| <!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"><meta name="keywords" content="rust, rustlang, rust-lang, once_cell"><title>once_cell - Rust</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceSerif4-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../FiraSans-Regular.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../FiraSans-Medium.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceCodePro-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceSerif4-Bold.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../SourceCodePro-Semibold.ttf.woff2"><link rel="stylesheet" href="../normalize.css"><link rel="stylesheet" href="../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" href="../ayu.css" disabled><link rel="stylesheet" href="../dark.css" disabled><link rel="stylesheet" href="../light.css" id="themeStyle"><script id="default-settings" ></script><script src="../storage.js"></script><script defer src="../crates.js"></script><script defer src="../main.js"></script><noscript><link rel="stylesheet" href="../noscript.css"></noscript><link rel="alternate icon" type="image/png" href="../favicon-16x16.png"><link rel="alternate icon" type="image/png" href="../favicon-32x32.png"><link rel="icon" type="image/svg+xml" href="../favicon.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]--><nav class="mobile-topbar"><button class="sidebar-menu-toggle">☰</button><a class="sidebar-logo" href="../once_cell/index.html"><div class="logo-container"><img class="rust-logo" src="../rust-logo.svg" alt="logo"></div></a><h2></h2></nav><nav class="sidebar"><a class="sidebar-logo" href="../once_cell/index.html"><div class="logo-container"><img class="rust-logo" src="../rust-logo.svg" alt="logo"></div></a><h2 class="location"><a href="#">Crate once_cell</a></h2><div class="sidebar-elems"><ul class="block"><li class="version">Version 1.18.0</li><li><a id="all-types" href="all.html">All Items</a></li></ul><section><ul class="block"><li><a href="#modules">Modules</a></li></ul></section></div></nav><main><div class="width-limiter"><nav class="sub"><form class="search-form"><div class="search-container"><span></span><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" title="help" tabindex="-1"><a href="../help.html">?</a></div><div id="settings-menu" tabindex="-1"><a href="../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../wheel.svg"></a></div></div></form></nav><section id="main-content" class="content"><div class="main-heading"><h1 class="fqn">Crate <a class="mod" href="#">once_cell</a><button id="copy-path" onclick="copy_path(this)" title="Copy item path to clipboard"><img src="../clipboard.svg" width="19" height="18" alt="Copy item path"></button></h1><span class="out-of-band"><a class="srclink" href="../src/once_cell/lib.rs.html#1-1414">source</a> · <a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class="inner">−</span>]</a></span></div><details class="rustdoc-toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><h2 id="overview"><a href="#overview">Overview</a></h2> |
| <p><code>once_cell</code> provides two new cell-like types, <a href="unsync/struct.OnceCell.html"><code>unsync::OnceCell</code></a> and |
| <a href="sync/struct.OnceCell.html"><code>sync::OnceCell</code></a>. A <code>OnceCell</code> might store arbitrary non-<code>Copy</code> types, can |
| be assigned to at most once and provides direct access to the stored |
| contents. The core API looks <em>roughly</em> like this (and there’s much more |
| inside, read on!):</p> |
| |
| <div class="example-wrap ignore"><div class='tooltip'>ⓘ</div><pre class="rust rust-example-rendered"><code><span class="kw">impl</span><T> OnceCell<T> { |
| <span class="kw">const fn </span>new() -> OnceCell<T> { ... } |
| <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> { ... } |
| <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> { ... } |
| }</code></pre></div> |
| <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 |
| only a shared reference. Because of the single assignment restriction <code>get</code> |
| can return a <code>&T</code> instead of <code>Ref<T></code> or <code>MutexGuard<T></code>.</p> |
| <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), |
| while the <code>unsync</code> one is not.</p> |
| <h2 id="recipes"><a href="#recipes">Recipes</a></h2> |
| <p><code>OnceCell</code> might be useful for a variety of patterns.</p> |
| <h3 id="safe-initialization-of-global-data"><a href="#safe-initialization-of-global-data">Safe Initialization of Global Data</a></h3> |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>std::{env, io}; |
| |
| <span class="kw">use </span>once_cell::sync::OnceCell; |
| |
| <span class="attribute">#[derive(Debug)] |
| </span><span class="kw">pub struct </span>Logger { |
| <span class="comment">// ... |
| </span>} |
| <span class="kw">static </span>INSTANCE: OnceCell<Logger> = OnceCell::new(); |
| |
| <span class="kw">impl </span>Logger { |
| <span class="kw">pub fn </span>global() -> <span class="kw-2">&</span><span class="lifetime">'static </span>Logger { |
| INSTANCE.get().expect(<span class="string">"logger is not initialized"</span>) |
| } |
| |
| <span class="kw">fn </span>from_cli(args: env::Args) -> <span class="prelude-ty">Result</span><Logger, std::io::Error> { |
| <span class="comment">// ... |
| </span>} |
| } |
| |
| <span class="kw">fn </span>main() { |
| <span class="kw">let </span>logger = Logger::from_cli(env::args()).unwrap(); |
| INSTANCE.set(logger).unwrap(); |
| <span class="comment">// use `Logger::global()` from now on |
| </span>}</code></pre></div> |
| <h3 id="lazy-initialized-global-data"><a href="#lazy-initialized-global-data">Lazy Initialized Global Data</a></h3> |
| <p>This is essentially the <code>lazy_static!</code> macro, but without a macro.</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>std::{sync::Mutex, collections::HashMap}; |
| |
| <span class="kw">use </span>once_cell::sync::OnceCell; |
| |
| <span class="kw">fn </span>global_data() -> <span class="kw-2">&</span><span class="lifetime">'static </span>Mutex<HashMap<i32, String>> { |
| <span class="kw">static </span>INSTANCE: OnceCell<Mutex<HashMap<i32, String>>> = OnceCell::new(); |
| INSTANCE.get_or_init(|| { |
| <span class="kw">let </span><span class="kw-2">mut </span>m = HashMap::new(); |
| m.insert(<span class="number">13</span>, <span class="string">"Spica"</span>.to_string()); |
| m.insert(<span class="number">74</span>, <span class="string">"Hoyten"</span>.to_string()); |
| Mutex::new(m) |
| }) |
| }</code></pre></div> |
| <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 |
| streamline this pattern:</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>std::{sync::Mutex, collections::HashMap}; |
| <span class="kw">use </span>once_cell::sync::Lazy; |
| |
| <span class="kw">static </span>GLOBAL_DATA: Lazy<Mutex<HashMap<i32, String>>> = Lazy::new(|| { |
| <span class="kw">let </span><span class="kw-2">mut </span>m = HashMap::new(); |
| m.insert(<span class="number">13</span>, <span class="string">"Spica"</span>.to_string()); |
| m.insert(<span class="number">74</span>, <span class="string">"Hoyten"</span>.to_string()); |
| Mutex::new(m) |
| }); |
| |
| <span class="kw">fn </span>main() { |
| <span class="macro">println!</span>(<span class="string">"{:?}"</span>, GLOBAL_DATA.lock().unwrap()); |
| }</code></pre></div> |
| <p>Note that the variable that holds <code>Lazy</code> is declared as <code>static</code>, <em>not</em> |
| <code>const</code>. This is important: using <code>const</code> instead compiles, but works wrong.</p> |
| <h3 id="general-purpose-lazy-evaluation"><a href="#general-purpose-lazy-evaluation">General purpose lazy evaluation</a></h3> |
| <p>Unlike <code>lazy_static!</code>, <code>Lazy</code> works with local variables.</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>once_cell::unsync::Lazy; |
| |
| <span class="kw">fn </span>main() { |
| <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>]; |
| <span class="kw">let </span>thunk = Lazy::new(|| { |
| ctx.iter().sum::<i32>() |
| }); |
| <span class="macro">assert_eq!</span>(<span class="kw-2">*</span>thunk, <span class="number">6</span>); |
| }</code></pre></div> |
| <p>If you need a lazy field in a struct, you probably should use <code>OnceCell</code> |
| directly, because that will allow you to access <code>self</code> during |
| initialization.</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>std::{fs, path::PathBuf}; |
| |
| <span class="kw">use </span>once_cell::unsync::OnceCell; |
| |
| <span class="kw">struct </span>Ctx { |
| config_path: PathBuf, |
| config: OnceCell<String>, |
| } |
| |
| <span class="kw">impl </span>Ctx { |
| <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> { |
| <span class="kw">let </span>cfg = <span class="self">self</span>.config.get_or_try_init(|| { |
| fs::read_to_string(<span class="kw-2">&</span><span class="self">self</span>.config_path) |
| })<span class="question-mark">?</span>; |
| <span class="prelude-val">Ok</span>(cfg.as_str()) |
| } |
| }</code></pre></div> |
| <h3 id="lazily-compiled-regex"><a href="#lazily-compiled-regex">Lazily Compiled Regex</a></h3> |
| <p>This is a <code>regex!</code> macro which takes a string literal and returns an |
| <em>expression</em> that evaluates to a <code>&'static Regex</code>:</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="macro">macro_rules! </span>regex { |
| (<span class="macro-nonterminal">$re</span>:literal $(,)<span class="question-mark">?</span>) => {{ |
| <span class="kw">static </span>RE: once_cell::sync::OnceCell<regex::Regex> = once_cell::sync::OnceCell::new(); |
| RE.get_or_init(|| regex::Regex::new(<span class="macro-nonterminal">$re</span>).unwrap()) |
| }}; |
| }</code></pre></div> |
| <p>This macro can be useful to avoid the “compile regex on every loop |
| iteration” problem.</p> |
| <h3 id="runtime-include_bytes"><a href="#runtime-include_bytes">Runtime <code>include_bytes!</code></a></h3> |
| <p>The <code>include_bytes</code> macro is useful to include test resources, but it slows |
| down test compilation a lot. An alternative is to load the resources at |
| runtime:</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>std::path::Path; |
| |
| <span class="kw">use </span>once_cell::sync::OnceCell; |
| |
| <span class="kw">pub struct </span>TestResource { |
| path: <span class="kw-2">&</span><span class="lifetime">'static </span>str, |
| cell: OnceCell<Vec<u8>>, |
| } |
| |
| <span class="kw">impl </span>TestResource { |
| <span class="kw">pub const fn </span>new(path: <span class="kw-2">&</span><span class="lifetime">'static </span>str) -> TestResource { |
| TestResource { path, cell: OnceCell::new() } |
| } |
| <span class="kw">pub fn </span>bytes(<span class="kw-2">&</span><span class="self">self</span>) -> <span class="kw-2">&</span>[u8] { |
| <span class="self">self</span>.cell.get_or_init(|| { |
| <span class="kw">let </span>dir = std::env::var(<span class="string">"CARGO_MANIFEST_DIR"</span>).unwrap(); |
| <span class="kw">let </span>path = Path::new(dir.as_str()).join(<span class="self">self</span>.path); |
| std::fs::read(<span class="kw-2">&</span>path).unwrap_or_else(|_err| { |
| <span class="macro">panic!</span>(<span class="string">"failed to load test resource: {}"</span>, path.display()) |
| }) |
| }).as_slice() |
| } |
| } |
| |
| <span class="kw">static </span>TEST_IMAGE: TestResource = TestResource::new(<span class="string">"test_data/lena.png"</span>); |
| |
| <span class="attribute">#[test] |
| </span><span class="kw">fn </span>test_sobel_filter() { |
| <span class="kw">let </span>rgb: <span class="kw-2">&</span>[u8] = TEST_IMAGE.bytes(); |
| <span class="comment">// ... |
| </span>}</code></pre></div> |
| <h3 id="lateinit"><a href="#lateinit"><code>lateinit</code></a></h3> |
| <p><code>LateInit</code> type for delayed initialization. It is reminiscent of Kotlin’s |
| <code>lateinit</code> keyword and allows construction of cyclic data structures:</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">use </span>once_cell::sync::OnceCell; |
| |
| <span class="kw">pub struct </span>LateInit<T> { cell: OnceCell<T> } |
| |
| <span class="kw">impl</span><T> LateInit<T> { |
| <span class="kw">pub fn </span>init(<span class="kw-2">&</span><span class="self">self</span>, value: T) { |
| <span class="macro">assert!</span>(<span class="self">self</span>.cell.set(value).is_ok()) |
| } |
| } |
| |
| <span class="kw">impl</span><T> Default <span class="kw">for </span>LateInit<T> { |
| <span class="kw">fn </span>default() -> <span class="self">Self </span>{ LateInit { cell: OnceCell::default() } } |
| } |
| |
| <span class="kw">impl</span><T> std::ops::Deref <span class="kw">for </span>LateInit<T> { |
| <span class="kw">type </span>Target = T; |
| <span class="kw">fn </span>deref(<span class="kw-2">&</span><span class="self">self</span>) -> <span class="kw-2">&</span>T { |
| <span class="self">self</span>.cell.get().unwrap() |
| } |
| } |
| |
| <span class="attribute">#[derive(Default)] |
| </span><span class="kw">struct </span>A<<span class="lifetime">'a</span>> { |
| b: LateInit<<span class="kw-2">&</span><span class="lifetime">'a </span>B<<span class="lifetime">'a</span>>>, |
| } |
| |
| <span class="attribute">#[derive(Default)] |
| </span><span class="kw">struct </span>B<<span class="lifetime">'a</span>> { |
| a: LateInit<<span class="kw-2">&</span><span class="lifetime">'a </span>A<<span class="lifetime">'a</span>>> |
| } |
| |
| |
| <span class="kw">fn </span>build_cycle() { |
| <span class="kw">let </span>a = A::default(); |
| <span class="kw">let </span>b = B::default(); |
| a.b.init(<span class="kw-2">&</span>b); |
| b.a.init(<span class="kw-2">&</span>a); |
| |
| <span class="kw">let </span>_a = <span class="kw-2">&</span>a.b.a.b.a; |
| }</code></pre></div> |
| <h2 id="comparison-with-std"><a href="#comparison-with-std">Comparison with std</a></h2><div><table><thead><tr><th><code>!Sync</code> types</th><th>Access Mode</th><th>Drawbacks</th></tr></thead><tbody> |
| <tr><td><code>Cell<T></code></td><td><code>T</code></td><td>requires <code>T: Copy</code> for <code>get</code></td></tr> |
| <tr><td><code>RefCell<T></code></td><td><code>RefMut<T></code> / <code>Ref<T></code></td><td>may panic at runtime</td></tr> |
| <tr><td><code>unsync::OnceCell<T></code></td><td><code>&T</code></td><td>assignable only once</td></tr> |
| </tbody></table> |
| </div><div><table><thead><tr><th><code>Sync</code> types</th><th>Access Mode</th><th>Drawbacks</th></tr></thead><tbody> |
| <tr><td><code>AtomicT</code></td><td><code>T</code></td><td>works only with certain <code>Copy</code> types</td></tr> |
| <tr><td><code>Mutex<T></code></td><td><code>MutexGuard<T></code></td><td>may deadlock at runtime, may block the thread</td></tr> |
| <tr><td><code>sync::OnceCell<T></code></td><td><code>&T</code></td><td>assignable only once, may block the thread</td></tr> |
| </tbody></table> |
| </div> |
| <p>Technically, calling <code>get_or_init</code> will also cause a panic or a deadlock if |
| it recursively calls itself. However, because the assignment can happen only |
| once, such cases should be more rare than equivalents with <code>RefCell</code> and |
| <code>Mutex</code>.</p> |
| <h2 id="minimum-supported-rustc-version"><a href="#minimum-supported-rustc-version">Minimum Supported <code>rustc</code> Version</a></h2> |
| <p>This crate’s minimum supported <code>rustc</code> version is <code>1.56.0</code>.</p> |
| <p>If only the <code>std</code> feature is enabled, MSRV will be updated conservatively, |
| supporting at least latest 8 versions of the compiler. When using other |
| features, like <code>parking_lot</code>, MSRV might be updated more frequently, up to |
| the latest stable. In both cases, increasing MSRV is <em>not</em> considered a |
| semver-breaking change.</p> |
| <h2 id="implementation-details"><a href="#implementation-details">Implementation details</a></h2> |
| <p>The implementation is based on the |
| <a href="https://github.com/rust-lang-nursery/lazy-static.rs/"><code>lazy_static</code></a> and |
| <a href="https://github.com/indiv0/lazycell/"><code>lazy_cell</code></a> crates and |
| <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 |
| those APIs.</p> |
| <p>To implement a sync flavor of <code>OnceCell</code>, this crates uses either a custom |
| re-implementation of <code>std::sync::Once</code> or <code>parking_lot::Mutex</code>. This is |
| controlled by the <code>parking_lot</code> feature (disabled by default). Performance |
| is the same for both cases, but the <code>parking_lot</code> based <code>OnceCell<T></code> is |
| smaller by up to 16 bytes.</p> |
| <p>This crate uses <code>unsafe</code>.</p> |
| <h2 id="faq"><a href="#faq">F.A.Q.</a></h2> |
| <p><strong>Should I use the sync or unsync flavor?</strong></p> |
| <p>Because Rust compiler checks thread safety for you, it’s impossible to |
| accidentally use <code>unsync</code> where <code>sync</code> is required. So, use <code>unsync</code> in |
| 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 href="#related-crates">Related crates</a></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="small-section-header"><a href="#modules">Modules</a></h2><div class="item-table"><div class="item-row"><div class="item-left module-item"><a class="mod" href="race/index.html" title="once_cell::race mod">race</a></div><div class="item-right docblock-short">Thread-safe, non-blocking, “first one wins” flavor of <code>OnceCell</code>.</div></div><div class="item-row"><div class="item-left module-item"><a class="mod" href="sync/index.html" title="once_cell::sync mod">sync</a></div><div class="item-right docblock-short">Thread-safe, blocking version of <code>OnceCell</code>.</div></div><div class="item-row"><div class="item-left module-item"><a class="mod" href="unsync/index.html" title="once_cell::unsync mod">unsync</a></div><div class="item-right docblock-short">Single-threaded version of <code>OnceCell</code>.</div></div></div></section></div></main><div id="rustdoc-vars" data-root-path="../" data-current-crate="once_cell" data-themes="ayu,dark,light" data-resource-suffix="" data-rustdoc-version="1.66.0-nightly (5c8bff74b 2022-10-21)" ></div></body></html> |
