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<ol class="chapter"><li class="expanded affix "><a href="title-page.html">The Rust Programming Language</a></li><li class="expanded affix "><a href="foreword.html">Foreword</a></li><li class="expanded affix "><a href="ch00-00-introduction.html">Introduction</a></li><li class="expanded "><a href="ch01-00-getting-started.html"><strong aria-hidden="true">1.</strong> Getting Started</a></li><li><ol class="section"><li class="expanded "><a href="ch01-01-installation.html"><strong aria-hidden="true">1.1.</strong> Installation</a></li><li class="expanded "><a href="ch01-02-hello-world.html"><strong aria-hidden="true">1.2.</strong> Hello, World!</a></li><li class="expanded "><a href="ch01-03-hello-cargo.html"><strong aria-hidden="true">1.3.</strong> Hello, Cargo!</a></li></ol></li><li class="expanded "><a href="ch02-00-guessing-game-tutorial.html"><strong aria-hidden="true">2.</strong> Programming a Guessing Game</a></li><li class="expanded "><a href="ch03-00-common-programming-concepts.html"><strong aria-hidden="true">3.</strong> Common Programming Concepts</a></li><li><ol class="section"><li class="expanded "><a href="ch03-01-variables-and-mutability.html"><strong aria-hidden="true">3.1.</strong> Variables and Mutability</a></li><li class="expanded "><a href="ch03-02-data-types.html"><strong aria-hidden="true">3.2.</strong> Data Types</a></li><li class="expanded "><a href="ch03-03-how-functions-work.html"><strong aria-hidden="true">3.3.</strong> Functions</a></li><li class="expanded "><a href="ch03-04-comments.html"><strong aria-hidden="true">3.4.</strong> Comments</a></li><li class="expanded "><a href="ch03-05-control-flow.html"><strong aria-hidden="true">3.5.</strong> Control Flow</a></li></ol></li><li class="expanded "><a href="ch04-00-understanding-ownership.html"><strong aria-hidden="true">4.</strong> Understanding Ownership</a></li><li><ol class="section"><li class="expanded "><a href="ch04-01-what-is-ownership.html"><strong aria-hidden="true">4.1.</strong> What is Ownership?</a></li><li class="expanded "><a href="ch04-02-references-and-borrowing.html"><strong aria-hidden="true">4.2.</strong> References and Borrowing</a></li><li class="expanded "><a href="ch04-03-slices.html"><strong aria-hidden="true">4.3.</strong> The Slice Type</a></li></ol></li><li class="expanded "><a href="ch05-00-structs.html"><strong aria-hidden="true">5.</strong> Using Structs to Structure Related Data</a></li><li><ol class="section"><li class="expanded "><a href="ch05-01-defining-structs.html"><strong aria-hidden="true">5.1.</strong> Defining and Instantiating Structs</a></li><li class="expanded "><a href="ch05-02-example-structs.html"><strong aria-hidden="true">5.2.</strong> An Example Program Using Structs</a></li><li class="expanded "><a href="ch05-03-method-syntax.html"><strong aria-hidden="true">5.3.</strong> Method Syntax</a></li></ol></li><li class="expanded "><a href="ch06-00-enums.html"><strong aria-hidden="true">6.</strong> Enums and Pattern Matching</a></li><li><ol class="section"><li class="expanded "><a href="ch06-01-defining-an-enum.html"><strong aria-hidden="true">6.1.</strong> Defining an Enum</a></li><li class="expanded "><a href="ch06-02-match.html"><strong aria-hidden="true">6.2.</strong> The match Control Flow Operator</a></li><li class="expanded "><a href="ch06-03-if-let.html"><strong aria-hidden="true">6.3.</strong> Concise Control Flow with if let</a></li></ol></li><li class="expanded "><a href="ch07-00-managing-growing-projects-with-packages-crates-and-modules.html"><strong aria-hidden="true">7.</strong> Managing Growing Projects with Packages, Crates, and Modules</a></li><li><ol class="section"><li class="expanded "><a href="ch07-01-packages-and-crates.html"><strong aria-hidden="true">7.1.</strong> Packages and Crates</a></li><li class="expanded "><a href="ch07-02-defining-modules-to-control-scope-and-privacy.html"><strong aria-hidden="true">7.2.</strong> Defining Modules to Control Scope and Privacy</a></li><li class="expanded "><a href="ch07-03-paths-for-referring-to-an-item-in-the-module-tree.html"><
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<main>
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<h1><a class="header" href="#smart-pointers" id="smart-pointers">Smart Pointers</a></h1>
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<p>A <em>pointer</em> is a general concept for a variable that contains an address in
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memory. This address refers to, or “points at,” some other data. The most
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common kind of pointer in Rust is a reference, which you learned about in
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Chapter 4. References are indicated by the <code>&</code> symbol and borrow the value they
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point to. They don’t have any special capabilities other than referring to
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data. Also, they don’t have any overhead and are the kind of pointer we use
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most often.</p>
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<p><em>Smart pointers</em>, on the other hand, are data structures that not only act like
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a pointer but also have additional metadata and capabilities. The concept of
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smart pointers isn’t unique to Rust: smart pointers originated in C++ and exist
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in other languages as well. In Rust, the different smart pointers defined in
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the standard library provide functionality beyond that provided by references.
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One example that we’ll explore in this chapter is the <em>reference counting</em>
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smart pointer type. This pointer enables you to have multiple owners of data by
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keeping track of the number of owners and, when no owners remain, cleaning up
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the data.</p>
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<p>In Rust, which uses the concept of ownership and borrowing, an additional
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difference between references and smart pointers is that references are
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pointers that only borrow data; in contrast, in many cases, smart pointers
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<em>own</em> the data they point to.</p>
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<p>We’ve already encountered a few smart pointers in this book, such as <code>String</code>
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and <code>Vec<T></code> in Chapter 8, although we didn’t call them smart pointers at the
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time. Both these types count as smart pointers because they own some memory and
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allow you to manipulate it. They also have metadata (such as their capacity)
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and extra capabilities or guarantees (such as with <code>String</code> ensuring its data
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will always be valid UTF-8).</p>
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<p>Smart pointers are usually implemented using structs. The characteristic that
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distinguishes a smart pointer from an ordinary struct is that smart pointers
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implement the <code>Deref</code> and <code>Drop</code> traits. The <code>Deref</code> trait allows an instance
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of the smart pointer struct to behave like a reference so you can write code
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that works with either references or smart pointers. The <code>Drop</code> trait allows
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you to customize the code that is run when an instance of the smart pointer
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goes out of scope. In this chapter, we’ll discuss both traits and demonstrate
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why they’re important to smart pointers.</p>
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<p>Given that the smart pointer pattern is a general design pattern used
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frequently in Rust, this chapter won’t cover every existing smart pointer. Many
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libraries have their own smart pointers, and you can even write your own. We’ll
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cover the most common smart pointers in the standard library:</p>
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<ul>
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<li><code>Box<T></code> for allocating values on the heap</li>
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<li><code>Rc<T></code>, a reference counting type that enables multiple ownership</li>
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<li><code>Ref<T></code> and <code>RefMut<T></code>, accessed through <code>RefCell<T></code>, a type that enforces
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the borrowing rules at runtime instead of compile time</li>
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</ul>
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<p>In addition, we’ll cover the <em>interior mutability</em> pattern where an immutable
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type exposes an API for mutating an interior value. We’ll also discuss
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<em>reference cycles</em>: how they can leak memory and how to prevent them.</p>
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<p>Let’s dive in!</p>
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</body>
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</html>
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