27 lines
6.2 KiB
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27 lines
6.2 KiB
HTML
<!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="API documentation for the Rust `singleton` macro in crate `cortex_m`."><meta name="keywords" content="rust, rustlang, rust-lang, singleton"><title>cortex_m::singleton - Rust</title><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="../dark.css"><link rel="stylesheet" type="text/css" href="../light.css" id="themeStyle"><script src="../storage.js"></script></head><body class="rustdoc macro"><!--[if lte IE 8]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><div class="sidebar-menu">☰</div><div class="sidebar-elems"><p class='location'><a href='index.html'>cortex_m</a></p><script>window.sidebarCurrent = {name: 'singleton', ty: 'macro', relpath: ''};</script><script defer src="sidebar-items.js"></script></div></nav><div class="theme-picker"><button id="theme-picker" aria-label="Pick another theme!"><img src="../brush.svg" width="18" alt="Pick another theme!"></button><div id="theme-choices"></div></div><script src="../theme.js"></script><nav class="sub"><form class="search-form js-only"><div class="search-container"><input class="search-input" name="search" autocomplete="off" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><a id="settings-menu" href="../settings.html"><img src="../wheel.svg" width="18" alt="Change settings"></a></div></form></nav><section id="main" class="content"><h1 class='fqn'><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'>−</span>]</a></span><a class='srclink' href='../src/cortex_m/macros.rs.html#53-77' title='goto source code'>[src]</a></span><span class='in-band'>Macro <a href='index.html'>cortex_m</a>::<wbr><a class="macro" href=''>singleton</a></span></h1><div class="docblock type-decl"><pre class="rust macro">
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<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">singleton</span> {
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(: <span class="macro-nonterminal">$</span><span class="macro-nonterminal">ty</span>:<span class="ident">ty</span> <span class="op">=</span> <span class="macro-nonterminal">$</span><span class="macro-nonterminal">expr</span>:<span class="ident">expr</span>) <span class="op">=></span> { ... };
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}</pre>
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</div><div class='docblock'><p>Macro to create a mutable reference to a statically allocated value</p>
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<p>This macro returns a value with type <code>Option<&'static mut $ty></code>. <code>Some($expr)</code> will be returned
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the first time the macro is executed; further calls will return <code>None</code>. To avoid <code>unwrap</code>ping a
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<code>None</code> variant the caller must ensure that the macro is called from a function that's executed
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at most once in the whole lifetime of the program.</p>
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<h1 id="example" class="section-header"><a href="#example">Example</a></h1>
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<pre class="rust rust-example-rendered">
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<span class="attribute">#[<span class="ident">macro_use</span>(<span class="ident">singleton</span>)]</span>
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<span class="kw">extern</span> <span class="kw">crate</span> <span class="ident">cortex_m</span>;
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<span class="kw">fn</span> <span class="ident">main</span>() {
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<span class="comment">// OK if `main` is executed only once</span>
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<span class="kw">let</span> <span class="ident">x</span>: <span class="kw-2">&</span><span class="lifetime">'static</span> <span class="kw-2">mut</span> <span class="ident">bool</span> <span class="op">=</span> <span class="macro">singleton</span><span class="macro">!</span>(: <span class="ident">bool</span> <span class="op">=</span> <span class="bool-val">false</span>).<span class="ident">unwrap</span>();
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<span class="kw">let</span> <span class="ident">y</span> <span class="op">=</span> <span class="ident">alias</span>();
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<span class="comment">// BAD this second call to `alias` will definitively `panic!`</span>
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<span class="kw">let</span> <span class="ident">y_alias</span> <span class="op">=</span> <span class="ident">alias</span>();
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}
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<span class="kw">fn</span> <span class="ident">alias</span>() <span class="op">-></span> <span class="kw-2">&</span><span class="lifetime">'static</span> <span class="kw-2">mut</span> <span class="ident">bool</span> {
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<span class="macro">singleton</span><span class="macro">!</span>(: <span class="ident">bool</span> <span class="op">=</span> <span class="bool-val">false</span>).<span class="ident">unwrap</span>()
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}</pre>
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