|
|
# 基本操作
|
|
|
|
|
|
> 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
|
|
|
|
|
|
在开始之前，大家需要先了解 unsafe 的[相关知识](https://course.rs/advance/unsafe/intro.html)。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：
|
|
|
```rust
|
|
|
impl<T> List<T> {
|
|
|
pub fn new() -> Self {
|
|
|
List { head: None, tail: None }
|
|
|
}
|
|
|
}
|
|
|
```
|
|
|
|
|
|
但是我们不再在 `tail` 中使用 `Option`:
|
|
|
```shell
|
|
|
> cargo build
|
|
|
|
|
|
error[E0308]: mismatched types
|
|
|
--> src/fifth.rs:15:34
|
|
|
|
|
|
|
15 | List { head: None, tail: None }
|
|
|
| ^^^^ expected *-ptr, found
|
|
|
| enum `std::option::Option`
|
|
|
|
|
|
|
= note: expected type `*mut fifth::Node<T>`
|
|
|
found type `std::option::Option<_>`
|
|
|
```
|
|
|
|
|
|
我们是可以使用 `Option` 包裹一层，但是 `*mut` 裸指针之所以裸，是因为它狂，它可以是 `null` ! 因此 `Option` 就变得没有意义:
|
|
|
```rust
|
|
|
use std::ptr;
|
|
|
|
|
|
// defns...
|
|
|
|
|
|
impl<T> List<T> {
|
|
|
pub fn new() -> Self {
|
|
|
List { head: None, tail: ptr::null_mut() }
|
|
|
}
|
|
|
}
|
|
|
```
|
|
|
|
|
|
如上所示，通过 `std::ptr::null_mut` 函数可以获取一个 `null`，当然，还可以使用 `0 as *mut _`，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 = , =
|
|
|
|
|
|
好了，现在是时候去重新实现 `push` ，之前获取的是 `Option<&mut Node<T>>` 成为我们的拦路虎，这次来看看如果是获取 `*mut Node<T>` 还会不会有类似的问题。
|
|
|
|
|
|
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。
|
|
|
|
|
|
```rust
|
|
|
let raw_tail: *mut _ = &mut *new_tail;
|
|
|
```
|
|
|
|
|
|
来看看 `push` 的实现:
|
|
|
```rust
|
|
|
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;
|
|
|
}
|
|
|
```
|
|
|
|
|
|
```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`
|
|
|
```
|
|
|
|
|
|
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 ):
|
|
|
```rust
|
|
|
*self.tail.next = Some(new_tail);
|
|
|
```
|
|
|
|
|
|
```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`
|
|
|
```
|
|
|
|
|
|
哦哦，运算符的优先级问题:
|
|
|
```rust
|
|
|
(*self.tail).next = Some(new_tail);
|
|
|
```
|
|
|
|
|
|
```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
|
|
|
```
|
|
|
|
|
|
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 `unsafe` 语句块。那么问题来了，是将某几行代码包在 `unsafe` 中还是将整个函数包在 `unsafe` 中呢？如果大家不知道哪个是正确答案的话，证明[之前的章节](https://course.rs/advance/unsafe/intro.html#控制-unsafe-的使用边界)还是没有仔细学，请回去再看一下，巩固巩固:)
|
|
|
|
|
|
```rust
|
|
|
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;
|
|
|
}
|
|
|
```
|
|
|
|
|
|
```shell
|
|
|
> cargo build
|
|
|
warning: field is never used: `elem`
|
|
|
--> src/fifth.rs:11:5
|
|
|
|
|
|
|
11 | elem: T,
|
|
|
| ^^^^^^^
|
|
|
|
|
|
|
= note: #[warn(dead_code)] on by default
|
|
|
```
|
|
|
|
|
|
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：**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**
|
|
|
|
|
|
呼，长出了一口气，终于成功实现了 `push` ，下面来看看 `pop`:
|
|
|
```rust
|
|
|
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
|
|
|
})
|
|
|
}
|
|
|
```
|
|
|
|
|
|
测试下:
|
|
|
```rust
|
|
|
#[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);
|
|
|
}
|
|
|
}
|
|
|
```
|
|
|
|
|
|
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 `pop` 中不会发生尾指针损坏( tail-pointer corruption )的情况。
|
|
|
|
|
|
```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
|
|
|
```
|
|
|
|
