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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="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" type="text/css" href="../normalize.css"><link rel="stylesheet" type="text/css" href="../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" type="text/css" href="../ayu.css" disabled><link rel="stylesheet" type="text/css" href="../dark.css" disabled><link rel="stylesheet" type="text/css" href="../light.css" id="themeStyle"><script id="default-settings" ></script><script src="../storage.js"></script><script src="../crates.js"></script><script defer src="../main.js"></script>
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</a><h2 class="location"><a href="#">Crate lazy_static</a></h2><div class="sidebar-elems"><div class="block"><ul><li class="version">Version 1.4.0</li><li><a id="all-types" href="all.html">All Items</a></li></div></ul><section><div class="block"><ul><li><a href="#macros">Macros</a></li><li><a href="#traits">Traits</a></li><li><a href="#functions">Functions</a></li></ul></div></section><div id="sidebar-vars" data-name="lazy_static" data-ty="mod" data-relpath=""></div><script defer src="sidebar-items.js"></script></div></nav><main><div class="width-limiter"><div class="sub-container"><a class="sub-logo-container" href="../lazy_static/index.html"><img class="rust-logo" src="../rust-logo.svg" alt="logo"></a><nav class="sub"><div class="theme-picker hidden"><button id="theme-picker" aria-label="Pick another theme!" aria-haspopup="menu" title="themes"><img width="22" height="22" alt="Pick another theme!" src="../brush.svg"></button><div id="theme-choices" role="menu"></div></div><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"><button type="button" id="help-button" title="help">?</button><a id="settings-menu" href="../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../wheel.svg"></a></div></form></nav></div><section id="main-content" class="content"><div class="main-heading">
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<h1 class="fqn"><span class="in-band">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></span></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>
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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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<h2 id="syntax"><a href="#syntax">Syntax</a></h2>
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<div class='information'><div class='tooltip ignore'>ⓘ</div></div><div class="example-wrap"><pre class="rust rust-example-rendered ignore"><code><span class="macro">lazy_static!</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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}</code></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"><code><span class="macro">lazy_static!</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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}</code></pre></div>
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<h2 id="semantics"><a href="#semantics">Semantics</a></h2>
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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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<h2 id="example"><a href="#example">Example</a></h2>
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<p>Using the macro:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><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::collections::HashMap</span>;
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<span class="macro">lazy_static!</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::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="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="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="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="string">"A expensive calculation on a static results in: {}."</span>, <span class="kw-2">*</span><span class="ident">NUMBER</span>);
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}</code></pre></div>
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<h2 id="implementation-details"><a href="#implementation-details">Implementation details</a></h2>
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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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<h2 id="cargo-features"><a href="#cargo-features">Cargo features</a></h2>
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<p>This crate provides one cargo feature:</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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</div></details><h2 id="macros" class="small-section-header"><a href="#macros">Macros</a></h2>
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<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 class="item-right docblock-short"></div></div></div><h2 id="traits" class="small-section-header"><a href="#traits">Traits</a></h2>
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<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"><p>Support trait for enabling a few common operation on lazy static values.</p>
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</div></div></div><h2 id="functions" class="small-section-header"><a href="#functions">Functions</a></h2>
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<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"><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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