<!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="../../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 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='https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.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="../../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'>Trait <a href='../index.html'>std</a>::<wbr><a href='index.html'>ops</a>::<wbr><a class="trait" href=''>FnOnce</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/core/ops/function.rs.html#216-224' title='goto source code'>[src]</a></span></h1> <pre class='rust trait'><div class="docblock attributes">#[lang = "fn_once"] </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 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/second-edition/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> <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> <h2 id="using-a-fnonce-parameter" class="section-header"><a href="#using-a-fnonce-parameter">Using a <code>FnOnce</code> parameter</a></h2> <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> <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'><span id='Output.t' class='invisible'><code>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></span></h3><div class='docblock'><p>The returned type after the call operator is used.</p> </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_once' class='method'><span id='call_once.v' class='invisible'><code>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></span></h3><div class='stability'><div class='stab unstable'><span class=microscope>🔬</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> <h2 id='implementors' class='small-section-header'> Implementors<a href='#implementors' class='anchor'></a> </h2> <ul class='item-list' id='implementors-list'> <li><div class='out-of-band'><a class='srclink' href='../../src/core/ops/function.rs.html#266-273' title='goto source code'>[src]</a></div><code>impl<'a, A, F> FnOnce<A> for <a class="primitive" href="../primitive.reference.html">&'a 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><span class="where fmt-newline"> type <a href='#associatedtype.Output' 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>;</span></code></li> <li><div class='out-of-band'><a class='srclink' href='../../src/core/ops/function.rs.html#246-254' title='goto source code'>[src]</a></div><code>impl<'a, A, F> FnOnce<A> for <a class="primitive" href="../primitive.reference.html">&'a </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><span class="where fmt-newline"> type <a href='#associatedtype.Output' 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>;</span></code></li> <li><div class='out-of-band'><a class='srclink' href='../../src/alloc/boxed.rs.html#804-810' title='goto source code'>[src]</a></div><code>impl<'a, A, R> FnOnce<A> for <a class="struct" href="../../std/boxed/struct.Box.html" title="struct std::boxed::Box">Box</a><<a class="trait" href="../../std/boxed/trait.FnBox.html" title="trait std::boxed::FnBox">FnBox</a><A, Output = R> + 'a + <a class="trait" href="../../std/marker/trait.Send.html" title="trait std::marker::Send">Send</a>><span class="where fmt-newline"> type <a href='#associatedtype.Output' class="type">Output</a> = R;</span></code></li> <li><div class='out-of-band'><a class='srclink' href='../../src/alloc/boxed.rs.html#794-800' title='goto source code'>[src]</a></div><code>impl<'a, A, R> FnOnce<A> for <a class="struct" href="../../std/boxed/struct.Box.html" title="struct std::boxed::Box">Box</a><<a class="trait" href="../../std/boxed/trait.FnBox.html" title="trait std::boxed::FnBox">FnBox</a><A, Output = R> + 'a><span class="where fmt-newline"> type <a href='#associatedtype.Output' class="type">Output</a> = R;</span></code></li> <li><div class='out-of-band'><a class='srclink' href='../../src/std/panic.rs.html#289-295' title='goto source code'>[src]</a></div><code>impl<R, F: <a class="trait" href="../../std/ops/trait.FnOnce.html" title="trait std::ops::FnOnce">FnOnce</a>() -> R> FnOnce<<a class="primitive" href="../primitive.unit.html">()</a>> for <a class="struct" href="../../std/panic/struct.AssertUnwindSafe.html" title="struct std::panic::AssertUnwindSafe">AssertUnwindSafe</a><F><span class="where fmt-newline"> type <a href='#associatedtype.Output' class="type">Output</a> = R;</span></code></li> </ul><script type="text/javascript" async src="../../implementors/core/ops/function/trait.FnOnce.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> </div> </div> </aside> <script> window.rootPath = "../../"; 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