<!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="../../normalize1.35.0.css"><link rel="stylesheet" type="text/css" href="../../rustdoc1.35.0.css" id="mainThemeStyle"><link rel="stylesheet" type="text/css" href="../../dark1.35.0.css"><link rel="stylesheet" type="text/css" href="../../light1.35.0.css" id="themeStyle"><script src="../../storage1.35.0.js"></script><noscript><link rel="stylesheet" href="../../noscript1.35.0.css"></noscript><link rel="shortcut icon" href="../../favicon1.35.0.ico"><style type="text/css">#crate-search{background-image:url("../../down-arrow1.35.0.svg");}</style></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='../../rust-logo1.35.0.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="#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="../../brush1.35.0.svg" width="18" alt="Pick another theme!"></button><div id="theme-choices"></div></div><script src="../../theme1.35.0.js"></script><nav class="sub"><form class="search-form js-only"><div class="search-container"><div><select id="crate-search"><option value="All crates">All crates</option></select><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"></div><a id="settings-menu" href="../../settings.html"><img src="../../wheel1.35.0.svg" width="18" alt="Change settings"></a></div></form></nav><section id="main" class="content"><h1 class='fqn'><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#143' title='goto source code'>[src]</a></span><span class='in-band'>Module <a href='../index.html'>std</a>::<wbr><a class="mod" href=''>boxed</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>
<p>For non-zero-sized values, a <a href="struct.Box.html"><code>Box</code></a> will use the [<code>Global</code>] allocator for
its allocation. It is valid to convert both ways between a <a href="struct.Box.html"><code>Box</code></a> and a
raw pointer allocated with the [<code>Global</code>] allocator, given that the
[<code>Layout</code>] used with the allocator is correct for the type. More precisely,
a <code>value: *mut T</code> that has been allocated with the [<code>Global</code>] allocator
with <code>Layout::for_value(&*value)</code> may be converted into a box using
<code>Box::<T>::from_raw(value)</code>. Conversely, the memory backing a <code>value: *mut T</code> obtained from <code>Box::<T>::into_raw</code> may be deallocated using the
[<code>Global</code>] allocator with <code>Layout::for_value(&*value)</code>.</p>
<h1 id="examples" class="section-header"><a href="#examples">Examples</a></h1>
<p>Move a value from the stack to the heap by creating a <a href="struct.Box.html"><code>Box</code></a>:</p>
<div class="example-wrap"><pre class="rust rust-example-rendered">
<span class="kw">let</span> <span class="ident">val</span>: <span class="ident">u8</span> <span class="op">=</span> <span class="number">5</span>;
<span class="kw">let</span> <span class="ident">boxed</span>: <span class="ident">Box</span><span class="op"><</span><span class="ident">u8</span><span class="op">></span> <span class="op">=</span> <span class="ident">Box</span>::<span class="ident">new</span>(<span class="ident">val</span>);<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0Afn%20main()%20%7B%0Alet%20val%3A%20u8%20%3D%205%3B%0Alet%20boxed%3A%20Box%3Cu8%3E%20%3D%20Box%3A%3Anew(val)%3B%0A%7D">Run</a></pre></div>
<p>Move a value from a <a href="struct.Box.html"><code>Box</code></a> back to the stack by <a href="../../std/ops/trait.Deref.html">dereferencing</a>:</p>
<div class="example-wrap"><pre class="rust rust-example-rendered">
<span class="kw">let</span> <span class="ident">boxed</span>: <span class="ident">Box</span><span class="op"><</span><span class="ident">u8</span><span class="op">></span> <span class="op">=</span> <span class="ident">Box</span>::<span class="ident">new</span>(<span class="number">5</span>);
<span class="kw">let</span> <span class="ident">val</span>: <span class="ident">u8</span> <span class="op">=</span> <span class="kw-2">*</span><span class="ident">boxed</span>;<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0Afn%20main()%20%7B%0Alet%20boxed%3A%20Box%3Cu8%3E%20%3D%20Box%3A%3Anew(5)%3B%0Alet%20val%3A%20u8%20%3D%20*boxed%3B%0A%7D">Run</a></pre></div>
<p>Creating a recursive data structure:</p>
<div class="example-wrap"><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">Run</a></pre></div>
<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><div class="example-wrap"><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></div>
<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='std::boxed::Box struct'>Box</a></td><td class='docblock-short'><p>A pointer type for heap allocation.</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='std::boxed::FnBox trait'>FnBox</a></td><td class='docblock-short'><span class="stab unstable">Experimental</span><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<dyn FnOnce()></code> in a data structure, you should use
<code>Box<dyn 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<dyn FnOnce()></code>
closures become directly usable.)</p>
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