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<section id='main' class="content mod">
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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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</a>
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</span><a id='src-0' class='srclink' href='../src/lazy_static/lib.rs.html#1-157' 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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<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='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>Metadata (such as doc comments) is allowed on each ref.</p>
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<h1 id='semantic' class='section-header'><a href='#semantic'>Semantic</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>. (Metadata ends 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.</p>
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<p>Like regular <code>static mut</code>s, this macro only works for types that fulfill the <code>Sync</code>
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trait.</p>
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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='op'>*</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='op'>*</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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<tr class=' module-item'>
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<td><a class='macro' href='macro.lazy_static!.html'
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title='lazy_static::lazy_static!'>lazy_static!</a></td>
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