<!DOCTYPE HTML> <html lang="en"> <head> <meta charset="UTF-8"> <title>Mutability - 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" class="active"><strong aria-hidden="true">3.11.</strong> Mutability</a></li><li><a href="structs.html"><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="mutability.html#mutability" id="mutability"><h1>Mutability</h1></a> <p>Mutability, the ability to change something, works a bit differently in Rust than in other languages. The first aspect of mutability is its non-default status:</p> <pre><code class="language-rust ignore">let x = 5; x = 6; // Error! </code></pre> <p>We can introduce mutability with the <code>mut</code> keyword:</p> <pre><pre class="playpen"><code class="language-rust"> # #![allow(unused_variables)] #fn main() { let mut x = 5; x = 6; // No problem! #}</code></pre></pre> <p>This is a mutable <a href="variable-bindings.html">variable binding</a>. When a binding is mutable, it means you’re allowed to change what the binding points to. So in the above example, it’s not so much that the value at <code>x</code> is changing, but that the binding changed from one <code>i32</code> to another.</p> <p>You can also create a <a href="references-and-borrowing.html">reference</a> to it, using <code>&x</code>, but if you want to use the reference to change it, you will need a mutable reference:</p> <pre><pre class="playpen"><code class="language-rust"> # #![allow(unused_variables)] #fn main() { let mut x = 5; let y = &mut x; #}</code></pre></pre> <p><code>y</code> is an immutable binding to a mutable reference, which means that you can’t bind 'y' to something else (<code>y = &mut z</code>), but <code>y</code> can be used to bind <code>x</code> to something else (<code>*y = 5</code>). A subtle distinction.</p> <p>Of course, if you need both:</p> <pre><pre class="playpen"><code class="language-rust"> # #![allow(unused_variables)] #fn main() { let mut x = 5; let mut y = &mut x; #}</code></pre></pre> <p>Now <code>y</code> can be bound to another value, and the value it’s referencing can be changed.</p> <p>It’s important to note that <code>mut</code> is part of a <a href="patterns.html">pattern</a>, so you can do things like this:</p> <pre><pre class="playpen"><code class="language-rust"> # #![allow(unused_variables)] #fn main() { let (mut x, y) = (5, 6); fn foo(mut x: i32) { # } #}</code></pre></pre> <p>Note that here, the <code>x</code> is mutable, but not the <code>y</code>.</p> <a class="header" href="mutability.html#interior-vs-exterior-mutability" id="interior-vs-exterior-mutability"><h1>Interior vs. Exterior Mutability</h1></a> <p>However, when we say something is ‘immutable’ in Rust, that doesn’t mean that it’s not able to be changed: we are referring to its ‘exterior mutability’ that in this case is immutable. Consider, for example, <a href="../../std/sync/struct.Arc.html"><code>Arc<T></code></a>:</p> <pre><pre class="playpen"><code class="language-rust"> # #![allow(unused_variables)] #fn main() { use std::sync::Arc; let x = Arc::new(5); let y = x.clone(); #}</code></pre></pre> <p>When we call <code>clone()</code>, the <code>Arc<T></code> needs to update the reference count. Yet we’ve not used any <code>mut</code>s here, <code>x</code> is an immutable binding, and we didn’t take <code>&mut 5</code> or anything. So what gives?</p> <p>To understand this, we have to go back to the core of Rust’s guiding philosophy, memory safety, and the mechanism by which Rust guarantees it, the <a href="ownership.html">ownership</a> system, and more specifically, <a href="references-and-borrowing.html#borrowing">borrowing</a>:</p> <blockquote> <p>You may have one or the other of these two kinds of borrows, but not both at the same time:</p> <ul> <li>one or more references (<code>&T</code>) to a resource,</li> <li>exactly one mutable reference (<code>&mut T</code>).</li> </ul> </blockquote> <p>So, that’s the real definition of ‘immutability’: is this safe to have two pointers to? In <code>Arc<T></code>’s case, yes: the mutation is entirely contained inside the structure itself. It’s not user facing. For this reason, it hands out <code>&T</code> with <code>clone()</code>. If it handed out <code>&mut T</code>s, though, that would be a problem.</p> <p>Other types, like the ones in the <a href="../../std/cell/index.html"><code>std::cell</code></a> module, have the opposite: interior mutability. For example:</p> <pre><pre class="playpen"><code class="language-rust"> # #![allow(unused_variables)] #fn main() { use std::cell::RefCell; let x = RefCell::new(42); let y = x.borrow_mut(); #}</code></pre></pre> <p>RefCell hands out <code>&mut</code> references to what’s inside of it with the <code>borrow_mut()</code> method. Isn’t that dangerous? What if we do:</p> <pre><code class="language-rust ignore">use std::cell::RefCell; let x = RefCell::new(42); let y = x.borrow_mut(); let z = x.borrow_mut(); # (y, z); </code></pre> <p>This will in fact panic, at runtime. This is what <code>RefCell</code> does: it enforces Rust’s borrowing rules at runtime, and <code>panic!</code>s if they’re violated. This allows us to get around another aspect of Rust’s mutability rules. Let’s talk about it first.</p> <a class="header" href="mutability.html#field-level-mutability" id="field-level-mutability"><h2>Field-level mutability</h2></a> <p>Mutability is a property of either a borrow (<code>&mut</code>) or a binding (<code>let mut</code>). This means that, for example, you cannot have a <a href="structs.html"><code>struct</code></a> with some fields mutable and some immutable:</p> <pre><code class="language-rust ignore">struct Point { x: i32, mut y: i32, // Nope. } </code></pre> <p>The mutability of a struct is in its binding:</p> <pre><code class="language-rust ignore">struct Point { x: i32, y: i32, } let mut a = Point { x: 5, y: 6 }; a.x = 10; let b = Point { x: 5, y: 6 }; b.x = 10; // Error: cannot assign to immutable field `b.x`. </code></pre> <p>However, by using <a href="../../std/cell/struct.Cell.html"><code>Cell<T></code></a>, you can emulate field-level mutability:</p> <pre><pre class="playpen"><code class="language-rust"> # #![allow(unused_variables)] #fn main() { use std::cell::Cell; struct Point { x: i32, y: Cell<i32>, } let point = Point { x: 5, y: Cell::new(6) }; point.y.set(7); println!("y: {:?}", point.y); #}</code></pre></pre> <p>This will print <code>y: Cell { value: 7 }</code>. We’ve successfully updated <code>y</code>.</p> </main> <nav class="nav-wrapper" aria-label="Page navigation"> <!-- Mobile navigation buttons --> <a rel="prev" href="lifetimes.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="structs.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="lifetimes.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="structs.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>