span class="boring">}</span></code></pre>
|
|
|
<p>来看看 <code>push</code> 的实现:</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(Node {
|
|
|
elem: elem,
|
|
|
next: None,
|
|
|
});
|
|
|
|
|
|
let raw_tail: *mut _ = &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><code class="language-shell">$ cargo build
|
|
|
|
|
|
error[E0609]: no field `next` on type `*mut fifth::Node<T>`
|
|
|
--> 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 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><code class="language-shell">$ cargo build
|
|
|
|
|
|
error[E0609]: no field `next` on type `*mut fifth::Node<T>`
|
|
|
--> 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 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><code class="language-shell">$ cargo build
|
|
|
|
|
|
error[E0133]: dereference of raw pointer is unsafe and requires
|
|
|
unsafe function or block
|
|
|
|
|
|
--> 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#控制-unsafe-的使用边界">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 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(Node {
|
|
|
elem: elem,
|
|
|
next: None,
|
|
|
});
|
|
|
|
|
|
let raw_tail: *mut _ = &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><code class="language-shell">$ cargo build
|
|
|
warning: field is never used: `elem`
|
|
|
--> 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 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> {
|
|
|
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>
|
|
|
<p>测试下:</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>
|
|
|
<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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<template id=fa-eye-slash><span class=fa-svg><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 640 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M38.8 5.1C28.4-3.1 13.3-1.2 5.1 9.2S-1.2 34.7 9.2 42.9l592 464c10.4 8.2 25.5 6.3 33.7-4.1s6.3-25.5-4.1-33.7L525.6 386.7c39.6-40.6 66.4-86.1 79.9-118.4c3.3-7.9 3.3-16.7 0-24.6c-14.9-35.7-46.2-87.7-93-131.1C465.5 68.8 400.8 32 320 32c-68.2 0-125 26.3-169.3 60.8L38.8 5.1zM223.1 149.5C248.6 126.2 282.7 112 320 112c79.5 0 144 64.5 144 144c0 24.9-6.3 48.3-17.4 68.7L408 294.5c5.2-11.8 8-24.8 8-38.5c0-53-43-96-96-96c-2.8 0-5.6 .1-8.4 .4c5.3 9.3 8.4 20.1 8.4 31.6c0 10.2-2.4 19.8-6.6 28.3l-90.3-70.8zm223.1 298L373 389.9c-16.4 6.5-34.3 10.1-53 10.1c-79.5 0-144-64.5-144-144c0-6.9 .5-13.6 1.4-20.2L83.1 161.5C60.3 191.2 44 220.8 34.5 243.7c-3.3 7.9-3.3 16.7 0 24.6c14.9 35.7 46.2 87.7 93 131.1C174.5 443.2 239.2 480 320 480c47.8 0 89.9-12.9 126.2-32.5z"/></svg></span></template>
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<template id=fa-play><span class=fa-svg><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 384 512"><!--! Font Awesome Free 6.2.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free (Icons: CC BY 4.0, Fonts: SIL OFL 1.1, Code: MIT License) Copyright 2022 Fonticons, Inc. --><path d="M73 39c-14.8-9.1-33.4-9.4-48.5-.9S0 62.6 0 80V432c0 17.4 9.4 33.4 24.5 41.9s33.7 8.1 48.5-.9L361 297c14.3-8.7 23-24.2 23-41s-8.7-32.2-23-41L73 39z"/></svg></span></template>
|
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