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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="API documentation for the Rust `once_cell` crate."><meta name="keywords" content="rust, rustlang, rust-lang, once_cell"><title>once_cell - Rust</title><link rel="stylesheet" type="text/css" href="../normalize.css"><link rel="stylesheet" type="text/css" href="../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" type="text/css" href="../light.css" id="themeStyle"><link rel="stylesheet" type="text/css" href="../dark.css" disabled ><link rel="stylesheet" type="text/css" href="../ayu.css" disabled ><script id="default-settings"></script><script src="../storage.js"></script><noscript><link rel="stylesheet" href="../noscript.css"></noscript><link rel="icon" type="image/svg+xml" href="../favicon.svg">
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<link rel="alternate icon" type="image/png" href="../favicon-32x32.png"><style type="text/css">#crate-search{background-image:url("../down-arrow.svg");}</style></head><body class="rustdoc mod"><!--[if lte IE 8]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><div class="sidebar-menu">☰</div><a href='../once_cell/index.html'><div class='logo-container rust-logo'><img src='../rust-logo.png' alt='logo'></div></a><p class="location">Crate once_cell</p><div class="block version"><p>Version 1.7.2</p></div><div class="sidebar-elems"><a id="all-types" href="all.html"><p>See all once_cell's items</p></a><div class="block items"><ul><li><a href="#modules">Modules</a></li></ul></div><p class="location"></p><div id="sidebar-vars" data-name="once_cell" data-ty="mod" data-relpath="../"></div></div></nav><div class="theme-picker"><button id="theme-picker" aria-label="Pick another theme!" aria-haspopup="menu"><img src="../brush.svg" width="18" alt="Pick another theme!"></button><div id="theme-choices" role="menu"></div></div><script src="../theme.js"></script><nav class="sub"><form class="search-form"><div class="search-container"><div><select id="crate-search"><option value="All crates">All crates</option></select><input class="search-input" name="search" disabled autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"></div><button type="button" class="help-button">?</button>
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<a id="settings-menu" href="../settings.html"><img src="../wheel.svg" width="18" alt="Change settings"></a></div></form></nav><section id="main" class="content"><h1 class="fqn"><span class="in-band">Crate <a class="mod" href="">once_cell</a></span><span class="out-of-band"><span id="render-detail"><a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class="inner">−</span>]</a></span><a class="srclink" href="../src/once_cell/lib.rs.html#1-1080" title="goto source code">[src]</a></span></h1><div class="docblock"><h1 id="overview" class="section-header"><a href="#overview">Overview</a></h1>
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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 <a href="sync/struct.OnceCell.html"><code>sync::OnceCell</code></a>. A <code>OnceCell</code>
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might store arbitrary non-<code>Copy</code> types, can be assigned to at most once and provides direct access
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to the stored contents. The core API looks <em>roughly</em> like this (and there's much more inside, read on!):</p>
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<div class='information'><div class='tooltip ignore'>ⓘ</div></div><div class="example-wrap"><pre class="rust rust-example-rendered ignore">
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<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="ident">OnceCell</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
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<span class="kw">fn</span> <span class="ident">new</span>() <span class="op">-</span><span class="op">></span> <span class="ident">OnceCell</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> { ... }
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<span class="kw">fn</span> <span class="ident">set</span>(<span class="kw-2">&</span><span class="self">self</span>, <span class="ident">value</span>: <span class="ident">T</span>) <span class="op">-</span><span class="op">></span> <span class="prelude-ty">Result</span><span class="op"><</span>(), <span class="ident">T</span><span class="op">></span> { ... }
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<span class="kw">fn</span> <span class="ident">get</span>(<span class="kw-2">&</span><span class="self">self</span>) <span class="op">-</span><span class="op">></span> <span class="prelude-ty">Option</span><span class="op"><</span><span class="kw-2">&</span><span class="ident">T</span><span class="op">></span> { ... }
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}</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 only a shared reference.
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Because of the single assignment restriction <code>get</code> can return a <code>&T</code> instead of <code>Ref<T></code>
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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), while the <code>unsync</code> one is not.</p>
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<h1 id="patterns" class="section-header"><a href="#patterns">Patterns</a></h1>
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<p><code>OnceCell</code> might be useful for a variety of patterns.</p>
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<h2 id="safe-initialization-of-global-data" class="section-header"><a href="#safe-initialization-of-global-data">Safe Initialization of global data</a></h2>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">use</span> <span class="ident">std</span>::{<span class="ident">env</span>, <span class="ident">io</span>};
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<span class="kw">use</span> <span class="ident">once_cell</span>::<span class="ident">sync</span>::<span class="ident">OnceCell</span>;
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<span class="attribute">#[<span class="ident">derive</span>(<span class="ident">Debug</span>)]</span>
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<span class="kw">pub</span> <span class="kw">struct</span> <span class="ident">Logger</span> {
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<span class="comment">// ...</span>
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}
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<span class="kw">static</span> <span class="ident">INSTANCE</span>: <span class="ident">OnceCell</span><span class="op"><</span><span class="ident">Logger</span><span class="op">></span> <span class="op">=</span> <span class="ident">OnceCell</span>::<span class="ident">new</span>();
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<span class="kw">impl</span> <span class="ident">Logger</span> {
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<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">global</span>() <span class="op">-</span><span class="op">></span> <span class="kw-2">&</span><span class="lifetime">'static</span> <span class="ident">Logger</span> {
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<span class="ident">INSTANCE</span>.<span class="ident">get</span>().<span class="ident">expect</span>(<span class="string">"logger is not initialized"</span>)
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}
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<span class="kw">fn</span> <span class="ident">from_cli</span>(<span class="ident">args</span>: <span class="ident">env</span>::<span class="ident">Args</span>) <span class="op">-</span><span class="op">></span> <span class="prelude-ty">Result</span><span class="op"><</span><span class="ident">Logger</span>, <span class="ident">std</span>::<span class="ident">io</span>::<span class="ident">Error</span><span class="op">></span> {
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<span class="comment">// ...</span>
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}
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}
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<span class="kw">fn</span> <span class="ident">main</span>() {
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<span class="kw">let</span> <span class="ident">logger</span> <span class="op">=</span> <span class="ident">Logger</span>::<span class="ident">from_cli</span>(<span class="ident">env</span>::<span class="ident">args</span>()).<span class="ident">unwrap</span>();
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<span class="ident">INSTANCE</span>.<span class="ident">set</span>(<span class="ident">logger</span>).<span class="ident">unwrap</span>();
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<span class="comment">// use `Logger::global()` from now on</span>
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}</pre></div>
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<h2 id="lazy-initialized-global-data" class="section-header"><a href="#lazy-initialized-global-data">Lazy initialized global data</a></h2>
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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">
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<span class="kw">use</span> <span class="ident">std</span>::{<span class="ident">sync</span>::<span class="ident">Mutex</span>, <span class="ident">collections</span>::<span class="ident">HashMap</span>};
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<span class="kw">use</span> <span class="ident">once_cell</span>::<span class="ident">sync</span>::<span class="ident">OnceCell</span>;
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<span class="kw">fn</span> <span class="ident">global_data</span>() <span class="op">-</span><span class="op">></span> <span class="kw-2">&</span><span class="lifetime">'static</span> <span class="ident">Mutex</span><span class="op"><</span><span class="ident">HashMap</span><span class="op"><</span><span class="ident">i32</span>, <span class="ident">String</span><span class="op">></span><span class="op">></span> {
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<span class="kw">static</span> <span class="ident">INSTANCE</span>: <span class="ident">OnceCell</span><span class="op"><</span><span class="ident">Mutex</span><span class="op"><</span><span class="ident">HashMap</span><span class="op"><</span><span class="ident">i32</span>, <span class="ident">String</span><span class="op">></span><span class="op">></span><span class="op">></span> <span class="op">=</span> <span class="ident">OnceCell</span>::<span class="ident">new</span>();
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<span class="ident">INSTANCE</span>.<span class="ident">get_or_init</span>(<span class="op">|</span><span class="op">|</span> {
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<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">m</span> <span class="op">=</span> <span class="ident">HashMap</span>::<span class="ident">new</span>();
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<span class="ident">m</span>.<span class="ident">insert</span>(<span class="number">13</span>, <span class="string">"Spica"</span>.<span class="ident">to_string</span>());
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<span class="ident">m</span>.<span class="ident">insert</span>(<span class="number">74</span>, <span class="string">"Hoyten"</span>.<span class="ident">to_string</span>());
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<span class="ident">Mutex</span>::<span class="ident">new</span>(<span class="ident">m</span>)
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})
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}</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 streamline this pattern:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">use</span> <span class="ident">std</span>::{<span class="ident">sync</span>::<span class="ident">Mutex</span>, <span class="ident">collections</span>::<span class="ident">HashMap</span>};
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<span class="kw">use</span> <span class="ident">once_cell</span>::<span class="ident">sync</span>::<span class="ident">Lazy</span>;
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<span class="kw">static</span> <span class="ident">GLOBAL_DATA</span>: <span class="ident">Lazy</span><span class="op"><</span><span class="ident">Mutex</span><span class="op"><</span><span class="ident">HashMap</span><span class="op"><</span><span class="ident">i32</span>, <span class="ident">String</span><span class="op">></span><span class="op">></span><span class="op">></span> <span class="op">=</span> <span class="ident">Lazy</span>::<span class="ident">new</span>(<span class="op">|</span><span class="op">|</span> {
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<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">m</span> <span class="op">=</span> <span class="ident">HashMap</span>::<span class="ident">new</span>();
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<span class="ident">m</span>.<span class="ident">insert</span>(<span class="number">13</span>, <span class="string">"Spica"</span>.<span class="ident">to_string</span>());
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<span class="ident">m</span>.<span class="ident">insert</span>(<span class="number">74</span>, <span class="string">"Hoyten"</span>.<span class="ident">to_string</span>());
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<span class="ident">Mutex</span>::<span class="ident">new</span>(<span class="ident">m</span>)
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});
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<span class="kw">fn</span> <span class="ident">main</span>() {
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<span class="macro">println</span><span class="macro">!</span>(<span class="string">"{:?}"</span>, <span class="ident">GLOBAL_DATA</span>.<span class="ident">lock</span>().<span class="ident">unwrap</span>());
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}</pre></div>
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<h2 id="general-purpose-lazy-evaluation" class="section-header"><a href="#general-purpose-lazy-evaluation">General purpose lazy evaluation</a></h2>
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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">
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<span class="kw">use</span> <span class="ident">once_cell</span>::<span class="ident">unsync</span>::<span class="ident">Lazy</span>;
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<span class="kw">fn</span> <span class="ident">main</span>() {
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<span class="kw">let</span> <span class="ident">ctx</span> <span class="op">=</span> <span class="macro">vec</span><span class="macro">!</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> <span class="ident">thunk</span> <span class="op">=</span> <span class="ident">Lazy</span>::<span class="ident">new</span>(<span class="op">|</span><span class="op">|</span> {
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<span class="ident">ctx</span>.<span class="ident">iter</span>().<span class="ident">sum</span>::<span class="op"><</span><span class="ident">i32</span><span class="op">></span>()
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});
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<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="kw-2">*</span><span class="ident">thunk</span>, <span class="number">6</span>);
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}</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 initialization.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">use</span> <span class="ident">std</span>::{<span class="ident">fs</span>, <span class="ident">path</span>::<span class="ident">PathBuf</span>};
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<span class="kw">use</span> <span class="ident">once_cell</span>::<span class="ident">unsync</span>::<span class="ident">OnceCell</span>;
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<span class="kw">struct</span> <span class="ident">Ctx</span> {
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<span class="ident">config_path</span>: <span class="ident">PathBuf</span>,
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<span class="ident">config</span>: <span class="ident">OnceCell</span><span class="op"><</span><span class="ident">String</span><span class="op">></span>,
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}
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<span class="kw">impl</span> <span class="ident">Ctx</span> {
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<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">get_config</span>(<span class="kw-2">&</span><span class="self">self</span>) <span class="op">-</span><span class="op">></span> <span class="prelude-ty">Result</span><span class="op"><</span><span class="kw-2">&</span><span class="ident">str</span>, <span class="ident">std</span>::<span class="ident">io</span>::<span class="ident">Error</span><span class="op">></span> {
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<span class="kw">let</span> <span class="ident">cfg</span> <span class="op">=</span> <span class="self">self</span>.<span class="ident">config</span>.<span class="ident">get_or_try_init</span>(<span class="op">|</span><span class="op">|</span> {
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<span class="ident">fs</span>::<span class="ident">read_to_string</span>(<span class="kw-2">&</span><span class="self">self</span>.<span class="ident">config_path</span>)
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})<span class="question-mark">?</span>;
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<span class="prelude-val">Ok</span>(<span class="ident">cfg</span>.<span class="ident">as_str</span>())
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}
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}</pre></div>
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<h2 id="building-block" class="section-header"><a href="#building-block">Building block</a></h2>
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<p>Naturally, it is possible to build other abstractions on top of <code>OnceCell</code>.
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For example, 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">
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<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">regex</span> {
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(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">re</span>:<span class="ident">literal</span> $(,)<span class="question-mark">?</span>) <span class="op">=</span><span class="op">></span> {{
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<span class="kw">static</span> <span class="ident">RE</span>: <span class="ident">once_cell</span>::<span class="ident">sync</span>::<span class="ident">OnceCell</span><span class="op"><</span><span class="ident">regex</span>::<span class="ident">Regex</span><span class="op">></span> <span class="op">=</span> <span class="ident">once_cell</span>::<span class="ident">sync</span>::<span class="ident">OnceCell</span>::<span class="ident">new</span>();
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<span class="ident">RE</span>.<span class="ident">get_or_init</span>(<span class="op">|</span><span class="op">|</span> <span class="ident">regex</span>::<span class="ident">Regex</span>::<span class="ident">new</span>(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">re</span>).<span class="ident">unwrap</span>())
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}};
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}</pre></div>
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<p>This macro can be useful to avoid the "compile regex on every loop iteration" problem.</p>
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<p>Another pattern would be a <code>LateInit</code> type for delayed initialization:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">use</span> <span class="ident">once_cell</span>::<span class="ident">sync</span>::<span class="ident">OnceCell</span>;
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<span class="attribute">#[<span class="ident">derive</span>(<span class="ident">Debug</span>)]</span>
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<span class="kw">pub</span> <span class="kw">struct</span> <span class="ident">LateInit</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> { <span class="ident">cell</span>: <span class="ident">OnceCell</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> }
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<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="ident">LateInit</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
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<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">init</span>(<span class="kw-2">&</span><span class="self">self</span>, <span class="ident">value</span>: <span class="ident">T</span>) {
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<span class="macro">assert</span><span class="macro">!</span>(<span class="self">self</span>.<span class="ident">cell</span>.<span class="ident">set</span>(<span class="ident">value</span>).<span class="ident">is_ok</span>())
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}
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}
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<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="ident">Default</span> <span class="kw">for</span> <span class="ident">LateInit</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
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<span class="kw">fn</span> <span class="ident">default</span>() <span class="op">-</span><span class="op">></span> <span class="self">Self</span> { <span class="ident">LateInit</span> { <span class="ident">cell</span>: <span class="ident">OnceCell</span>::<span class="ident">default</span>() } }
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}
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<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="ident">std</span>::<span class="ident">ops</span>::<span class="ident">Deref</span> <span class="kw">for</span> <span class="ident">LateInit</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
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<span class="kw">type</span> <span class="ident">Target</span> <span class="op">=</span> <span class="ident">T</span>;
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<span class="kw">fn</span> <span class="ident">deref</span>(<span class="kw-2">&</span><span class="self">self</span>) <span class="op">-</span><span class="op">></span> <span class="kw-2">&</span><span class="ident">T</span> {
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<span class="self">self</span>.<span class="ident">cell</span>.<span class="ident">get</span>().<span class="ident">unwrap</span>()
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}
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}
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<span class="attribute">#[<span class="ident">derive</span>(<span class="ident">Default</span>, <span class="ident">Debug</span>)]</span>
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<span class="kw">struct</span> <span class="ident">A</span><span class="op"><</span><span class="lifetime">'a</span><span class="op">></span> {
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<span class="ident">b</span>: <span class="ident">LateInit</span><span class="op"><</span><span class="kw-2">&</span><span class="lifetime">'a</span> <span class="ident">B</span><span class="op"><</span><span class="lifetime">'a</span><span class="op">></span><span class="op">></span>,
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}
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<span class="attribute">#[<span class="ident">derive</span>(<span class="ident">Default</span>, <span class="ident">Debug</span>)]</span>
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<span class="kw">struct</span> <span class="ident">B</span><span class="op"><</span><span class="lifetime">'a</span><span class="op">></span> {
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<span class="ident">a</span>: <span class="ident">LateInit</span><span class="op"><</span><span class="kw-2">&</span><span class="lifetime">'a</span> <span class="ident">A</span><span class="op"><</span><span class="lifetime">'a</span><span class="op">></span><span class="op">></span>
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}
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<span class="kw">fn</span> <span class="ident">build_cycle</span>() {
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<span class="kw">let</span> <span class="ident">a</span> <span class="op">=</span> <span class="ident">A</span>::<span class="ident">default</span>();
|
||
<span class="kw">let</span> <span class="ident">b</span> <span class="op">=</span> <span class="ident">B</span>::<span class="ident">default</span>();
|
||
<span class="ident">a</span>.<span class="ident">b</span>.<span class="ident">init</span>(<span class="kw-2">&</span><span class="ident">b</span>);
|
||
<span class="ident">b</span>.<span class="ident">a</span>.<span class="ident">init</span>(<span class="kw-2">&</span><span class="ident">a</span>);
|
||
<span class="macro">println</span><span class="macro">!</span>(<span class="string">"{:?}"</span>, <span class="ident">a</span>.<span class="ident">b</span>.<span class="ident">a</span>.<span class="ident">b</span>.<span class="ident">a</span>);
|
||
}</pre></div>
|
||
<h1 id="comparison-with-std" class="section-header"><a href="#comparison-with-std">Comparison with std</a></h1><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>
|
||
<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>
|
||
<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>
|
||
<h1 id="minimum-supported-rustc-version" class="section-header"><a href="#minimum-supported-rustc-version">Minimum Supported <code>rustc</code> Version</a></h1>
|
||
<p>This crate's minimum supported <code>rustc</code> version is <code>1.36.0</code>.</p>
|
||
<p>If only the <code>std</code> feature is enabled, MSRV will be updated conservatively.
|
||
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>
|
||
<h1 id="implementation-details" class="section-header"><a href="#implementation-details">Implementation details</a></h1>
|
||
<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>
|
||
<h1 id="faq" class="section-header"><a href="#faq">F.A.Q.</a></h1>
|
||
<p><strong>Should I use lazy_static or once_cell?</strong></p>
|
||
<p>To the first approximation, <code>once_cell</code> is both more flexible and more convenient than <code>lazy_static</code>
|
||
and should be preferred.</p>
|
||
<p>Unlike <code>once_cell</code>, <code>lazy_static</code> supports spinlock-based implementation of blocking which works with
|
||
<code>#![no_std]</code>.</p>
|
||
<p><code>lazy_static</code> has received significantly more real world testing, but <code>once_cell</code> is also a widely
|
||
used crate.</p>
|
||
<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>
|
||
<h1 id="related-crates" class="section-header"><a href="#related-crates">Related crates</a></h1>
|
||
<ul>
|
||
<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>
|
||
</ul>
|
||
<p>Most of this crate's functionality is available in <code>std</code> in nightly Rust.
|
||
See the <a href="https://github.com/rust-lang/rust/issues/74465">tracking issue</a>.</p>
|
||
</div><h2 id="modules" class="section-header"><a href="#modules">Modules</a></h2>
|
||
<table><tr class="module-item"><td><a class="mod" href="race/index.html" title="once_cell::race mod">race</a></td><td class="docblock-short"><p>"First one wins" flavor of <code>OnceCell</code>.</p>
|
||
</td></tr><tr class="module-item"><td><a class="mod" href="sync/index.html" title="once_cell::sync mod">sync</a></td><td class="docblock-short"></td></tr><tr class="module-item"><td><a class="mod" href="unsync/index.html" title="once_cell::unsync mod">unsync</a></td><td class="docblock-short"></td></tr></table></section><section id="search" class="content hidden"></section><section class="footer"></section><div id="rustdoc-vars" data-root-path="../" data-current-crate="once_cell"></div>
|
||
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