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< h1 id = "cell-和-refcell" > < a class = "header" href = "#cell-和-refcell" > Cell 和 RefCell< / a > < / h1 >
< p > Rust 的编译器之严格， ， < / p >
< p > 但是严格是一把双刃剑，带来安全提升的同时，损失了灵活性，有时甚至会让用户痛苦不堪、怨声载道。因此 Rust 提供了 < code > Cell< / code > 和 < code > RefCell< / code > 用于内部可变性，简而言之，可以在拥有不可变引用的同时修改目标数据，对于正常的代码实现来说，这个是不可能做到的（要么一个可变借用，要么多个不可变借用）。< / p >
< blockquote >
< p > 内部可变性的实现是因为 Rust 使用了 < code > unsafe< / code > 来做到这一点，但是对于使用者来说，这些都是透明的，因为这些不安全代码都被封装到了安全的 API 中< / p >
< / blockquote >
< h2 id = "cell" > < a class = "header" href = "#cell" > Cell< / a > < / h2 >
< p > < code > Cell< / code > 和 < code > RefCell< / code > 在功能上没有区别，区别在于 < code > Cell< T> < / code > 适用于 < code > T< / code > 实现 < code > Copy< / code > 的情况：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use std::cell::Cell;
fn main() {
let c = Cell::new("asdf");
let one = c.get();
c.set("qwer");
let two = c.get();
println!("{},{}", one, two);
}< / code > < / pre >
< p > 以上代码展示了 < code > Cell< / code > 的基本用法，有几点值得注意：< / p >
< ul >
< li > “asdf” 是 < code > & str< / code > 类型，它实现了 < code > Copy< / code > 特征< / li >
< li > < code > c.get< / code > 用来取值，< code > c.set< / code > 用来设置新值< / li >
< / ul >
< p > 取到值保存在 < code > one< / code > 变量后，还能同时进行修改，这个违背了 Rust 的借用规则，但是由于 < code > Cell< / code > 的存在，我们很优雅地做到了这一点，但是如果你尝试在 < code > Cell< / code > 中存放< code > String< / code > ： < / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > let c = Cell::new(String::from("asdf"));
< span class = "boring" > }< / span > < / code > < / pre >
< p > 编译器会立刻报错，因为 < code > String< / code > 没有实现 < code > Copy< / code > 特征：< / p >
< pre > < code class = "language-console" > | pub struct String {
| ----------------- doesn't satisfy `String: Copy`
|
= note: the following trait bounds were not satisfied:
`String: Copy`
< / code > < / pre >
< h2 id = "refcell" > < a class = "header" href = "#refcell" > RefCell< / a > < / h2 >
< p > 由于 < code > Cell< / code > 类型针对的是实现了 < code > Copy< / code > 特征的值类型，因此在实际开发中，< code > Cell< / code > 使用的并不多，因为我们要解决的往往是可变、不可变引用共存导致的问题，此时就需要借助于 < code > RefCell< / code > 来达成目的。< / p >
< p > 我们可以将所有权、借用规则与这些智能指针做一个对比：< / p >
< div class = "table-wrapper" >
< table >
< thead >
< tr > < th > Rust 规则< / th > < th > 智能指针带来的额外规则< / th > < / tr >
< / thead >
< tbody >
< tr > < td > 一个数据只有一个所有者< / td > < td > < code > Rc/Arc< / code > 让一个数据可以拥有多个所有者< / td > < / tr >
< tr > < td > 要么多个不可变借用，要么一个可变借用< / td > < td > < code > RefCell< / code > 实现编译期可变、不可变引用共存< / td > < / tr >
< tr > < td > 违背规则导致< strong > 编译错误< / strong > < / td > < td > 违背规则导致< strong > 运行时< code > panic< / code > < / strong > < / td > < / tr >
< / tbody >
< / table >
< / div >
< p > 可以看出，< code > Rc/Arc< / code > 和 < code > RefCell< / code > 合在一起，解决了 Rust 中严苛的所有权和借用规则带来的某些场景下难使用的问题。但是它们并不是银弹，例如 < code > RefCell< / code > 实际上并没有解决可变引用和引用可以共存的问题，只是将报错从编译期推迟到运行时，从编译器错误变成了 < code > panic< / code > 异常：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use std::cell::RefCell;
fn main() {
let s = RefCell::new(String::from("hello, world"));
let s1 = s.borrow();
let s2 = s.borrow_mut();
println!("{},{}", s1, s2);
}< / code > < / pre >
< p > 上面代码在编译期不会报任何错误，你可以顺利运行程序：< / p >
< pre > < code class = "language-console" > thread 'main' panicked at 'already borrowed: BorrowMutError', src/main.rs:6:16
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
< / code > < / pre >
< p > 但是依然会因为违背了借用规则导致了运行期 < code > panic< / code > ，这非常像中国的天网，它也许会被罪犯蒙蔽一时，但是并不会被蒙蔽一世，任何导致安全风险的存在都将不能被容忍，法网恢恢，疏而不漏。< / p >
< h4 id = "refcell-为何存在" > < a class = "header" href = "#refcell-为何存在" > RefCell 为何存在< / a > < / h4 >
< p > 相信肯定有读者有疑问了，这么做有任何意义吗？还不如在编译期报错，至少能提前发现问题，而且性能还更好。< / p >
< p > 存在即合理，究其根因，在于 Rust 编译期的< strong > 宁可错杀，绝不放过< / strong > 的原则，当编译器不能确定你的代码是否正确时，就统统会判定为错误，因此难免会导致一些误报。< / p >
< p > 而 < code > RefCell< / code > 正是< strong > 用于你确信代码是正确的，而编译器却发生了误判时< / strong > 。< / p >
< p > 对于大型的复杂程序，也可以选择使用 < code > RefCell< / code > 来让事情简化。例如在 Rust 编译器的< a href = "https://github.com/rust-lang/rust/blob/620d1ee5346bee10ba7ce129b2e20d6e59f0377d/src/librustc/middle/ty.rs#L803-L987" > < code > ctxt结构体< / code > < / a > 中有大量的 < code > RefCell< / code > 类型的 < code > map< / code > 字段，主要的原因是：这些 < code > map< / code > 会被分散在各个地方的代码片段所广泛使用或修改。由于这种分散在各处的使用方式，导致了管理可变和不可变成为一件非常复杂的任务（甚至不可能），你很容易就碰到编译器抛出来的各种错误。而且 < code > RefCell< / code > 的运行时错误在这种情况下也变得非常可爱：一旦有人做了不正确的使用，代码会 < code > panic< / code > ，然后告诉我们哪些借用冲突了。< / p >
< p > 总之，当你确信编译器误报但不知道该如何解决时，或者你有一个引用类型，需要被四处使用和修改然后导致借用关系难以管理时，都可以优先考虑使用 < code > RefCell< / code > 。< / p >
< h4 id = "refcell-简单总结" > < a class = "header" href = "#refcell-简单总结" > RefCell 简单总结< / a > < / h4 >
< ul >
< li > 与 < code > Cell< / code > 用于可 < code > Copy< / code > 的值不同，< code > RefCell< / code > 用于引用< / li >
< li > < code > RefCell< / code > 只是将借用规则从编译期推迟到程序运行期，并不能帮你绕过这个规则< / li >
< li > < code > RefCell< / code > 适用于编译期误报或者一个引用被在多处代码使用、修改以至于难于管理借用关系时< / li >
< li > 使用 < code > RefCell< / code > 时，违背借用规则会导致运行期的 < code > panic< / code > < / li >
< / ul >
< h2 id = "选择-cell-还是-refcell" > < a class = "header" href = "#选择-cell-还是-refcell" > 选择 < code > Cell< / code > 还是 < code > RefCell< / code > < / a > < / h2 >
< p > 根据本文的内容，我们可以大概总结下两者的区别：< / p >
< ul >
< li > < code > Cell< / code > 只适用于 < code > Copy< / code > 类型，用于提供值，而 < code > RefCell< / code > 用于提供引用< / li >
< li > < code > Cell< / code > 不会 < code > panic< / code > ，而 < code > RefCell< / code > 会< / li >
< / ul >
< h4 id = "性能比较" > < a class = "header" href = "#性能比较" > 性能比较< / a > < / h4 >
< p > < code > Cell< / code > 没有额外的性能损耗，例如以下两段代码的性能其实是一致的：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > // code snipet 1
let x = Cell::new(1);
let y = & x;
let z = & x;
x.set(2);
y.set(3);
z.set(4);
println!("{}", x.get());
// code snipet 2
let mut x = 1;
let y = & mut x;
let z = & mut x;
x = 2;
*y = 3;
*z = 4;
println!("{}", x);
< span class = "boring" > }< / span > < / code > < / pre >
< p > 虽然性能一致，但代码 < code > 1< / code > 拥有代码 < code > 2< / code > 不具有的优势：它能编译成功:)< / p >
< p > 与 < code > Cell< / code > 的 < code > zero cost< / code > 不同，< code > RefCell< / code > 其实是有一点运行期开销的，原因是它包含了一个字节大小的“借用状态”指示器，该指示器在每次运行时借用时都会被修改，进而产生一点开销。< / p >
< p > 总之，当非要使用内部可变性时，首选 < code > Cell< / code > ，只有你的类型没有实现 < code > Copy< / code > 时，才去选择 < code > RefCell< / code > 。< / p >
< h2 id = "内部可变性" > < a class = "header" href = "#内部可变性" > 内部可变性< / a > < / h2 >
< p > 之前我们提到 < code > RefCell< / code > 具有内部可变性，何为内部可变性？简单来说，对一个不可变的值进行可变借用，但这个并不符合 Rust 的基本借用规则：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > fn main() {
let x = 5;
let y = & mut x;
}< / code > < / pre >
< p > 上面的代码会报错，因为我们不能对一个不可变的值进行可变借用，这会破坏 Rust 的安全性保证，相反，你可以对一个可变值进行不可变借用。原因是：当值不可变时，可能会有多个不可变的引用指向它，此时若将其中一个修改为可变的，会造成可变引用与不可变引用共存的情况；而当值可变时，最多只会有一个可变引用指向它，将其修改为不可变，那么最终依然是只有一个不可变的引用指向它。< / p >
< p > 虽然基本借用规则是 Rust 的基石，然而在某些场景中，一个值可以在其方法内部被修改，同时对于其它代码不可变，是很有用的：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > // 定义在外部库中的特征
pub trait Messenger {
fn send(& self, msg: String);
}
// --------------------------
// 我们的代码中的数据结构和实现
struct MsgQueue {
msg_cache: Vec< String> ,
}
impl Messenger for MsgQueue {
fn send(& self, msg: String) {
self.msg_cache.push(msg)
}
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 如上所示，外部库中定义了一个消息发送器特征 < code > Messenger< / code > ，它只有一个发送消息的功能：< code > fn send(& self, msg: String)< / code > ，因为发送消息不需要修改自身，因此原作者在定义时，使用了 < code > & self< / code > 的不可变借用，这个无可厚非。< / p >
< p > 我们要在自己的代码中使用该特征实现一个异步消息队列，出于性能的考虑，消息先写到本地缓存(内存)中，然后批量发送出去，因此在 < code > send< / code > 方法中，需要将消息先行插入到本地缓存 < code > msg_cache< / code > 中。但是问题来了，该 < code > send< / code > 方法的签名是 < code > & self< / code > ，因此上述代码会报错：< / p >
< pre > < code class = "language-console" > error[E0596]: cannot borrow `self.msg_cache` as mutable, as it is behind a `& ` reference
--> src/main.rs:11:9
|
2 | fn send(& self, msg: String);
| ----- help: consider changing that to be a mutable reference: `& mut self`
...
11 | self.msg_cache.push(msg)
| ^^^^^^^^^^^^^^^^^^ `self` is a `& ` reference, so the data it refers to cannot be borrowed as mutable
< / code > < / pre >
< p > 在报错的同时，编译器大聪明还善意地给出了提示：将 < code > & self< / code > 修改为 < code > & mut self< / code > ，但是。。。我们实现的特征是定义在外部库中，因此该签名根本不能修改。值此危急关头， < code > RefCell< / code > 闪亮登场：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use std::cell::RefCell;
pub trait Messenger {
fn send(& self, msg: String);
}
pub struct MsgQueue {
msg_cache: RefCell< Vec< String> > ,
}
impl Messenger for MsgQueue {
fn send(& self, msg: String) {
self.msg_cache.borrow_mut().push(msg)
}
}
fn main() {
let mq = MsgQueue {
msg_cache: RefCell::new(Vec::new()),
};
mq.send("hello, world".to_string());
}< / code > < / pre >
< p > 这个 MQ 功能很弱，但是并不妨碍我们演示内部可变性的核心用法：通过包裹一层 < code > RefCell< / code > ，成功的让 < code > & self< / code > 中的 < code > msg_cache< / code > 成为一个可变值，然后实现对其的修改。< / p >
< h2 id = "rc--refcell-组合使用" > < a class = "header" href = "#rc--refcell-组合使用" > Rc + RefCell 组合使用< / a > < / h2 >
< p > 在 Rust 中，一个常见的组合就是 < code > Rc< / code > 和 < code > RefCell< / code > 在一起使用，前者可以实现一个数据拥有多个所有者，后者可以实现数据的可变性：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use std::cell::RefCell;
use std::rc::Rc;
fn main() {
let s = Rc::new(RefCell::new("我很善变，还拥有多个主人".to_string()));
let s1 = s.clone();
let s2 = s.clone();
// let mut s2 = s.borrow_mut();
s2.borrow_mut().push_str(", oh yeah!");
println!("{:?}\n{:?}\n{:?}", s, s1, s2);
}
< / code > < / pre >
< p > 上面代码中，我们使用 < code > RefCell< String> < / code > 包裹一个字符串，同时通过 < code > Rc< / code > 创建了它的三个所有者：< code > s< / code > 、< code > s1< / code > 和< code > s2< / code > ，并且通过其中一个所有者 < code > s2< / code > 对字符串内容进行了修改。< / p >
< p > 由于 < code > Rc< / code > 的所有者们共享同一个底层的数据，因此当一个所有者修改了数据时，会导致全部所有者持有的数据都发生了变化。< / p >
< p > 程序的运行结果也在预料之中：< / p >
< pre > < code class = "language-console" > RefCell { value: "我很善变，还拥有多个主人, oh yeah!" }
RefCell { value: "我很善变，还拥有多个主人, oh yeah!" }
RefCell { value: "我很善变，还拥有多个主人, oh yeah!" }
< / code > < / pre >
< h4 id = "性能损耗" > < a class = "header" href = "#性能损耗" > 性能损耗< / a > < / h4 >
< p > 相信这两者组合在一起使用时，很多人会好奇到底性能如何，下面我们来简单分析下。< / p >
< p > 首先给出一个大概的结论，这两者结合在一起使用的性能其实非常高，大致相当于没有线程安全版本的 C++ < code > std::shared_ptr< / code > 指针， ， < / p >
< h4 id = "内存损耗" > < a class = "header" href = "#内存损耗" > 内存损耗< / a > < / h4 >
< p > 两者结合的数据结构与下面类似：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > struct Wrapper< T> {
// Rc
strong_count: usize,
weak_count: usize,
// Refcell
borrow_count: isize,
// 包裹的数据
item: T,
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 从上面可以看出，从对内存的影响来看，仅仅多分配了三个< code > usize/isize< / code > ，并没有其它额外的负担。< / p >
< h4 id = "cpu-损耗" > < a class = "header" href = "#cpu-损耗" > CPU 损耗< / a > < / h4 >
< p > 从 CPU 来看，损耗如下：< / p >
< ul >
< li > 对 < code > Rc< T> < / code > 解引用是免费的（编译期），但是 < code > *< / code > 带来的间接取值并不免费< / li >
< li > 克隆 < code > Rc< T> < / code > 需要将当前的引用计数跟 < code > 0< / code > 和 < code > usize::Max< / code > 进行一次比较，然后将计数值加 1< / li >
< li > 释放（ ） < code > Rc< T> < / code > 需要将计数值减 1， < code > 0< / code > 进行一次比较< / li >
< li > 对 < code > RefCell< / code > 进行不可变借用，需要将 < code > isize< / code > 类型的借用计数加 1， < code > 0< / code > 进行比较< / li >
< li > 对 < code > RefCell < / code > 的不可变借用进行释放，需要将 < code > isize< / code > 减 1< / li >
< li > 对 < code > RefCell< / code > 的可变借用大致流程跟上面差不多，但是需要先跟 < code > 0< / code > 比较，然后再减 1< / li >
< li > 对 < code > RefCell< / code > 的可变借用进行释放，需要将 < code > isize< / code > 加 1< / li >
< / ul >
< p > 其实这些细节不必过于关注，只要知道 CPU 消耗也非常低，甚至编译器还会对此进行进一步优化！< / p >
< h4 id = "cpu-缓存-miss" > < a class = "header" href = "#cpu-缓存-miss" > CPU 缓存 Miss< / a > < / h4 >
< p > 唯一需要担心的可能就是这种组合数据结构对于 CPU 缓存是否亲和，这个我们无法证明，只能提出来存在这个可能性，最终的性能影响还需要在实际场景中进行测试。< / p >
< p > 总之，分析这两者组合的性能还挺复杂的，大概总结下：< / p >
< ul >
< li > 从表面来看，它们带来的内存和 CPU 损耗都不大< / li >
< li > 但是由于 < code > Rc< / code > 额外的引入了一次间接取值（< code > *< / code > ），在少数场景下可能会造成性能上的显著损失< / li >
< li > CPU 缓存可能也不够亲和< / li >
< / ul >
< h2 id = "通过-cellfrom_mut-解决借用冲突" > < a class = "header" href = "#通过-cellfrom_mut-解决借用冲突" > 通过 < code > Cell::from_mut< / code > 解决借用冲突< / a > < / h2 >
< p > 在 Rust 1.37 版本中新增了两个非常实用的方法：< / p >
< ul >
< li > Cell::from_mut， < code > & mut T< / code > 转为 < code > & Cell< T> < / code > < / li >
< li > Cell::as_slice_of_cells， < code > & Cell< [T]> < / code > 转为 < code > & [Cell< T> ]< / code > < / li >
< / ul >
< p > 这里我们不做深入的介绍，但是来看看如何使用这两个方法来解决一个常见的借用冲突问题：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > fn is_even(i: i32) -> bool {
i % 2 == 0
}
fn retain_even(nums: & mut Vec< i32> ) {
let mut i = 0;
for num in nums.iter().filter(|& num| is_even(*num)) {
nums[i] = *num;
i += 1;
}
nums.truncate(i);
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 以上代码会报错：< / p >
< pre > < code class = "language-console" > error[E0502]: cannot borrow `*nums` as mutable because it is also borrowed as immutable
--> src/main.rs:8:9
|
7 | for num in nums.iter().filter(|& num| is_even(*num)) {
| ----------------------------------------
| |
| immutable borrow occurs here
| immutable borrow later used here
8 | nums[i] = *num;
| ^^^^ mutable borrow occurs here
< / code > < / pre >
< p > 很明显，报错是因为同时借用了不可变与可变引用，你可以通过索引的方式来避免这个问题：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > fn retain_even(nums: & mut Vec< i32> ) {
let mut i = 0;
for j in 0..nums.len() {
if is_even(nums[j]) {
nums[i] = nums[j];
i += 1;
}
}
nums.truncate(i);
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 但是这样就违背我们的初衷了，毕竟迭代器会让代码更加简洁，那么还有其它的办法吗？< / p >
< p > 这时就可以使用 < code > Cell< / code > 新增的这两个方法：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > < span class = "boring" > #![allow(unused)]
< / span > < span class = "boring" > fn main() {
< / span > use std::cell::Cell;
fn retain_even(nums: & mut Vec< i32> ) {
let slice: & [Cell< i32> ] = Cell::from_mut(& mut nums[..])
.as_slice_of_cells();
let mut i = 0;
for num in slice.iter().filter(|num| is_even(num.get())) {
slice[i].set(num.get());
i += 1;
}
nums.truncate(i);
}
< span class = "boring" > }< / span > < / code > < / pre >
< p > 此时代码将不会报错，因为 < code > Cell< / code > 上的 < code > set< / code > 方法获取的是不可变引用 < code > pub fn set(& self, val: T)< / code > 。< / p >
< p > 当然，以上代码的本质还是对 < code > Cell< / code > 的运用，只不过这两个方法可以很方便的帮我们把 < code > & mut [T]< / code > 类型转换成 < code > & [Cell< T> ]< / code > 类型。< / p >
< h2 id = "总结" > < a class = "header" href = "#总结" > 总结< / a > < / h2 >
< p > < code > Cell< / code > 和 < code > RefCell< / code > 都为我们带来了内部可变性这个重要特性，同时还将借用规则的检查从编译期推迟到运行期，但是这个检查并不能被绕过，该来早晚还是会来，< code > RefCell< / code > 在运行期的报错会造成 < code > panic< / code > 。< / p >
< p > < code > RefCell< / code > 适用于编译器误报或者一个引用被在多个代码中使用、修改以至于难于管理借用关系时，还有就是需要内部可变性时。< / p >
< p > 从性能上看，< code > RefCell< / code > 由于是非线程安全的，因此无需保证原子性，性能虽然有一点损耗，但是依然非常好，而 < code > Cell< / code > 则完全不存在任何额外的性能损耗。< / p >
< p > < code > Rc< / code > 跟 < code > RefCell< / code > 结合使用可以实现多个所有者共享同一份数据，非常好用，但是潜在的性能损耗也要考虑进去，建议对于热点代码使用时，做好 < code > benchmark< / code > 。< / p >
< / main >
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