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< h1 id = "drop-and-panic-safety" > < a class = "header" href = "#drop-and-panic-safety" > Drop and Panic Safety< / a > < / h1 >
< p > 嘿，你注意到这些注释了吗：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > // Note that we don't need to mess around with `take` anymore
// because everything is Copy and there are no dtors that will
// run if we mess up... right? :) Riiiight? :)))
< span class = "boring" > }< / span > < / code > < / pre >
< p > 这对吗？< / p >
< p > 你忘记你正在读那本书了吗？当然这是错误的（部分上是）。< / p >
< p > 让我们再次看看 pop_front 内部:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > // Bring the Box back to life so we can move out its value and
// Drop it (Box continues to magically understand this for us).
let boxed_node = Box::from_raw(node.as_ptr());
let result = boxed_node.elem;
// Make the next node into the new front.
self.front = boxed_node.back;
if let Some(new) = self.front {
// Cleanup its reference to the removed node
(*new.as_ptr()).front = None;
} else {
// If the front is now null, then this list is now empty!
debug_assert!(self.len == 1);
self.back = None;
}
self.len -= 1;
result
// Box gets implicitly freed here, knows there is no T.
< span class = "boring" > }< / span > < / code > < / pre >
< p > 你看到 bug 了吗? 真可怕, 是这一行:< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > debug_assert!(self.len == 1);
< span class = "boring" > }< / span > < / code > < / pre >
< p > 大多数情况下，你不需要考虑或担心恐慌，但一旦你开始编写真正不安全的代码，并在 “invariants(不可变性) “上大做文章，你就需要对恐慌保持高度警惕！< / p >
< p > 我们必须谈谈 < a href = "https://doc.rust-lang.org/nightly/nomicon/exception-safety.html" > < em > 异常安全< / em > < / a > （又名恐慌安全、解除安全……）。< / p >
< p > 情况是这样的：在默认情况下，恐慌会被 unwinding。unwind 只是 “让每个函数立即返回 “的一种花哨说法。你可能会想：“好吧，如果每个函数都返回，那么程序就要结束了，何必在乎它呢？“但你错了！< / p >
< p > 我们必须关注有两个原因：当函数返回时，析构函数会运行，而且可以捕获 unwind。在这两种情况下， ， ， ， ！ < / p >
< p > 让我们想一想，到这一行时，我们的集合处于什么状态：< / p >
< p > 我们将 boxed_node 放在栈上， ， ， < / p >
< p > 有趣的是，这行也有类似的问题，但它要安全得多：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > self.len -= 1;
< span class = "boring" > }< / span > < / code > < / pre >
< p > 默认情况下， < / p >
< p > 在实现过程中， ， < code > & mut Self< / code > ， ， < / p >
< p > 我们有两种方法可以让我们的代码更健壮：< / p >
< ul >
< li > 更积极地使用 Option::take 这样的操作，因为它们更 “事务性”， < / li >
< li > 放弃 debug_asserts， ， < / li >
< / ul >
< p > 原则上， ， ， ， ， < / p >
< p > 这是我们的全部实现：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use std::ptr::NonNull;
use std::marker::PhantomData;
pub struct LinkedList< T> {
front: Link< T> ,
back: Link< T> ,
len: usize,
_boo: PhantomData< T> ,
}
type Link< T> = Option< NonNull< Node< T> > > ;
struct Node< T> {
front: Link< T> ,
back: Link< T> ,
elem: T,
}
impl< T> LinkedList< T> {
pub fn new() -> Self {
Self {
front: None,
back: None,
len: 0,
_boo: PhantomData,
}
}
pub fn push_front(& mut self, elem: T) {
// SAFETY: it's a linked-list, what do you want?
unsafe {
let new = NonNull::new_unchecked(Box::into_raw(Box::new(Node {
front: None,
back: None,
elem,
})));
if let Some(old) = self.front {
// Put the new front before the old one
(*old.as_ptr()).front = Some(new);
(*new.as_ptr()).back = Some(old);
} else {
// If there's no front, then we're the empty list and need
// to set the back too.
self.back = Some(new);
}
// These things always happen!
self.front = Some(new);
self.len += 1;
}
}
pub fn pop_front(& mut self) -> Option< T> {
unsafe {
// Only have to do stuff if there is a front node to pop.
self.front.map(|node| {
// Bring the Box back to life so we can move out its value and
// Drop it (Box continues to magically understand this for us).
let boxed_node = Box::from_raw(node.as_ptr());
let result = boxed_node.elem;
// Make the next node into the new front.
self.front = boxed_node.back;
if let Some(new) = self.front {
// Cleanup its reference to the removed node
(*new.as_ptr()).front = None;
} else {
// If the front is now null, then this list is now empty!
self.back = None;
}
self.len -= 1;
result
// Box gets implicitly freed here, knows there is no T.
})
}
}
pub fn len(& self) -> usize {
self.len
}
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 这还有什么可以引发恐慌？老实说，要知道这些需要你是 Rust 专家，不过幸好我是！< / p >
< p > 在这段代码中，我能看到的唯一可能引起恐慌的地方是 < code > Box::new< / code > （用于内存不足的情况）和 < code > len< / code > 运算。所有这些都在我们方法的最末端或最开始，所以，我们是安全的！< / p >
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