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<h1 id="智能指针"><a class="header" href="#智能指针">智能指针</a></h1>
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<!-- https://github.com/rust-lang/book/blob/main/src/ch15-00-smart-pointers.md -->
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<!-- commit ecef81cbc6f0c2d1c8a67409329b0641258c04c2 -->
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<p><strong>指针</strong>（<em>pointer</em>）是一个包含内存地址的变量的通用概念。这个地址会引用，或者说 “指向”（points at）一些其它数据。Rust 中最常见的指针是第四章介绍的<strong>引用</strong>（<em>reference</em>）。引用使用 <code>&</code> 符号来表示，而且会借用它们指向的值。它们除了引用数据之外没有任何其他特殊功能，也没有额外开销。</p>
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<p>另一方面，<strong>智能指针</strong>（<em>smart pointers</em>）是一类数据结构，它们的表现类似指针，但是也拥有额外的元数据和功能。智能指针的概念并不为 Rust 所独有；其起源于 C++ 并存在于其它语言中。Rust 标准库中定义了多种不同的智能指针，它们提供了远超引用的功能。为了探索其基本概念，我们来看一些智能指针的例子，这包括<strong>引用计数</strong> （<em>reference counting</em>）智能指针类型。这种指针允许数据有多个所有者，它会记录所有者的数量，当没有所有者时清理数据。</p>
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<p>在 Rust 中因为引用和借用的概念，引用和智能指针有一个额外的区别：引用只会借用数据，而智能指针在很多时候<strong>拥有</strong>它们指向的数据。</p>
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<p>智能指针通常使用结构体实现。智能指针不同于普通结构体的地方在于它实现了 <code>Deref</code> 和 <code>Drop</code> trait。<code>Deref</code> trait 允许智能指针结构体实例表现得像引用一样，这样就可以编写既用于引用、又用于智能指针的代码。<code>Drop</code> trait 允许我们自定义当智能指针离开作用域时运行的代码。本章会讨论这些 trait 以及为什么它们对智能指针很重要。</p>
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<p>考虑到智能指针是一个在 Rust 中经常使用的通用设计模式，本章的内容并不会涉及到所有现存的智能指针。很多库都有自己的智能指针，而且你也可以编写属于你自己的智能指针。这里会讲到标准库中最常用的几种：</p>
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<ul>
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<li><code>Box<T></code>，用于在堆上分配值</li>
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<li><code>Rc<T></code>，一个引用计数类型，其数据可以有多个所有者</li>
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<li><code>Ref<T></code> 和 <code>RefMut<T></code>，它们是通过 <code>RefCell<T></code> 访问的。而 <code>RefCell<T></code> 是一个在运行时而非编译时执行借用规则的类型。</li>
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</ul>
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<p>另外我们会讲到<strong>内部可变性</strong>（<em>interior mutability</em>）模式，这种模式允许一个不可变的类型暴露出能够修改其内部值的 API。我们也会讨论<strong>引用循环</strong>（<em>reference cycles</em>）是怎样导致内存泄漏的，以及如何避免它们。</p>
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<p>让我们开始吧！</p>
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