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<ul class="chapter"><li><a href="ch01-00-introduction.html"><strong>1.</strong> 介绍</a></li><li><ul class="section"><li><a href="ch01-01-installation.html"><strong>1.1.</strong> 安装</a></li><li><a href="ch01-02-hello-world.html"><strong>1.2.</strong> Hello, World!</a></li></ul></li><li><a href="ch02-00-guessing-game-tutorial.html"><strong>2.</strong> 猜猜看教程</a></li><li><a href="ch03-00-common-programming-concepts.html"><strong>3.</strong> 通用编程概念</a></li><li><ul class="section"><li><a href="ch03-01-variables-and-mutability.html"><strong>3.1.</strong> 变量和可变性</a></li><li><a href="ch03-02-data-types.html"><strong>3.2.</strong> 数据类型</a></li><li><a href="ch03-03-how-functions-work.html"><strong>3.3.</strong> 函数如何工作</a></li><li><a href="ch03-04-comments.html"><strong>3.4.</strong> 注释</a></li><li><a href="ch03-05-control-flow.html"><strong>3.5.</strong> 控制流</a></li></ul></li><li><a href="ch04-00-understanding-ownership.html"><strong>4.</strong> 认识所有权</a></li><li><ul class="section"><li><a href="ch04-01-what-is-ownership.html"><strong>4.1.</strong> 什么是所有权</a></li><li><a href="ch04-02-references-and-borrowing.html"><strong>4.2.</strong> 引用 & 借用</a></li><li><a href="ch04-03-slices.html"><strong>4.3.</strong> Slices</a></li></ul></li><li><a href="ch05-00-structs.html"><strong>5.</strong> 结构体</a></li><li><ul class="section"><li><a href="ch05-01-method-syntax.html"><strong>5.1.</strong> 方法语法</a></li></ul></li><li><a href="ch06-00-enums.html"><strong>6.</strong> 枚举和模式匹配</a></li><li><ul class="section"><li><a href="ch06-01-defining-an-enum.html"><strong>6.1.</strong> 定义枚举</a></li><li><a href="ch06-02-match.html"><strong>6.2.</strong> <code>match</code>控制流运算符</a></li><li><a href="ch06-03-if-let.html"><strong>6.3.</strong> <code>if let</code>简单控制流</a></li></ul></li><li><a href="ch07-00-modules.html"><strong>7.</strong> 模块</a></li><li><ul class="section"><li><a href="ch07-01-mod-and-the-filesystem.html"><strong>7.1.</strong> <code>mod</code>和文件系统</a></li><li><a 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<code>panic!</code>还是不<code>panic!</code></a></li></ul></li><li><a href="ch10-00-generics.html"><strong>10.</strong> 泛型、trait 和生命周期</a></li><li><ul class="section"><li><a href="ch10-01-syntax.html"><strong>10.1.</strong> 泛型数据类型</a></li><li><a href="ch10-02-traits.html"><strong>10.2.</strong> trait：定义共享的行为</a></li><li><a href="ch10-03-lifetime-syntax.html"><strong>10.3.</strong> 生命周期与引用有效性</a></li></ul></li><li><a href="ch11-00-testing.html"><strong>11.</strong> 测试</a></li><li><ul class="section"><li><a href="ch11-01-writing-tests.html"><strong>11.1.</strong> 编写测试</a></li><li><a href="ch11-02-running-tests.html"><strong>11.2.</strong> 运行测试</a></li><li><a href="ch11-03-test-organization.html"><strong>11.3.</strong> 测试的组织结构</a></li></ul></li><li><a href="ch12-00-an-io-project.html"><strong>12.</strong> 一个 I/O 项目</a></li><li><ul class="section"><li><a href="ch12-01-accepting-command-line-arguments.html"><strong>12.1.</strong> 接受命令行参数</a></li><li><a href="ch12-02-reading-a-file.html"><strong>12.2.</strong> 读取文件</a></li><li><a href="ch12-03-improving-error-handling-and-modularity.html"><strong>12.3.</strong> 增强错误处理和模块化</a></li><li><a href="ch12-04-testing-the-librarys-functionality.html"><strong>12.4.</strong> 测试库的功能</a></li><li><a href="ch12-05-working-with-environment-variables.html"><strong>12.5.</strong> 处理环境变量</a></li><li><a href="ch12-06-writing-to-stderr-instead-of-stdout.html"><strong>12.6.</strong> 输出到<code>stderr</code>而不是<code>stdout</code></a></li></ul></li><li><a href="ch13-00-functional-features.html"><strong>13.</strong> Rust 中的函数式语言功能</a></li><li><ul class="section"><li><a href="ch13-01-closures.html"><strong>13.1.</strong> 闭包</a></li><li><a href="ch13-02-iterators.html"><strong>13.2.</strong> 迭代器</a></li><li><a href="ch13-03-improving-our-io-project.html"><strong>13.3.</strong> 改进 I/O 项目</a></li><li><a href="ch13-04-performance.html"><strong>13.4.</strong> 性能</a></li></ul></li><li><a href="ch14-00-more-about-cargo.html"><strong>14.</strong> 更多关于 Cargo 和 Crates.io</a></li><li><ul class="section"><li><a href="ch14-01-release-profiles.html"><strong>14.1.</strong> 发布配置</a></li><li><a href="ch14-02-publishing-to-crates-io.html"><strong>14.2.</strong> 将 crate 发布到 Crates.io</a></li><li><a href="ch14-03-cargo-workspaces.html"><strong>14.3.</strong> Cargo 工作空间</a></li><li><a href="ch14-04-installing-binaries.html"><strong>14.4.</strong> 使用<code>cargo install</code>从 Crates.io 安装文件</a></li><li><a href="ch14-05-extending-cargo.html"><strong>14.5.</strong> Cargo 自定义扩展命令</a></li></ul></li><li><a href="ch15-00-smart-pointers.html"><strong>15.</strong> 智能指针</a></li><li><ul class="section"><li><a href="ch15-01-box.html"><strong>15.1.</strong> <code>Box<T></code>用于已知大小的堆上数据</a></li><li><a href="ch15-02-deref.html"><strong>15.2.</strong> <code>Deref</code> Trait 允许通过引用访问数据</a></li><li><a href="ch15-03-drop.html"><strong>15.3.</strong> <code>Drop</code> Trait 运行清理代码</a></li><li><a href="ch15-04-rc.html"><strong>15.4.</strong> <code>Rc<T></code> 引用计数智能指针</a></li><li><a href="ch15-05-interior-mutability.html"><strong>15.5.</strong> <code>RefCell<T></code>和内部可变性模式</a></li><li><a href="ch15-06-reference-cycles.html" class="active"><strong>15.6.</strong> 引用循环和内存泄漏是安全的</a></li></ul></li></ul>
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<h1 class="menu-title">Rust 程序设计语言 简体中文版</h1>
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<a class="header" href="#引用循环和内存泄漏是安全的" name="引用循环和内存泄漏是安全的"><h2>引用循环和内存泄漏是安全的</h2></a>
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<blockquote>
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<p><a href="https://github.com/rust-lang/book/blob/master/second-edition/src/ch15-06-reference-cycles.md">ch15-06-reference-cycles.md</a>
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<br>
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commit c49e5ee8859f8eb8f8867cbeafbdf5b802aa5894</p>
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</blockquote>
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<p>我们讨论过 Rust 做出的一些保证，例如永远也不会遇到一个空值，而且数据竞争也会在编译时被阻止。Rust 的内存安全保证也使其更难以制造从不被清理的内存，这被称为<strong>内存泄露</strong>。然而 Rust 并不是<strong>不可能</strong>出现内存泄漏，避免内存泄露<strong>并</strong>不是 Rust 的保证之一。换句话说，内存泄露是安全的。</p>
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<p>在使用<code>Rc<T></code>和<code>RefCell<T></code>时，有可能创建循环引用，这时各个项相互引用并形成环。这是不好的因为每一项的引用计数将永远也到不了 0，其值也永远也不会被丢弃。让我们看看这是如何发生的以及如何避免它。</p>
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<p>在列表 15-16 中，我们将使用列表 15-5 中<code>List</code>定义的另一个变体。我们将回到储存<code>i32</code>值作为<code>Cons</code>成员的第一个元素。现在<code>Cons</code>成员的第二个元素是<code>RefCell<Rc<List>></code>：这时就不能修改<code>i32</code>值了，但是能够修改<code>Cons</code>成员指向的哪个<code>List</code>。还需要增加一个<code>tail</code>方法来方便我们在拥有一个<code>Cons</code>成员时访问第二个项：</p>
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<p><span class="filename">Filename: src/main.rs</span></p>
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<pre><code class="language-rust,ignore">#[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(_, ref 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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</code></pre>
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<p><span class="caption">Listing 15-16: A cons list definition that holds a
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<code>RefCell</code> so that we can modify what a <code>Cons</code> variant is referring to</span></p>
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<p>接下来，在列表 15-17 中，我们将在变量<code>a</code>中创建一个<code>List</code>值，其内部是一个<code>5, Nil</code>的列表。接着在变量<code>b</code>创建一个值 10 和指向<code>a</code>中列表的<code>List</code>值。最后修改<code>a</code>指向<code>b</code>而不是<code>Nil</code>，这会创建一个循环：</p>
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<p><span class="filename">Filename: src/main.rs</span></p>
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<pre><code class="language-rust"># #[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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#
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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(_, ref 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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#
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use List::{Cons, Nil};
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use std::rc::Rc;
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use std::cell::RefCell;
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fn main() {
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let a = Rc::new(Cons(5, RefCell::new(Rc::new(Nil))));
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println!("a initial rc count = {}", Rc::strong_count(&a));
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println!("a next item = {:?}", a.tail());
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let b = Rc::new(Cons(10, RefCell::new(a.clone())));
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println!("a rc count after b creation = {}", Rc::strong_count(&a));
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println!("b initial rc count = {}", Rc::strong_count(&b));
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println!("b next item = {:?}", b.tail());
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if let Some(ref link) = a.tail() {
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*link.borrow_mut() = b.clone();
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}
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println!("b rc count after changing a = {}", Rc::strong_count(&b));
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println!("a rc count after changing a = {}", Rc::strong_count(&a));
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// Uncomment the next line to see that we have a cycle; it will
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// overflow the stack
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// println!("a next item = {:?}", a.tail());
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}
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</code></pre>
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<p><span class="caption">Listing 15-17: Creating a reference cycle of two <code>List</code>
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values pointing to each other</span></p>
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<p>使用<code>tail</code>方法来获取<code>a</code>中<code>RefCell</code>的引用，并将其放入变量<code>link</code>中。接着对<code>RefCell</code>使用<code>borrow_mut</code>方法将其中的值从存放<code>Nil</code>值的<code>Rc</code>改为<code>b</code>中的<code>Rc</code>。这创建了一个看起来像图 15-18 所示的引用循环：</p>
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<p><img alt="Reference cycle of lists" src="img/trpl15-04.svg" class="center" style="width: 50%;" /></p>
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<p><span class="caption">Figure 15-18: A reference cycle of lists <code>a</code> and <code>b</code>
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pointing to each other</span></p>
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