<!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 `transmute_copy` fn in crate `std`."><meta name="keywords" content="rust, rustlang, rust-lang, transmute_copy"><title>std::mem::transmute_copy - 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><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='../../std/index.html'><img src='https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png' alt='logo' width='100'></a><div class="sidebar-elems"><p class='location'><a href='../index.html'>std</a>::<wbr><a href='index.html'>mem</a></p><script>window.sidebarCurrent = {name: 'transmute_copy', 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 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"><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='in-band'>Function <a href='../index.html'>std</a>::<wbr><a href='index.html'>mem</a>::<wbr><a class="fn" href=''>transmute_copy</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#835-837' title='goto source code'>[src]</a></span></h1><pre class='rust fn'>pub unsafe fn transmute_copy<T, U>(src: <a class="primitive" href="../primitive.reference.html">&</a>T) -> U</pre><div class='docblock'><p>Interprets <code>src</code> as having type <code>&U</code>, and then reads <code>src</code> without moving
the contained value.</p>
<p>This function will unsafely assume the pointer <code>src</code> is valid for
<a href="fn.size_of.html"><code>size_of::<U></code></a> bytes by transmuting <code>&T</code> to <code>&U</code> and then reading
the <code>&U</code>. It will also unsafely create a copy of the contained value instead of
moving out of <code>src</code>.</p>
<p>It is not a compile-time error if <code>T</code> and <code>U</code> have different sizes, but it
is highly encouraged to only invoke this function where <code>T</code> and <code>U</code> have the
same size. This function triggers <a href="../../reference/behavior-considered-undefined.html">undefined behavior</a> if <code>U</code> is larger than
<code>T</code>.</p>
<h1 id="examples" class="section-header"><a href="#examples">Examples</a></h1>
<pre class="rust rust-example-rendered">
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">mem</span>;
<span class="attribute">#[<span class="ident">repr</span>(<span class="ident">packed</span>)]</span>
<span class="kw">struct</span> <span class="ident">Foo</span> {
<span class="ident">bar</span>: <span class="ident">u8</span>,
}
<span class="kw">let</span> <span class="ident">foo_slice</span> <span class="op">=</span> [<span class="number">10u8</span>];
<span class="kw">unsafe</span> {
<span class="comment">// Copy the data from 'foo_slice' and treat it as a 'Foo'</span>
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">foo_struct</span>: <span class="ident">Foo</span> <span class="op">=</span> <span class="ident">mem</span>::<span class="ident">transmute_copy</span>(<span class="kw-2">&</span><span class="ident">foo_slice</span>);
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">foo_struct</span>.<span class="ident">bar</span>, <span class="number">10</span>);
<span class="comment">// Modify the copied data</span>
<span class="ident">foo_struct</span>.<span class="ident">bar</span> <span class="op">=</span> <span class="number">20</span>;
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">foo_struct</span>.<span class="ident">bar</span>, <span class="number">20</span>);
}
<span class="comment">// The contents of 'foo_slice' should not have changed</span>
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">foo_slice</span>, [<span class="number">10</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%3Amem%3B%0A%0A%23%5Brepr(packed)%5D%0Astruct%20Foo%20%7B%0A%20%20%20%20bar%3A%20u8%2C%0A%7D%0A%0Alet%20foo_slice%20%3D%20%5B10u8%5D%3B%0A%0Aunsafe%20%7B%0A%20%20%20%20%2F%2F%20Copy%20the%20data%20from%20'foo_slice'%20and%20treat%20it%20as%20a%20'Foo'%0A%20%20%20%20let%20mut%20foo_struct%3A%20Foo%20%3D%20mem%3A%3Atransmute_copy(%26foo_slice)%3B%0A%20%20%20%20assert_eq!(foo_struct.bar%2C%2010)%3B%0A%0A%20%20%20%20%2F%2F%20Modify%20the%20copied%20data%0A%20%20%20%20foo_struct.bar%20%3D%2020%3B%0A%20%20%20%20assert_eq!(foo_struct.bar%2C%2020)%3B%0A%7D%0A%0A%2F%2F%20The%20contents%20of%20'foo_slice'%20should%20not%20have%20changed%0Aassert_eq!(foo_slice%2C%20%5B10%5D)%3B%0A%7D">Run</a></pre>
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