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<!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="Waits for either one of two differently-typed futures to complete."><title>select in futures_util::future - Rust</title><script>if(window.location.protocol!=="file:")document.head.insertAdjacentHTML("beforeend","SourceSerif4-Regular-46f98efaafac5295.ttf.woff2,FiraSans-Regular-018c141bf0843ffd.woff2,FiraSans-Medium-8f9a781e4970d388.woff2,SourceCodePro-Regular-562dcc5011b6de7d.ttf.woff2,SourceCodePro-Semibold-d899c5a5c4aeb14a.ttf.woff2".split(",").map(f=>`<link rel="preload" as="font" type="font/woff2" crossorigin href="../../static.files/${f}">`).join(""))</script><link rel="stylesheet" href="../../static.files/normalize-76eba96aa4d2e634.css"><link rel="stylesheet" href="../../static.files/rustdoc-492a78a4a87dcc01.css"><meta name="rustdoc-vars" data-root-path="../../" data-static-root-path="../../static.files/" data-current-crate="futures_util" data-themes="" data-resource-suffix="" data-rustdoc-version="1.82.0 (f6e511eec 2024-10-15)" data-channel="1.82.0" data-search-js="search-a99f1315e7cc5121.js" data-settings-js="settings-4313503d2e1961c2.js" ><script src="../../static.files/storage-118b08c4c78b968e.js"></script><script defer src="sidebar-items.js"></script><script defer src="../../static.files/main-921df33f47b8780c.js"></script><noscript><link rel="stylesheet" href="../../static.files/noscript-3b12f09e550e0385.css"></noscript><link rel="alternate icon" type="image/png" href="../../static.files/favicon-32x32-422f7d1d52889060.png"><link rel="icon" type="image/svg+xml" href="../../static.files/favicon-2c020d218678b618.svg"></head><body class="rustdoc fn"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="mobile-topbar"><button class="sidebar-menu-toggle" title="show sidebar"></button></nav><nav class="sidebar"><div class="sidebar-crate"><h2><a href="../../futures_util/index.html">futures_<wbr>util</a><span class="version">0.3.31</span></h2></div><div class="sidebar-elems"><h2><a href="index.html">In futures_<wbr>util::<wbr>future</a></h2></div></nav><div class="sidebar-resizer"></div><main><div class="width-limiter"><rustdoc-search></rustdoc-search><section id="main-content" class="content"><div class="main-heading"><h1>Function <a href="../index.html">futures_util</a>::<wbr><a href="index.html">future</a>::<wbr><a class="fn" href="#">select</a><button id="copy-path" title="Copy item path to clipboard">Copy item path</button></h1><span class="out-of-band"><a class="src" href="../../src/futures_util/future/select.rs.html#84-92">source</a> · <button id="toggle-all-docs" title="collapse all docs">[<span>−</span>]</button></span></div><pre class="rust item-decl"><code>pub fn select<A, B>(future1: A, future2: B) -> <a class="struct" href="struct.Select.html" title="struct futures_util::future::Select">Select</a><A, B> <a href="#" class="tooltip" data-notable-ty="Select<A, B>">ⓘ</a><div class="where">where
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A: <a class="trait" href="https://doc.rust-lang.org/1.82.0/core/future/future/trait.Future.html" title="trait core::future::future::Future">Future</a> + <a class="trait" href="https://doc.rust-lang.org/1.82.0/core/marker/trait.Unpin.html" title="trait core::marker::Unpin">Unpin</a>,
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B: <a class="trait" href="https://doc.rust-lang.org/1.82.0/core/future/future/trait.Future.html" title="trait core::future::future::Future">Future</a> + <a class="trait" href="https://doc.rust-lang.org/1.82.0/core/marker/trait.Unpin.html" title="trait core::marker::Unpin">Unpin</a>,</div></code></pre><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Waits for either one of two differently-typed futures to complete.</p>
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<p>This function will return a new future which awaits for either one of both
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futures to complete. The returned future will finish with both the value
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resolved and a future representing the completion of the other work.</p>
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<p>Note that this function consumes the receiving futures and returns a
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wrapped version of them.</p>
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<p>Also note that if both this and the second future have the same
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output type you can use the <code>Either::factor_first</code> method to
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conveniently extract out the value at the end.</p>
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<h2 id="examples"><a class="doc-anchor" href="#examples">§</a>Examples</h2>
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<p>A simple example</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>futures::{
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pin_mut,
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future::Either,
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future::<span class="self">self</span>,
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};
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<span class="comment">// These two futures have different types even though their outputs have the same type.
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</span><span class="kw">let </span>future1 = <span class="kw">async </span>{
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future::pending::<()>().<span class="kw">await</span>; <span class="comment">// will never finish
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</span><span class="number">1
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</span>};
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<span class="kw">let </span>future2 = <span class="kw">async </span>{
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future::ready(<span class="number">2</span>).<span class="kw">await
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</span>};
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<span class="comment">// 'select' requires Future + Unpin bounds
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</span><span class="macro">pin_mut!</span>(future1);
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<span class="macro">pin_mut!</span>(future2);
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<span class="kw">let </span>value = <span class="kw">match </span>future::select(future1, future2).<span class="kw">await </span>{
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Either::Left((value1, <span class="kw">_</span>)) => value1, <span class="comment">// `value1` is resolved from `future1`
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// `_` represents `future2`
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</span>Either::Right((value2, <span class="kw">_</span>)) => value2, <span class="comment">// `value2` is resolved from `future2`
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// `_` represents `future1`
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</span>};
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<span class="macro">assert!</span>(value == <span class="number">2</span>);</code></pre></div>
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<p>A more complex example</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>futures::future::{<span class="self">self</span>, Either, Future, FutureExt};
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<span class="comment">// A poor-man's join implemented on top of select
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</span><span class="kw">fn </span>join<A, B>(a: A, b: B) -> <span class="kw">impl </span>Future<Output=(A::Output, B::Output)>
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<span class="kw">where </span>A: Future + Unpin,
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B: Future + Unpin,
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{
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future::select(a, b).then(|either| {
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<span class="kw">match </span>either {
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Either::Left((x, b)) => b.map(<span class="kw">move </span>|y| (x, y)).left_future(),
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Either::Right((y, a)) => a.map(<span class="kw">move </span>|x| (x, y)).right_future(),
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}
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})
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}</code></pre></div>
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</div></details><script type="text/json" id="notable-traits-data">{"Select<A, B>":"<h3>Notable traits for <code><a class=\"struct\" href=\"struct.Select.html\" title=\"struct futures_util::future::Select\">Select</a><A, B></code></h3><pre><code><div class=\"where\">impl<A, B> <a class=\"trait\" href=\"https://doc.rust-lang.org/1.82.0/core/future/future/trait.Future.html\" title=\"trait core::future::future::Future\">Future</a> for <a class=\"struct\" href=\"struct.Select.html\" title=\"struct futures_util::future::Select\">Select</a><A, B><div class=\"where\">where\n A: <a class=\"trait\" href=\"https://doc.rust-lang.org/1.82.0/core/future/future/trait.Future.html\" title=\"trait core::future::future::Future\">Future</a> + <a class=\"trait\" href=\"https://doc.rust-lang.org/1.82.0/core/marker/trait.Unpin.html\" title=\"trait core::marker::Unpin\">Unpin</a>,\n B: <a class=\"trait\" href=\"https://doc.rust-lang.org/1.82.0/core/future/future/trait.Future.html\" title=\"trait core::future::future::Future\">Future</a> + <a class=\"trait\" href=\"https://doc.rust-lang.org/1.82.0/core/marker/trait.Unpin.html\" title=\"trait core::marker::Unpin\">Unpin</a>,</div></div><div class=\"where\"> type <a href=\"https://doc.rust-lang.org/1.82.0/core/future/future/trait.Future.html#associatedtype.Output\" class=\"associatedtype\">Output</a> = <a class=\"enum\" href=\"enum.Either.html\" title=\"enum futures_util::future::Either\">Either</a><(A::<a class=\"associatedtype\" href=\"https://doc.rust-lang.org/1.82.0/core/future/future/trait.Future.html#associatedtype.Output\" title=\"type core::future::future::Future::Output\">Output</a>, B), (B::<a class=\"associatedtype\" href=\"https://doc.rust-lang.org/1.82.0/core/future/future/trait.Future.html#associatedtype.Output\" title=\"type core::future::future::Future::Output\">Output</a>, A)>;</div>"}</script></section></div></main></body></html>
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