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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 `lazy_static` crate."><meta name="keywords" content="rust, rustlang, rust-lang, lazy_static"><title>lazy_static - 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="../dark.css"><link rel="stylesheet" type="text/css" href="../light.css" id="themeStyle"><script src="../storage.js"></script><noscript><link rel="stylesheet" href="../noscript.css"></noscript><link rel="shortcut icon" href="../favicon.ico"><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='../lazy_static/index.html'><div class='logo-container'><img src='../rust-logo.png' alt='logo'></div></a><p class='location'>Crate lazy_static</p><div class="sidebar-elems"><a id='all-types' href='all.html'><p>See all lazy_static's items</p></a><div class="block items"><ul><li><a href="#macros">Macros</a></li><li><a href="#traits">Traits</a></li><li><a href="#functions">Functions</a></li></ul></div><p class='location'></p><script>window.sidebarCurrent = {name: 'lazy_static', ty: 'mod', relpath: '../'};</script></div></nav><div class="theme-picker"><button id="theme-picker" aria-label="Pick another theme!"><img src="../brush.svg" width="18" alt="Pick another theme!"></button><div id="theme-choices"></div></div><script src="../theme.js"></script><nav class="sub"><form class="search-form js-only"><div class="search-container"><div><select id="crate-search"><option value="All crates">All crates</option></select><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"></div><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='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/lazy_static/lib.rs.html#8-211' title='goto source code'>[src]</a></span><span class='in-band'>Crate <a class="mod" href=''>lazy_static</a></span></h1><div class='docblock'><p>A macro for declaring lazily evaluated statics.</p>
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<p>Using this macro, it is possible to have <code>static</code>s that require code to be
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executed at runtime in order to be initialized.
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This includes anything requiring heap allocations, like vectors or hash maps,
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as well as anything that requires function calls to be computed.</p>
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<h1 id="syntax" class="section-header"><a href="#syntax">Syntax</a></h1>
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<div class='information'><div class='tooltip ignore'>ⓘ<span class='tooltiptext'>This example is not tested</span></div></div><div class="example-wrap"><pre class="rust rust-example-rendered ignore">
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<span class="macro">lazy_static</span><span class="macro">!</span> {
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[<span class="kw">pub</span>] <span class="kw">static</span> <span class="kw-2">ref</span> <span class="ident">NAME_1</span>: <span class="ident">TYPE_1</span> <span class="op">=</span> <span class="ident">EXPR_1</span>;
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[<span class="kw">pub</span>] <span class="kw">static</span> <span class="kw-2">ref</span> <span class="ident">NAME_2</span>: <span class="ident">TYPE_2</span> <span class="op">=</span> <span class="ident">EXPR_2</span>;
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...
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[<span class="kw">pub</span>] <span class="kw">static</span> <span class="kw-2">ref</span> <span class="ident">NAME_N</span>: <span class="ident">TYPE_N</span> <span class="op">=</span> <span class="ident">EXPR_N</span>;
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}</pre></div>
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<p>Attributes (including doc comments) are supported as well:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="macro">lazy_static</span><span class="macro">!</span> {
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<span class="doccomment">/// This is an example for using doc comment attributes</span>
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<span class="kw">static</span> <span class="kw-2">ref</span> <span class="ident">EXAMPLE</span>: <span class="ident">u8</span> <span class="op">=</span> <span class="number">42</span>;
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}</pre></div>
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<h1 id="semantics" class="section-header"><a href="#semantics">Semantics</a></h1>
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<p>For a given <code>static ref NAME: TYPE = EXPR;</code>, the macro generates a unique type that
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implements <code>Deref<TYPE></code> and stores it in a static with name <code>NAME</code>. (Attributes end up
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attaching to this type.)</p>
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<p>On first deref, <code>EXPR</code> gets evaluated and stored internally, such that all further derefs
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can return a reference to the same object. Note that this can lead to deadlocks
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if you have multiple lazy statics that depend on each other in their initialization.</p>
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<p>Apart from the lazy initialization, the resulting "static ref" variables
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have generally the same properties as regular "static" variables:</p>
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<ul>
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<li>Any type in them needs to fulfill the <code>Sync</code> trait.</li>
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<li>If the type has a destructor, then it will not run when the process exits.</li>
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</ul>
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<h1 id="example" class="section-header"><a href="#example">Example</a></h1>
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<p>Using the macro:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="attribute">#[<span class="ident">macro_use</span>]</span>
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<span class="kw">extern</span> <span class="kw">crate</span> <span class="ident">lazy_static</span>;
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<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">collections</span>::<span class="ident">HashMap</span>;
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<span class="macro">lazy_static</span><span class="macro">!</span> {
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<span class="kw">static</span> <span class="kw-2">ref</span> <span class="ident">HASHMAP</span>: <span class="ident">HashMap</span><span class="op"><</span><span class="ident">u32</span>, <span class="kw-2">&</span><span class="lifetime">'static</span> <span class="ident">str</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">0</span>, <span class="string">"foo"</span>);
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<span class="ident">m</span>.<span class="ident">insert</span>(<span class="number">1</span>, <span class="string">"bar"</span>);
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<span class="ident">m</span>.<span class="ident">insert</span>(<span class="number">2</span>, <span class="string">"baz"</span>);
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<span class="ident">m</span>
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};
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<span class="kw">static</span> <span class="kw-2">ref</span> <span class="ident">COUNT</span>: <span class="ident">usize</span> <span class="op">=</span> <span class="ident">HASHMAP</span>.<span class="ident">len</span>();
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<span class="kw">static</span> <span class="kw-2">ref</span> <span class="ident">NUMBER</span>: <span class="ident">u32</span> <span class="op">=</span> <span class="ident">times_two</span>(<span class="number">21</span>);
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}
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<span class="kw">fn</span> <span class="ident">times_two</span>(<span class="ident">n</span>: <span class="ident">u32</span>) <span class="op">-</span><span class="op">></span> <span class="ident">u32</span> { <span class="ident">n</span> <span class="op">*</span> <span class="number">2</span> }
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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">"The map has {} entries."</span>, <span class="kw-2">*</span><span class="ident">COUNT</span>);
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<span class="macro">println</span><span class="macro">!</span>(<span class="string">"The entry for `0` is \"{}\"."</span>, <span class="ident">HASHMAP</span>.<span class="ident">get</span>(<span class="kw-2">&</span><span class="number">0</span>).<span class="ident">unwrap</span>());
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<span class="macro">println</span><span class="macro">!</span>(<span class="string">"A expensive calculation on a static results in: {}."</span>, <span class="kw-2">*</span><span class="ident">NUMBER</span>);
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}</pre></div>
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<h1 id="implementation-details" class="section-header"><a href="#implementation-details">Implementation details</a></h1>
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<p>The <code>Deref</code> implementation uses a hidden static variable that is guarded by an atomic check on each access.</p>
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<h1 id="cargo-features" class="section-header"><a href="#cargo-features">Cargo features</a></h1>
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<p>This crate provides two cargo features:</p>
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<ul>
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<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>
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</ul>
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<p>Both features depend on unstable language features, which means
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no guarantees can be made about them in regard to SemVer stability.</p>
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</div><h2 id='macros' class='section-header'><a href="#macros">Macros</a></h2>
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<table><tr class='module-item'><td><a class="macro" href="macro.lazy_static.html" title='lazy_static::lazy_static macro'>lazy_static</a></td><td class='docblock-short'></td></tr></table><h2 id='traits' class='section-header'><a href="#traits">Traits</a></h2>
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<table><tr class='module-item'><td><a class="trait" href="trait.LazyStatic.html" title='lazy_static::LazyStatic trait'>LazyStatic</a></td><td class='docblock-short'><p>Support trait for enabling a few common operation on lazy static values.</p>
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</td></tr></table><h2 id='functions' class='section-header'><a href="#functions">Functions</a></h2>
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<table><tr class='module-item'><td><a class="fn" href="fn.initialize.html" title='lazy_static::initialize fn'>initialize</a></td><td class='docblock-short'><p>Takes a shared reference to a lazy static and initializes
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it if it has not been already.</p>
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</td></tr></table></section><section id="search" class="content hidden"></section><section class="footer"></section><aside id="help" class="hidden"><div><h1 class="hidden">Help</h1><div class="shortcuts"><h2>Keyboard Shortcuts</h2><dl><dt><kbd>?</kbd></dt><dd>Show this help dialog</dd><dt><kbd>S</kbd></dt><dd>Focus the search field</dd><dt><kbd>↑</kbd></dt><dd>Move up in search results</dd><dt><kbd>↓</kbd></dt><dd>Move down in search results</dd><dt><kbd>↹</kbd></dt><dd>Switch tab</dd><dt><kbd>⏎</kbd></dt><dd>Go to active search result</dd><dt><kbd>+</kbd></dt><dd>Expand all sections</dd><dt><kbd>-</kbd></dt><dd>Collapse all sections</dd></dl></div><div class="infos"><h2>Search Tricks</h2><p>Prefix searches with a type followed by a colon (e.g., <code>fn:</code>) to restrict the search to a given type.</p><p>Accepted types are: <code>fn</code>, <code>mod</code>, <code>struct</code>, <code>enum</code>, <code>trait</code>, <code>type</code>, <code>macro</code>, and <code>const</code>.</p><p>Search functions by type signature (e.g., <code>vec -> usize</code> or <code>* -> vec</code>)</p><p>Search multiple things at once by splitting your query with comma (e.g., <code>str,u8</code> or <code>String,struct:Vec,test</code>)</p></div></div></aside><script>window.rootPath = "../";window.currentCrate = "lazy_static";</script><script src="../aliases.js"></script><script src="../main.js"></script><script defer src="../search-index.js"></script></body></html>
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