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<h1 id="drop-释放资源"><a class="header" href="#drop-释放资源">Drop 释放资源</a></h1>
<p>在 Rust 中，我们之所以可以一拳打跑 GC 的同时一脚踢翻手动资源回收，主要就归功于 <code>Drop</code> 特征，同时它也是智能指针的必备特征之一。</p>
<h2 id="学习目标"><a class="header" href="#学习目标">学习目标</a></h2>
<p>如何自动和手动释放资源及执行指定的收尾工作</p>
<h2 id="rust-中的资源回收"><a class="header" href="#rust-中的资源回收">Rust 中的资源回收</a></h2>
<p>在一些无 GC 语言中，程序员在一个变量无需再被使用时，需要手动释放它占用的内存资源，如果忘记了，那么就会发生内存泄漏，最终臭名昭著的 <code>OOM</code> 问题可能就会发生。</p>
<p>而在 Rust 中，你可以指定在一个变量超出作用域时，执行一段特定的代码，最终编译器将帮你自动插入这段收尾代码。这样，就无需在每一个使用该变量的地方，都写一段代码来进行收尾工作和资源释放。不禁让人感叹，Rust 的大腿真粗，香！</p>
<p>没错，指定这样一段收尾工作靠的就是咱这章的主角 - <code>Drop</code> 特征。</p>
<h2 id="一个不那么简单的-drop-例子"><a class="header" href="#一个不那么简单的-drop-例子">一个不那么简单的 Drop 例子</a></h2>
<pre><pre class="playground"><code class="language-rust edition2021">struct HasDrop1;
struct HasDrop2;
impl Drop for HasDrop1 {
fn drop(&amp;mut self) {
println!(&quot;Dropping HasDrop1!&quot;);
}
}
impl Drop for HasDrop2 {
fn drop(&amp;mut self) {
println!(&quot;Dropping HasDrop2!&quot;);
}
}
struct HasTwoDrops {
one: HasDrop1,
two: HasDrop2,
}
impl Drop for HasTwoDrops {
fn drop(&amp;mut self) {
println!(&quot;Dropping HasTwoDrops!&quot;);
}
}
struct Foo;
impl Drop for Foo {
fn drop(&amp;mut self) {
println!(&quot;Dropping Foo!&quot;)
}
}
fn main() {
let _x = HasTwoDrops {
two: HasDrop2,
one: HasDrop1,
};
let _foo = Foo;
println!(&quot;Running!&quot;);
}</code></pre></pre>
<p>上面代码虽然长，但是目的其实很单纯，就是为了观察不同情况下变量级别的、结构体内部字段的 <code>Drop</code>，有几点值得注意：</p>
<ul>
<li><code>Drop</code> 特征中的 <code>drop</code> 方法借用了目标的可变引用，而不是拿走了所有权，这里先设置一个悬念，后边会讲</li>
<li>结构体中每个字段都有自己的 <code>Drop</code></li>
</ul>
<p>来看看输出：</p>
<pre><code class="language-console">Running!
Dropping Foo!
Dropping HasTwoDrops!
Dropping HasDrop1!
Dropping HasDrop2!
</code></pre>
<p>嗯，结果符合预期，每个资源都成功的执行了收尾工作，虽然 <code>println!</code> 这种收尾工作毫无意义 =,=</p>
<h4 id="drop-的顺序"><a class="header" href="#drop-的顺序">Drop 的顺序</a></h4>
<p>观察以上输出，我们可以得出以下关于 <code>Drop</code> 顺序的结论</p>
<ul>
<li><strong>变量级别，按照逆序的方式</strong>，<code>_x</code> 在 <code>_foo</code> 之前创建，因此 <code>_x</code> 在 <code>_foo</code> 之后被 <code>drop</code></li>
<li><strong>结构体内部，按照顺序的方式</strong>，结构体 <code>_x</code> 中的字段按照定义中的顺序依次 <code>drop</code></li>
</ul>
<h4 id="没有实现-drop-的结构体"><a class="header" href="#没有实现-drop-的结构体">没有实现 Drop 的结构体</a></h4>
<p>实际上，就算你不为 <code>_x</code> 结构体实现 <code>Drop</code> 特征，它内部的两个字段依然会调用 <code>drop</code>，移除以下代码，并观察输出：</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>impl Drop for HasTwoDrops {
fn drop(&amp;mut self) {
println!(&quot;Dropping HasTwoDrops!&quot;);
}
}
<span class="boring">}</span></code></pre></pre>
<p>原因在于，Rust 自动为几乎所有类型都实现了 <code>Drop</code> 特征，因此就算你不手动为结构体实现 <code>Drop</code>，它依然会调用默认实现的 <code>drop</code> 函数，同时再调用每个字段的 <code>drop</code> 方法，最终打印出：</p>
<pre><code class="language-cnosole">Dropping HasDrop1!
Dropping HasDrop2!
</code></pre>
<h2 id="手动回收"><a class="header" href="#手动回收">手动回收</a></h2>
<p>当使用智能指针来管理锁的时候，你可能希望提前释放这个锁，然后让其它代码能及时获得锁，此时就需要提前去手动 <code>drop</code>。
但是在之前我们提到一个悬念，<code>Drop::drop</code> 只是借用了目标值的可变引用，所以，就算你提前调用了 <code>drop</code>，后面的代码依然可以使用目标值，但是这就会访问一个并不存在的值，非常不安全，好在 Rust 会阻止你：</p>
<pre><pre class="playground"><code class="language-rust edition2021">#[derive(Debug)]
struct Foo;
impl Drop for Foo {
fn drop(&amp;mut self) {
println!(&quot;Dropping Foo!&quot;)
}
}
fn main() {
let foo = Foo;
foo.drop();
println!(&quot;Running!:{:?}&quot;, foo);
}</code></pre></pre>
<p>报错如下：</p>
<pre><code class="language-console">error[E0040]: explicit use of destructor method
--&gt; src/main.rs:37:9
|
37 | foo.drop();
| ----^^^^--
| | |
| | explicit destructor calls not allowed
| help: consider using `drop` function: `drop(foo)`
</code></pre>
<p>如上所示，编译器直接阻止了我们调用 <code>Drop</code> 特征的 <code>drop</code> 方法，原因是对于 Rust 而言，不允许显式地调用析构函数（这是一个用来清理实例的通用编程概念）。好在在报错的同时，编译器还给出了一个提示：使用 <code>drop</code> 函数。</p>
<p>针对编译器提示的 <code>drop</code> 函数，我们可以大胆推测下：它能够拿走目标值的所有权。现在来看看这个猜测正确与否，以下是 <code>std::mem::drop</code> 函数的签名：</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>pub fn drop&lt;T&gt;(_x: T)
<span class="boring">}</span></code></pre></pre>
<p>如上所示，<code>drop</code> 函数确实拿走了目标值的所有权，来验证下：</p>
<pre><pre class="playground"><code class="language-rust edition2021">fn main() {
let foo = Foo;
drop(foo);
// 以下代码会报错：借用了所有权被转移的值
// println!(&quot;Running!:{:?}&quot;, foo);
}</code></pre></pre>
<p>Bingo，完美拿走了所有权，而且这种实现保证了后续的使用必定会导致编译错误，因此非常安全！</p>
<p>细心的同学可能已经注意到，这里直接调用了 <code>drop</code> 函数，并没有引入任何模块信息，原因是该函数在<a href="https://course.rs/appendix/prelude.html"><code>std::prelude</code></a>里。</p>
<blockquote>
<p>事实上，能被显式调用的drop(_x)函数只是个空函数，在拿走目标值的所有权后没有任何操作。而由于其持有目标值的所有权，在drop(_x)函数结束之际，编译器会执行_x真正的析构函数，从而完成释放资源的操作。换句话说，drop(_x)函数只是帮助目标值的所有者提前离开了作用域。https://doc.rust-lang.org/std/mem/fn.drop.html</p>
</blockquote>
<h2 id="drop-使用场景"><a class="header" href="#drop-使用场景">Drop 使用场景</a></h2>
<p>对于 Drop 而言，主要有两个功能：</p>
<ul>
<li>回收内存资源</li>
<li>执行一些收尾工作</li>
</ul>
<p>对于第二点，在之前我们已经详细介绍过，因此这里主要对第一点进行下简单说明。</p>
<p>在绝大多数情况下，我们都无需手动去 <code>drop</code> 以回收内存资源，因为 Rust 会自动帮我们完成这些工作，它甚至会对复杂类型的每个字段都单独的调用 <code>drop</code> 进行回收！但是确实有极少数情况，需要你自己来回收资源的，例如文件描述符、网络 socket 等，当这些值超出作用域不再使用时，就需要进行关闭以释放相关的资源，在这些情况下，就需要使用者自己来解决 <code>Drop</code> 的问题。</p>
<h2 id="互斥的-copy-和-drop"><a class="header" href="#互斥的-copy-和-drop">互斥的 Copy 和 Drop</a></h2>
<p>我们无法为一个类型同时实现 <code>Copy</code> 和 <code>Drop</code> 特征。因为实现了 <code>Copy</code> 特征的类型会被编译器隐式的复制，因此非常难以预测析构函数执行的时间和频率。因此这些实现了 <code>Copy</code> 的类型无法拥有析构函数。</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>#[derive(Copy)]
struct Foo;
impl Drop for Foo {
fn drop(&amp;mut self) {
println!(&quot;Dropping Foo!&quot;)
}
}
<span class="boring">}</span></code></pre></pre>
<p>以上代码报错如下：</p>
<pre><code class="language-console">error[E0184]: the trait `Copy` may not be implemented for this type; the type has a destructor
--&gt; src/main.rs:24:10
|
24 | #[derive(Copy)]
| ^^^^ Copy not allowed on types with destructors
</code></pre>
<h2 id="总结"><a class="header" href="#总结">总结</a></h2>
<p><code>Drop</code> 可以用于许多方面，来使得资源清理及收尾工作变得方便和安全，甚至可以用其创建我们自己的内存分配器！通过 <code>Drop</code> 特征和 Rust 所有权系统，你无需担心之后的代码清理，Rust 会自动考虑这些问题。</p>
<p>我们也无需担心意外的清理掉仍在使用的值，这会造成编译器错误：所有权系统确保引用总是有效的，也会确保 <code>drop</code> 只会在值不再被使用时被调用一次。</p>
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