<!DOCTYPE html> <html lang="en"> <head> <meta charset="utf-8"> <meta name="viewport" content="width=device-width, initial-scale=1.0"> <meta name="generator" content="rustdoc"> <meta name="description" content="API documentation for the Rust `boxed` mod in crate `std`."> <meta name="keywords" content="rust, rustlang, rust-lang, boxed"> <title>std::boxed - Rust</title> <link rel="stylesheet" type="text/css" href="../../normalize.css"> <link rel="stylesheet" type="text/css" href="../../rustdoc.css" id="mainThemeStyle"> <link rel="stylesheet" type="text/css" href="../../dark.css"> <link rel="stylesheet" type="text/css" href="../../main.css" id="themeStyle"> <script src="../../storage.js"></script> <link rel="shortcut icon" href="https://doc.rust-lang.org/favicon.ico"> </head> <body class="rustdoc mod"> <!--[if lte IE 8]> <div class="warning"> This old browser is unsupported and will most likely display funky things. </div> <![endif]--> <nav class="sidebar"> <div class="sidebar-menu">☰</div> <a href='../../std/index.html'><img src='https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png' alt='logo' width='100'></a> <p class='location'>Module boxed</p><div class="sidebar-elems"><div class="block items"><ul><li><a href="#structs">Structs</a></li><li><a href="#constants">Constants</a></li><li><a href="#traits">Traits</a></li></ul></div><p class='location'><a href='../index.html'>std</a></p><script>window.sidebarCurrent = {name: 'boxed', ty: 'mod', relpath: '../'};</script><script defer src="../sidebar-items.js"></script></div> </nav> <div class="theme-picker"> <button id="theme-picker" aria-label="Pick another theme!"> <img src="../../brush.svg" width="18" alt="Pick another theme!"> </button> <div id="theme-choices"></div> </div> <script src="../../theme.js"></script> <nav class="sub"> <form class="search-form js-only"> <div class="search-container"> <input class="search-input" name="search" autocomplete="off" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"> </div> </form> </nav> <section id='main' class="content"> <h1 class='fqn'><span class='in-band'>Module <a href='../index.html'>std</a>::<wbr><a class="mod" href=''>boxed</a></span><span class='out-of-band'><span class='since' title='Stable since Rust version 1.0.0'>1.0.0</span><span id='render-detail'> <a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs"> [<span class='inner'>−</span>] </a> </span><a class='srclink' href='../../src/alloc/lib.rs.html#163' title='goto source code'>[src]</a></span></h1> <div class='docblock'><p>A pointer type for heap allocation.</p> <p><code>Box<T></code>, casually referred to as a 'box', provides the simplest form of heap allocation in Rust. Boxes provide ownership for this allocation, and drop their contents when they go out of scope.</p> <h1 id="examples" class="section-header"><a href="#examples">Examples</a></h1> <p>Creating a box:</p> <pre class="rust rust-example-rendered"> <span class="kw">let</span> <span class="ident">x</span> <span class="op">=</span> <span class="ident">Box</span>::<span class="ident">new</span>(<span class="number">5</span>);<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0Afn%20main()%20%7B%0Alet%20x%20%3D%20Box%3A%3Anew(5)%3B%0A%7D">Run</a></pre> <p>Creating a recursive data structure:</p> <pre class="rust rust-example-rendered"> <span class="attribute">#[<span class="ident">derive</span>(<span class="ident">Debug</span>)]</span> <span class="kw">enum</span> <span class="ident">List</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> { <span class="ident">Cons</span>(<span class="ident">T</span>, <span class="ident">Box</span><span class="op"><</span><span class="ident">List</span><span class="op"><</span><span class="ident">T</span><span class="op">>></span>), <span class="ident">Nil</span>, } <span class="kw">fn</span> <span class="ident">main</span>() { <span class="kw">let</span> <span class="ident">list</span>: <span class="ident">List</span><span class="op"><</span><span class="ident">i32</span><span class="op">></span> <span class="op">=</span> <span class="ident">List</span>::<span class="ident">Cons</span>(<span class="number">1</span>, <span class="ident">Box</span>::<span class="ident">new</span>(<span class="ident">List</span>::<span class="ident">Cons</span>(<span class="number">2</span>, <span class="ident">Box</span>::<span class="ident">new</span>(<span class="ident">List</span>::<span class="ident">Nil</span>)))); <span class="macro">println</span><span class="macro">!</span>(<span class="string">"{:?}"</span>, <span class="ident">list</span>); }<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0A%23%5Bderive(Debug)%5D%0Aenum%20List%3CT%3E%20%7B%0A%20%20%20%20Cons(T%2C%20Box%3CList%3CT%3E%3E)%2C%0A%20%20%20%20Nil%2C%0A%7D%0A%0Afn%20main()%20%7B%0A%20%20%20%20let%20list%3A%20List%3Ci32%3E%20%3D%20List%3A%3ACons(1%2C%20Box%3A%3Anew(List%3A%3ACons(2%2C%20Box%3A%3Anew(List%3A%3ANil))))%3B%0A%20%20%20%20println!(%22%7B%3A%3F%7D%22%2C%20list)%3B%0A%7D%0A">Run</a></pre> <p>This will print <code>Cons(1, Cons(2, Nil))</code>.</p> <p>Recursive structures must be boxed, because if the definition of <code>Cons</code> looked like this:</p> <div class='information'><div class='tooltip compile_fail'>ⓘ<span class='tooltiptext'>This example deliberately fails to compile</span></div></div><pre class="rust rust-example-rendered compile_fail"> <span class="ident">Cons</span>(<span class="ident">T</span>, <span class="ident">List</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>),<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0Afn%20main()%20%7B%0Aenum%20List%3CT%3E%20%7B%0ACons(T%2C%20List%3CT%3E)%2C%0A%7D%0A%7D">Run</a></pre> <p>It wouldn't work. This is because the size of a <code>List</code> depends on how many elements are in the list, and so we don't know how much memory to allocate for a <code>Cons</code>. By introducing a <code>Box</code>, which has a defined size, we know how big <code>Cons</code> needs to be.</p> </div><h2 id='structs' class='section-header'><a href="#structs">Structs</a></h2> <table> <tr class=' module-item'> <td><a class="struct" href="struct.Box.html" title='struct std::boxed::Box'>Box</a></td> <td class='docblock-short'> <p>A pointer type for heap allocation.</p> </td> </tr> <tr class='unstable module-item'> <td><a class="struct" href="struct.ExchangeHeapSingleton.html" title='struct std::boxed::ExchangeHeapSingleton'>ExchangeHeapSingleton</a></td> <td class='docblock-short'> [<div class='stab unstable'>Experimental</div>] <p>This the singleton type used solely for <code>boxed::HEAP</code>.</p> </td> </tr> <tr class='unstable module-item'> <td><a class="struct" href="struct.IntermediateBox.html" title='struct std::boxed::IntermediateBox'>IntermediateBox</a></td> <td class='docblock-short'> [<div class='stab unstable'>Experimental</div>] <p><code>IntermediateBox</code> represents uninitialized backing storage for <code>Box</code>.</p> </td> </tr></table><h2 id='constants' class='section-header'><a href="#constants">Constants</a></h2> <table> <tr class='unstable module-item'> <td><a class="constant" href="constant.HEAP.html" title='constant std::boxed::HEAP'>HEAP</a></td> <td class='docblock-short'> [<div class='stab unstable'>Experimental</div>] <p>A value that represents the heap. This is the default place that the <code>box</code> keyword allocates into when no place is supplied.</p> </td> </tr></table><h2 id='traits' class='section-header'><a href="#traits">Traits</a></h2> <table> <tr class='unstable module-item'> <td><a class="trait" href="trait.FnBox.html" title='trait std::boxed::FnBox'>FnBox</a></td> <td class='docblock-short'> [<div class='stab unstable'>Experimental</div>] <p><code>FnBox</code> is a version of the <code>FnOnce</code> intended for use with boxed closure objects. The idea is that where one would normally store a <code>Box<FnOnce()></code> in a data structure, you should use <code>Box<FnBox()></code>. The two traits behave essentially the same, except that a <code>FnBox</code> closure can only be called if it is boxed. (Note that <code>FnBox</code> may be deprecated in the future if <code>Box<FnOnce()></code> closures become directly usable.)</p> </td> </tr></table></section> <section id='search' class="content hidden"></section> <section class="footer"></section> <aside id="help" class="hidden"> <div> <h1 class="hidden">Help</h1> <div class="shortcuts"> <h2>Keyboard Shortcuts</h2> <dl> <dt><kbd>?</kbd></dt> <dd>Show this help dialog</dd> <dt><kbd>S</kbd></dt> <dd>Focus the search field</dd> <dt><kbd>↑</kbd></dt> <dd>Move up in search results</dd> <dt><kbd>↓</kbd></dt> <dd>Move down in search results</dd> <dt><kbd>↹</kbd></dt> <dd>Switch tab</dd> <dt><kbd>⏎</kbd></dt> <dd>Go to active search result</dd> <dt><kbd>+</kbd></dt> <dd>Expand all sections</dd> <dt><kbd>-</kbd></dt> <dd>Collapse all sections</dd> </dl> </div> <div class="infos"> <h2>Search Tricks</h2> <p> Prefix searches with a type followed by a colon (e.g. <code>fn:</code>) to restrict the search to a given type. </p> <p> Accepted types are: <code>fn</code>, <code>mod</code>, <code>struct</code>, <code>enum</code>, <code>trait</code>, <code>type</code>, <code>macro</code>, and <code>const</code>. </p> <p> Search functions by type signature (e.g. <code>vec -> usize</code> or <code>* -> vec</code>) </p> </div> </div> </aside> <script> window.rootPath = "../../"; window.currentCrate = "std"; </script> <script src="../../main.js"></script> <script defer src="../../search-index.js"></script> </body> </html>