284 lines
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284 lines
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HTML
<!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="Overview"><title>once_cell - Rust</title><script>if(window.location.protocol!=="file:")document.head.insertAdjacentHTML("beforeend","SourceSerif4-Regular-6b053e98.ttf.woff2,FiraSans-Italic-81dc35de.woff2,FiraSans-Regular-0fe48ade.woff2,FiraSans-MediumItalic-ccf7e434.woff2,FiraSans-Medium-e1aa3f0a.woff2,SourceCodePro-Regular-8badfe75.ttf.woff2,SourceCodePro-Semibold-aa29a496.ttf.woff2".split(",").map(f=>`<link rel="preload" as="font" type="font/woff2"href="../static.files/${f}">`).join(""))</script><link rel="stylesheet" href="../static.files/normalize-9960930a.css"><link rel="stylesheet" href="../static.files/rustdoc-ca0dd0c4.css"><meta name="rustdoc-vars" data-root-path="../" data-static-root-path="../static.files/" data-current-crate="once_cell" data-themes="" data-resource-suffix="" data-rustdoc-version="1.93.1 (01f6ddf75 2026-02-11) (Arch Linux rust 1:1.93.1-1)" data-channel="1.93.1" data-search-js="search-9e2438ea.js" data-stringdex-js="stringdex-a3946164.js" data-settings-js="settings-c38705f0.js" ><script src="../static.files/storage-e2aeef58.js"></script><script defer src="../crates.js"></script><script defer src="../static.files/main-a410ff4d.js"></script><noscript><link rel="stylesheet" href="../static.files/noscript-263c88ec.css"></noscript><link rel="alternate icon" type="image/png" href="../static.files/favicon-32x32-eab170b8.png"><link rel="icon" type="image/svg+xml" href="../static.files/favicon-044be391.svg"></head><body class="rustdoc mod crate"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><rustdoc-topbar><h2><a href="#">Crate once_cell</a></h2></rustdoc-topbar><nav class="sidebar"><div class="sidebar-crate"><h2><a href="../once_cell/index.html">once_<wbr>cell</a><span class="version">1.21.3</span></h2></div><div class="sidebar-elems"><ul class="block"><li><a id="all-types" href="all.html">All Items</a></li></ul><section id="rustdoc-toc"><h3><a href="#">Sections</a></h3><ul class="block top-toc"><li><a href="#overview" title="Overview">Overview</a></li><li><a href="#recipes" title="Recipes">Recipes</a><ul><li><a href="#safe-initialization-of-global-data" title="Safe Initialization of Global Data">Safe Initialization of Global Data</a></li><li><a href="#lazy-initialized-global-data" title="Lazy Initialized Global Data">Lazy Initialized Global Data</a></li><li><a href="#general-purpose-lazy-evaluation" title="General purpose lazy evaluation">General purpose lazy evaluation</a></li><li><a href="#lazily-compiled-regex" title="Lazily Compiled Regex">Lazily Compiled Regex</a></li><li><a href="#runtime-include_bytes" title="Runtime `include_bytes!`">Runtime <code>include_bytes!</code></a></li><li><a href="#lateinit" title="`lateinit`"><code>lateinit</code></a></li></ul></li><li><a href="#comparison-with-std" title="Comparison with std">Comparison with std</a></li><li><a href="#minimum-supported-rustc-version" title="Minimum Supported `rustc` Version">Minimum Supported <code>rustc</code> Version</a></li><li><a href="#implementation-details" title="Implementation details">Implementation details</a></li><li><a href="#faq" title="F.A.Q.">F.A.Q.</a></li><li><a href="#related-crates" title="Related crates">Related crates</a></li></ul><h3><a href="#modules">Crate Items</a></h3><ul class="block"><li><a href="#modules" title="Modules">Modules</a></li></ul></section><div id="rustdoc-modnav"></div></div></nav><div class="sidebar-resizer" title="Drag to resize sidebar"></div><main><div class="width-limiter"><section id="main-content" class="content"><div class="main-heading"><h1>Crate <span>once_<wbr>cell</span> <button id="copy-path" title="Copy item path to clipboard">Copy item path</button></h1><rustdoc-toolbar></rustdoc-toolbar><span class="sub-heading"><a class="src" href="../src/once_cell/lib.rs.html#1-1412">Source</a> </span></div><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><h2 id="overview"><a class="doc-anchor" href="#overview">§</a>Overview</h2>
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<p><code>once_cell</code> provides two new cell-like types, <a href="unsync/struct.OnceCell.html"><code>unsync::OnceCell</code></a> and
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<a href="sync/struct.OnceCell.html"><code>sync::OnceCell</code></a>. A <code>OnceCell</code> might store arbitrary non-<code>Copy</code> types, can
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be assigned to at most once and provides direct access to the stored
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contents. The core API looks <em>roughly</em> like this (and there’s much more
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inside, read on!):</p>
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<div class="example-wrap ignore"><a href="#" class="tooltip" title="This example is not tested">ⓘ</a><pre class="rust rust-example-rendered"><code><span class="kw">impl</span><T> OnceCell<T> {
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<span class="kw">const fn </span>new() -> OnceCell<T> { ... }
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<span class="kw">fn </span>set(<span class="kw-2">&</span><span class="self">self</span>, value: T) -> <span class="prelude-ty">Result</span><(), T> { ... }
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<span class="kw">fn </span>get(<span class="kw-2">&</span><span class="self">self</span>) -> <span class="prelude-ty">Option</span><<span class="kw-2">&</span>T> { ... }
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}</code></pre></div>
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<p>Note that, like with <a href="https://doc.rust-lang.org/std/cell/struct.RefCell.html"><code>RefCell</code></a> and <a href="https://doc.rust-lang.org/std/sync/struct.Mutex.html"><code>Mutex</code></a>, the <code>set</code> method requires
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only a shared reference. Because of the single assignment restriction <code>get</code>
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can return a <code>&T</code> instead of <code>Ref<T></code> or <code>MutexGuard<T></code>.</p>
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<p>The <code>sync</code> flavor is thread-safe (that is, implements the <a href="https://doc.rust-lang.org/std/marker/trait.Sync.html"><code>Sync</code></a> trait),
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while the <code>unsync</code> one is not.</p>
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<h2 id="recipes"><a class="doc-anchor" href="#recipes">§</a>Recipes</h2>
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<p><code>OnceCell</code> might be useful for a variety of patterns.</p>
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<h3 id="safe-initialization-of-global-data"><a class="doc-anchor" href="#safe-initialization-of-global-data">§</a>Safe Initialization of Global Data</h3>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>std::{env, io};
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<span class="kw">use </span>once_cell::sync::OnceCell;
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<span class="attr">#[derive(Debug)]
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</span><span class="kw">pub struct </span>Logger {
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<span class="comment">// ...
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</span>}
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<span class="kw">static </span>INSTANCE: OnceCell<Logger> = OnceCell::new();
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<span class="kw">impl </span>Logger {
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<span class="kw">pub fn </span>global() -> <span class="kw-2">&</span><span class="lifetime">'static </span>Logger {
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INSTANCE.get().expect(<span class="string">"logger is not initialized"</span>)
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}
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<span class="kw">fn </span>from_cli(args: env::Args) -> <span class="prelude-ty">Result</span><Logger, std::io::Error> {
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<span class="comment">// ...
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</span>}
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}
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<span class="kw">fn </span>main() {
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<span class="kw">let </span>logger = Logger::from_cli(env::args()).unwrap();
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INSTANCE.set(logger).unwrap();
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<span class="comment">// use `Logger::global()` from now on
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</span>}</code></pre></div><h3 id="lazy-initialized-global-data"><a class="doc-anchor" href="#lazy-initialized-global-data">§</a>Lazy Initialized Global Data</h3>
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<p>This is essentially the <code>lazy_static!</code> macro, but without a macro.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>std::{sync::Mutex, collections::HashMap};
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<span class="kw">use </span>once_cell::sync::OnceCell;
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<span class="kw">fn </span>global_data() -> <span class="kw-2">&</span><span class="lifetime">'static </span>Mutex<HashMap<i32, String>> {
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<span class="kw">static </span>INSTANCE: OnceCell<Mutex<HashMap<i32, String>>> = OnceCell::new();
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INSTANCE.get_or_init(|| {
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<span class="kw">let </span><span class="kw-2">mut </span>m = HashMap::new();
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m.insert(<span class="number">13</span>, <span class="string">"Spica"</span>.to_string());
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m.insert(<span class="number">74</span>, <span class="string">"Hoyten"</span>.to_string());
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Mutex::new(m)
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})
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}</code></pre></div>
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<p>There are also the <a href="sync/struct.Lazy.html"><code>sync::Lazy</code></a> and <a href="unsync/struct.Lazy.html"><code>unsync::Lazy</code></a> convenience types to
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streamline this pattern:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>std::{sync::Mutex, collections::HashMap};
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<span class="kw">use </span>once_cell::sync::Lazy;
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<span class="kw">static </span>GLOBAL_DATA: Lazy<Mutex<HashMap<i32, String>>> = Lazy::new(|| {
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<span class="kw">let </span><span class="kw-2">mut </span>m = HashMap::new();
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m.insert(<span class="number">13</span>, <span class="string">"Spica"</span>.to_string());
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m.insert(<span class="number">74</span>, <span class="string">"Hoyten"</span>.to_string());
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Mutex::new(m)
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});
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<span class="kw">fn </span>main() {
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<span class="macro">println!</span>(<span class="string">"{:?}"</span>, GLOBAL_DATA.lock().unwrap());
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}</code></pre></div>
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<p>Note that the variable that holds <code>Lazy</code> is declared as <code>static</code>, <em>not</em>
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<code>const</code>. This is important: using <code>const</code> instead compiles, but works wrong.</p>
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<h3 id="general-purpose-lazy-evaluation"><a class="doc-anchor" href="#general-purpose-lazy-evaluation">§</a>General purpose lazy evaluation</h3>
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<p>Unlike <code>lazy_static!</code>, <code>Lazy</code> works with local variables.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>once_cell::unsync::Lazy;
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<span class="kw">fn </span>main() {
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<span class="kw">let </span>ctx = <span class="macro">vec!</span>[<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>];
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<span class="kw">let </span>thunk = Lazy::new(|| {
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ctx.iter().sum::<i32>()
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});
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<span class="macro">assert_eq!</span>(<span class="kw-2">*</span>thunk, <span class="number">6</span>);
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}</code></pre></div>
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<p>If you need a lazy field in a struct, you probably should use <code>OnceCell</code>
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directly, because that will allow you to access <code>self</code> during
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initialization.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>std::{fs, path::PathBuf};
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<span class="kw">use </span>once_cell::unsync::OnceCell;
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<span class="kw">struct </span>Ctx {
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config_path: PathBuf,
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config: OnceCell<String>,
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}
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<span class="kw">impl </span>Ctx {
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<span class="kw">pub fn </span>get_config(<span class="kw-2">&</span><span class="self">self</span>) -> <span class="prelude-ty">Result</span><<span class="kw-2">&</span>str, std::io::Error> {
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<span class="kw">let </span>cfg = <span class="self">self</span>.config.get_or_try_init(|| {
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fs::read_to_string(<span class="kw-2">&</span><span class="self">self</span>.config_path)
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})<span class="question-mark">?</span>;
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<span class="prelude-val">Ok</span>(cfg.as_str())
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}
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}</code></pre></div><h3 id="lazily-compiled-regex"><a class="doc-anchor" href="#lazily-compiled-regex">§</a>Lazily Compiled Regex</h3>
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<p>This is a <code>regex!</code> macro which takes a string literal and returns an
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<em>expression</em> that evaluates to a <code>&'static Regex</code>:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="macro">macro_rules!</span> regex {
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(<span class="macro-nonterminal">$re</span>:literal $(,)<span class="question-mark">?</span>) => {{
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<span class="kw">static </span>RE: once_cell::sync::OnceCell<regex::Regex> = once_cell::sync::OnceCell::new();
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RE.get_or_init(|| regex::Regex::new(<span class="macro-nonterminal">$re</span>).unwrap())
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}};
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}</code></pre></div>
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<p>This macro can be useful to avoid the “compile regex on every loop
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iteration” problem.</p>
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<h3 id="runtime-include_bytes"><a class="doc-anchor" href="#runtime-include_bytes">§</a>Runtime <code>include_bytes!</code></h3>
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<p>The <code>include_bytes</code> macro is useful to include test resources, but it slows
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down test compilation a lot. An alternative is to load the resources at
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runtime:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>std::path::Path;
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<span class="kw">use </span>once_cell::sync::OnceCell;
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<span class="kw">pub struct </span>TestResource {
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path: <span class="kw-2">&</span><span class="lifetime">'static </span>str,
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cell: OnceCell<Vec<u8>>,
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}
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<span class="kw">impl </span>TestResource {
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<span class="kw">pub const fn </span>new(path: <span class="kw-2">&</span><span class="lifetime">'static </span>str) -> TestResource {
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TestResource { path, cell: OnceCell::new() }
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}
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<span class="kw">pub fn </span>bytes(<span class="kw-2">&</span><span class="self">self</span>) -> <span class="kw-2">&</span>[u8] {
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<span class="self">self</span>.cell.get_or_init(|| {
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<span class="kw">let </span>dir = std::env::var(<span class="string">"CARGO_MANIFEST_DIR"</span>).unwrap();
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<span class="kw">let </span>path = Path::new(dir.as_str()).join(<span class="self">self</span>.path);
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std::fs::read(<span class="kw-2">&</span>path).unwrap_or_else(|_err| {
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<span class="macro">panic!</span>(<span class="string">"failed to load test resource: {}"</span>, path.display())
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})
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}).as_slice()
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}
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}
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<span class="kw">static </span>TEST_IMAGE: TestResource = TestResource::new(<span class="string">"test_data/lena.png"</span>);
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<span class="attr">#[test]
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</span><span class="kw">fn </span>test_sobel_filter() {
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<span class="kw">let </span>rgb: <span class="kw-2">&</span>[u8] = TEST_IMAGE.bytes();
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<span class="comment">// ...
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</span>}</code></pre></div><h3 id="lateinit"><a class="doc-anchor" href="#lateinit">§</a><code>lateinit</code></h3>
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<p><code>LateInit</code> type for delayed initialization. It is reminiscent of Kotlin’s
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<code>lateinit</code> keyword and allows construction of cyclic data structures:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>once_cell::sync::OnceCell;
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<span class="kw">pub struct </span>LateInit<T> { cell: OnceCell<T> }
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<span class="kw">impl</span><T> LateInit<T> {
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<span class="kw">pub fn </span>init(<span class="kw-2">&</span><span class="self">self</span>, value: T) {
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<span class="macro">assert!</span>(<span class="self">self</span>.cell.set(value).is_ok())
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}
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}
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<span class="kw">impl</span><T> Default <span class="kw">for </span>LateInit<T> {
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<span class="kw">fn </span>default() -> <span class="self">Self </span>{ LateInit { cell: OnceCell::default() } }
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}
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<span class="kw">impl</span><T> std::ops::Deref <span class="kw">for </span>LateInit<T> {
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<span class="kw">type </span>Target = T;
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<span class="kw">fn </span>deref(<span class="kw-2">&</span><span class="self">self</span>) -> <span class="kw-2">&</span>T {
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<span class="self">self</span>.cell.get().unwrap()
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}
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}
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<span class="attr">#[derive(Default)]
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</span><span class="kw">struct </span>A<<span class="lifetime">'a</span>> {
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b: LateInit<<span class="kw-2">&</span><span class="lifetime">'a </span>B<<span class="lifetime">'a</span>>>,
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}
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<span class="attr">#[derive(Default)]
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</span><span class="kw">struct </span>B<<span class="lifetime">'a</span>> {
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a: LateInit<<span class="kw-2">&</span><span class="lifetime">'a </span>A<<span class="lifetime">'a</span>>>
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}
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<span class="kw">fn </span>build_cycle() {
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<span class="kw">let </span>a = A::default();
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<span class="kw">let </span>b = B::default();
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a.b.init(<span class="kw-2">&</span>b);
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b.a.init(<span class="kw-2">&</span>a);
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<span class="kw">let </span>_a = <span class="kw-2">&</span>a.b.a.b.a;
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}</code></pre></div><h2 id="comparison-with-std"><a class="doc-anchor" href="#comparison-with-std">§</a>Comparison with std</h2><div><table><thead><tr><th><code>!Sync</code> types</th><th>Access Mode</th><th>Drawbacks</th></tr></thead><tbody>
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<tr><td><code>Cell<T></code></td><td><code>T</code></td><td>requires <code>T: Copy</code> for <code>get</code></td></tr>
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<tr><td><code>RefCell<T></code></td><td><code>RefMut<T></code> / <code>Ref<T></code></td><td>may panic at runtime</td></tr>
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<tr><td><code>unsync::OnceCell<T></code></td><td><code>&T</code></td><td>assignable only once</td></tr>
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</tbody></table>
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</div><div><table><thead><tr><th><code>Sync</code> types</th><th>Access Mode</th><th>Drawbacks</th></tr></thead><tbody>
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<tr><td><code>AtomicT</code></td><td><code>T</code></td><td>works only with certain <code>Copy</code> types</td></tr>
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<tr><td><code>Mutex<T></code></td><td><code>MutexGuard<T></code></td><td>may deadlock at runtime, may block the thread</td></tr>
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<tr><td><code>sync::OnceCell<T></code></td><td><code>&T</code></td><td>assignable only once, may block the thread</td></tr>
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</tbody></table>
|
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</div>
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<p>Technically, calling <code>get_or_init</code> will also cause a panic or a deadlock if
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it recursively calls itself. However, because the assignment can happen only
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once, such cases should be more rare than equivalents with <code>RefCell</code> and
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<code>Mutex</code>.</p>
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<h2 id="minimum-supported-rustc-version"><a class="doc-anchor" href="#minimum-supported-rustc-version">§</a>Minimum Supported <code>rustc</code> Version</h2>
|
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<p>If only the <code>std</code>, <code>alloc</code>, or <code>race</code> features are enabled, MSRV will be
|
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updated conservatively, supporting at least latest 8 versions of the compiler.
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When using other features, like <code>parking_lot</code>, MSRV might be updated more
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frequently, up to the latest stable. In both cases, increasing MSRV is <em>not</em>
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considered a semver-breaking change and requires only a minor version bump.</p>
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<h2 id="implementation-details"><a class="doc-anchor" href="#implementation-details">§</a>Implementation details</h2>
|
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<p>The implementation is based on the
|
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<a href="https://github.com/rust-lang-nursery/lazy-static.rs/"><code>lazy_static</code></a> and
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<a href="https://github.com/indiv0/lazycell/"><code>lazy_cell</code></a> crates and
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<a href="https://doc.rust-lang.org/std/sync/struct.Once.html"><code>std::sync::Once</code></a>. In some sense, <code>once_cell</code> just streamlines and unifies
|
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those APIs.</p>
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<p>To implement a sync flavor of <code>OnceCell</code>, this crates uses either a custom
|
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re-implementation of <code>std::sync::Once</code> or <code>parking_lot::Mutex</code>. This is
|
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controlled by the <code>parking_lot</code> feature (disabled by default). Performance
|
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is the same for both cases, but the <code>parking_lot</code> based <code>OnceCell<T></code> is
|
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smaller by up to 16 bytes.</p>
|
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<p>This crate uses <code>unsafe</code>.</p>
|
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<h2 id="faq"><a class="doc-anchor" href="#faq">§</a>F.A.Q.</h2>
|
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<p><strong>Should I use the sync or unsync flavor?</strong></p>
|
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<p>Because Rust compiler checks thread safety for you, it’s impossible to
|
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accidentally use <code>unsync</code> where <code>sync</code> is required. So, use <code>unsync</code> in
|
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single-threaded code and <code>sync</code> in multi-threaded. It’s easy to switch
|
||
between the two if code becomes multi-threaded later.</p>
|
||
<p>At the moment, <code>unsync</code> has an additional benefit that reentrant
|
||
initialization causes a panic, which might be easier to debug than a
|
||
deadlock.</p>
|
||
<p><strong>Does this crate support async?</strong></p>
|
||
<p>No, but you can use
|
||
<a href="https://crates.io/crates/async_once_cell"><code>async_once_cell</code></a> instead.</p>
|
||
<p><strong>Does this crate support <code>no_std</code>?</strong></p>
|
||
<p>Yes, but with caveats. <code>OnceCell</code> is a synchronization primitive which
|
||
<em>semantically</em> relies on blocking. <code>OnceCell</code> guarantees that at most one
|
||
<code>f</code> will be called to compute the value. If two threads of execution call
|
||
<code>get_or_init</code> concurrently, one of them has to wait.</p>
|
||
<p>Waiting fundamentally requires OS support. Execution environment needs to
|
||
understand who waits on whom to prevent deadlocks due to priority inversion.
|
||
You <em>could</em> make code to compile by blindly using pure spinlocks, but the
|
||
runtime behavior would be subtly wrong.</p>
|
||
<p>Given these constraints, <code>once_cell</code> provides the following options:</p>
|
||
<ul>
|
||
<li>The <code>race</code> module provides similar, but distinct synchronization primitive
|
||
which is compatible with <code>no_std</code>. With <code>race</code>, the <code>f</code> function can be
|
||
called multiple times by different threads, but only one thread will win
|
||
to install the value.</li>
|
||
<li><code>critical-section</code> feature (with a <code>-</code>, not <code>_</code>) uses <code>critical_section</code>
|
||
to implement blocking.</li>
|
||
</ul>
|
||
<p><strong>Can I bring my own mutex?</strong></p>
|
||
<p>There is <a href="https://crates.io/crates/generic_once_cell">generic_once_cell</a> to
|
||
allow just that.</p>
|
||
<p><strong>Should I use <code>std::cell::OnceCell</code>, <code>once_cell</code>, or <code>lazy_static</code>?</strong></p>
|
||
<p>If you can use <code>std</code> version (your MSRV is at least 1.70, and you don’t need
|
||
extra features <code>once_cell</code> provides), use <code>std</code>. Otherwise, use <code>once_cell</code>.
|
||
Don’t use <code>lazy_static</code>.</p>
|
||
<h2 id="related-crates"><a class="doc-anchor" href="#related-crates">§</a>Related crates</h2>
|
||
<ul>
|
||
<li>Most of this crate’s functionality is available in <code>std</code> starting with
|
||
Rust 1.70. See <code>std::cell::OnceCell</code> and <code>std::sync::OnceLock</code>.</li>
|
||
<li><a href="https://github.com/niklasf/double-checked-cell">double-checked-cell</a></li>
|
||
<li><a href="https://crates.io/crates/lazy-init">lazy-init</a></li>
|
||
<li><a href="https://crates.io/crates/lazycell">lazycell</a></li>
|
||
<li><a href="https://crates.io/crates/mitochondria">mitochondria</a></li>
|
||
<li><a href="https://crates.io/crates/lazy_static">lazy_static</a></li>
|
||
<li><a href="https://crates.io/crates/async_once_cell">async_once_cell</a></li>
|
||
<li><a href="https://crates.io/crates/generic_once_cell">generic_once_cell</a> (bring
|
||
your own mutex)</li>
|
||
</ul>
|
||
</div></details><h2 id="modules" class="section-header">Modules<a href="#modules" class="anchor">§</a></h2><dl class="item-table"><dt><a class="mod" href="race/index.html" title="mod once_cell::race">race</a></dt><dd>Thread-safe, non-blocking, “first one wins” flavor of <code>OnceCell</code>.</dd><dt><a class="mod" href="sync/index.html" title="mod once_cell::sync">sync</a></dt><dd>Thread-safe, blocking version of <code>OnceCell</code>.</dd><dt><a class="mod" href="unsync/index.html" title="mod once_cell::unsync">unsync</a></dt><dd>Single-threaded version of <code>OnceCell</code>.</dd></dl></section></div></main></body></html> |