<!DOCTYPE HTML> <html lang="en"> <head> <meta charset="UTF-8"> <title>Enums - The Rust Programming Language</title> <meta content="text/html; charset=utf-8" http-equiv="Content-Type"> <meta name="description" content=""> <meta name="viewport" content="width=device-width, initial-scale=1"> <base href=""> <link rel="stylesheet" href="book.css"> <link href="https://fonts.googleapis.com/css?family=Open+Sans:300italic,400italic,600italic,700italic,800italic,400,300,600,700,800" rel="stylesheet" type="text/css"> <link href="https://fonts.googleapis.com/css?family=Source+Code+Pro:500" rel="stylesheet" type="text/css"> <link rel="shortcut icon" href="favicon.png"> <!-- Font Awesome --> <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/font-awesome/4.3.0/css/font-awesome.min.css"> <link rel="stylesheet" href="highlight.css"> <link rel="stylesheet" href="tomorrow-night.css"> <!-- MathJax --> <script type="text/javascript" src="https://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML"></script> <!-- Fetch JQuery from CDN but have a local fallback --> <script src="https://code.jquery.com/jquery-2.1.4.min.js"></script> <script> if (typeof jQuery == 'undefined') { document.write(unescape("%3Cscript src='jquery.js'%3E%3C/script%3E")); } </script> </head> <body class="light"> <!-- Set the theme before any content is loaded, prevents flash --> <script type="text/javascript"> var theme = localStorage.getItem('theme'); if (theme == null) { theme = 'light'; } $('body').removeClass().addClass(theme); </script> <!-- Hide / unhide sidebar before it is displayed --> <script type="text/javascript"> var sidebar = localStorage.getItem('sidebar'); if (sidebar === "hidden") { $("html").addClass("sidebar-hidden") } else if (sidebar === "visible") { $("html").addClass("sidebar-visible") } </script> <div id="sidebar" class="sidebar"> <ul class="chapter"><li class="affix"><a href="README.html">Introduction</a></li><li><a href="getting-started.html"><strong>1.</strong> Getting Started</a></li><li><a href="guessing-game.html"><strong>2.</strong> Tutorial: Guessing Game</a></li><li><a href="syntax-and-semantics.html"><strong>3.</strong> Syntax and Semantics</a></li><li><ul class="section"><li><a href="variable-bindings.html"><strong>3.1.</strong> Variable Bindings</a></li><li><a href="functions.html"><strong>3.2.</strong> Functions</a></li><li><a href="primitive-types.html"><strong>3.3.</strong> Primitive Types</a></li><li><a href="comments.html"><strong>3.4.</strong> Comments</a></li><li><a href="if.html"><strong>3.5.</strong> if</a></li><li><a href="loops.html"><strong>3.6.</strong> Loops</a></li><li><a href="vectors.html"><strong>3.7.</strong> Vectors</a></li><li><a href="ownership.html"><strong>3.8.</strong> Ownership</a></li><li><a href="references-and-borrowing.html"><strong>3.9.</strong> References and Borrowing</a></li><li><a href="lifetimes.html"><strong>3.10.</strong> Lifetimes</a></li><li><a href="mutability.html"><strong>3.11.</strong> Mutability</a></li><li><a href="structs.html"><strong>3.12.</strong> Structs</a></li><li><a href="enums.html" class="active"><strong>3.13.</strong> Enums</a></li><li><a href="match.html"><strong>3.14.</strong> Match</a></li><li><a href="patterns.html"><strong>3.15.</strong> Patterns</a></li><li><a href="method-syntax.html"><strong>3.16.</strong> Method Syntax</a></li><li><a href="strings.html"><strong>3.17.</strong> Strings</a></li><li><a href="generics.html"><strong>3.18.</strong> Generics</a></li><li><a href="traits.html"><strong>3.19.</strong> Traits</a></li><li><a href="drop.html"><strong>3.20.</strong> Drop</a></li><li><a href="if-let.html"><strong>3.21.</strong> if let</a></li><li><a href="trait-objects.html"><strong>3.22.</strong> Trait Objects</a></li><li><a href="closures.html"><strong>3.23.</strong> Closures</a></li><li><a href="ufcs.html"><strong>3.24.</strong> Universal Function Call Syntax</a></li><li><a href="crates-and-modules.html"><strong>3.25.</strong> Crates and Modules</a></li><li><a href="const-and-static.html"><strong>3.26.</strong> <code>const</code> and <code>static</code></a></li><li><a href="attributes.html"><strong>3.27.</strong> Attributes</a></li><li><a href="type-aliases.html"><strong>3.28.</strong> <code>type</code> aliases</a></li><li><a href="casting-between-types.html"><strong>3.29.</strong> Casting between types</a></li><li><a href="associated-types.html"><strong>3.30.</strong> Associated Types</a></li><li><a href="unsized-types.html"><strong>3.31.</strong> Unsized Types</a></li><li><a href="operators-and-overloading.html"><strong>3.32.</strong> Operators and Overloading</a></li><li><a href="deref-coercions.html"><strong>3.33.</strong> Deref coercions</a></li><li><a href="macros.html"><strong>3.34.</strong> Macros</a></li><li><a href="raw-pointers.html"><strong>3.35.</strong> Raw Pointers</a></li><li><a href="unsafe.html"><strong>3.36.</strong> <code>unsafe</code></a></li></ul></li><li><a href="effective-rust.html"><strong>4.</strong> Effective Rust</a></li><li><ul class="section"><li><a href="the-stack-and-the-heap.html"><strong>4.1.</strong> The Stack and the Heap</a></li><li><a href="testing.html"><strong>4.2.</strong> Testing</a></li><li><a href="conditional-compilation.html"><strong>4.3.</strong> Conditional Compilation</a></li><li><a href="documentation.html"><strong>4.4.</strong> Documentation</a></li><li><a href="iterators.html"><strong>4.5.</strong> Iterators</a></li><li><a href="concurrency.html"><strong>4.6.</strong> Concurrency</a></li><li><a href="error-handling.html"><strong>4.7.</strong> Error Handling</a></li><li><a href="choosing-your-guarantees.html"><strong>4.8.</strong> Choosing your Guarantees</a></li><li><a href="ffi.html"><strong>4.9.</strong> FFI</a></li><li><a href="borrow-and-asref.html"><strong>4.10.</strong> Borrow and AsRef</a></li><li><a href="release-channels.html"><strong>4.11.</strong> Release Channels</a></li><li><a href="using-rust-without-the-standard-library.html"><strong>4.12.</strong> Using Rust without the standard library</a></li><li><a href="procedural-macros.html"><strong>4.13.</strong> Procedural Macros (and custom derive)</a></li></ul></li><li><a href="glossary.html"><strong>5.</strong> Glossary</a></li><li><a href="syntax-index.html"><strong>6.</strong> Syntax Index</a></li><li><a href="bibliography.html"><strong>7.</strong> Bibliography</a></li></ul> </div> <div id="page-wrapper" class="page-wrapper"> <div class="page"> <div id="menu-bar" class="menu-bar"> <div class="left-buttons"> <i id="sidebar-toggle" class="fa fa-bars"></i> <i id="theme-toggle" class="fa fa-paint-brush"></i> </div> <h1 class="menu-title">The Rust Programming Language</h1> <div class="right-buttons"> <i id="print-button" class="fa fa-print" title="Print this book"></i> </div> </div> <div id="content" class="content"> <a class="header" href="enums.html#enums" id="enums"><h1>Enums</h1></a> <p>An <code>enum</code> in Rust is a type that represents data that is one of several possible variants. Each variant in the <code>enum</code> can optionally have data associated with it:</p> <pre><pre class="playpen"><code class="language-rust"># #![allow(unused_variables)] #fn main() { enum Message { Quit, ChangeColor(i32, i32, i32), Move { x: i32, y: i32 }, Write(String), } #}</code></pre></pre> <p>The syntax for defining variants resembles the syntaxes used to define structs: you can have variants with no data (like unit-like structs), variants with named data, and variants with unnamed data (like tuple structs). Unlike separate struct definitions, however, an <code>enum</code> is a single type. A value of the enum can match any of the variants. For this reason, an enum is sometimes called a ‘sum type’: the set of possible values of the enum is the sum of the sets of possible values for each variant.</p> <p>We use the <code>::</code> syntax to use the name of each variant: they’re scoped by the name of the <code>enum</code> itself. This allows both of these to work:</p> <pre><pre class="playpen"><code class="language-rust"># #![allow(unused_variables)] #fn main() { # enum Message { # Move { x: i32, y: i32 }, # } let x: Message = Message::Move { x: 3, y: 4 }; enum BoardGameTurn { Move { squares: i32 }, Pass, } let y: BoardGameTurn = BoardGameTurn::Move { squares: 1 }; #}</code></pre></pre> <p>Both variants are named <code>Move</code>, but since they’re scoped to the name of the enum, they can both be used without conflict.</p> <p>A value of an <code>enum</code> type contains information about which variant it is, in addition to any data associated with that variant. This is sometimes referred to as a ‘tagged union’, since the data includes a ‘tag’ indicating what type it is. The compiler uses this information to enforce that you’re accessing the data in the enum safely. For instance, you can’t simply try to destructure a value as if it were one of the possible variants:</p> <pre><code class="language-rust ignore">fn process_color_change(msg: Message) { let Message::ChangeColor(r, g, b) = msg; // This causes a compile-time error. } </code></pre> <p>Not supporting these operations may seem rather limiting, but it’s a limitation which we can overcome. There are two ways: by implementing equality ourselves, or by pattern matching variants with <a href="match.html"><code>match</code></a> expressions, which you’ll learn in the next section. We don’t know enough about Rust to implement equality yet, but we’ll find out in the <a href="traits.html"><code>traits</code></a> section.</p> <a class="header" href="enums.html#constructors-as-functions" id="constructors-as-functions"><h1>Constructors as functions</h1></a> <p>An <code>enum</code> constructor can also be used like a function. For example:</p> <pre><pre class="playpen"><code class="language-rust"># #![allow(unused_variables)] #fn main() { # enum Message { # Write(String), # } let m = Message::Write("Hello, world".to_string()); #}</code></pre></pre> <p>is the same as</p> <pre><pre class="playpen"><code class="language-rust"># #![allow(unused_variables)] #fn main() { # enum Message { # Write(String), # } fn foo(x: String) -> Message { Message::Write(x) } let x = foo("Hello, world".to_string()); #}</code></pre></pre> <p>This is not immediately useful to us, but when we get to <a href="closures.html"><code>closures</code></a>, we’ll talk about passing functions as arguments to other functions. For example, with <a href="iterators.html"><code>iterators</code></a>, we can do this to convert a vector of <code>String</code>s into a vector of <code>Message::Write</code>s:</p> <pre><pre class="playpen"><code class="language-rust"># #![allow(unused_variables)] #fn main() { # enum Message { # Write(String), # } let v = vec!["Hello".to_string(), "World".to_string()]; let v1: Vec<Message> = v.into_iter().map(Message::Write).collect(); #}</code></pre></pre> </div> <!-- Mobile navigation buttons --> <a href="structs.html" class="mobile-nav-chapters previous"> <i class="fa fa-angle-left"></i> </a> <a href="match.html" class="mobile-nav-chapters next"> <i class="fa fa-angle-right"></i> </a> </div> <a href="structs.html" class="nav-chapters previous" title="You can navigate through the chapters using the arrow keys"> <i class="fa fa-angle-left"></i> </a> <a href="match.html" class="nav-chapters next" title="You can navigate through the chapters using the arrow keys"> <i class="fa fa-angle-right"></i> </a> </div> <!-- Local fallback for Font Awesome --> <script> if ($(".fa").css("font-family") !== "FontAwesome") { $('<link rel="stylesheet" type="text/css" href="_FontAwesome/css/font-awesome.css">').prependTo('head'); } </script> <!-- Livereload script (if served using the cli tool) --> <script src="highlight.js"></script> <script src="book.js"></script> </body> </html>