42 lines
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42 lines
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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 `needs_drop` fn in crate `nom`."><meta name="keywords" content="rust, rustlang, rust-lang, needs_drop"><title>nom::lib::std::mem::needs_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="../../../../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 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='../../../../nom/index.html'><div class='logo-container'><img src='../../../../rust-logo.png' alt='logo'></div></a><div class="sidebar-elems"><p class='location'><a href='../../../index.html'>nom</a>::<wbr><a href='../../index.html'>lib</a>::<wbr><a href='../index.html'>std</a>::<wbr><a href='index.html'>mem</a></p><script>window.sidebarCurrent = {name: 'needs_drop', ty: 'fn', 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"><div class="search-container"><div><select id="crate-search"><option value="All crates">All crates</option></select><input class="search-input" name="search" disabled 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 class='since' title='Stable since Rust version 1.21.0'>1.21.0</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='https://doc.rust-lang.org/nightly/src/core/mem/mod.rs.html#452-454' title='goto source code'>[src]</a></span><span class='in-band'>Function <a href='../../../index.html'>nom</a>::<wbr><a href='../../index.html'>lib</a>::<wbr><a href='../index.html'>std</a>::<wbr><a href='index.html'>mem</a>::<wbr><a class="fn" href=''>needs_drop</a></span></h1><pre class='rust fn'>pub const fn needs_drop<T>() -> <a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.bool.html">bool</a></pre><div class='docblock'><p>Returns <code>true</code> if dropping values of type <code>T</code> matters.</p>
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<p>This is purely an optimization hint, and may be implemented conservatively:
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it may return <code>true</code> for types that don't actually need to be dropped.
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As such always returning <code>true</code> would be a valid implementation of
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this function. However if this function actually returns <code>false</code>, then you
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can be certain dropping <code>T</code> has no side effect.</p>
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<p>Low level implementations of things like collections, which need to manually
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drop their data, should use this function to avoid unnecessarily
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trying to drop all their contents when they are destroyed. This might not
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make a difference in release builds (where a loop that has no side-effects
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is easily detected and eliminated), but is often a big win for debug builds.</p>
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<p>Note that <a href="../ptr/fn.drop_in_place.html"><code>drop_in_place</code></a> already performs this check, so if your workload
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can be reduced to some small number of <a href="../ptr/fn.drop_in_place.html"><code>drop_in_place</code></a> calls, using this is
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unnecessary. In particular note that you can <a href="../ptr/fn.drop_in_place.html"><code>drop_in_place</code></a> a slice, and that
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will do a single needs_drop check for all the values.</p>
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<p>Types like Vec therefore just <code>drop_in_place(&mut self[..])</code> without using
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<code>needs_drop</code> explicitly. Types like <a href="../../std/collections/struct.HashMap.html"><code>HashMap</code></a>, on the other hand, have to drop
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values one at a time and should use this API.</p>
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<h1 id="examples" class="section-header"><a href="#examples">Examples</a></h1>
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<p>Here's an example of how a collection might make use of <code>needs_drop</code>:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">use</span> <span class="ident">std</span>::{<span class="ident">mem</span>, <span class="ident">ptr</span>};
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<span class="kw">pub</span> <span class="kw">struct</span> <span class="ident">MyCollection</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
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<span class="comment">/* ... */</span>
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}
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<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="ident">Drop</span> <span class="kw">for</span> <span class="ident">MyCollection</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
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<span class="kw">fn</span> <span class="ident">drop</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>) {
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<span class="kw">unsafe</span> {
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<span class="comment">// drop the data</span>
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<span class="kw">if</span> <span class="ident">mem</span>::<span class="ident">needs_drop</span>::<span class="op"><</span><span class="ident">T</span><span class="op">></span>() {
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<span class="kw">for</span> <span class="ident">x</span> <span class="kw">in</span> <span class="self">self</span>.<span class="ident">iter_mut</span>() {
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<span class="ident">ptr</span>::<span class="ident">drop_in_place</span>(<span class="ident">x</span>);
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}
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}
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<span class="self">self</span>.<span class="ident">free_buffer</span>();
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}
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}
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}</pre></div>
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