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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 `replace` fn in crate `nom`."><meta name="keywords" content="rust, rustlang, rust-lang, replace"><title>nom::lib::std::mem::replace - 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: 'replace', 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 1.0.0'>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='https://doc.rust-lang.org/nightly/src/core/mem/mod.rs.html#680-683' 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=''>replace</a></span></h1><pre class='rust fn'>pub fn replace<T>(dest: <a class="primitive" href="https://doc.rust-lang.org/nightly/std/primitive.reference.html">&mut </a>T, src: T) -> T</pre><div class='docblock'><p>Moves <code>src</code> into the referenced <code>dest</code>, returning the previous <code>dest</code> value.</p>
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<p>Neither value is dropped.</p>
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<h1 id="examples" class="section-header"><a href="#examples">Examples</a></h1>
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<p>A simple example:</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>;
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<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">v</span>: <span class="ident">Vec</span><span class="op"><</span><span class="ident">i32</span><span class="op">></span> <span class="op">=</span> <span class="macro">vec</span><span class="macro">!</span>[<span class="number">1</span>, <span class="number">2</span>];
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<span class="kw">let</span> <span class="ident">old_v</span> <span class="op">=</span> <span class="ident">mem</span>::<span class="ident">replace</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">v</span>, <span class="macro">vec</span><span class="macro">!</span>[<span class="number">3</span>, <span class="number">4</span>, <span class="number">5</span>]);
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<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="macro">vec</span><span class="macro">!</span>[<span class="number">1</span>, <span class="number">2</span>], <span class="ident">old_v</span>);
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<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="macro">vec</span><span class="macro">!</span>[<span class="number">3</span>, <span class="number">4</span>, <span class="number">5</span>], <span class="ident">v</span>);</pre></div>
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<p><code>replace</code> allows consumption of a struct field by replacing it with another value.
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Without <code>replace</code> you can run into issues like these:</p>
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<div class='information'><div class='tooltip compile_fail'>ⓘ<span class='tooltiptext'>This example deliberately fails to compile</span></div></div><div class="example-wrap"><pre class="rust rust-example-rendered compile_fail">
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<span class="kw">struct</span> <span class="ident">Buffer</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> { <span class="ident">buf</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">impl</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="ident">Buffer</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">replace_index</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>, <span class="ident">i</span>: <span class="ident">usize</span>, <span class="ident">v</span>: <span class="ident">T</span>) <span class="op">-</span><span class="op">></span> <span class="ident">T</span> {
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<span class="comment">// error: cannot move out of dereference of `&mut`-pointer</span>
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<span class="kw">let</span> <span class="ident">t</span> <span class="op">=</span> <span class="self">self</span>.<span class="ident">buf</span>[<span class="ident">i</span>];
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<span class="self">self</span>.<span class="ident">buf</span>[<span class="ident">i</span>] <span class="op">=</span> <span class="ident">v</span>;
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<span class="ident">t</span>
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}
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}</pre></div>
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<p>Note that <code>T</code> does not necessarily implement <a href="../../std/clone/trait.Clone.html"><code>Clone</code></a>, so we can't even clone <code>self.buf[i]</code> to
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avoid the move. But <code>replace</code> can be used to disassociate the original value at that index from
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<code>self</code>, allowing it to be returned:</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>;
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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">Buffer</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">replace_index</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>, <span class="ident">i</span>: <span class="ident">usize</span>, <span class="ident">v</span>: <span class="ident">T</span>) <span class="op">-</span><span class="op">></span> <span class="ident">T</span> {
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<span class="ident">mem</span>::<span class="ident">replace</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>.<span class="ident">buf</span>[<span class="ident">i</span>], <span class="ident">v</span>)
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}
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}
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<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">buffer</span> <span class="op">=</span> <span class="ident">Buffer</span> { <span class="ident">buf</span>: <span class="macro">vec</span><span class="macro">!</span>[<span class="number">0</span>, <span class="number">1</span>] };
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<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">buffer</span>.<span class="ident">buf</span>[<span class="number">0</span>], <span class="number">0</span>);
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<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">buffer</span>.<span class="ident">replace_index</span>(<span class="number">0</span>, <span class="number">2</span>), <span class="number">0</span>);
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<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">buffer</span>.<span class="ident">buf</span>[<span class="number">0</span>], <span class="number">2</span>);</pre></div>
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