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+ <section id='main' class="content mod">
+<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'>
+ <a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">
+ [<span class='inner'>&#x2212;</span>]
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+ </span><a id='src-0' class='srclink' href='../src/lazy_static/lib.rs.html#1-124' title='goto source code'>[src]</a></span></h1>
+<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>
+
+<h1 id="syntax" class='section-header'><a
+ href="#syntax">Syntax</a></h1><pre class='rust rust-example-rendered'>
+<span class='macro'>lazy_static</span><span class='macro'>!</span> {
+ [<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>;
+ [<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>;
+ ...
+ [<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>;
+}
+</pre>
+
+<h1 id="semantic" class='section-header'><a
+ href="#semantic">Semantic</a></h1>
+<p>For a given <code>static ref NAME: TYPE = EXPR;</code>, the macro generates a
+unique type that implements <code>Deref&lt;TYPE&gt;</code> and stores it in a static with name <code>NAME</code>.</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.</p>
+
+<p>Like regular <code>static mut</code>s, this macro only works for types that fulfill the <code>Sync</code>
+trait.</p>
+
+<h1 id="example" class='section-header'><a
+ href="#example">Example</a></h1>
+<p>Using the macro:</p>
+<pre class='rust rust-example-rendered'>
+<span class='attribute'>#[<span class='ident'>macro_use</span>]</span>
+<span class='kw'>extern</span> <span class='kw'>crate</span> <span class='ident'>lazy_static</span>;
+
+<span class='kw'>use</span> <span class='ident'>std</span>::<span class='ident'>collections</span>::<span class='ident'>HashMap</span>;
+
+<span class='macro'>lazy_static</span><span class='macro'>!</span> {
+ <span class='kw'>static</span> <span class='kw-2'>ref</span> <span class='ident'>HASHMAP</span>: <span class='ident'>HashMap</span><span class='op'>&lt;</span><span class='ident'>u32</span>, <span class='kw-2'>&amp;</span><span class='lifetime'>&#39;static</span> <span class='ident'>str</span><span class='op'>&gt;</span> <span class='op'>=</span> {
+ <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>();
+ <span class='ident'>m</span>.<span class='ident'>insert</span>(<span class='number'>0</span>, <span class='string'>&quot;foo&quot;</span>);
+ <span class='ident'>m</span>.<span class='ident'>insert</span>(<span class='number'>1</span>, <span class='string'>&quot;bar&quot;</span>);
+ <span class='ident'>m</span>.<span class='ident'>insert</span>(<span class='number'>2</span>, <span class='string'>&quot;baz&quot;</span>);
+ <span class='ident'>m</span>
+ };
+ <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>();
+ <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>);
+}
+
+<span class='kw'>fn</span> <span class='ident'>times_two</span>(<span class='ident'>n</span>: <span class='ident'>u32</span>) <span class='op'>-&gt;</span> <span class='ident'>u32</span> { <span class='ident'>n</span> <span class='op'>*</span> <span class='number'>2</span> }
+
+<span class='kw'>fn</span> <span class='ident'>main</span>() {
+ <span class='macro'>println</span><span class='macro'>!</span>(<span class='string'>&quot;The map has {} entries.&quot;</span>, <span class='op'>*</span><span class='ident'>COUNT</span>);
+ <span class='macro'>println</span><span class='macro'>!</span>(<span class='string'>&quot;The entry for `0` is \&quot;{}\&quot;.&quot;</span>, <span class='ident'>HASHMAP</span>.<span class='ident'>get</span>(<span class='kw-2'>&amp;</span><span class='number'>0</span>).<span class='ident'>unwrap</span>());
+ <span class='macro'>println</span><span class='macro'>!</span>(<span class='string'>&quot;A expensive calculation on a static results in: {}.&quot;</span>, <span class='op'>*</span><span class='ident'>NUMBER</span>);
+}
+</pre>
+
+<h1 id="implementation-details" class='section-header'><a
+ href="#implementation-details">Implementation details</a></h1>
+<p>The <code>Deref</code> implementation uses a hidden <code>static mut</code> that is guarded by a atomic check
+using the <code>sync::Once</code> abstraction. All lazily evaluated values are currently
+put in a heap allocated box, due to the Rust language currently not providing any way to
+define uninitialized <code>static mut</code> values.</p>
+</div><h2 id='macros' class='section-header'><a href="#macros">Macros</a></h2>
+<table>
+ <tr class=' module-item'>
+ <td><a class='macro' href='macro.lazy_static!.html'
+ title='lazy_static::lazy_static!'>lazy_static!</a></td>
+ <td class='docblock short'>
+
+ </td>
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