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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="Polls multiple futures and streams simultaneously, executing the branch for the future that finishes first. If multiple futures are ready, one will be pseudo-randomly selected at runtime. Futures directly passed to `select!` must be `Unpin` and implement `FusedFuture`."><title>select in futures - Rust</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/SourceSerif4-Regular-46f98efaafac5295.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/FiraSans-Regular-018c141bf0843ffd.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/FiraSans-Medium-8f9a781e4970d388.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/SourceCodePro-Regular-562dcc5011b6de7d.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/SourceCodePro-Semibold-d899c5a5c4aeb14a.ttf.woff2"><link rel="stylesheet" href="../static.files/normalize-76eba96aa4d2e634.css"><link rel="stylesheet" href="../static.files/rustdoc-ac92e1bbe349e143.css"><meta name="rustdoc-vars" data-root-path="../" data-static-root-path="../static.files/" data-current-crate="futures" data-themes="" data-resource-suffix="" data-rustdoc-version="1.76.0 (07dca489a 2024-02-04)" data-channel="1.76.0" data-search-js="search-2b6ce74ff89ae146.js" data-settings-js="settings-4313503d2e1961c2.js" ><script src="../static.files/storage-f2adc0d6ca4d09fb.js"></script><script defer src="sidebar-items.js"></script><script defer src="../static.files/main-305769736d49e732.js"></script><noscript><link rel="stylesheet" href="../static.files/noscript-feafe1bb7466e4bd.css"></noscript><link rel="alternate icon" type="image/png" href="../static.files/favicon-16x16-8b506e7a72182f1c.png"><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 macro"><!--[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">☰</button></nav><nav class="sidebar"><div class="sidebar-crate"><h2><a href="../futures/index.html">futures</a><span class="version">0.3.28</span></h2></div><div class="sidebar-elems"></div></nav><div class="sidebar-resizer"></div>
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<main><div class="width-limiter"><nav class="sub"><form class="search-form"><span></span><div id="sidebar-button" tabindex="-1"><a href="../futures/all.html" title="show sidebar"></a></div><input class="search-input" name="search" aria-label="Run search in the documentation" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" tabindex="-1"><a href="../help.html" title="help">?</a></div><div id="settings-menu" tabindex="-1"><a href="../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../static.files/wheel-7b819b6101059cd0.svg"></a></div></form></nav><section id="main-content" class="content"><div class="main-heading"><h1>Macro <a href="index.html">futures</a>::<wbr><a class="macro" href="#">select</a><button id="copy-path" title="Copy item path to clipboard"><img src="../static.files/clipboard-7571035ce49a181d.svg" width="19" height="18" alt="Copy item path"></button></h1><span class="out-of-band"><a class="src" href="../src/futures_util/async_await/select_mod.rs.html#318">source</a> · <button id="toggle-all-docs" title="collapse all docs">[<span>−</span>]</button></span></div><pre class="rust item-decl"><span class="macro">macro_rules! </span>select {
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($(<span class="macro-nonterminal">$tokens</span>:tt)<span class="kw-2">*</span>) => { ... };
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}</pre><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Polls multiple futures and streams simultaneously, executing the branch
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for the future that finishes first. If multiple futures are ready,
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one will be pseudo-randomly selected at runtime. Futures directly
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passed to <code>select!</code> must be <code>Unpin</code> and implement <code>FusedFuture</code>.</p>
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<p>If an expression which yields a <code>Future</code> is passed to <code>select!</code>
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(e.g. an <code>async fn</code> call) instead of a <code>Future</code> by name the <code>Unpin</code>
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requirement is relaxed, since the macro will pin the resulting <code>Future</code>
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on the stack. However the <code>Future</code> returned by the expression must
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still implement <code>FusedFuture</code>.</p>
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<p>Futures and streams which are not already fused can be fused using the
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<code>.fuse()</code> method. Note, though, that fusing a future or stream directly
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in the call to <code>select!</code> will not be enough to prevent it from being
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polled after completion if the <code>select!</code> call is in a loop, so when
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<code>select!</code>ing in a loop, users should take care to <code>fuse()</code> outside of
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the loop.</p>
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<p><code>select!</code> can be used as an expression and will return the return
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value of the selected branch. For this reason the return type of every
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branch in a <code>select!</code> must be the same.</p>
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<p>This macro is only usable inside of async functions, closures, and blocks.
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It is also gated behind the <code>async-await</code> feature of this library, which is
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activated by default.</p>
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<h2 id="examples"><a href="#examples">Examples</a></h2>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>futures::future;
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<span class="kw">use </span>futures::select;
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<span class="kw">let </span><span class="kw-2">mut </span>a = future::ready(<span class="number">4</span>);
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<span class="kw">let </span><span class="kw-2">mut </span>b = future::pending::<()>();
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<span class="kw">let </span>res = <span class="macro">select! </span>{
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a_res = a => a_res + <span class="number">1</span>,
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<span class="kw">_ </span>= b => <span class="number">0</span>,
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};
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<span class="macro">assert_eq!</span>(res, <span class="number">5</span>);</code></pre></div>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>futures::future;
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<span class="kw">use </span>futures::stream::{<span class="self">self</span>, StreamExt};
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<span class="kw">use </span>futures::select;
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<span class="kw">let </span><span class="kw-2">mut </span>st = stream::iter(<span class="macro">vec!</span>[<span class="number">2</span>]).fuse();
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<span class="kw">let </span><span class="kw-2">mut </span>fut = future::pending::<()>();
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<span class="macro">select! </span>{
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x = st.next() => <span class="macro">assert_eq!</span>(<span class="prelude-val">Some</span>(<span class="number">2</span>), x),
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<span class="kw">_ </span>= fut => <span class="macro">panic!</span>(),
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};</code></pre></div>
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<p>As described earlier, <code>select</code> can directly select on expressions
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which return <code>Future</code>s - even if those do not implement <code>Unpin</code>:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>futures::future::FutureExt;
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<span class="kw">use </span>futures::select;
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<span class="comment">// Calling the following async fn returns a Future which does not
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// implement Unpin
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</span><span class="kw">async fn </span>async_identity_fn(arg: usize) -> usize {
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arg
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}
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<span class="kw">let </span>res = <span class="macro">select! </span>{
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a_res = async_identity_fn(<span class="number">62</span>).fuse() => a_res + <span class="number">1</span>,
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b_res = async_identity_fn(<span class="number">13</span>).fuse() => b_res,
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};
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<span class="macro">assert!</span>(res == <span class="number">63 </span>|| res == <span class="number">13</span>);</code></pre></div>
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<p>If a similar async function is called outside of <code>select</code> to produce
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a <code>Future</code>, the <code>Future</code> must be pinned in order to be able to pass
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it to <code>select</code>. This can be achieved via <code>Box::pin</code> for pinning a
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<code>Future</code> on the heap or the <code>pin_mut!</code> macro for pinning a <code>Future</code>
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on the stack.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>futures::future::FutureExt;
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<span class="kw">use </span>futures::select;
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<span class="kw">use </span>futures::pin_mut;
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<span class="comment">// Calling the following async fn returns a Future which does not
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// implement Unpin
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</span><span class="kw">async fn </span>async_identity_fn(arg: usize) -> usize {
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arg
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}
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<span class="kw">let </span>fut_1 = async_identity_fn(<span class="number">1</span>).fuse();
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<span class="kw">let </span>fut_2 = async_identity_fn(<span class="number">2</span>).fuse();
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<span class="kw">let </span><span class="kw-2">mut </span>fut_1 = Box::pin(fut_1); <span class="comment">// Pins the Future on the heap
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</span><span class="macro">pin_mut!</span>(fut_2); <span class="comment">// Pins the Future on the stack
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</span><span class="kw">let </span>res = <span class="macro">select! </span>{
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a_res = fut_1 => a_res,
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b_res = fut_2 => b_res,
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};
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<span class="macro">assert!</span>(res == <span class="number">1 </span>|| res == <span class="number">2</span>);</code></pre></div>
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<p><code>select</code> also accepts a <code>complete</code> branch and a <code>default</code> branch.
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<code>complete</code> will run if all futures and streams have already been
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exhausted. <code>default</code> will run if no futures or streams are
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immediately ready. <code>complete</code> takes priority over <code>default</code> in
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the case where all futures have completed.
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A motivating use-case for passing <code>Future</code>s by name as well as for
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<code>complete</code> blocks is to call <code>select!</code> in a loop, which is
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demonstrated in the following example:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>futures::future;
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<span class="kw">use </span>futures::select;
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<span class="kw">let </span><span class="kw-2">mut </span>a_fut = future::ready(<span class="number">4</span>);
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<span class="kw">let </span><span class="kw-2">mut </span>b_fut = future::ready(<span class="number">6</span>);
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<span class="kw">let </span><span class="kw-2">mut </span>total = <span class="number">0</span>;
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<span class="kw">loop </span>{
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<span class="macro">select! </span>{
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a = a_fut => total += a,
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b = b_fut => total += b,
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complete => <span class="kw">break</span>,
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default => <span class="macro">panic!</span>(), <span class="comment">// never runs (futures run first, then complete)
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</span>};
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}
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<span class="macro">assert_eq!</span>(total, <span class="number">10</span>);</code></pre></div>
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<p>Note that the futures that have been matched over can still be mutated
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from inside the <code>select!</code> block’s branches. This can be used to implement
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more complex behavior such as timer resets or writing into the head of
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a stream.</p>
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</div></details></section></div></main></body></html> |