<!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 `FnOnce` trait in crate `std`."><meta name="keywords" content="rust, rustlang, rust-lang, FnOnce"><title>std::ops::FnOnce - 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 FnOnce</p><div class="sidebar-elems"><div class="block items"><a class="sidebar-title" href="#associated-types">Associated Types</a><div class="sidebar-links"><a href="#associatedtype.Output">Output</a></div><a class="sidebar-title" href="#required-methods">Required Methods</a><div class="sidebar-links"><a href="#tymethod.call_once">call_once</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'>ops</a></p><script>window.sidebarCurrent = {name: 'FnOnce', 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 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/core/ops/function.rs.html#224-232' title='goto source code'>[src]</a></span><span class='in-band'>Trait <a href='../index.html'>std</a>::<wbr><a href='index.html'>ops</a>::<wbr><a class="trait" href=''>FnOnce</a></span></h1><div class="docblock type-decl hidden-by-usual-hider"><pre class='rust trait'><div class="docblock attributes">#[lang = "fn_once"]
#[must_use = "closures are lazy and do nothing unless called"]
</div>pub trait FnOnce<Args> {
type <a href='#associatedtype.Output' class="type">Output</a>;
extern "rust-call" fn <a href='#tymethod.call_once' class='fnname'>call_once</a>(self, args: Args) -> Self::<a class="type" href="../../std/ops/trait.FnOnce.html#associatedtype.Output" title="type std::ops::FnOnce::Output">Output</a>;
}</pre></div><div class='docblock'><p>The version of the call operator that takes a by-value receiver.</p>
<p>Instances of <code>FnOnce</code> can be called, but might not be callable multiple
times. Because of this, if the only thing known about a type is that it
implements <code>FnOnce</code>, it can only be called once.</p>
<p><code>FnOnce</code> is implemented automatically by closure that might consume captured
variables, as well as all types that implement <a href="trait.FnMut.html"><code>FnMut</code></a>, e.g., (safe)
<a href="../../std/primitive.fn.html">function pointers</a> (since <code>FnOnce</code> is a supertrait of <a href="trait.FnMut.html"><code>FnMut</code></a>).</p>
<p>Since both <a href="trait.Fn.html"><code>Fn</code></a> and <a href="trait.FnMut.html"><code>FnMut</code></a> are subtraits of <code>FnOnce</code>, any instance of
<a href="trait.Fn.html"><code>Fn</code></a> or <a href="trait.FnMut.html"><code>FnMut</code></a> can be used where a <code>FnOnce</code> is expected.</p>
<p>Use <code>FnOnce</code> as a bound when you want to accept a parameter of function-like
type and only need to call it once. If you need to call the parameter
repeatedly, use <a href="trait.FnMut.html"><code>FnMut</code></a> as a bound; if you also need it to not mutate
state, use <a href="trait.Fn.html"><code>Fn</code></a>.</p>
<p>See the <a href="../../book/ch13-01-closures.html">chapter on closures in <em>The Rust Programming Language</em></a> for
some more information on this topic.</p>
<p>Also of note is the special syntax for <code>Fn</code> traits (e.g.
<code>Fn(usize, bool) -> usize</code>). Those interested in the technical details of
this can refer to <a href="../../nomicon/hrtb.html">the relevant section in the <em>Rustonomicon</em></a>.</p>
<h1 id="examples" class="section-header"><a href="#examples">Examples</a></h1><h2 id="calling-a-by-value-closure" class="section-header"><a href="#calling-a-by-value-closure">Calling a by-value closure</a></h2>
<div class="example-wrap"><pre class="rust rust-example-rendered">
<span class="kw">let</span> <span class="ident">x</span> <span class="op">=</span> <span class="number">5</span>;
<span class="kw">let</span> <span class="ident">square_x</span> <span class="op">=</span> <span class="kw">move</span> <span class="op">||</span> <span class="ident">x</span> <span class="op">*</span> <span class="ident">x</span>;
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">square_x</span>(), <span class="number">25</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%205%3B%0Alet%20square_x%20%3D%20move%20%7C%7C%20x%20*%20x%3B%0Aassert_eq!(square_x()%2C%2025)%3B%0A%7D">Run</a></pre></div>
<h2 id="using-a-fnonce-parameter" class="section-header"><a href="#using-a-fnonce-parameter">Using a <code>FnOnce</code> parameter</a></h2>
<div class="example-wrap"><pre class="rust rust-example-rendered">
<span class="kw">fn</span> <span class="ident">consume_with_relish</span><span class="op"><</span><span class="ident">F</span><span class="op">></span>(<span class="ident">func</span>: <span class="ident">F</span>)
<span class="kw">where</span> <span class="ident">F</span>: <span class="ident">FnOnce</span>() <span class="op">-></span> <span class="ident">String</span>
{
<span class="comment">// `func` consumes its captured variables, so it cannot be run more</span>
<span class="comment">// than once.</span>
<span class="macro">println</span><span class="macro">!</span>(<span class="string">"Consumed: {}"</span>, <span class="ident">func</span>());
<span class="macro">println</span><span class="macro">!</span>(<span class="string">"Delicious!"</span>);
<span class="comment">// Attempting to invoke `func()` again will throw a `use of moved</span>
<span class="comment">// value` error for `func`.</span>
}
<span class="kw">let</span> <span class="ident">x</span> <span class="op">=</span> <span class="ident">String</span>::<span class="ident">from</span>(<span class="string">"x"</span>);
<span class="kw">let</span> <span class="ident">consume_and_return_x</span> <span class="op">=</span> <span class="kw">move</span> <span class="op">||</span> <span class="ident">x</span>;
<span class="ident">consume_with_relish</span>(<span class="ident">consume_and_return_x</span>);
<span class="comment">// `consume_and_return_x` can no longer be invoked at this point</span><a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0Afn%20main()%20%7B%0Afn%20consume_with_relish%3CF%3E(func%3A%20F)%0A%20%20%20%20where%20F%3A%20FnOnce()%20-%3E%20String%0A%7B%0A%20%20%20%20%2F%2F%20%60func%60%20consumes%20its%20captured%20variables%2C%20so%20it%20cannot%20be%20run%20more%0A%20%20%20%20%2F%2F%20than%20once.%0A%20%20%20%20println!(%22Consumed%3A%20%7B%7D%22%2C%20func())%3B%0A%0A%20%20%20%20println!(%22Delicious!%22)%3B%0A%0A%20%20%20%20%2F%2F%20Attempting%20to%20invoke%20%60func()%60%20again%20will%20throw%20a%20%60use%20of%20moved%0A%20%20%20%20%2F%2F%20value%60%20error%20for%20%60func%60.%0A%7D%0A%0Alet%20x%20%3D%20String%3A%3Afrom(%22x%22)%3B%0Alet%20consume_and_return_x%20%3D%20move%20%7C%7C%20x%3B%0Aconsume_with_relish(consume_and_return_x)%3B%0A%0A%2F%2F%20%60consume_and_return_x%60%20can%20no%20longer%20be%20invoked%20at%20this%20point%0A%7D">Run</a></pre></div>
</div>
<h2 id='associated-types' class='small-section-header'>Associated Types<a href='#associated-types' class='anchor'></a></h2><div class='methods'><h3 id='associatedtype.Output' class='method'><code id='Output.t'>type <a href='#associatedtype.Output' class="type">Output</a></code><div class='since' title='Stable since Rust version 1.12.0'>1.12.0</div></h3><div class='docblock'><p>The returned type after the call operator is used.</p>
</div></div><span class='loading-content'>Loading content...</span>
<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_once' class='method'><code id='call_once.v'>extern "rust-call" fn <a href='#tymethod.call_once' class='fnname'>call_once</a>(self, args: Args) -> 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'><span class='emoji'>🔬</span> This is a nightly-only experimental API. (<code>fn_traits</code> <a href="https://github.com/rust-lang/rust/issues/29625">#29625</a>)</div></div><div class='docblock'><p>Performs the call operation.</p>
</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-FnOnce%3CA%3E' class='impl'><code class='in-band'>impl<'_, A, F> FnOnce<A> for <a class="primitive" href="../primitive.reference.html">&'_ </a>F <span class="where fmt-newline">where<br> F: <a class="trait" href="../../std/ops/trait.Fn.html" title="trait std::ops::Fn">Fn</a><A> + ?<a class="trait" href="../../std/marker/trait.Sized.html" title="trait std::marker::Sized">Sized</a>, </span></code><a href='#impl-FnOnce%3CA%3E' class='anchor'></a><a class='srclink' href='../../src/core/ops/function.rs.html#254-262' title='goto source code'>[src]</a></h3><div class='impl-items'><h4 id='associatedtype.Output-1' class="type"><code id='Output.t-1'>type <a href='#associatedtype.Output-1' class="type">Output</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></h4><h4 id='method.call_once' class="method hidden"><code id='call_once.v-1'>extern "rust-call" fn <a href='#method.call_once' class='fnname'>call_once</a>(self, 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/core/ops/function.rs.html#259-261' title='goto source code'>[src]</a></h4><div class='stability hidden'><div class='stab unstable'><span class='emoji'>🔬</span> This is a nightly-only experimental API. (<code>fn_traits</code> <a href="https://github.com/rust-lang/rust/issues/29625">#29625</a>)</div></div></div><h3 id='impl-FnOnce%3CA%3E-1' class='impl'><code class='in-band'>impl<'_, A, F> FnOnce<A> for <a class="primitive" href="../primitive.reference.html">&'_ mut </a>F <span class="where fmt-newline">where<br> F: <a class="trait" href="../../std/ops/trait.FnMut.html" title="trait std::ops::FnMut">FnMut</a><A> + ?<a class="trait" href="../../std/marker/trait.Sized.html" title="trait std::marker::Sized">Sized</a>, </span></code><a href='#impl-FnOnce%3CA%3E-1' class='anchor'></a><a class='srclink' href='../../src/core/ops/function.rs.html#274-281' title='goto source code'>[src]</a></h3><div class='impl-items'><h4 id='associatedtype.Output-2' class="type"><code id='Output.t-2'>type <a href='#associatedtype.Output-2' class="type">Output</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></h4><h4 id='method.call_once-1' class="method hidden"><code id='call_once.v-2'>extern "rust-call" fn <a href='#method.call_once-1' class='fnname'>call_once</a>(self, 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/core/ops/function.rs.html#278-280' title='goto source code'>[src]</a></h4><div class='stability hidden'><div class='stab unstable'><span class='emoji'>🔬</span> This is a nightly-only experimental API. (<code>fn_traits</code> <a href="https://github.com/rust-lang/rust/issues/29625">#29625</a>)</div></div></div><h3 id='impl-FnOnce%3CA%3E-2' class='impl'><code class='in-band'>impl<A, F> FnOnce<A> for <a class="struct" href="../../std/boxed/struct.Box.html" title="struct std::boxed::Box">Box</a><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> + ?<a class="trait" href="../../std/marker/trait.Sized.html" title="trait std::marker::Sized">Sized</a>, </span></code><a href='#impl-FnOnce%3CA%3E-2' class='anchor'></a><a class='srclink' href='../../src/alloc/boxed.rs.html#698-704' title='goto source code'>[src]</a></h3><div class='impl-items'><h4 id='associatedtype.Output-3' class="type"><code id='Output.t-3'>type <a href='#associatedtype.Output-3' class="type">Output</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></h4><h4 id='method.call_once-2' class="method hidden"><div class="important-traits"><div class='tooltip'>ⓘ<span class='tooltiptext'>Important traits for <a class="struct" href="../../std/boxed/struct.Box.html" title="struct std::boxed::Box">Box</a><I></span></div><div class="content hidden"><h3 class="important">Important traits for <a class="struct" href="../../std/boxed/struct.Box.html" title="struct std::boxed::Box">Box</a><I></h3><code class="content"><span class="where fmt-newline">impl<I> <a class="trait" href="../../std/iter/trait.Iterator.html" title="trait std::iter::Iterator">Iterator</a> for <a class="struct" href="../../std/boxed/struct.Box.html" title="struct std::boxed::Box">Box</a><I> <span class="where fmt-newline">where<br> I: <a class="trait" href="../../std/iter/trait.Iterator.html" title="trait std::iter::Iterator">Iterator</a> + ?<a class="trait" href="../../std/marker/trait.Sized.html" title="trait std::marker::Sized">Sized</a>, </span></span><span class="where fmt-newline"> type <a href='../../std/iter/trait.Iterator.html#associatedtype.Item' class="type">Item</a> = <I as <a class="trait" href="../../std/iter/trait.Iterator.html" title="trait std::iter::Iterator">Iterator</a>>::<a class="type" href="../../std/iter/trait.Iterator.html#associatedtype.Item" title="type std::iter::Iterator::Item">Item</a>;</span><span class="where fmt-newline">impl<F> <a class="trait" href="../../std/future/trait.Future.html" title="trait std::future::Future">Future</a> for <a class="struct" href="../../std/boxed/struct.Box.html" title="struct std::boxed::Box">Box</a><F> <span class="where fmt-newline">where<br> F: <a class="trait" href="../../std/marker/trait.Unpin.html" title="trait std::marker::Unpin">Unpin</a> + <a class="trait" href="../../std/future/trait.Future.html" title="trait std::future::Future">Future</a> + ?<a class="trait" href="../../std/marker/trait.Sized.html" title="trait std::marker::Sized">Sized</a>, </span></span><span class="where fmt-newline"> type <a href='../../std/future/trait.Future.html#associatedtype.Output' class="type">Output</a> = <F as <a class="trait" href="../../std/future/trait.Future.html" title="trait std::future::Future">Future</a>>::<a class="type" href="../../std/future/trait.Future.html#associatedtype.Output" title="type std::future::Future::Output">Output</a>;</span><span class="where fmt-newline">impl<R: <a class="trait" href="../../std/io/trait.Read.html" title="trait std::io::Read">Read</a> + ?<a class="trait" href="../../std/marker/trait.Sized.html" title="trait std::marker::Sized">Sized</a>> <a class="trait" href="../../std/io/trait.Read.html" title="trait std::io::Read">Read</a> for <a class="struct" href="../../std/boxed/struct.Box.html" title="struct std::boxed::Box">Box</a><R></span><span class="where fmt-newline">impl<W: <a class="trait" href="../../std/io/trait.Write.html" title="trait std::io::Write">Write</a> + ?<a class="trait" href="../../std/marker/trait.Sized.html" title="trait std::marker::Sized">Sized</a>> <a class="trait" href="../../std/io/trait.Write.html" title="trait std::io::Write">Write</a> for <a class="struct" href="../../std/boxed/struct.Box.html" title="struct std::boxed::Box">Box</a><W></span></code></div></div><code id='call_once.v-3'>extern "rust-call" fn <a href='#method.call_once-2' class='fnname'>call_once</a>(self, args: A) -> <<a class="struct" href="../../std/boxed/struct.Box.html" title="struct std::boxed::Box">Box</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#701-703' title='goto source code'>[src]</a></h4><div class='stability hidden'><div class='stab unstable'><span class='emoji'>🔬</span> This is a nightly-only experimental API. (<code>fn_traits</code> <a href="https://github.com/rust-lang/rust/issues/29625">#29625</a>)</div></div></div><h3 id='impl-FnOnce()' class='impl'><code class='in-band'>impl<R, F: <a class="trait" href="../../std/ops/trait.FnOnce.html" title="trait std::ops::FnOnce">FnOnce</a>() -> R> FnOnce() for <a class="struct" href="../../std/panic/struct.AssertUnwindSafe.html" title="struct std::panic::AssertUnwindSafe">AssertUnwindSafe</a><F></code><a href='#impl-FnOnce()' class='anchor'></a><a class='srclink' href='../../src/std/panic.rs.html#305-311' title='goto source code'>[src]</a></h3><div class='impl-items'><h4 id='associatedtype.Output-4' class="type"><code id='Output.t-4'>type <a href='#associatedtype.Output-4' class="type">Output</a> = R</code></h4><h4 id='method.call_once-3' class="method hidden"><code id='call_once.v-4'>extern "rust-call" fn <a href='#method.call_once-3' class='fnname'>call_once</a>(self, _args: <a class="primitive" href="../primitive.unit.html">()</a>) -> R</code><a class='srclink' href='../../src/std/panic.rs.html#308-310' title='goto source code'>[src]</a></h4><div class='stability hidden'><div class='stab unstable'><span class='emoji'>🔬</span> This is a nightly-only experimental API. (<code>fn_traits</code> <a href="https://github.com/rust-lang/rust/issues/29625">#29625</a>)</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
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