<!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 `drop` fn in crate `core`."> <meta name="keywords" content="rust, rustlang, rust-lang, drop"> <title>core::mem::drop - 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="../../main.css" id="themeStyle"> <script src="../../storage.js"></script> <link rel="shortcut icon" href="https://doc.rust-lang.org/favicon.ico"> </head> <body class="rustdoc fn"> <!--[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='../../core/index.html'><img src='https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png' alt='logo' width='100'></a> <p class='location'><a href='../index.html'>core</a>::<wbr><a href='index.html'>mem</a></p><script>window.sidebarCurrent = {name: 'drop', ty: 'fn', relpath: ''};</script><script defer src="sidebar-items.js"></script> </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"> </div> </form> </nav> <section id='main' class="content"> <h1 class='fqn'><span class='in-band'>Function <a href='../index.html'>core</a>::<wbr><a href='index.html'>mem</a>::<wbr><a class="fn" href=''>drop</a></span><span class='out-of-band'><span class='since' title='Stable since Rust version 1.0.0'>1.0.0</span><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/core/mem.rs.html#787' title='goto source code'>[src]</a></span></h1> <pre class='rust fn'>pub fn drop<T>(_x: T)</pre><div class='docblock'><p>Disposes of a value.</p> <p>While this does call the argument's implementation of <a href="../ops/trait.Drop.html"><code>Drop</code></a>, it will not release any borrows, as borrows are based on lexical scope.</p> <p>This effectively does nothing for <a href="../../book/first-edition/ownership.html#copy-types">types which implement <code>Copy</code></a>, e.g. integers. Such values are copied and <em>then</em> moved into the function, so the value persists after this function call.</p> <p>This function is not magic; it is literally defined as</p> <pre class="rust rust-example-rendered"> <span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">drop</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>(<span class="ident">_x</span>: <span class="ident">T</span>) { }<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0Afn%20main()%20%7B%0Apub%20fn%20drop%3CT%3E(_x%3A%20T)%20%7B%20%7D%0A%7D">Run</a></pre> <p>Because <code>_x</code> is moved into the function, it is automatically dropped before the function returns.</p> <h1 id="examples" class="section-header"><a href="#examples">Examples</a></h1> <p>Basic usage:</p> <pre class="rust rust-example-rendered"> <span class="kw">let</span> <span class="ident">v</span> <span class="op">=</span> <span class="macro">vec</span><span class="macro">!</span>[<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>]; <span class="ident">drop</span>(<span class="ident">v</span>); <span class="comment">// explicitly drop the vector</span><a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0Afn%20main()%20%7B%0Alet%20v%20%3D%20vec!%5B1%2C%202%2C%203%5D%3B%0A%0Adrop(v)%3B%20%2F%2F%20explicitly%20drop%20the%20vector%0A%7D">Run</a></pre> <p>Borrows are based on lexical scope, so this produces an error:</p> <div class='information'><div class='tooltip compile_fail'>ⓘ<span class='tooltiptext'>This example deliberately fails to compile</span></div></div><pre class="rust rust-example-rendered compile_fail"> <span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">v</span> <span class="op">=</span> <span class="macro">vec</span><span class="macro">!</span>[<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>]; <span class="kw">let</span> <span class="ident">x</span> <span class="op">=</span> <span class="kw-2">&</span><span class="ident">v</span>[<span class="number">0</span>]; <span class="ident">drop</span>(<span class="ident">x</span>); <span class="comment">// explicitly drop the reference, but the borrow still exists</span> <span class="ident">v</span>.<span class="ident">push</span>(<span class="number">4</span>); <span class="comment">// error: cannot borrow `v` as mutable because it is also</span> <span class="comment">// borrowed as immutable</span><a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0Afn%20main()%20%7B%0Alet%20mut%20v%20%3D%20vec!%5B1%2C%202%2C%203%5D%3B%0Alet%20x%20%3D%20%26v%5B0%5D%3B%0A%0Adrop(x)%3B%20%2F%2F%20explicitly%20drop%20the%20reference%2C%20but%20the%20borrow%20still%20exists%0A%0Av.push(4)%3B%20%2F%2F%20error%3A%20cannot%20borrow%20%60v%60%20as%20mutable%20because%20it%20is%20also%0A%20%20%20%20%20%20%20%20%20%20%20%2F%2F%20borrowed%20as%20immutable%0A%7D">Run</a></pre> <p>An inner scope is needed to fix this:</p> <pre class="rust rust-example-rendered"> <span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">v</span> <span class="op">=</span> <span class="macro">vec</span><span class="macro">!</span>[<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>]; { <span class="kw">let</span> <span class="ident">x</span> <span class="op">=</span> <span class="kw-2">&</span><span class="ident">v</span>[<span class="number">0</span>]; <span class="ident">drop</span>(<span class="ident">x</span>); <span class="comment">// this is now redundant, as `x` is going out of scope anyway</span> } <span class="ident">v</span>.<span class="ident">push</span>(<span class="number">4</span>); <span class="comment">// no problems</span><a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0Afn%20main()%20%7B%0Alet%20mut%20v%20%3D%20vec!%5B1%2C%202%2C%203%5D%3B%0A%0A%7B%0A%20%20%20%20let%20x%20%3D%20%26v%5B0%5D%3B%0A%0A%20%20%20%20drop(x)%3B%20%2F%2F%20this%20is%20now%20redundant%2C%20as%20%60x%60%20is%20going%20out%20of%20scope%20anyway%0A%7D%0A%0Av.push(4)%3B%20%2F%2F%20no%20problems%0A%7D">Run</a></pre> <p>Since <a href="../../std/cell/struct.RefCell.html"><code>RefCell</code></a> enforces the borrow rules at runtime, <code>drop</code> can release a <a href="../../std/cell/struct.RefCell.html"><code>RefCell</code></a> borrow:</p> <pre class="rust rust-example-rendered"> <span class="kw">use</span> <span class="ident">std</span>::<span class="ident">cell</span>::<span class="ident">RefCell</span>; <span class="kw">let</span> <span class="ident">x</span> <span class="op">=</span> <span class="ident">RefCell</span>::<span class="ident">new</span>(<span class="number">1</span>); <span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">mutable_borrow</span> <span class="op">=</span> <span class="ident">x</span>.<span class="ident">borrow_mut</span>(); <span class="kw-2">*</span><span class="ident">mutable_borrow</span> <span class="op">=</span> <span class="number">1</span>; <span class="ident">drop</span>(<span class="ident">mutable_borrow</span>); <span class="comment">// relinquish the mutable borrow on this slot</span> <span class="kw">let</span> <span class="ident">borrow</span> <span class="op">=</span> <span class="ident">x</span>.<span class="ident">borrow</span>(); <span class="macro">println</span><span class="macro">!</span>(<span class="string">"{}"</span>, <span class="kw-2">*</span><span class="ident">borrow</span>);<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0Afn%20main()%20%7B%0Ause%20std%3A%3Acell%3A%3ARefCell%3B%0A%0Alet%20x%20%3D%20RefCell%3A%3Anew(1)%3B%0A%0Alet%20mut%20mutable_borrow%20%3D%20x.borrow_mut()%3B%0A*mutable_borrow%20%3D%201%3B%0A%0Adrop(mutable_borrow)%3B%20%2F%2F%20relinquish%20the%20mutable%20borrow%20on%20this%20slot%0A%0Alet%20borrow%20%3D%20x.borrow()%3B%0Aprintln!(%22%7B%7D%22%2C%20*borrow)%3B%0A%7D">Run</a></pre> <p>Integers and other types implementing <a href="../../std/marker/trait.Copy.html"><code>Copy</code></a> are unaffected by <code>drop</code>.</p> <pre class="rust rust-example-rendered"> <span class="attribute">#[<span class="ident">derive</span>(<span class="ident">Copy</span>, <span class="ident">Clone</span>)]</span> <span class="kw">struct</span> <span class="ident">Foo</span>(<span class="ident">u8</span>); <span class="kw">let</span> <span class="ident">x</span> <span class="op">=</span> <span class="number">1</span>; <span class="kw">let</span> <span class="ident">y</span> <span class="op">=</span> <span class="ident">Foo</span>(<span class="number">2</span>); <span class="ident">drop</span>(<span class="ident">x</span>); <span class="comment">// a copy of `x` is moved and dropped</span> <span class="ident">drop</span>(<span class="ident">y</span>); <span class="comment">// a copy of `y` is moved and dropped</span> <span class="macro">println</span><span class="macro">!</span>(<span class="string">"x: {}, y: {}"</span>, <span class="ident">x</span>, <span class="ident">y</span>.<span class="number">0</span>); <span class="comment">// still available</span><a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=%23!%5Ballow(unused)%5D%0Afn%20main()%20%7B%0A%23%5Bderive(Copy%2C%20Clone)%5D%0Astruct%20Foo(u8)%3B%0A%0Alet%20x%20%3D%201%3B%0Alet%20y%20%3D%20Foo(2)%3B%0Adrop(x)%3B%20%2F%2F%20a%20copy%20of%20%60x%60%20is%20moved%20and%20dropped%0Adrop(y)%3B%20%2F%2F%20a%20copy%20of%20%60y%60%20is%20moved%20and%20dropped%0A%0Aprintln!(%22x%3A%20%7B%7D%2C%20y%3A%20%7B%7D%22%2C%20x%2C%20y.0)%3B%20%2F%2F%20still%20available%0A%7D">Run</a></pre> </div></section> <section id='search' class="content hidden"></section> <section class="footer"></section> <aside id="help" class="hidden"> <div> <h1 class="hidden">Help</h1> <div class="shortcuts"> <h2>Keyboard Shortcuts</h2> <dl> <dt><kbd>?</kbd></dt> <dd>Show this help dialog</dd> <dt><kbd>S</kbd></dt> <dd>Focus the search field</dd> <dt><kbd>↑</kbd></dt> <dd>Move up in search results</dd> <dt><kbd>↓</kbd></dt> <dd>Move down in search results</dd> <dt><kbd>↹</kbd></dt> <dd>Switch tab</dd> <dt><kbd>⏎</kbd></dt> <dd>Go to active search result</dd> <dt><kbd>+</kbd></dt> <dd>Expand all sections</dd> <dt><kbd>-</kbd></dt> <dd>Collapse all sections</dd> </dl> </div> <div class="infos"> <h2>Search Tricks</h2> <p> Prefix searches with a type followed by a colon (e.g. <code>fn:</code>) to restrict the search to a given type. </p> <p> Accepted types are: <code>fn</code>, <code>mod</code>, <code>struct</code>, <code>enum</code>, <code>trait</code>, <code>type</code>, <code>macro</code>, and <code>const</code>. </p> <p> Search functions by type signature (e.g. <code>vec -> usize</code> or <code>* -> vec</code>) </p> </div> </div> </aside> <script> window.rootPath = "../../"; 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