<!DOCTYPE HTML> <html lang="en"> <head> <meta charset="UTF-8"> <title>Structs - 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"> <meta name="theme-color" content="#ffffff" /> <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"> <link rel="stylesheet" href="ayu-highlight.css"> <!-- Custom 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href="variable-bindings.html"><strong aria-hidden="true">3.1.</strong> Variable Bindings</a></li><li><a href="functions.html"><strong aria-hidden="true">3.2.</strong> Functions</a></li><li><a href="primitive-types.html"><strong aria-hidden="true">3.3.</strong> Primitive Types</a></li><li><a href="comments.html"><strong aria-hidden="true">3.4.</strong> Comments</a></li><li><a href="if.html"><strong aria-hidden="true">3.5.</strong> if</a></li><li><a href="loops.html"><strong aria-hidden="true">3.6.</strong> Loops</a></li><li><a href="vectors.html"><strong aria-hidden="true">3.7.</strong> Vectors</a></li><li><a href="ownership.html"><strong aria-hidden="true">3.8.</strong> Ownership</a></li><li><a href="references-and-borrowing.html"><strong aria-hidden="true">3.9.</strong> References and Borrowing</a></li><li><a href="lifetimes.html"><strong aria-hidden="true">3.10.</strong> Lifetimes</a></li><li><a href="mutability.html"><strong aria-hidden="true">3.11.</strong> Mutability</a></li><li><a href="structs.html" class="active"><strong aria-hidden="true">3.12.</strong> Structs</a></li><li><a href="enums.html"><strong aria-hidden="true">3.13.</strong> Enums</a></li><li><a href="match.html"><strong aria-hidden="true">3.14.</strong> Match</a></li><li><a href="patterns.html"><strong aria-hidden="true">3.15.</strong> Patterns</a></li><li><a href="method-syntax.html"><strong aria-hidden="true">3.16.</strong> Method Syntax</a></li><li><a href="strings.html"><strong aria-hidden="true">3.17.</strong> Strings</a></li><li><a href="generics.html"><strong aria-hidden="true">3.18.</strong> Generics</a></li><li><a href="traits.html"><strong aria-hidden="true">3.19.</strong> Traits</a></li><li><a href="drop.html"><strong aria-hidden="true">3.20.</strong> Drop</a></li><li><a href="if-let.html"><strong aria-hidden="true">3.21.</strong> if let</a></li><li><a href="trait-objects.html"><strong aria-hidden="true">3.22.</strong> Trait Objects</a></li><li><a href="closures.html"><strong aria-hidden="true">3.23.</strong> Closures</a></li><li><a href="ufcs.html"><strong aria-hidden="true">3.24.</strong> Universal Function Call Syntax</a></li><li><a href="crates-and-modules.html"><strong aria-hidden="true">3.25.</strong> Crates and Modules</a></li><li><a href="const-and-static.html"><strong aria-hidden="true">3.26.</strong> const and static</a></li><li><a href="attributes.html"><strong aria-hidden="true">3.27.</strong> Attributes</a></li><li><a href="type-aliases.html"><strong aria-hidden="true">3.28.</strong> type aliases</a></li><li><a href="casting-between-types.html"><strong aria-hidden="true">3.29.</strong> Casting between types</a></li><li><a href="associated-types.html"><strong aria-hidden="true">3.30.</strong> Associated Types</a></li><li><a href="unsized-types.html"><strong aria-hidden="true">3.31.</strong> Unsized Types</a></li><li><a href="operators-and-overloading.html"><strong aria-hidden="true">3.32.</strong> Operators and Overloading</a></li><li><a href="deref-coercions.html"><strong aria-hidden="true">3.33.</strong> Deref coercions</a></li><li><a href="macros.html"><strong aria-hidden="true">3.34.</strong> Macros</a></li><li><a href="raw-pointers.html"><strong aria-hidden="true">3.35.</strong> Raw Pointers</a></li><li><a href="unsafe.html"><strong aria-hidden="true">3.36.</strong> unsafe</a></li></ol></li><li><a href="effective-rust.html"><strong aria-hidden="true">4.</strong> Effective Rust</a></li><li><ol class="section"><li><a href="the-stack-and-the-heap.html"><strong aria-hidden="true">4.1.</strong> The Stack and the Heap</a></li><li><a href="testing.html"><strong aria-hidden="true">4.2.</strong> Testing</a></li><li><a href="conditional-compilation.html"><strong aria-hidden="true">4.3.</strong> Conditional Compilation</a></li><li><a href="documentation.html"><strong aria-hidden="true">4.4.</strong> Documentation</a></li><li><a href="iterators.html"><strong aria-hidden="true">4.5.</strong> Iterators</a></li><li><a href="concurrency.html"><strong aria-hidden="true">4.6.</strong> Concurrency</a></li><li><a href="error-handling.html"><strong aria-hidden="true">4.7.</strong> Error Handling</a></li><li><a href="choosing-your-guarantees.html"><strong aria-hidden="true">4.8.</strong> Choosing your Guarantees</a></li><li><a href="ffi.html"><strong aria-hidden="true">4.9.</strong> FFI</a></li><li><a href="borrow-and-asref.html"><strong aria-hidden="true">4.10.</strong> Borrow and AsRef</a></li><li><a href="release-channels.html"><strong aria-hidden="true">4.11.</strong> Release Channels</a></li><li><a href="using-rust-without-the-standard-library.html"><strong aria-hidden="true">4.12.</strong> Using Rust without the standard library</a></li><li><a href="procedural-macros.html"><strong aria-hidden="true">4.13.</strong> Procedural Macros (and custom derive)</a></li></ol></li><li><a href="glossary.html"><strong aria-hidden="true">5.</strong> Glossary</a></li><li><a href="syntax-index.html"><strong aria-hidden="true">6.</strong> Syntax Index</a></li><li><a href="bibliography.html"><strong aria-hidden="true">7.</strong> Bibliography</a></li></ol> </nav> <div id="page-wrapper" class="page-wrapper"> <div class="page"> <div id="menu-bar" class="menu-bar"> <div id="menu-bar-sticky-container"> <div class="left-buttons"> <button id="sidebar-toggle" class="icon-button" type="button" title="Toggle Table of Contents" aria-label="Toggle Table of Contents" aria-controls="sidebar"> <i class="fa fa-bars"></i> </button> <button id="theme-toggle" class="icon-button" type="button" title="Change theme" aria-label="Change theme" aria-haspopup="true" aria-expanded="false" aria-controls="theme-list"> <i class="fa fa-paint-brush"></i> </button> <ul id="theme-list" class="theme-popup" aria-label="submenu"> <li><button class="theme" id="light">Light <span class="default">(default)</span></button></li> <li><button class="theme" id="rust">Rust</button></li> <li><button class="theme" id="coal">Coal</button></li> <li><button class="theme" id="navy">Navy</button></li> <li><button class="theme" id="ayu">Ayu</button></li> </ul> </div> <h1 class="menu-title">The Rust Programming Language</h1> <div class="right-buttons"> <a href="print.html" title="Print this book" aria-label="Print this book"> <i id="print-button" class="fa fa-print"></i> </a> </div> </div> </div> <!-- Apply ARIA attributes after the sidebar and the sidebar toggle button are added to the DOM --> <script type="text/javascript"> document.getElementById('sidebar-toggle').setAttribute('aria-expanded', sidebar === 'visible'); document.getElementById('sidebar').setAttribute('aria-hidden', sidebar !== 'visible'); Array.from(document.querySelectorAll('#sidebar a')).forEach(function(link) { link.setAttribute('tabIndex', sidebar === 'visible' ? 0 : -1); }); </script> <div id="content" class="content"> <main> <a class="header" href="structs.html#structs" id="structs"><h1>Structs</h1></a> <p><code>struct</code>s are a way of creating more complex data types. For example, if we were doing calculations involving coordinates in 2D space, we would need both an <code>x</code> and a <code>y</code> value:</p> <pre><pre class="playpen"><code class="language-rust"> # #![allow(unused_variables)] #fn main() { let origin_x = 0; let origin_y = 0; #}</code></pre></pre> <p>A <code>struct</code> lets us combine these two into a single, unified datatype with <code>x</code> and <code>y</code> as field labels:</p> <pre><pre class="playpen"><code class="language-rust">struct Point { x: i32, y: i32, } fn main() { let origin = Point { x: 0, y: 0 }; // origin: Point println!("The origin is at ({}, {})", origin.x, origin.y); } </code></pre></pre> <p>There’s a lot going on here, so let’s break it down. We declare a <code>struct</code> with the <code>struct</code> keyword, and then with a name. By convention, <code>struct</code>s begin with a capital letter and are camel cased: <code>PointInSpace</code>, not <code>Point_In_Space</code>.</p> <p>We can create an instance of our <code>struct</code> via <code>let</code>, as usual, but we use a <code>key: value</code> style syntax to set each field. The order doesn’t need to be the same as in the original declaration.</p> <p>Finally, because fields have names, we can access them through dot notation: <code>origin.x</code>.</p> <p>The values in <code>struct</code>s are immutable by default, like other bindings in Rust. Use <code>mut</code> to make them mutable:</p> <pre><pre class="playpen"><code class="language-rust">struct Point { x: i32, y: i32, } fn main() { let mut point = Point { x: 0, y: 0 }; point.x = 5; println!("The point is at ({}, {})", point.x, point.y); } </code></pre></pre> <p>This will print <code>The point is at (5, 0)</code>.</p> <p>Rust does not support field mutability at the language level, so you cannot write something like this:</p> <pre><code class="language-rust ignore">struct Point { mut x: i32, // This causes an error. y: i32, } </code></pre> <p>Mutability is a property of the binding, not of the structure itself. If you’re used to field-level mutability, this may seem strange at first, but it significantly simplifies things. It even lets you make things mutable on a temporary basis:</p> <pre><code class="language-rust ignore">struct Point { x: i32, y: i32, } fn main() { let mut point = Point { x: 0, y: 0 }; point.x = 5; let point = point; // `point` is now immutable. point.y = 6; // This causes an error. } </code></pre> <p>Your structure can still contain <code>&mut</code> references, which will let you do some kinds of mutation:</p> <pre><pre class="playpen"><code class="language-rust">struct Point { x: i32, y: i32, } struct PointRef<'a> { x: &'a mut i32, y: &'a mut i32, } fn main() { let mut point = Point { x: 0, y: 0 }; { let r = PointRef { x: &mut point.x, y: &mut point.y }; *r.x = 5; *r.y = 6; } assert_eq!(5, point.x); assert_eq!(6, point.y); } </code></pre></pre> <p>Initialization of a data structure (struct, enum, union) can be simplified when fields of the data structure are initialized with variables of the same names as the fields.</p> <pre><pre class="playpen"><code class="language-rust">#[derive(Debug)] struct Person<'a> { name: &'a str, age: u8 } fn main() { // Create struct with field init shorthand let name = "Peter"; let age = 27; let peter = Person { name, age }; // Debug-print struct println!("{:?}", peter); } </code></pre></pre> <a class="header" href="structs.html#update-syntax" id="update-syntax"><h1>Update syntax</h1></a> <p>A <code>struct</code> can include <code>..</code> to indicate that you want to use a copy of some other <code>struct</code> for some of the values. For example:</p> <pre><pre class="playpen"><code class="language-rust"> # #![allow(unused_variables)] #fn main() { struct Point3d { x: i32, y: i32, z: i32, } let mut point = Point3d { x: 0, y: 0, z: 0 }; point = Point3d { y: 1, .. point }; #}</code></pre></pre> <p>This gives <code>point</code> a new <code>y</code>, but keeps the old <code>x</code> and <code>z</code> values. It doesn’t have to be the same <code>struct</code> either, you can use this syntax when making new ones, and it will copy the values you don’t specify:</p> <pre><pre class="playpen"><code class="language-rust"> # #![allow(unused_variables)] #fn main() { # struct Point3d { # x: i32, # y: i32, # z: i32, # } let origin = Point3d { x: 0, y: 0, z: 0 }; let point = Point3d { z: 1, x: 2, .. origin }; #}</code></pre></pre> <a class="header" href="structs.html#tuple-structs" id="tuple-structs"><h1>Tuple structs</h1></a> <p>Rust has another data type that’s like a hybrid between a <a href="primitive-types.html#tuples">tuple</a> and a <code>struct</code>, called a ‘tuple struct’. Tuple structs have a name, but their fields don't. They are declared with the <code>struct</code> keyword, and then with a name followed by a tuple:</p> <pre><pre class="playpen"><code class="language-rust"> # #![allow(unused_variables)] #fn main() { struct Color(i32, i32, i32); struct Point(i32, i32, i32); let black = Color(0, 0, 0); let origin = Point(0, 0, 0); #}</code></pre></pre> <p>Here, <code>black</code> and <code>origin</code> are not the same type, even though they contain the same values.</p> <p>The members of a tuple struct may be accessed by dot notation or destructuring <code>let</code>, just like regular tuples:</p> <pre><pre class="playpen"><code class="language-rust"> # #![allow(unused_variables)] #fn main() { # struct Color(i32, i32, i32); # struct Point(i32, i32, i32); # let black = Color(0, 0, 0); # let origin = Point(0, 0, 0); let black_r = black.0; let Point(_, origin_y, origin_z) = origin; #}</code></pre></pre> <p>Patterns like <code>Point(_, origin_y, origin_z)</code> are also used in <a href="match.html">match expressions</a>.</p> <p>One case when a tuple struct is very useful is when it has only one element. We call this the ‘newtype’ pattern, because it allows you to create a new type that is distinct from its contained value and also expresses its own semantic meaning:</p> <pre><pre class="playpen"><code class="language-rust"> # #![allow(unused_variables)] #fn main() { struct Inches(i32); let length = Inches(10); let Inches(integer_length) = length; println!("length is {} inches", integer_length); #}</code></pre></pre> <p>As above, you can extract the inner integer type through a destructuring <code>let</code>. In this case, the <code>let Inches(integer_length)</code> assigns <code>10</code> to <code>integer_length</code>. We could have used dot notation to do the same thing:</p> <pre><pre class="playpen"><code class="language-rust"> # #![allow(unused_variables)] #fn main() { # struct Inches(i32); # let length = Inches(10); let integer_length = length.0; #}</code></pre></pre> <p>It's always possible to use a <code>struct</code> instead of a tuple struct, and can be clearer. We could write <code>Color</code> and <code>Point</code> like this instead:</p> <pre><pre class="playpen"><code class="language-rust"> # #![allow(unused_variables)] #fn main() { struct Color { red: i32, blue: i32, green: i32, } struct Point { x: i32, y: i32, z: i32, } #}</code></pre></pre> <p>Good names are important, and while values in a tuple struct can be referenced with dot notation as well, a <code>struct</code> gives us actual names, rather than positions.</p> <a class="header" href="structs.html#unit-like-structs" id="unit-like-structs"><h1>Unit-like structs</h1></a> <p>You can define a <code>struct</code> with no members at all:</p> <pre><code class="language-rust ignore">struct Electron {} // Use empty braces... struct Proton; // ...or just a semicolon. // Use the same notation when creating an instance. let x = Electron {}; let y = Proton; let z = Electron; // Error </code></pre> <p>Such a <code>struct</code> is called ‘unit-like’ because it resembles the empty tuple, <code>()</code>, sometimes called ‘unit’. Like a tuple struct, it defines a new type.</p> <p>This is rarely useful on its own (although sometimes it can serve as a marker type), but in combination with other features, it can become useful. For instance, a library may ask you to create a structure that implements a certain <a href="traits.html">trait</a> to handle events. If you don’t have any data you need to store in the structure, you can create a unit-like <code>struct</code>.</p> </main> <nav class="nav-wrapper" aria-label="Page navigation"> <!-- Mobile navigation buttons --> <a rel="prev" href="mutability.html" class="mobile-nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left"> <i class="fa fa-angle-left"></i> </a> <a rel="next" href="enums.html" class="mobile-nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right"> <i class="fa fa-angle-right"></i> </a> <div style="clear: both"></div> </nav> </div> </div> <nav class="nav-wide-wrapper" aria-label="Page navigation"> <a href="mutability.html" class="nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left"> <i class="fa fa-angle-left"></i> </a> <a href="enums.html" class="nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right"> <i class="fa fa-angle-right"></i> </a> </nav> </div> <!-- Local fallback for Font Awesome --> <script> if (getComputedStyle(document.querySelector(".fa")).fontFamily !== "FontAwesome") { var link = document.createElement('link'); link.rel = 'stylesheet'; link.type = 'text/css'; link.href = '_FontAwesome/css/font-awesome.css'; document.head.insertBefore(link, document.head.firstChild) } </script> <script src="highlight.js"></script> <script src="book.js"></script> <!-- Custom JS script --> </body> </html>