| <!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="A macro for declaring lazily evaluated statics."><meta name="keywords" content="rust, rustlang, rust-lang, lazy_static"><title>lazy_static - 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="../lazy_static/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="../lazy_static/index.html"><div class="logo-container"><img class="rust-logo" src="../rust-logo.svg" alt="logo"></div></a><h2 class="location"><a href="#">Crate lazy_static</a></h2><div class="sidebar-elems"><ul class="block"><li class="version">Version 1.4.0</li><li><a id="all-types" href="all.html">All Items</a></li></ul><section><ul class="block"><li><a href="#macros">Macros</a></li><li><a href="#traits">Traits</a></li><li><a href="#functions">Functions</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="#">lazy_static</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/lazy_static/lib.rs.html#8-215">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"><p>A macro for declaring lazily evaluated statics.</p> |
| <p>Using this macro, it is possible to have <code>static</code>s that require code to be |
| executed at runtime in order to be initialized. |
| This includes anything requiring heap allocations, like vectors or hash maps, |
| as well as anything that requires function calls to be computed.</p> |
| <h2 id="syntax"><a href="#syntax">Syntax</a></h2> |
| <div class="example-wrap ignore"><div class='tooltip'>ⓘ</div><pre class="rust rust-example-rendered"><code><span class="macro">lazy_static! </span>{ |
| [<span class="kw">pub</span>] <span class="kw">static </span><span class="kw-2">ref </span>NAME_1: TYPE_1 = EXPR_1; |
| [<span class="kw">pub</span>] <span class="kw">static </span><span class="kw-2">ref </span>NAME_2: TYPE_2 = EXPR_2; |
| ... |
| [<span class="kw">pub</span>] <span class="kw">static </span><span class="kw-2">ref </span>NAME_N: TYPE_N = EXPR_N; |
| }</code></pre></div> |
| <p>Attributes (including doc comments) are supported as well:</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="macro">lazy_static! </span>{ |
| <span class="doccomment">/// This is an example for using doc comment attributes |
| </span><span class="kw">static </span><span class="kw-2">ref </span>EXAMPLE: u8 = <span class="number">42</span>; |
| }</code></pre></div> |
| <h2 id="semantics"><a href="#semantics">Semantics</a></h2> |
| <p>For a given <code>static ref NAME: TYPE = EXPR;</code>, the macro generates a unique type that |
| implements <code>Deref<TYPE></code> and stores it in a static with name <code>NAME</code>. (Attributes end up |
| attaching to this type.)</p> |
| <p>On first deref, <code>EXPR</code> gets evaluated and stored internally, such that all further derefs |
| can return a reference to the same object. Note that this can lead to deadlocks |
| if you have multiple lazy statics that depend on each other in their initialization.</p> |
| <p>Apart from the lazy initialization, the resulting “static ref” variables |
| have generally the same properties as regular “static” variables:</p> |
| <ul> |
| <li>Any type in them needs to fulfill the <code>Sync</code> trait.</li> |
| <li>If the type has a destructor, then it will not run when the process exits.</li> |
| </ul> |
| <h2 id="example"><a href="#example">Example</a></h2> |
| <p>Using the macro:</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="attribute">#[macro_use] |
| </span><span class="kw">extern crate </span>lazy_static; |
| |
| <span class="kw">use </span>std::collections::HashMap; |
| |
| <span class="macro">lazy_static! </span>{ |
| <span class="kw">static </span><span class="kw-2">ref </span>HASHMAP: HashMap<u32, <span class="kw-2">&</span><span class="lifetime">'static </span>str> = { |
| <span class="kw">let </span><span class="kw-2">mut </span>m = HashMap::new(); |
| m.insert(<span class="number">0</span>, <span class="string">"foo"</span>); |
| m.insert(<span class="number">1</span>, <span class="string">"bar"</span>); |
| m.insert(<span class="number">2</span>, <span class="string">"baz"</span>); |
| m |
| }; |
| <span class="kw">static </span><span class="kw-2">ref </span>COUNT: usize = HASHMAP.len(); |
| <span class="kw">static </span><span class="kw-2">ref </span>NUMBER: u32 = times_two(<span class="number">21</span>); |
| } |
| |
| <span class="kw">fn </span>times_two(n: u32) -> u32 { n * <span class="number">2 </span>} |
| |
| <span class="kw">fn </span>main() { |
| <span class="macro">println!</span>(<span class="string">"The map has {} entries."</span>, <span class="kw-2">*</span>COUNT); |
| <span class="macro">println!</span>(<span class="string">"The entry for `0` is \"{}\"."</span>, HASHMAP.get(<span class="kw-2">&</span><span class="number">0</span>).unwrap()); |
| <span class="macro">println!</span>(<span class="string">"A expensive calculation on a static results in: {}."</span>, <span class="kw-2">*</span>NUMBER); |
| }</code></pre></div> |
| <h2 id="implementation-details"><a href="#implementation-details">Implementation details</a></h2> |
| <p>The <code>Deref</code> implementation uses a hidden static variable that is guarded by an atomic check on each access.</p> |
| <h2 id="cargo-features"><a href="#cargo-features">Cargo features</a></h2> |
| <p>This crate provides one cargo feature:</p> |
| <ul> |
| <li><code>spin_no_std</code>: This allows using this crate in a no-std environment, by depending on the standalone <code>spin</code> crate.</li> |
| </ul> |
| </div></details><h2 id="macros" class="small-section-header"><a href="#macros">Macros</a></h2><div class="item-table"><div class="item-row"><div class="item-left module-item"><a class="macro" href="macro.lazy_static.html" title="lazy_static::lazy_static macro">lazy_static</a></div></div></div><h2 id="traits" class="small-section-header"><a href="#traits">Traits</a></h2><div class="item-table"><div class="item-row"><div class="item-left module-item"><a class="trait" href="trait.LazyStatic.html" title="lazy_static::LazyStatic trait">LazyStatic</a></div><div class="item-right docblock-short">Support trait for enabling a few common operation on lazy static values.</div></div></div><h2 id="functions" class="small-section-header"><a href="#functions">Functions</a></h2><div class="item-table"><div class="item-row"><div class="item-left module-item"><a class="fn" href="fn.initialize.html" title="lazy_static::initialize fn">initialize</a></div><div class="item-right docblock-short">Takes a shared reference to a lazy static and initializes |
| it if it has not been already.</div></div></div></section></div></main><div id="rustdoc-vars" data-root-path="../" data-current-crate="lazy_static" data-themes="ayu,dark,light" data-resource-suffix="" data-rustdoc-version="1.66.0-nightly (5c8bff74b 2022-10-21)" ></div></body></html> |
