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<!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 `scopeguard` crate."><meta name="keywords" content="rust, rustlang, rust-lang, scopeguard"><title>scopeguard - 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><noscript><link rel="stylesheet" href="../noscript.css"></noscript><link rel="shortcut icon" href="../favicon.ico"><style type="text/css">#crate-search{background-image:url("../down-arrow.svg");}</style></head><body class="rustdoc mod"><!--[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><a href='../scopeguard/index.html'><div class='logo-container'><img src='../rust-logo.png' alt='logo'></div></a><p class='location'>Crate scopeguard</p><div class="sidebar-elems"><a id='all-types' href='all.html'><p>See all scopeguard's items</p></a><div class="block items"><ul><li><a href="#macros">Macros</a></li><li><a href="#structs">Structs</a></li><li><a href="#enums">Enums</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: 'scopeguard', ty: 'mod', relpath: '../'};</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"><div><select id="crate-search"><option value="All crates">All crates</option></select><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"></div><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/scopeguard/lib.rs.html#1-409' title='goto source code'>[src]</a></span><span class='in-band'>Crate <a class="mod" href=''>scopeguard</a></span></h1><div class='docblock'><p>A scope guard will run a given closure when it goes out of scope,
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even if the code between panics.
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(as long as panic doesn't abort)</p>
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<h1 id="examples" class="section-header"><a href="#examples">Examples</a></h1><h2 id="defer" class="section-header"><a href="#defer"><code>defer!</code></a></h2>
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<p>Use the <code>defer</code> macro to run an operation at scope exit,
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either regular scope exit or during unwinding from a panic.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="attribute">#[<span class="ident">macro_use</span>(<span class="ident">defer</span>)]</span> <span class="kw">extern</span> <span class="kw">crate</span> <span class="ident">scopeguard</span>;
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<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">cell</span>::<span class="ident">Cell</span>;
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<span class="kw">fn</span> <span class="ident">main</span>() {
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<span class="comment">// use a cell to observe drops during and after the scope guard is active</span>
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<span class="kw">let</span> <span class="ident">drop_counter</span> <span class="op">=</span> <span class="ident">Cell</span>::<span class="ident">new</span>(<span class="number">0</span>);
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{
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<span class="comment">// Create a scope guard using `defer!` for the current scope</span>
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<span class="macro">defer</span><span class="macro">!</span> {{
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<span class="ident">drop_counter</span>.<span class="ident">set</span>(<span class="number">1</span> <span class="op">+</span> <span class="ident">drop_counter</span>.<span class="ident">get</span>());
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}};
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<span class="comment">// Do regular operations here in the meantime.</span>
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<span class="comment">// Just before scope exit: it hasn't run yet.</span>
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<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">drop_counter</span>.<span class="ident">get</span>(), <span class="number">0</span>);
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<span class="comment">// The following scope end is where the defer closure is called</span>
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}
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<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">drop_counter</span>.<span class="ident">get</span>(), <span class="number">1</span>);
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}</pre></div>
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<h2 id="scope-guard-with-value" class="section-header"><a href="#scope-guard-with-value">Scope Guard with Value</a></h2>
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<p>If the scope guard closure needs to access an outer value that is also
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mutated outside of the scope guard, then you may want to use the scope guard
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with a value. The guard works like a smart pointer, so the inner value can
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be accessed by reference or by mutable reference.</p>
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<h3 id="1-the-guard-owns-a-file" class="section-header"><a href="#1-the-guard-owns-a-file">1. The guard owns a file</a></h3>
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<p>In this example, the scope guard owns a file and ensures pending writes are
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synced at scope exit.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">extern</span> <span class="kw">crate</span> <span class="ident">scopeguard</span>;
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<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">fs</span>::<span class="ident">File</span>;
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<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">io</span>::{<span class="self">self</span>, <span class="ident">Write</span>};
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<span class="kw">fn</span> <span class="ident">try_main</span>() <span class="op">-</span><span class="op">></span> <span class="ident">io</span>::<span class="prelude-ty">Result</span><span class="op"><</span>()<span class="op">></span> {
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<span class="kw">let</span> <span class="ident">f</span> <span class="op">=</span> <span class="ident">File</span>::<span class="ident">create</span>(<span class="string">"newfile.txt"</span>)<span class="question-mark">?</span>;
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<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">file</span> <span class="op">=</span> <span class="ident">scopeguard</span>::<span class="ident">guard</span>(<span class="ident">f</span>, <span class="op">|</span><span class="ident">f</span><span class="op">|</span> {
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<span class="comment">// ensure we flush file at return or panic</span>
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<span class="kw">let</span> <span class="kw">_</span> <span class="op">=</span> <span class="ident">f</span>.<span class="ident">sync_all</span>();
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});
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<span class="comment">// Access the file through the scope guard itself</span>
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<span class="ident">file</span>.<span class="ident">write</span>(<span class="string">b"test me\n"</span>).<span class="ident">map</span>(<span class="op">|</span><span class="kw">_</span><span class="op">|</span> ())
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}
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<span class="kw">fn</span> <span class="ident">main</span>() {
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<span class="ident">try_main</span>().<span class="ident">unwrap</span>();
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}
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</pre></div>
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<h3 id="2-the-guard-restores-an-invariant-on-scope-exit" class="section-header"><a href="#2-the-guard-restores-an-invariant-on-scope-exit">2. The guard restores an invariant on scope exit</a></h3>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">extern</span> <span class="kw">crate</span> <span class="ident">scopeguard</span>;
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<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">mem</span>::<span class="ident">ManuallyDrop</span>;
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<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">ptr</span>;
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<span class="comment">// This function, just for this example, takes the first element</span>
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<span class="comment">// and inserts it into the assumed sorted tail of the vector.</span>
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<span class="comment">//</span>
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<span class="comment">// For optimization purposes we temporarily violate an invariant of the</span>
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<span class="comment">// Vec, that it owns all of its elements.</span>
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<span class="comment">// </span>
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<span class="comment">// The safe approach is to use swap, which means two writes to memory,</span>
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<span class="comment">// the optimization is to use a “hole” which uses only one write of memory</span>
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<span class="comment">// for each position it moves.</span>
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<span class="comment">//</span>
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<span class="comment">// We *must* use a scope guard to run this code safely. We</span>
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<span class="comment">// are running arbitrary user code (comparison operators) that may panic.</span>
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<span class="comment">// The scope guard ensures we restore the invariant after successful</span>
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<span class="comment">// exit or during unwinding from panic.</span>
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<span class="kw">fn</span> <span class="ident">insertion_sort_first</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>(<span class="ident">v</span>: <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">Vec</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>)
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<span class="kw">where</span> <span class="ident">T</span>: <span class="ident">PartialOrd</span>
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{
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<span class="kw">struct</span> <span class="ident">Hole</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">T</span>: <span class="lifetime">'a</span><span class="op">></span> {
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<span class="ident">v</span>: <span class="kw-2">&</span><span class="lifetime">'a</span> <span class="kw-2">mut</span> <span class="ident">Vec</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>,
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<span class="ident">index</span>: <span class="ident">usize</span>,
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<span class="ident">value</span>: <span class="ident">ManuallyDrop</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>,
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}
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<span class="kw">unsafe</span> {
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<span class="comment">// Create a moved-from location in the vector, a “hole”.</span>
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<span class="kw">let</span> <span class="ident">value</span> <span class="op">=</span> <span class="ident">ptr</span>::<span class="ident">read</span>(<span class="kw-2">&</span><span class="ident">v</span>[<span class="number">0</span>]);
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<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">hole</span> <span class="op">=</span> <span class="ident">Hole</span> { <span class="ident">v</span>: <span class="ident">v</span>, <span class="ident">index</span>: <span class="number">0</span>, <span class="ident">value</span>: <span class="ident">ManuallyDrop</span>::<span class="ident">new</span>(<span class="ident">value</span>) };
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<span class="comment">// Use a scope guard with a value.</span>
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<span class="comment">// At scope exit, plug the hole so that the vector is fully</span>
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<span class="comment">// initialized again.</span>
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<span class="comment">// The scope guard owns the hole, but we can access it through the guard.</span>
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<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">hole_guard</span> <span class="op">=</span> <span class="ident">scopeguard</span>::<span class="ident">guard</span>(<span class="ident">hole</span>, <span class="op">|</span><span class="ident">hole</span><span class="op">|</span> {
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<span class="comment">// plug the hole in the vector with the value that was // taken out</span>
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<span class="kw">let</span> <span class="ident">index</span> <span class="op">=</span> <span class="ident">hole</span>.<span class="ident">index</span>;
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<span class="ident">ptr</span>::<span class="ident">copy_nonoverlapping</span>(<span class="kw-2">&</span><span class="kw-2">*</span><span class="ident">hole</span>.<span class="ident">value</span>, <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">hole</span>.<span class="ident">v</span>[<span class="ident">index</span>], <span class="number">1</span>);
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});
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<span class="comment">// run algorithm that moves the hole in the vector here</span>
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<span class="comment">// move the hole until it's in a sorted position</span>
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<span class="kw">for</span> <span class="ident">i</span> <span class="kw">in</span> <span class="number">1</span>..<span class="ident">hole_guard</span>.<span class="ident">v</span>.<span class="ident">len</span>() {
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<span class="kw">if</span> <span class="kw-2">*</span><span class="ident">hole_guard</span>.<span class="ident">value</span> <span class="op">></span><span class="op">=</span> <span class="ident">hole_guard</span>.<span class="ident">v</span>[<span class="ident">i</span>] {
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<span class="comment">// move the element back and the hole forward</span>
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<span class="kw">let</span> <span class="ident">index</span> <span class="op">=</span> <span class="ident">hole_guard</span>.<span class="ident">index</span>;
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<span class="ident">ptr</span>::<span class="ident">copy_nonoverlapping</span>(<span class="kw-2">&</span><span class="ident">hole_guard</span>.<span class="ident">v</span>[<span class="ident">index</span> <span class="op">+</span> <span class="number">1</span>], <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">hole_guard</span>.<span class="ident">v</span>[<span class="ident">index</span>], <span class="number">1</span>);
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<span class="ident">hole_guard</span>.<span class="ident">index</span> <span class="op">+</span><span class="op">=</span> <span class="number">1</span>;
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} <span class="kw">else</span> {
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<span class="kw">break</span>;
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}
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}
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<span class="comment">// When the scope exits here, the Vec becomes whole again!</span>
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}
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}
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<span class="kw">fn</span> <span class="ident">main</span>() {
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<span class="kw">let</span> <span class="ident">string</span> <span class="op">=</span> <span class="ident">String</span>::<span class="ident">from</span>;
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<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">data</span> <span class="op">=</span> <span class="macro">vec</span><span class="macro">!</span>[<span class="ident">string</span>(<span class="string">"c"</span>), <span class="ident">string</span>(<span class="string">"a"</span>), <span class="ident">string</span>(<span class="string">"b"</span>), <span class="ident">string</span>(<span class="string">"d"</span>)];
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<span class="ident">insertion_sort_first</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">data</span>);
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<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">data</span>, <span class="macro">vec</span><span class="macro">!</span>[<span class="string">"a"</span>, <span class="string">"b"</span>, <span class="string">"c"</span>, <span class="string">"d"</span>]);
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}
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</pre></div>
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<h1 id="crate-features" class="section-header"><a href="#crate-features">Crate features:</a></h1>
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<ul>
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<li><code>use_std</code>
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<ul>
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<li>Enabled by default. Enables the <code>OnUnwind</code> strategy.</li>
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<li>Disable to use <code>no_std</code>.</li>
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</ul>
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</li>
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</ul>
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</div><h2 id='macros' class='section-header'><a href="#macros">Macros</a></h2>
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<table><tr class='module-item'><td><a class="macro" href="macro.defer.html" title='scopeguard::defer macro'>defer</a></td><td class='docblock-short'><p>Macro to create a <code>ScopeGuard</code> (always run).</p>
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</td></tr><tr class='module-item'><td><a class="macro" href="macro.defer_on_unwind.html" title='scopeguard::defer_on_unwind macro'>defer_on_unwind</a></td><td class='docblock-short'><p>Macro to create a <code>ScopeGuard</code> (run on unwinding from panic).</p>
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</td></tr></table><h2 id='structs' class='section-header'><a href="#structs">Structs</a></h2>
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<table><tr class='module-item'><td><a class="struct" href="struct.ScopeGuard.html" title='scopeguard::ScopeGuard struct'>ScopeGuard</a></td><td class='docblock-short'><p><code>ScopeGuard</code> is a scope guard that may own a protected value.</p>
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</td></tr></table><h2 id='enums' class='section-header'><a href="#enums">Enums</a></h2>
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<table><tr class='module-item'><td><a class="enum" href="enum.Always.html" title='scopeguard::Always enum'>Always</a></td><td class='docblock-short'><p>Always run on scope exit.</p>
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</td></tr></table><h2 id='traits' class='section-header'><a href="#traits">Traits</a></h2>
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<table><tr class='module-item'><td><a class="trait" href="trait.Strategy.html" title='scopeguard::Strategy trait'>Strategy</a></td><td class='docblock-short'></td></tr></table><h2 id='functions' class='section-header'><a href="#functions">Functions</a></h2>
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<table><tr class='module-item'><td><a class="fn" href="fn.guard.html" title='scopeguard::guard fn'>guard</a></td><td class='docblock-short'><p>Create a new <code>ScopeGuard</code> owning <code>v</code> and with deferred closure <code>dropfn</code>.</p>
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