新增两个unsafe队列的章节

pull/668/head
sunface 3 years ago
parent b01f8269af
commit 6a6361d26c

@ -220,8 +220,8 @@
- [栈借用](too-many-lists/unsafe-queue/stacked-borrow.md)
- [测试栈借用](too-many-lists/unsafe-queue/testing-stacked-borrow.md)
- [数据布局2](too-many-lists/unsafe-queue/layout2.md)
- [额外的操作](too-many-lists/unsafe-queue/extra-junk.md)
- [最终代码](too-many-lists/unsafe-queue/final-code.md)
- [Rust 性能优化 todo](profiling/intro.md)
- [深入内存 todo](profiling/memory/intro.md)
- [指针和引用 todo](profiling/memory/pointer-ref.md)

@ -0,0 +1,351 @@
# 额外的操作
在搞定 `push`、`pop` 后,剩下的基本跟栈链表的实现没有啥区别。只有会改变链表长度的操作才会使用<ruby><rt>tail</rt></ruby>指针。
当然,现在一切都是裸指针,因此我们要重写代码来使用它们,在此过程中必须要确保没有遗漏地修改所有地方。
首先,先从栈链表实现中拷贝以下代码:
```rust
// ...
pub struct IntoIter<T>(List<T>);
pub struct Iter<'a, T> {
next: Option<&'a Node<T>>,
}
pub struct IterMut<'a, T> {
next: Option<&'a mut Node<T>>,
}
```
这里的 `Iter``IterMut` 并没有实现裸指针,先来修改下:
```rust
pub struct IntoIter<T>(List<T>);
pub struct Iter<'a, T> {
next: *mut Node<T>,
}
pub struct IterMut<'a, T> {
next: *mut Node<T>,
}
impl<T> List<T> {
pub fn into_iter(self) -> IntoIter<T> {
IntoIter(self)
}
pub fn iter(&self) -> Iter<'_, T> {
Iter { next: self.head }
}
pub fn iter_mut(&mut self) -> IterMut<'_, T> {
IterMut { next: self.head }
}
}
```
看起来不错!
```text
error[E0392]: parameter `'a` is never used
--> src\fifth.rs:17:17
|
17 | pub struct Iter<'a, T> {
| ^^ unused parameter
|
= help: consider removing `'a`, referring to it in a field,
or using a marker such as `PhantomData`
error[E0392]: parameter `'a` is never used
--> src\fifth.rs:21:20
|
21 | pub struct IterMut<'a, T> {
| ^^ unused parameter
|
= help: consider removing `'a`, referring to it in a field,
or using a marker such as `PhantomData`
```
咦?这里的 [PhantomData](https://doc.rust-lang.org/std/marker/struct.PhantomData.html) 是什么?
> PhantomData 是<ruby>零大小<rt>zero sized</rt></ruby>的类型
>
> 在你的类型中添加一个 `PhantomData<T>` 字段,可以告诉编译器你的类型对 `T` 进行了使用,虽然并没有。说白了,就是让编译器不再给出 `T` 未被使用的警告或者错误。
>
> 如果想要更深入的了解,可以看下 [Nomicon](https://doc.rust-lang.org/nightly/nomicon/)
大概最适用于 PhantomData 的场景就是一个结构体拥有未使用的生命周期,典型的就是在 unsafe 中使用。
总之,之前的错误是可以通过 PhantomData 来解决的,但是我想将这个秘密武器留到下一章中的双向链表,它才是真正的需要。
那现在只能破坏我们之前的豪言壮语了,灰溜溜的继续使用引用貌似也是不错的选择。能使用引用的原因是:我们可以创建一个迭代器,在其中使用安全引用,然后再丢弃迭代器。一旦迭代器被丢弃后,就可以继续使用 `push``pop` 了。
事实上,在迭代期间,我们还是需要解引用大量的裸指针,但是可以把引用看作裸指针的再借用。
偷偷的说一句:对于这个方法,我不敢保证一定能成功,先来试试吧..
```rust
pub struct IntoIter<T>(List<T>);
pub struct Iter<'a, T> {
next: Option<&'a Node<T>>,
}
pub struct IterMut<'a, T> {
next: Option<&'a mut Node<T>>,
}
impl<T> List<T> {
pub fn into_iter(self) -> IntoIter<T> {
IntoIter(self)
}
pub fn iter(&self) -> Iter<'_, T> {
unsafe {
Iter { next: self.head.as_ref() }
}
}
pub fn iter_mut(&mut self) -> IterMut<'_, T> {
unsafe {
IterMut { next: self.head.as_mut() }
}
}
}
```
为了存储引用,这里使用 `Option` 来包裹,并通过 [`ptr::as_ref`](https://doc.rust-lang.org/std/primitive.pointer.html#method.as_ref-1) 和 [`ptr::as_mut`](https://doc.rust-lang.org/std/primitive.pointer.html#method.as_mut) 来将裸指针转换成引用。
通常,我会尽量避免使用 `as_ref` 这类方法,因为它们在做一些不可思议的转换!但是上面却是极少数可以使用的场景之一。
这两个方法的使用往往会伴随很多警告,其中最有趣的是:
> 你必须要遵循混叠(Aliasing)的规则,原因是返回的生命周期 `'a` 只是任意选择的,并不能代表数据真实的生命周期。特别的,在这段生命周期的过程中,指针指向的内存区域绝不能被其它指针所访问。
好消息是,我们貌似不存在这个问题,因为混叠是我们一直在讨论和避免的问题。除此之外,还有一个恶魔:
```rust
pub unsafe fn as_mut<'a>(self) -> Option<&'a mut T>
```
大家注意到这个凭空出现的 `'a` 吗?这里 `self` 是一个值类型,按照生命周期的规则,`'a` 无根之木,它就是[无界生命周期](https://course.rs/advance/lifetime/advance.html#无界生命周期)。
兄弟们,我很紧张,但是该继续的还是得继续,让我们从栈链表中再复制一些代码过来:
```rust
impl<T> Iterator for IntoIter<T> {
type Item = T;
fn next(&mut self) -> Option<Self::Item> {
self.0.pop()
}
}
impl<'a, T> Iterator for Iter<'a, T> {
type Item = &'a T;
fn next(&mut self) -> Option<Self::Item> {
self.next.map(|node| {
self.next = node.next.as_deref();
&node.elem
})
}
}
impl<'a, T> Iterator for IterMut<'a, T> {
type Item = &'a mut T;
fn next(&mut self) -> Option<Self::Item> {
self.next.take().map(|node| {
self.next = node.next.as_deref_mut();
&mut node.elem
})
}
}
```
验证下测试用例:
```rust
cargo test
running 15 tests
test fifth::test::basics ... ok
test fifth::test::into_iter ... ok
test fifth::test::iter ... ok
test fifth::test::iter_mut ... ok
test first::test::basics ... ok
test fourth::test::basics ... ok
test fourth::test::into_iter ... ok
test fourth::test::peek ... ok
test second::test::basics ... 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::iter ... ok
test third::test::basics ... ok
test result: ok. 15 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out;
```
还有 miri:
```text
MIRIFLAGS="-Zmiri-tag-raw-pointers" cargo +nightly-2022-01-21 miri test
running 15 tests
test fifth::test::basics ... ok
test fifth::test::into_iter ... ok
test fifth::test::iter ... ok
test fifth::test::iter_mut ... ok
test first::test::basics ... ok
test fourth::test::basics ... ok
test fourth::test::into_iter ... ok
test fourth::test::peek ... ok
test second::test::basics ... 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. 15 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
```
嗯,还有 `peek``peek_mut` 的实现:
```rust
pub fn peek(&self) -> Option<&T> {
unsafe {
self.head.as_ref()
}
}
pub fn peek_mut(&mut self) -> Option<&mut T> {
unsafe {
self.head.as_mut()
}
}
```
实现这么简单,运行起来肯定没问题:
```text
$ cargo build
error[E0308]: mismatched types
--> src\fifth.rs:66:13
|
25 | impl<T> List<T> {
| - this type parameter
...
64 | pub fn peek(&self) -> Option<&T> {
| ---------- expected `Option<&T>`
| because of return type
65 | unsafe {
66 | self.head.as_ref()
| ^^^^^^^^^^^^^^^^^^ expected type parameter `T`,
| found struct `fifth::Node`
|
= note: expected enum `Option<&T>`
found enum `Option<&fifth::Node<T>>`
```
这个简单map 以下就可以了:
```rust
pub fn peek(&self) -> Option<&T> {
unsafe {
self.head.as_ref().map(|node| &node.elem)
}
}
pub fn peek_mut(&mut self) -> Option<&mut T> {
unsafe {
self.head.as_mut().map(|node| &mut node.elem)
}
}
```
我感觉有很多错误正在赶来的路上,因此大家需要提高警惕,要么先写一个测试吧:把我们的 API 都混合在一起,让 miri 来享用 - miri food!
```rust
#[test]
fn miri_food() {
let mut list = List::new();
list.push(1);
list.push(2);
list.push(3);
assert!(list.pop() == Some(1));
list.push(4);
assert!(list.pop() == Some(2));
list.push(5);
assert!(list.peek() == Some(&3));
list.push(6);
list.peek_mut().map(|x| *x *= 10);
assert!(list.peek() == Some(&30));
assert!(list.pop() == Some(30));
for elem in list.iter_mut() {
*elem *= 100;
}
let mut iter = list.iter();
assert_eq!(iter.next(), Some(&400));
assert_eq!(iter.next(), Some(&500));
assert_eq!(iter.next(), Some(&600));
assert_eq!(iter.next(), None);
assert_eq!(iter.next(), None);
assert!(list.pop() == Some(400));
list.peek_mut().map(|x| *x *= 10);
assert!(list.peek() == Some(&5000));
list.push(7);
// Drop it on the ground and let the dtor exercise itself
}
```
```text
cargo test
running 16 tests
test fifth::test::basics ... ok
test fifth::test::into_iter ... ok
test fifth::test::iter ... ok
test fifth::test::iter_mut ... ok
test fifth::test::miri_food ... ok
test first::test::basics ... ok
test fourth::test::basics ... ok
test fourth::test::into_iter ... ok
test fourth::test::peek ... ok
test second::test::into_iter ... ok
test second::test::basics ... ok
test second::test::iter_mut ... ok
test second::test::peek ... ok
test third::test::iter ... ok
test second::test::iter ... ok
test third::test::basics ... ok
test result: ok. 16 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
MIRIFLAGS="-Zmiri-tag-raw-pointers" cargo +nightly-2022-01-21 miri test
running 16 tests
test fifth::test::basics ... ok
test fifth::test::into_iter ... ok
test fifth::test::iter ... ok
test fifth::test::iter_mut ... ok
test fifth::test::miri_food ... ok
test first::test::basics ... ok
test fourth::test::basics ... ok
test fourth::test::into_iter ... ok
test fourth::test::peek ... ok
test second::test::into_iter ... ok
test second::test::basics ... ok
test second::test::iter_mut ... ok
test second::test::peek ... ok
test third::test::iter ... ok
test second::test::iter ... ok
test third::test::basics ... ok
test result: ok. 16 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
```
完美。

@ -0,0 +1,253 @@
# 最终代码
得益于不安全代码的引入,新的实现可以获得线性的性能提升,同时我们还设法复用了栈链表的很多代码。
当然,这个过程中,我们还引入了新的概念,例如借用栈,相信直到现在有些同学还晕乎乎的。不管如何,我们不用再去写一大堆嵌套来嵌套去的 `Rc``RefCell`
下面来看看咱们这个不安全链表的全貌吧。
```rust
use std::ptr;
pub struct List<T> {
head: Link<T>,
tail: *mut Node<T>,
}
type Link<T> = *mut Node<T>;
struct Node<T> {
elem: T,
next: Link<T>,
}
pub struct IntoIter<T>(List<T>);
pub struct Iter<'a, T> {
next: Option<&'a Node<T>>,
}
pub struct IterMut<'a, T> {
next: Option<&'a mut Node<T>>,
}
impl<T> List<T> {
pub fn new() -> Self {
List { head: ptr::null_mut(), tail: ptr::null_mut() }
}
pub fn push(&mut self, elem: T) {
unsafe {
let new_tail = Box::into_raw(Box::new(Node {
elem: elem,
next: ptr::null_mut(),
}));
if !self.tail.is_null() {
(*self.tail).next = new_tail;
} else {
self.head = new_tail;
}
self.tail = new_tail;
}
}
pub fn pop(&mut self) -> Option<T> {
unsafe {
if self.head.is_null() {
None
} else {
let head = Box::from_raw(self.head);
self.head = head.next;
if self.head.is_null() {
self.tail = ptr::null_mut();
}
Some(head.elem)
}
}
}
pub fn peek(&self) -> Option<&T> {
unsafe {
self.head.as_ref().map(|node| &node.elem)
}
}
pub fn peek_mut(&mut self) -> Option<&mut T> {
unsafe {
self.head.as_mut().map(|node| &mut node.elem)
}
}
pub fn into_iter(self) -> IntoIter<T> {
IntoIter(self)
}
pub fn iter(&self) -> Iter<'_, T> {
unsafe {
Iter { next: self.head.as_ref() }
}
}
pub fn iter_mut(&mut self) -> IterMut<'_, T> {
unsafe {
IterMut { next: self.head.as_mut() }
}
}
}
impl<T> Drop for List<T> {
fn drop(&mut self) {
while let Some(_) = self.pop() { }
}
}
impl<T> Iterator for IntoIter<T> {
type Item = T;
fn next(&mut self) -> Option<Self::Item> {
self.0.pop()
}
}
impl<'a, T> Iterator for Iter<'a, T> {
type Item = &'a T;
fn next(&mut self) -> Option<Self::Item> {
unsafe {
self.next.map(|node| {
self.next = node.next.as_ref();
&node.elem
})
}
}
}
impl<'a, T> Iterator for IterMut<'a, T> {
type Item = &'a mut T;
fn next(&mut self) -> Option<Self::Item> {
unsafe {
self.next.take().map(|node| {
self.next = node.next.as_mut();
&mut node.elem
})
}
}
}
#[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);
}
#[test]
fn into_iter() {
let mut list = List::new();
list.push(1); list.push(2); list.push(3);
let mut iter = list.into_iter();
assert_eq!(iter.next(), Some(1));
assert_eq!(iter.next(), Some(2));
assert_eq!(iter.next(), Some(3));
assert_eq!(iter.next(), None);
}
#[test]
fn iter() {
let mut list = List::new();
list.push(1); list.push(2); list.push(3);
let mut iter = list.iter();
assert_eq!(iter.next(), Some(&1));
assert_eq!(iter.next(), Some(&2));
assert_eq!(iter.next(), Some(&3));
assert_eq!(iter.next(), None);
}
#[test]
fn iter_mut() {
let mut list = List::new();
list.push(1); list.push(2); list.push(3);
let mut iter = list.iter_mut();
assert_eq!(iter.next(), Some(&mut 1));
assert_eq!(iter.next(), Some(&mut 2));
assert_eq!(iter.next(), Some(&mut 3));
assert_eq!(iter.next(), None);
}
#[test]
fn miri_food() {
let mut list = List::new();
list.push(1);
list.push(2);
list.push(3);
assert!(list.pop() == Some(1));
list.push(4);
assert!(list.pop() == Some(2));
list.push(5);
assert!(list.peek() == Some(&3));
list.push(6);
list.peek_mut().map(|x| *x *= 10);
assert!(list.peek() == Some(&30));
assert!(list.pop() == Some(30));
for elem in list.iter_mut() {
*elem *= 100;
}
let mut iter = list.iter();
assert_eq!(iter.next(), Some(&400));
assert_eq!(iter.next(), Some(&500));
assert_eq!(iter.next(), Some(&600));
assert_eq!(iter.next(), None);
assert_eq!(iter.next(), None);
assert!(list.pop() == Some(400));
list.peek_mut().map(|x| *x *= 10);
assert!(list.peek() == Some(&5000));
list.push(7);
// Drop it on the ground and let the dtor exercise itself
}
}
```

@ -1,6 +1,12 @@
# ChangeLog
记录一些值得注意的变更。
## 2022-03-27
- 新增章节: [不错的unsafe队列 - 额外的操作](https://course.rs/too-many-lists/unsafe-queue/extra-junk.html)
- 新增章节: [不错的unsafe队列 - 最终代码](https://course.rs/too-many-lists/unsafe-queue/final-code.html)
## 2022-03-25
- 将 gitalk 更换成 giscuz 评论系统,同时优化了样式展现和默认语言

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