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<div class="sidebar-menu">☰</div>
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<p class='location'>Crate scopeguard</p><div class="sidebar-elems"><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>
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<section id='main' class="content">
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<h1 class='fqn'><span class='in-band'>Crate <a class="mod" href=''>scopeguard</a></span><span class='out-of-band'><span id='render-detail'>
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<a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">
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[<span class='inner'>−</span>]
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</span><a class='srclink' href='../src/scopeguard/lib.rs.html#1-409' title='goto source code'>[src]</a></span></h1>
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<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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<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>
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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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<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="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="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="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>
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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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<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="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="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>
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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>
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<tr class=' module-item'>
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<td><a class="macro" href="macro.defer.html"
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title='macro scopeguard::defer'>defer</a></td>
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<td class='docblock-short'>
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<p>Macro to create a <code>ScopeGuard</code> (always run).</p>
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</td>
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</tr>
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<tr class=' module-item'>
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<td><a class="macro" href="macro.defer_on_unwind.html"
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title='macro scopeguard::defer_on_unwind'>defer_on_unwind</a></td>
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<td class='docblock-short'>
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<p>Macro to create a <code>ScopeGuard</code> (run on unwinding from panic).</p>
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</td>
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</tr></table><h2 id='structs' class='section-header'><a href="#structs">Structs</a></h2>
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<table>
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||
<tr class=' module-item'>
|
||
<td><a class="struct" href="struct.ScopeGuard.html"
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title='struct scopeguard::ScopeGuard'>ScopeGuard</a></td>
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||
<td class='docblock-short'>
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||
<p><code>ScopeGuard</code> is a scope guard that may own a protected value.</p>
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||
</td>
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||
</tr></table><h2 id='enums' class='section-header'><a href="#enums">Enums</a></h2>
|
||
<table>
|
||
<tr class=' module-item'>
|
||
<td><a class="enum" href="enum.Always.html"
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||
title='enum scopeguard::Always'>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='trait scopeguard::Strategy'>Strategy</a></td>
|
||
<td class='docblock-short'>
|
||
|
||
</td>
|
||
</tr></table><h2 id='functions' class='section-header'><a href="#functions">Functions</a></h2>
|
||
<table>
|
||
<tr class=' module-item'>
|
||
<td><a class="fn" href="fn.guard.html"
|
||
title='fn scopeguard::guard'>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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</td>
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</tr></table></section>
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<h2>Search Tricks</h2>
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Prefix searches with a type followed by a colon (e.g.
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Accepted types are: <code>fn</code>, <code>mod</code>,
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Search functions by type signature (e.g.
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