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< h1 id = "weak-与循环引用" > < a class = "header" href = "#weak-与循环引用" > Weak 与循环引用< / a > < / h1 >
< p > Rust 的安全性是众所周知的，但是不代表它不会内存泄漏。一个典型的例子就是同时使用 < code > Rc< T> < / code > 和 < code > RefCell< T> < / code > 创建循环引用，最终这些引用的计数都无法被归零，因此 < code > Rc< T> < / code > 拥有的值也不会被释放清理。< / p >
< h2 id = "何为循环引用" > < a class = "header" href = "#何为循环引用" > 何为循环引用< / a > < / h2 >
< p > 关于内存泄漏，如果你没有充足的 Rust 经验，可能都无法造出一份代码来再现它：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use crate::List::{Cons, Nil};
use std::cell::RefCell;
use std::rc::Rc;
#[derive(Debug)]
enum List {
Cons(i32, RefCell< Rc< List> > ),
Nil,
}
impl List {
fn tail(& self) -> Option< & RefCell< Rc< List> > > {
match self {
Cons(_, item) => Some(item),
Nil => None,
}
}
}
fn main() {}< / code > < / pre >
< p > 这里我们创建一个有些复杂的枚举类型 < code > List< / code > ，这个类型很有意思，它的每个值都指向了另一个 < code > List< / code > ，此外，得益于 < code > Rc< / code > 的使用还允许多个值指向一个 < code > List< / code > ： < / p >
< img alt = "" src = "https://pica.zhimg.com/80/v2-0db007dfb4167ebc22f50cf5b5a85f53_1440w.png" class = "center" / >
< p > 如上图所示，每个矩形框节点都是一个 < code > List< / code > 类型，它们或者是拥有值且指向另一个 < code > List< / code > 的< code > Cons< / code > ，或者是一个没有值的终结点 < code > Nil< / code > 。同时，由于 < code > RefCell< / code > 的使用，每个 < code > List< / code > 所指向的 < code > List< / code > 还能够被修改。< / p >
< p > 下面来使用一下这个复杂的 < code > List< / code > 枚举：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > fn main() {
let a = Rc::new(Cons(5, RefCell::new(Rc::new(Nil))));
println!("a的初始化rc计数 = {}", Rc::strong_count(& a));
println!("a指向的节点 = {:?}", a.tail());
// 创建`b`到`a`的引用
let b = Rc::new(Cons(10, RefCell::new(Rc::clone(& a))));
println!("在b创建后， & a));
println!("b的初始化rc计数 = {}", Rc::strong_count(& b));
println!("b指向的节点 = {:?}", b.tail());
// 利用RefCell的可变性，
if let Some(link) = a.tail() {
*link.borrow_mut() = Rc::clone(& b);
}
println!("在更改a后， & b));
println!("在更改a后， & a));
// 下面一行println!将导致循环引用
// 我们可怜的8MB大小的main线程栈空间将被它冲垮，
// println!("a next item = {:?}", a.tail());
}< / code > < / pre >
< p > 这个类型定义看着复杂，使用起来更复杂！不过排除这些因素，我们可以清晰看出：< / p >
< ol >
< li > 在创建了 < code > a< / code > 后，紧接着就使用 < code > a< / code > 创建了 < code > b< / code > ，因此 < code > b< / code > 引用了 < code > a< / code > < / li >
< li > 然后我们又利用 < code > Rc< / code > 克隆了 < code > b< / code > ，然后通过 < code > RefCell< / code > 的可变性，让 < code > a< / code > 引用了 < code > b< / code > < / li >
< / ol >
< p > 至此我们成功创建了循环引用< code > a< / code > -> < code > b< / code > -> < code > a< / code > -> < code > b< / code > ····< / p >
< p > 先来观察下引用计数：< / p >
< pre > < code class = "language-console" > a的初始化rc计数 = 1
a指向的节点 = Some(RefCell { value: Nil })
在b创建后，
b的初始化rc计数 = 1
b指向的节点 = Some(RefCell { value: Cons(5, RefCell { value: Nil }) })
在更改a后，
在更改a后，
< / code > < / pre >
< p > 在 < code > main< / code > 函数结束前，< code > a< / code > 和 < code > b< / code > 的引用计数均是 < code > 2< / code > ，随后 < code > b< / code > 触发 < code > Drop< / code > ，此时引用计数会变为 < code > 1< / code > ，并不会归 < code > 0< / code > ，因此 < code > b< / code > 所指向内存不会被释放，同理可得 < code > a< / code > 指向的内存也不会被释放，最终发生了内存泄漏。< / p >
< p > 下面一张图很好的展示了这种引用循环关系：
< img alt = "" src = "https://pic1.zhimg.com/80/v2-2dbfc981f05019bf70bf81c93f956c35_1440w.png" class = "center" / > < / p >
< p > 现在我们还需要轻轻的推一下，让塔米诺骨牌轰然倒塌。反注释最后一行代码，试着运行下：< / p >
< pre > < code class = "language-console" > RefCell { value: Cons(5, RefCell { value: Cons(10, RefCell { value: Cons(5, RefCell { value: Cons(10, RefCell { value: Cons(5, RefCell { value: Cons(10, RefCell {
...无穷无尽
thread 'main' has overflowed its stack
fatal runtime error: stack overflow
< / code > < / pre >
< p > 通过 < code > a.tail< / code > 的调用， < code > a -> b -> a ···< / code > 的所有内容，但是在不懈的努力后，< code > main< / code > 线程终于不堪重负，发生了< a href = "https://course.rs/pitfalls/stack-overflow.html" > 栈溢出< / a > 。< / p >
< p > 以上的代码可能并不会造成什么大的问题，但是在一个更加复杂的程序中，类似的问题可能会造成你的程序不断地分配内存、泄漏内存，最终程序会不幸< strong > OOM< / strong > ，当然这其中的 CPU 损耗也不可小觑。< / p >
< p > 总之，创建循环引用并不简单，但是也并不是完全遇不到，当你使用 < code > RefCell< Rc< T> > < / code > 或者类似的类型嵌套组合（具备内部可变性和引用计数）时，就要打起万分精神，前面可能是深渊！< / p >
< p > 那么问题来了？ 如果我们确实需要实现上面的功能，该怎么办？答案是使用 < code > Weak< / code > 。< / p >
< h2 id = "weak" > < a class = "header" href = "#weak" > Weak< / a > < / h2 >
< p > < code > Weak< / code > 非常类似于 < code > Rc< / code > ，但是与 < code > Rc< / code > 持有所有权不同，< code > Weak< / code > 不持有所有权，它仅仅保存一份指向数据的弱引用：如果你想要访问数据，需要通过 < code > Weak< / code > 指针的 < code > upgrade< / code > 方法实现，该方法返回一个类型为 < code > Option< Rc< T> > < / code > 的值。< / p >
< p > 看到这个返回，相信大家就懂了：何为弱引用？就是< strong > 不保证引用关系依然存在< / strong > ，如果不存在，就返回一个 < code > None< / code > ！ < / p >
< p > 因为 < code > Weak< / code > 引用不计入所有权，因此它< strong > 无法阻止所引用的内存值被释放掉< / strong > ，而且 < code > Weak< / code > 本身不对值的存在性做任何担保，引用的值还存在就返回 < code > Some< / code > ，不存在就返回 < code > None< / code > 。< / p >
< h4 id = "weak-与-rc-对比" > < a class = "header" href = "#weak-与-rc-对比" > Weak 与 Rc 对比< / a > < / h4 >
< p > 我们来将 < code > Weak< / code > 与 < code > Rc< / code > 进行以下简单对比：< / p >
< div class = "table-wrapper" >
< table >
< thead >
< tr > < th > < code > Weak< / code > < / th > < th > < code > Rc< / code > < / th > < / tr >
< / thead >
< tbody >
< tr > < td > 不计数< / td > < td > 引用计数< / td > < / tr >
< tr > < td > 不拥有所有权< / td > < td > 拥有值的所有权< / td > < / tr >
< tr > < td > 不阻止值被释放(drop)< / td > < td > 所有权计数归零，才能 drop< / td > < / tr >
< tr > < td > 引用的值存在返回 < code > Some< / code > ，不存在返回 < code > None < / code > < / td > < td > 引用的值必定存在< / td > < / tr >
< tr > < td > 通过 < code > upgrade< / code > 取到 < code > Option< Rc< T> > < / code > ，然后再取值< / td > < td > 通过 < code > Deref< / code > 自动解引用，取值无需任何操作< / td > < / tr >
< / tbody >
< / table >
< / div >
< p > 通过这个对比，可以非常清晰的看出 < code > Weak< / code > 为何这么弱，而这种弱恰恰非常适合我们实现以下的场景：< / p >
< ul >
< li > 持有一个 < code > Rc< / code > 对象的临时引用，并且不在乎引用的值是否依然存在< / li >
< li > 阻止 < code > Rc< / code > 导致的循环引用，因为 < code > Rc< / code > 的所有权机制，会导致多个 < code > Rc< / code > 都无法计数归零< / li >
< / ul >
< p > 使用方式简单总结下：< strong > 对于父子引用关系，可以让父节点通过 < code > Rc< / code > 来引用子节点，然后让子节点通过 < code > Weak< / code > 来引用父节点< / strong > 。< / p >
< h4 id = "weak-总结" > < a class = "header" href = "#weak-总结" > Weak 总结< / a > < / h4 >
< p > 因为 < code > Weak< / code > 本身并不是很好理解，因此我们再来帮大家梳理总结下，然后再通过一个例子，来彻底掌握。< / p >
< p > < code > Weak< / code > 通过 < code > use std::rc::Weak< / code > 来引入，它具有以下特点:< / p >
< ul >
< li > 可访问，但没有所有权，不增加引用计数，因此不会影响被引用值的释放回收< / li >
< li > 可由 < code > Rc< T> < / code > 调用 < code > downgrade< / code > 方法转换成 < code > Weak< T> < / code > < / li >
< li > < code > Weak< T> < / code > 可使用 < code > upgrade< / code > 方法转换成 < code > Option< Rc< T> > < / code > ，如果资源已经被释放，则 < code > Option< / code > 的值是 < code > None< / code > < / li >
< li > 常用于解决循环引用的问题< / li >
< / ul >
< p > 一个简单的例子：< / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use std::rc::Rc;
fn main() {
// 创建Rc，
let five = Rc::new(5);
// 通过Rc，
let weak_five = Rc::downgrade(& five);
// Weak引用的资源依然存在，
let strong_five: Option< Rc< _> > = weak_five.upgrade();
assert_eq!(*strong_five.unwrap(), 5);
// 手动释放资源`five`
drop(five);
// Weak引用的资源已不存在，
let strong_five: Option< Rc< _> > = weak_five.upgrade();
assert_eq!(strong_five, None);
}< / code > < / pre >
< p > 需要承认的是，使用 < code > Weak< / code > 让 Rust 本来就堪忧的代码可读性又下降了不少，但是。。。真香，因为可以解决循环引用了。< / p >
< h2 id = "使用-weak-解决循环引用" > < a class = "header" href = "#使用-weak-解决循环引用" > 使用 Weak 解决循环引用< / a > < / h2 >
< p > 理论知识已经足够，现在用两个例子来模拟下真实场景下可能会遇到的循环引用。< / p >
< h4 id = "工具间的故事" > < a class = "header" href = "#工具间的故事" > 工具间的故事< / a > < / h4 >
< p > 工具间里，每个工具都有其主人，且多个工具可以拥有一个主人；同时一个主人也可以拥有多个工具，在这种场景下，就很容易形成循环引用，好在我们有 < code > Weak< / code > ： < / p >
< pre class = "playground" > < code class = "language-rust edition2021" > use std::rc::Rc;
use std::rc::Weak;
use std::cell::RefCell;
// 主人
struct Owner {
name: String,
gadgets: RefCell< Vec< Weak< Gadget> > > ,
}
// 工具
struct Gadget {
id: i32,
owner: Rc< Owner> ,
}
fn main() {
// 创建一个 Owner
// 需要注意，该 Owner 也拥有多个 `gadgets`
let gadget_owner : Rc< Owner> = Rc::new(
Owner {
name: "Gadget Man".to_string(),
gadgets: RefCell::new(Vec::new()),
}
);
// 创建工具，同时与主人进行关联：创建两个 gadget，
let gadget1 = Rc::new(Gadget{id: 1, owner: gadget_owner.clone()});
let gadget2 = Rc::new(Gadget{id: 2, owner: gadget_owner.clone()});
// 为主人更新它所拥有的工具
// 因为之前使用了 `Rc`，现在必须要使用 `Weak`，否则就会循环引用
gadget_owner.gadgets.borrow_mut().push(Rc::downgrade(& gadget1));
gadget_owner.gadgets.borrow_mut().push(Rc::downgrade(& gadget2));
// 遍历 gadget_owner 的 gadgets 字段
for gadget_opt in gadget_owner.gadgets.borrow().iter() {
// gadget_opt 是一个 Weak< Gadget> 。 因为 weak 指针不能保证他所引用的对象
// 仍然存在。所以我们需要显式的调用 upgrade() 来通过其返回值(Option< _> )来判
// 断其所指向的对象是否存在。
// 当然，
let gadget = gadget_opt.upgrade().unwrap();
println!("Gadget {} owned by {}", gadget.id, gadget.owner.name);
}
// 在 main 函数的最后， ，
// 具体是，因为这几个结构体之间没有了强引用（`Rc< T> `），所以，当他们销毁的时候。
// 首先 gadget2 和 gadget1 被销毁。
// 然后因为 gadget_owner 的引用数量为 0，
// 循环引用问题也就避免了
}< / code > < / pre >
< h4 id = "tree-数据结构" > < a class = "header" href = "#tree-数据结构" > tree 数据结构< / a > < / h4 >
< pre class = "playground" > < code class = "language-rust edition2021" > use std::cell::RefCell;
use std::rc::{Rc, Weak};
#[derive(Debug)]
struct Node {
value: i32,
parent: RefCell< Weak< Node> > ,
children: RefCell< Vec< Rc< Node> > > ,
}
fn main() {
let leaf = Rc::new(Node {
value: 3,
parent: RefCell::new(Weak::new()),
children: RefCell::new(vec![]),
});
println!(
"leaf strong = {}, weak = {}",
Rc::strong_count(& leaf),
Rc::weak_count(& leaf),
);
{
let branch = Rc::new(Node {
value: 5,
parent: RefCell::new(Weak::new()),
children: RefCell::new(vec![Rc::clone(& leaf)]),
});
*leaf.parent.borrow_mut() = Rc::downgrade(& branch);
println!(
"branch strong = {}, weak = {}",
Rc::strong_count(& branch),
Rc::weak_count(& branch),
);
println!(
"leaf strong = {}, weak = {}",
Rc::strong_count(& leaf),
Rc::weak_count(& leaf),
);
}
println!("leaf parent = {:?}", leaf.parent.borrow().upgrade());
println!(
"leaf strong = {}, weak = {}",
Rc::strong_count(& leaf),
Rc::weak_count(& leaf),
);
}
< / code > < / pre >
< p > 这个例子就留给读者自己解读和分析，我们就不画蛇添足了:） < / p >
< h2 id = "unsafe-解决循环引用" > < a class = "header" href = "#unsafe-解决循环引用" > unsafe 解决循环引用< / a > < / h2 >
< p > 除了使用 Rust 标准库提供的这些类型，你还可以使用 < code > unsafe< / code > 里的裸指针来解决这些棘手的问题，但是由于我们还没有讲解 < code > unsafe< / code > ，因此这里就不进行展开，只附上< a href = "https://github.com/sunface/rust-algos/blob/fbcdccf3e8178a9039329562c0de0fd01a3372fb/src/unsafe/self-ref.md" > 源码链接< / a > ，挺长的，需要耐心 o_o< / p >
< p > 虽然 < code > unsafe< / code > 不安全，但是在各种库的代码中依然很常见用它来实现自引用结构，主要优点如下:< / p >
< ul >
< li > 性能高，毕竟直接用裸指针操作< / li >
< li > 代码更简单更符合直觉: 对比下 < code > Option< Rc< RefCell< Node> > > < / code > < / li >
< / ul >
< h2 id = "总结" > < a class = "header" href = "#总结" > 总结< / a > < / h2 >
< p > 本文深入讲解了何为循环引用以及如何使用 < code > Weak< / code > 来解决，同时还结合 < code > Rc< / code > 、< code > RefCell< / code > 、< code > Weak< / code > 等实现了两个有实战价值的例子，让大家对智能指针的使用更加融会贯通。< / p >
< p > 至此，智能指针一章即将结束（严格来说还有一个 < code > Mutex< / code > 放在多线程一章讲解），而 Rust 语言本身的学习之旅也即将结束，后面我们将深入多线程、项目工程、应用实践、性能分析等特色专题，来一睹 Rust 在这些领域的风采。< / p >
< / main >
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