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<title>lazy_static - Rust</title>
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<nav class="sidebar">
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<p class='location'>Crate lazy_static</p><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>
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<section id='main' class="content">
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<h1 class='fqn'><span class='in-band'>Crate <a class="mod" href=''>lazy_static</a></span><span class='out-of-band'><span id='render-detail'>
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<a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">
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[<span class='inner'>−</span>]
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</span><a class='srclink' href='../src/lazy_static/lib.rs.html#8-212' title='goto source code'>[src]</a></span></h1>
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<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><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>
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<p>Attributes (including doc comments) are supported as well:</p>
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<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>
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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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<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="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>
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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 a atomic check on each access. On stable Rust, the macro may need to allocate each static on the heap.</p>
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</div><h2 id='macros' class='section-header'><a href="#macros">Macros</a></h2>
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<table>
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<tr class=' module-item'>
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<td><a class="macro" href="macro.__lazy_static_create.html"
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title='macro lazy_static::__lazy_static_create'>__lazy_static_create</a></td>
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<td class='docblock-short'>
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</td>
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</tr>
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<tr class=' module-item'>
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<td><a class="macro" href="macro.lazy_static.html"
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title='macro lazy_static::lazy_static'>lazy_static</a></td>
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<td class='docblock-short'>
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</td>
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</tr></table><h2 id='traits' class='section-header'><a href="#traits">Traits</a></h2>
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<table>
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<tr class=' module-item'>
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<td><a class="trait" href="trait.LazyStatic.html"
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title='trait lazy_static::LazyStatic'>LazyStatic</a></td>
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<td class='docblock-short'>
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<p>Support trait for enabling a few common operation on lazy static values.</p>
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</td>
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</tr></table><h2 id='functions' class='section-header'><a href="#functions">Functions</a></h2>
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<table>
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<tr class=' module-item'>
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<td><a class="fn" href="fn.initialize.html"
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title='fn lazy_static::initialize'>initialize</a></td>
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<td class='docblock-short'>
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<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>
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