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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="A scope guard will run a given closure when it goes out of scope, even if the code between panics. (as long as panic doesn’t abort)"><title>scopeguard - Rust</title><script>if(window.location.protocol!=="file:")document.head.insertAdjacentHTML("beforeend","SourceSerif4-Regular-46f98efaafac5295.ttf.woff2,FiraSans-Regular-018c141bf0843ffd.woff2,FiraSans-Medium-8f9a781e4970d388.woff2,SourceCodePro-Regular-562dcc5011b6de7d.ttf.woff2,SourceCodePro-Semibold-d899c5a5c4aeb14a.ttf.woff2".split(",").map(f=>`<link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/${f}">`).join(""))</script><link rel="stylesheet" href="../static.files/normalize-76eba96aa4d2e634.css"><link rel="stylesheet" href="../static.files/rustdoc-492a78a4a87dcc01.css"><meta name="rustdoc-vars" data-root-path="../" data-static-root-path="../static.files/" data-current-crate="scopeguard" data-themes="" data-resource-suffix="" data-rustdoc-version="1.82.0 (f6e511eec 2024-10-15)" data-channel="1.82.0" data-search-js="search-a99f1315e7cc5121.js" data-settings-js="settings-4313503d2e1961c2.js" ><script src="../static.files/storage-118b08c4c78b968e.js"></script><script defer src="../crates.js"></script><script defer src="../static.files/main-921df33f47b8780c.js"></script><noscript><link rel="stylesheet" href="../static.files/noscript-3b12f09e550e0385.css"></noscript><link rel="alternate icon" type="image/png" href="../static.files/favicon-32x32-422f7d1d52889060.png"><link rel="icon" type="image/svg+xml" href="../static.files/favicon-2c020d218678b618.svg"></head><body class="rustdoc mod crate"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="mobile-topbar"><button class="sidebar-menu-toggle" title="show sidebar"></button></nav><nav class="sidebar"><div class="sidebar-crate"><h2><a href="../scopeguard/index.html">scopeguard</a><span class="version">1.2.0</span></h2></div><div class="sidebar-elems"><ul class="block"><li><a id="all-types" href="all.html">All Items</a></li></ul><section><ul class="block"><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></section></div></nav><div class="sidebar-resizer"></div><main><div class="width-limiter"><rustdoc-search></rustdoc-search><section id="main-content" class="content"><div class="main-heading"><h1>Crate <a class="mod" href="#">scopeguard</a><button id="copy-path" title="Copy item path to clipboard">Copy item path</button></h1><span class="out-of-band"><a class="src" href="../src/scopeguard/lib.rs.html#1-595">source</a> · <button id="toggle-all-docs" title="collapse all docs">[<span>−</span>]</button></span></div><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><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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<h2 id="examples"><a class="doc-anchor" href="#examples">§</a>Examples</h2><h3 id="hello-world"><a class="doc-anchor" href="#hello-world">§</a>Hello World</h3>
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<p>This example creates a scope guard with an example function:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">extern crate </span>scopeguard;
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<span class="kw">fn </span>f() {
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<span class="kw">let </span>_guard = scopeguard::guard((), |<span class="kw">_</span>| {
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<span class="macro">println!</span>(<span class="string">"Hello Scope Exit!"</span>);
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});
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<span class="comment">// rest of the code here.
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// Here, at the end of `_guard`'s scope, the guard's closure is called.
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// It is also called if we exit this scope through unwinding instead.
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</span>}</code></pre></div>
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<h3 id="defer"><a class="doc-anchor" href="#defer">§</a><code>defer!</code></h3>
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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"><code><span class="attr">#[macro_use(defer)] </span><span class="kw">extern crate </span>scopeguard;
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<span class="kw">use </span>std::cell::Cell;
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<span class="kw">fn </span>main() {
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<span class="comment">// use a cell to observe drops during and after the scope guard is active
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</span><span class="kw">let </span>drop_counter = Cell::new(<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
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</span><span class="macro">defer!</span> {
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drop_counter.set(<span class="number">1 </span>+ drop_counter.get());
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}
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<span class="comment">// Do regular operations here in the meantime.
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// Just before scope exit: it hasn't run yet.
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</span><span class="macro">assert_eq!</span>(drop_counter.get(), <span class="number">0</span>);
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<span class="comment">// The following scope end is where the defer closure is called
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</span>}
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<span class="macro">assert_eq!</span>(drop_counter.get(), <span class="number">1</span>);
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}</code></pre></div>
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<h3 id="scope-guard-with-value"><a class="doc-anchor" href="#scope-guard-with-value">§</a>Scope Guard with Value</h3>
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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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<h4 id="1-the-guard-owns-a-file"><a class="doc-anchor" href="#1-the-guard-owns-a-file">§</a>1. The guard owns a file</h4>
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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"><code><span class="kw">extern crate </span>scopeguard;
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<span class="kw">use </span>std::fs::<span class="kw-2">*</span>;
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<span class="kw">use </span>std::io::{<span class="self">self</span>, Write};
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<span class="kw">fn </span>try_main() -> io::Result<()> {
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<span class="kw">let </span>f = File::create(<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>file = scopeguard::guard(f, |f| {
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<span class="comment">// ensure we flush file at return or panic
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</span><span class="kw">let _ </span>= f.sync_all();
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});
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<span class="comment">// Access the file through the scope guard itself
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</span>file.write_all(<span class="string">b"test me\n"</span>).map(|<span class="kw">_</span>| ())
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}
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<span class="kw">fn </span>main() {
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try_main().unwrap();
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}
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</code></pre></div>
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<h4 id="2-the-guard-restores-an-invariant-on-scope-exit"><a class="doc-anchor" href="#2-the-guard-restores-an-invariant-on-scope-exit">§</a>2. The guard restores an invariant on scope exit</h4>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">extern crate </span>scopeguard;
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<span class="kw">use </span>std::mem::ManuallyDrop;
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<span class="kw">use </span>std::ptr;
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<span class="comment">// This function, just for this example, takes the first element
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// and inserts it into the assumed sorted tail of the vector.
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//
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// For optimization purposes we temporarily violate an invariant of the
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// Vec, that it owns all of its elements.
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//
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// The safe approach is to use swap, which means two writes to memory,
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// the optimization is to use a “hole” which uses only one write of memory
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// for each position it moves.
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//
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// We *must* use a scope guard to run this code safely. We
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// are running arbitrary user code (comparison operators) that may panic.
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// The scope guard ensures we restore the invariant after successful
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// exit or during unwinding from panic.
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</span><span class="kw">fn </span>insertion_sort_first<T>(v: <span class="kw-2">&mut </span>Vec<T>)
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<span class="kw">where </span>T: PartialOrd
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{
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<span class="kw">struct </span>Hole<<span class="lifetime">'a</span>, T: <span class="lifetime">'a</span>> {
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v: <span class="kw-2">&</span><span class="lifetime">'a </span><span class="kw-2">mut </span>Vec<T>,
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index: usize,
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value: ManuallyDrop<T>,
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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”.
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</span><span class="kw">let </span>value = ptr::read(<span class="kw-2">&</span>v[<span class="number">0</span>]);
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<span class="kw">let </span><span class="kw-2">mut </span>hole = Hole { v: v, index: <span class="number">0</span>, value: ManuallyDrop::new(value) };
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<span class="comment">// Use a scope guard with a value.
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// At scope exit, plug the hole so that the vector is fully
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// initialized again.
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// The scope guard owns the hole, but we can access it through the guard.
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</span><span class="kw">let </span><span class="kw-2">mut </span>hole_guard = scopeguard::guard(hole, |hole| {
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<span class="comment">// plug the hole in the vector with the value that was // taken out
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</span><span class="kw">let </span>index = hole.index;
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ptr::copy_nonoverlapping(<span class="kw-2">&*</span>hole.value, <span class="kw-2">&mut </span>hole.v[index], <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
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// move the hole until it's in a sorted position
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</span><span class="kw">for </span>i <span class="kw">in </span><span class="number">1</span>..hole_guard.v.len() {
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<span class="kw">if </span><span class="kw-2">*</span>hole_guard.value >= hole_guard.v[i] {
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<span class="comment">// move the element back and the hole forward
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</span><span class="kw">let </span>index = hole_guard.index;
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hole_guard.v.swap(index, index + <span class="number">1</span>);
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hole_guard.index += <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!
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</span>}
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}
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<span class="kw">fn </span>main() {
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<span class="kw">let </span>string = String::from;
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<span class="kw">let </span><span class="kw-2">mut </span>data = <span class="macro">vec!</span>[string(<span class="string">"c"</span>), string(<span class="string">"a"</span>), string(<span class="string">"b"</span>), string(<span class="string">"d"</span>)];
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insertion_sort_first(<span class="kw-2">&mut </span>data);
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<span class="macro">assert_eq!</span>(data, <span class="macro">vec!</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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</code></pre></div>
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<h2 id="crate-features"><a class="doc-anchor" href="#crate-features">§</a>Crate Features</h2>
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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> and <code>OnSuccess</code> strategies.</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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<h2 id="rust-version"><a class="doc-anchor" href="#rust-version">§</a>Rust Version</h2>
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<p>This version of the crate requires Rust 1.20 or later.</p>
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<p>The scopeguard 1.x release series will use a carefully considered version
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upgrade policy, where in a later 1.x version, we will raise the minimum
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required Rust version.</p>
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</div></details><h2 id="macros" class="section-header">Macros<a href="#macros" class="anchor">§</a></h2><ul class="item-table"><li><div class="item-name"><a class="macro" href="macro.defer.html" title="macro scopeguard::defer">defer</a></div><div class="desc docblock-short">Macro to create a <code>ScopeGuard</code> (always run).</div></li></ul><h2 id="structs" class="section-header">Structs<a href="#structs" class="anchor">§</a></h2><ul class="item-table"><li><div class="item-name"><a class="struct" href="struct.ScopeGuard.html" title="struct scopeguard::ScopeGuard">Scope<wbr>Guard</a></div><div class="desc docblock-short"><code>ScopeGuard</code> is a scope guard that may own a protected value.</div></li></ul><h2 id="enums" class="section-header">Enums<a href="#enums" class="anchor">§</a></h2><ul class="item-table"><li><div class="item-name"><a class="enum" href="enum.Always.html" title="enum scopeguard::Always">Always</a></div><div class="desc docblock-short">Always run on scope exit.</div></li></ul><h2 id="traits" class="section-header">Traits<a href="#traits" class="anchor">§</a></h2><ul class="item-table"><li><div class="item-name"><a class="trait" href="trait.Strategy.html" title="trait scopeguard::Strategy">Strategy</a></div><div class="desc docblock-short">Controls in which cases the associated code should be run</div></li></ul><h2 id="functions" class="section-header">Functions<a href="#functions" class="anchor">§</a></h2><ul class="item-table"><li><div class="item-name"><a class="fn" href="fn.guard.html" title="fn scopeguard::guard">guard</a></div><div class="desc docblock-short">Create a new <code>ScopeGuard</code> owning <code>v</code> and with deferred closure <code>dropfn</code>.</div></li></ul></section></div></main></body></html>
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