<!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 `FnBox` trait in crate `std`."><meta name="keywords" content="rust, rustlang, rust-lang, FnBox"><title>std::boxed::FnBox - 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 trait"><!--[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'>Trait FnBox</p><div class="sidebar-elems"><div class="block items"><a class="sidebar-title" href="#required-methods">Required Methods</a><div class="sidebar-links"><a href="#tymethod.call_box">call_box</a></div><a class="sidebar-title" href="#implementors">Implementors</a></div><p class='location'><a href='../index.html'>std</a>::<wbr><a href='index.html'>boxed</a></p><script>window.sidebarCurrent = {name: 'FnBox', ty: 'trait', 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 '></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/boxed.rs.html#760-762' title='goto source code'>[src]</a></span><span class='in-band'>Trait <a href='../index.html'>std</a>::<wbr><a href='index.html'>boxed</a>::<wbr><a class="trait" href=''>FnBox</a></span></h1><div class="docblock type-decl hidden-by-usual-hider"><pre class='rust trait'>pub trait FnBox<A>: <a class="trait" href="../../std/ops/trait.FnOnce.html" title="trait std::ops::FnOnce">FnOnce</a><A> {
fn <a href='#tymethod.call_box' class='fnname'>call_box</a>(self: <a class="struct" href="../../std/boxed/struct.Box.html" title="struct std::boxed::Box">Box</a><Self>, args: A) -> Self::<a class="type" href="../../std/ops/trait.FnOnce.html#associatedtype.Output" title="type std::ops::FnOnce::Output">Output</a>;
}</pre></div><div class='stability'><div class='stab unstable'><details><summary><span class='emoji'>🔬</span> This is a nightly-only experimental API. (<code>fnbox</code> <a href="https://github.com/rust-lang/rust/issues/28796">#28796</a>)</summary><p>will be deprecated if and when <code>Box<FnOnce></code> becomes usable</p>
</details></div></div><div class='docblock'><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>
<h1 id="examples" class="section-header"><a href="#examples">Examples</a></h1>
<p>Here is a snippet of code which creates a hashmap full of boxed
once closures and then removes them one by one, calling each
closure as it is removed. Note that the type of the closures
stored in the map is <code>Box<dyn FnBox() -> i32></code> and not <code>Box<dyn FnOnce() -> i32></code>.</p>
<div class="example-wrap"><pre class="rust rust-example-rendered">
<span class="attribute">#![<span class="ident">feature</span>(<span class="ident">fnbox</span>)]</span>
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">boxed</span>::<span class="ident">FnBox</span>;
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">collections</span>::<span class="ident">HashMap</span>;
<span class="kw">fn</span> <span class="ident">make_map</span>() <span class="op">-></span> <span class="ident">HashMap</span><span class="op"><</span><span class="ident">i32</span>, <span class="ident">Box</span><span class="op"><</span><span class="kw">dyn</span> <span class="ident">FnBox</span>() <span class="op">-></span> <span class="ident">i32</span><span class="op">>></span> {
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">map</span>: <span class="ident">HashMap</span><span class="op"><</span><span class="ident">i32</span>, <span class="ident">Box</span><span class="op"><</span><span class="kw">dyn</span> <span class="ident">FnBox</span>() <span class="op">-></span> <span class="ident">i32</span><span class="op">>></span> <span class="op">=</span> <span class="ident">HashMap</span>::<span class="ident">new</span>();
<span class="ident">map</span>.<span class="ident">insert</span>(<span class="number">1</span>, <span class="ident">Box</span>::<span class="ident">new</span>(<span class="op">||</span> <span class="number">22</span>));
<span class="ident">map</span>.<span class="ident">insert</span>(<span class="number">2</span>, <span class="ident">Box</span>::<span class="ident">new</span>(<span class="op">||</span> <span class="number">44</span>));
<span class="ident">map</span>
}
<span class="kw">fn</span> <span class="ident">main</span>() {
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">map</span> <span class="op">=</span> <span class="ident">make_map</span>();
<span class="kw">for</span> <span class="ident">i</span> <span class="kw">in</span> <span class="kw-2">&</span>[<span class="number">1</span>, <span class="number">2</span>] {
<span class="kw">let</span> <span class="ident">f</span> <span class="op">=</span> <span class="ident">map</span>.<span class="ident">remove</span>(<span class="kw-2">&</span><span class="ident">i</span>).<span class="ident">unwrap</span>();
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">f</span>(), <span class="ident">i</span> <span class="op">*</span> <span class="number">22</span>);
}
}<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0A%23!%5Bfeature(fnbox)%5D%0A%0Ause%20std%3A%3Aboxed%3A%3AFnBox%3B%0Ause%20std%3A%3Acollections%3A%3AHashMap%3B%0A%0Afn%20make_map()%20-%3E%20HashMap%3Ci32%2C%20Box%3Cdyn%20FnBox()%20-%3E%20i32%3E%3E%20%7B%0A%20%20%20%20let%20mut%20map%3A%20HashMap%3Ci32%2C%20Box%3Cdyn%20FnBox()%20-%3E%20i32%3E%3E%20%3D%20HashMap%3A%3Anew()%3B%0A%20%20%20%20map.insert(1%2C%20Box%3A%3Anew(%7C%7C%2022))%3B%0A%20%20%20%20map.insert(2%2C%20Box%3A%3Anew(%7C%7C%2044))%3B%0A%20%20%20%20map%0A%7D%0A%0Afn%20main()%20%7B%0A%20%20%20%20let%20mut%20map%20%3D%20make_map()%3B%0A%20%20%20%20for%20i%20in%20%26%5B1%2C%202%5D%20%7B%0A%20%20%20%20%20%20%20%20let%20f%20%3D%20map.remove(%26i).unwrap()%3B%0A%20%20%20%20%20%20%20%20assert_eq!(f()%2C%20i%20*%2022)%3B%0A%20%20%20%20%7D%0A%7D&version=nightly">Run</a></pre></div>
</div>
<h2 id='required-methods' class='small-section-header'>Required methods<a href='#required-methods' class='anchor'></a></h2><div class='methods'><h3 id='tymethod.call_box' class='method'><code id='call_box.v'>fn <a href='#tymethod.call_box' class='fnname'>call_box</a>(self: <a class="struct" href="../../std/boxed/struct.Box.html" title="struct std::boxed::Box">Box</a><Self>, args: A) -> Self::<a class="type" href="../../std/ops/trait.FnOnce.html#associatedtype.Output" title="type std::ops::FnOnce::Output">Output</a></code></h3><div class='stability'><div class='stab unstable'><details><summary><span class='emoji'>🔬</span> This is a nightly-only experimental API. (<code>fnbox</code> <a href="https://github.com/rust-lang/rust/issues/28796">#28796</a>)</summary><p>will be deprecated if and when <code>Box<FnOnce></code> becomes usable</p>
</details></div></div></div><span class='loading-content'>Loading content...</span>
<h2 id='implementors' class='small-section-header'>Implementors<a href='#implementors' class='anchor'></a></h2><div class='item-list' id='implementors-list'><h3 id='impl-FnBox%3CA%3E' class='impl'><code class='in-band'>impl<A, F> FnBox<A> for F <span class="where fmt-newline">where<br> F: <a class="trait" href="../../std/ops/trait.FnOnce.html" title="trait std::ops::FnOnce">FnOnce</a><A>, </span></code><a href='#impl-FnBox%3CA%3E' class='anchor'></a><a class='srclink' href='../../src/alloc/boxed.rs.html#766-772' title='goto source code'>[src]</a></h3><div class='impl-items'><h4 id='method.call_box' class="method hidden"><code id='call_box.v-1'>fn <a href='#method.call_box' class='fnname'>call_box</a>(self: <a class="struct" href="../../std/boxed/struct.Box.html" title="struct std::boxed::Box">Box</a><F>, args: A) -> <F as <a class="trait" href="../../std/ops/trait.FnOnce.html" title="trait std::ops::FnOnce">FnOnce</a><A>>::<a class="type" href="../../std/ops/trait.FnOnce.html#associatedtype.Output" title="type std::ops::FnOnce::Output">Output</a></code><a class='srclink' href='../../src/alloc/boxed.rs.html#769-771' title='goto source code'>[src]</a></h4><div class='stability hidden'><div class='stab unstable'><details><summary><span class='emoji'>🔬</span> This is a nightly-only experimental API. (<code>fnbox</code> <a href="https://github.com/rust-lang/rust/issues/28796">#28796</a>)</summary><p>will be deprecated if and when <code>Box<FnOnce></code> becomes usable</p>
</details></div></div></div></div><span class='loading-content'>Loading content...</span><script type="text/javascript">window.inlined_types=new Set([]);</script><script type="text/javascript" async
src="../../implementors/alloc/boxed/trait.FnBox.js">
</script></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><p>Search multiple things at once by splitting your query with comma (e.g., <code>str,u8</code> or <code>String,struct:Vec,test</code>)</p></div></div></aside><script>window.rootPath = "../../";window.currentCrate = "std";</script><script src="../../aliases.js"></script><script src="../../main1.35.0.js"></script><script defer src="../../search-index.js"></script></body></html>
distrib
>
Mageia
>
7
>
armv7hl
>
media
>
core-release
>
by-pkgid
>
0c2243f8a1696816431e7210e991fa52
>
files
>
14346
