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<main>
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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>
|
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<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>
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<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>
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<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>
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<pre class="playground"><code class="language-rust edition2021">use crate::List::{Cons, Nil};
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use std::cell::RefCell;
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use std::rc::Rc;
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#[derive(Debug)]
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enum List {
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Cons(i32, RefCell<Rc<List>>),
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Nil,
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}
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impl List {
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fn tail(&self) -> Option<&RefCell<Rc<List>>> {
|
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match self {
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Cons(_, item) => Some(item),
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Nil => None,
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}
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}
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}
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fn main() {}</code></pre>
|
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<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>
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<img alt="" src="https://pica.zhimg.com/80/v2-0db007dfb4167ebc22f50cf5b5a85f53_1440w.png" class="center" />
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|
<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的rc计数 = {}", Rc::strong_count(&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，创建了`a`到`b`的引用
|
|
|
if let Some(link) = a.tail() {
|
|
|
*link.borrow_mut() = Rc::clone(&b);
|
|
|
}
|
|
|
|
|
|
println!("在更改a后，b的rc计数 = {}", Rc::strong_count(&b));
|
|
|
println!("在更改a后，a的rc计数 = {}", Rc::strong_count(&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，最终造成栈溢出
|
|
|
// 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，a的rc计数 = 2
|
|
|
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，b的rc计数 = 2
|
|
|
在更改a后，a的rc计数 = 2
|
|
|
</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，Rust 试图打印出 <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，持有一个值5
|
|
|
let five = Rc::new(5);
|
|
|
|
|
|
// 通过Rc，创建一个Weak指针
|
|
|
let weak_five = Rc::downgrade(&five);
|
|
|
|
|
|
// Weak引用的资源依然存在，取到值5
|
|
|
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，因此返回None
|
|
|
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，他们分别持有 gadget_owner 的一个引用。
|
|
|
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，Option 为 None 的时候这个引用原对象就不存在了。
|
|
|
let gadget = gadget_opt.upgrade().unwrap();
|
|
|
println!("Gadget {} owned by {}", gadget.id, gadget.owner.name);
|
|
|
}
|
|
|
|
|
|
// 在 main 函数的最后，gadget_owner，gadget1 和 gadget2 都被销毁。
|
|
|
// 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，所以这个对象可以被销毁了。
|
|
|
// 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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