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wip add ch15-02
2017-03-13 23:27:01 +08:00
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<title>引用循环和内存泄漏是安全的 - Rust 程序设计语言 简体中文版</title>
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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 &amp; 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 href="ch07-02-controlling-visibility-with-pub.html"><strong>7.2.</strong> 使用<code>pub</code>控制可见性</a></li><li><a href="ch07-03-importing-names-with-use.html"><strong>7.3.</strong> 使用<code>use</code>导入命名</a></li></ul></li><li><a href="ch08-00-common-collections.html"><strong>8.</strong> 通用集合类型</a></li><li><ul class="section"><li><a href="ch08-01-vectors.html"><strong>8.1.</strong> vector</a></li><li><a href="ch08-02-strings.html"><strong>8.2.</strong> 字符串</a></li><li><a href="ch08-03-hash-maps.html"><strong>8.3.</strong> 哈希 map</a></li></ul></li><li><a href="ch09-00-error-handling.html"><strong>9.</strong> 错误处理</a></li><li><ul class="section"><li><a href="ch09-01-unrecoverable-errors-with-panic.html"><strong>9.1.</strong> <code>panic!</code>与不可恢复的错误</a></li><li><a href="ch09-02-recoverable-errors-with-result.html"><strong>9.2.</strong> <code>Result</code>与可恢复的错误</a></li><li><a href="ch09-03-to-panic-or-not-to-panic.html"><strong>9.3.</strong> <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.h
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wip add ch15-06
2017-03-17 00:00:53 +08:00
<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>
<blockquote>
<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>
<br>
commit c49e5ee8859f8eb8f8867cbeafbdf5b802aa5894</p>
</blockquote>
<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>
<p>在使用<code>Rc&lt;T&gt;</code>和<code>RefCell&lt;T&gt;</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>
<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&lt;Rc&lt;List&gt;&gt;</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>
<p><span class="filename">Filename: src/main.rs</span></p>
<pre><code class="language-rust,ignore">#[derive(Debug)]
enum List {
Cons(i32, RefCell&lt;Rc&lt;List&gt;&gt;),
Nil,
}
impl List {
fn tail(&amp;self) -&gt; Option&lt;&amp;RefCell&lt;Rc&lt;List&gt;&gt;&gt; {
match *self {
Cons(_, ref item) =&gt; Some(item),
Nil =&gt; None,
}
}
}
</code></pre>
<p><span class="caption">Listing 15-16: A cons list definition that holds a
<code>RefCell</code> so that we can modify what a <code>Cons</code> variant is referring to</span></p>
<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>
<p><span class="filename">Filename: src/main.rs</span></p>
<pre><code class="language-rust"># #[derive(Debug)]
# enum List {
# Cons(i32, RefCell&lt;Rc&lt;List&gt;&gt;),
# Nil,
# }
#
# impl List {
# fn tail(&amp;self) -&gt; Option&lt;&amp;RefCell&lt;Rc&lt;List&gt;&gt;&gt; {
# match *self {
# Cons(_, ref item) =&gt; Some(item),
# Nil =&gt; None,
# }
# }
# }
#
use List::{Cons, Nil};
use std::rc::Rc;
use std::cell::RefCell;
fn main() {
let a = Rc::new(Cons(5, RefCell::new(Rc::new(Nil))));
println!(&quot;a initial rc count = {}&quot;, Rc::strong_count(&amp;a));
println!(&quot;a next item = {:?}&quot;, a.tail());
let b = Rc::new(Cons(10, RefCell::new(a.clone())));
println!(&quot;a rc count after b creation = {}&quot;, Rc::strong_count(&amp;a));
println!(&quot;b initial rc count = {}&quot;, Rc::strong_count(&amp;b));
println!(&quot;b next item = {:?}&quot;, b.tail());
if let Some(ref link) = a.tail() {
*link.borrow_mut() = b.clone();
}
println!(&quot;b rc count after changing a = {}&quot;, Rc::strong_count(&amp;b));
println!(&quot;a rc count after changing a = {}&quot;, Rc::strong_count(&amp;a));
// Uncomment the next line to see that we have a cycle; it will
// overflow the stack
// println!(&quot;a next item = {:?}&quot;, a.tail());
}
</code></pre>
<p><span class="caption">Listing 15-17: Creating a reference cycle of two <code>List</code>
values pointing to each other</span></p>
<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>
<p><img alt="Reference cycle of lists" src="img/trpl15-04.svg" class="center" style="width: 50%;" /></p>
<p><span class="caption">Figure 15-18: A reference cycle of lists <code>a</code> and <code>b</code>
pointing to each other</span></p>
wip add ch15-02
2017-03-13 23:27:01 +08:00
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