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<p class='location'>Trait FnOnce</p><div class="block items"><ul><li><a href="#associated-types">Associated Types</a></li><li><a href="#required-methods">Required Methods</a></li><li><a href="#implementors">Implementors</a></li></ul></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>
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<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>]
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</span><a class='srclink' href='../../src/core/ops.rs.html#2676-2684' 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="trait" href="../../std/ops/trait.FnOnce.html" title="trait std::ops::FnOnce">Output</a>;
}</pre><div class='docblock'><p>A version of the call operator that takes a by-value receiver.</p>
<h1 id='examples' class='section-header'><a href='#examples'>Examples</a></h1>
<p>By-value closures automatically implement this trait, which allows them to
be invoked.</p>
<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=fn%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>
<p>By-value Closures can also be passed to higher-level functions through a
<code>FnOnce</code> parameter.</p>
<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=fn%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'>Associated Types</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'>Required Methods</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="trait" href="../../std/ops/trait.FnOnce.html" title="trait std::ops::FnOnce">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>This is called when the call operator is used.</p>
</div></div>
<h2 id='implementors'>Implementors</h2>
<ul class='item-list' id='implementors-list'>
<li><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>></code></li>
<li><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></code></li>
<li><code>impl<'a, A, F> FnOnce<A> for &'a mut 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></li>
<li><code>impl<'a, A, F> FnOnce<A> for &'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></li>
<li><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.tuple.html">()</a>> for <a class="struct" href="../../std/panic/struct.AssertUnwindSafe.html" title="struct std::panic::AssertUnwindSafe">AssertUnwindSafe</a><F></code></li>
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