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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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<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<TYPE></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'><</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> {
<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'>"foo"</span>);
<span class='ident'>m</span>.<span class='ident'>insert</span>(<span class='number'>1</span>, <span class='string'>"bar"</span>);
<span class='ident'>m</span>.<span class='ident'>insert</span>(<span class='number'>2</span>, <span class='string'>"baz"</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'>-></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'>"The map has {} entries."</span>, <span class='op'>*</span><span class='ident'>COUNT</span>);
<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>());
<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>);
}
</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>
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<td><a class='macro' href='macro.lazy_static!.html'
title='lazy_static::lazy_static!'>lazy_static!</a></td>
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