<!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 `FnMut` trait in crate `core`."><meta name="keywords" content="rust, rustlang, rust-lang, FnMut"><title>core::ops::FnMut - 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="../../light.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='../../core/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 FnMut</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_mut">call_mut</a></div><a class="sidebar-title" href="#implementors">Implementors</a></div><p class='location'><a href='../index.html'>core</a>::<wbr><a href='index.html'>ops</a></p><script>window.sidebarCurrent = {name: 'FnMut', 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"><a id="settings-menu" href="../../settings.html"><img src="../../wheel.svg" width="18" alt="Change settings"></a></div></form></nav><section id="main" class="content"><h1 class='fqn'><span class='in-band'>Trait <a href='../index.html'>core</a>::<wbr><a href='index.html'>ops</a>::<wbr><a class="trait" href=''>FnMut</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#143-147' title='goto source code'>[src]</a></span></h1><div class="docblock type-decl"><pre class='rust trait'><div class="docblock attributes">#[lang = "fn_mut"]
</div>pub trait FnMut<Args>: <a class="trait" href="../../core/ops/trait.FnOnce.html" title="trait core::ops::FnOnce">FnOnce</a><Args> {
extern "rust-call" fn <a href='#tymethod.call_mut' class='fnname'>call_mut</a>(&mut self, args: Args) -> Self::<a class="type" href="../../core/ops/trait.FnOnce.html#associatedtype.Output" title="type core::ops::FnOnce::Output">Output</a>;
}</pre></div><div class='docblock'><p>The version of the call operator that takes a mutable receiver.</p>
<p>Instances of <code>FnMut</code> can be called repeatedly and may mutate state.</p>
<p><code>FnMut</code> is implemented automatically by closures which take mutable
references to captured variables, as well as all types that implement
<a href="trait.Fn.html"><code>Fn</code></a>, e.g. (safe) <a href="../../std/primitive.fn.html">function pointers</a> (since <code>FnMut</code> is a supertrait of
<a href="trait.Fn.html"><code>Fn</code></a>). Additionally, for any type <code>F</code> that implements <code>FnMut</code>, <code>&mut F</code>
implements <code>FnMut</code>, too.</p>
<p>Since <a href="trait.FnOnce.html"><code>FnOnce</code></a> is a supertrait of <code>FnMut</code>, any instance of <code>FnMut</code> can be
used where a <a href="trait.FnOnce.html"><code>FnOnce</code></a> is expected, and since <a href="trait.Fn.html"><code>Fn</code></a> is a subtrait of
<code>FnMut</code>, any instance of <a href="trait.Fn.html"><code>Fn</code></a> can be used where <code>FnMut</code> is expected.</p>
<p>Use <code>FnMut</code> as a bound when you want to accept a parameter of function-like
type and need to call it repeatedly, while allowing it to mutate state.
If you don't want the parameter to mutate state, use <a href="trait.Fn.html"><code>Fn</code></a> as a
bound; if you don't need to call it repeatedly, use <a href="trait.FnOnce.html"><code>FnOnce</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-mutably-capturing-closure" class="section-header"><a href="#calling-a-mutably-capturing-closure">Calling a mutably capturing closure</a></h2>
<pre class="rust rust-example-rendered">
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">x</span> <span class="op">=</span> <span class="number">5</span>;
{
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">square_x</span> <span class="op">=</span> <span class="op">||</span> <span class="ident">x</span> <span class="op">*=</span> <span class="ident">x</span>;
<span class="ident">square_x</span>();
}
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">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%20mut%20x%20%3D%205%3B%0A%7B%0A%20%20%20%20let%20mut%20square_x%20%3D%20%7C%7C%20x%20*%3D%20x%3B%0A%20%20%20%20square_x()%3B%0A%7D%0Aassert_eq!(x%2C%2025)%3B%0A%7D">Run</a></pre>
<h2 id="using-a-fnmut-parameter" class="section-header"><a href="#using-a-fnmut-parameter">Using a <code>FnMut</code> parameter</a></h2>
<pre class="rust rust-example-rendered">
<span class="kw">fn</span> <span class="ident">do_twice</span><span class="op"><</span><span class="ident">F</span><span class="op">></span>(<span class="kw-2">mut</span> <span class="ident">func</span>: <span class="ident">F</span>)
<span class="kw">where</span> <span class="ident">F</span>: <span class="ident">FnMut</span>()
{
<span class="ident">func</span>();
<span class="ident">func</span>();
}
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">x</span>: <span class="ident">usize</span> <span class="op">=</span> <span class="number">1</span>;
{
<span class="kw">let</span> <span class="ident">add_two_to_x</span> <span class="op">=</span> <span class="op">||</span> <span class="ident">x</span> <span class="op">+=</span> <span class="number">2</span>;
<span class="ident">do_twice</span>(<span class="ident">add_two_to_x</span>);
}
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">x</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%0Afn%20do_twice%3CF%3E(mut%20func%3A%20F)%0A%20%20%20%20where%20F%3A%20FnMut()%0A%7B%0A%20%20%20%20func()%3B%0A%20%20%20%20func()%3B%0A%7D%0A%0Alet%20mut%20x%3A%20usize%20%3D%201%3B%0A%7B%0A%20%20%20%20let%20add_two_to_x%20%3D%20%7C%7C%20x%20%2B%3D%202%3B%0A%20%20%20%20do_twice(add_two_to_x)%3B%0A%7D%0A%0Aassert_eq!(x%2C%205)%3B%0A%7D">Run</a></pre>
</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_mut' class='method'><span id='call_mut.v' class='invisible'><code>extern "rust-call" fn <a href='#tymethod.call_mut' class='fnname'>call_mut</a>(&mut self, args: Args) -> Self::<a class="type" href="../../core/ops/trait.FnOnce.html#associatedtype.Output" title="type core::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><table class='table-display'><tbody><tr><td><code>impl<'a, A, F: ?<a class="trait" href="../../core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a>> FnMut<A> for &'a F <span class="where fmt-newline">where<br> F: <a class="trait" href="../../core/ops/trait.Fn.html" title="trait core::ops::Fn">Fn</a><A>, </span></code><td><div class='out-of-band'><a class='srclink' href='../../src/core/ops/function.rs.html#237-243' title='goto source code'>[src]</a></div></td></tr></tbody></table></li>
<li><table class='table-display'><tbody><tr><td><code>impl<'a, A, F: ?<a class="trait" href="../../core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a>> FnMut<A> for &'a mut F <span class="where fmt-newline">where<br> F: <a class="trait" href="../../core/ops/trait.FnMut.html" title="trait core::ops::FnMut">FnMut</a><A>, </span></code><td><div class='out-of-band'><a class='srclink' href='../../src/core/ops/function.rs.html#257-263' title='goto source code'>[src]</a></div></td></tr></tbody></table></li>
</ul><script type="text/javascript">window.inlined_types=new Set([]);</script><script type="text/javascript" async
src="../../implementors/core/ops/trait.FnMut.js">
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