<!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 `UnsafeCell` struct in crate `core`."><meta name="keywords" content="rust, rustlang, rust-lang, UnsafeCell"><title>core::cell::UnsafeCell - 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 struct"><!--[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'>Struct UnsafeCell</p><div class="sidebar-elems"><div class="block items"><a class="sidebar-title" href="#methods">Methods</a><div class="sidebar-links"><a href="#method.new">new</a><a href="#method.into_inner">into_inner</a><a href="#method.get">get</a></div><a class="sidebar-title" href="#implementations">Trait Implementations</a><div class="sidebar-links"><a href="#impl-Sync">!Sync</a><a href="#impl-Default">Default</a><a href="#impl-From%3CT%3E">From<T></a><a href="#impl-CoerceUnsized%3CUnsafeCell%3CU%3E%3E">CoerceUnsized<UnsafeCell<U>></a><a href="#impl-Debug">Debug</a></div><a class="sidebar-title" href="#synthetic-implementations">Auto Trait Implementations</a><div class="sidebar-links"><a href="#impl-Send">Send</a></div></div><p class='location'><a href='../index.html'>core</a>::<wbr><a href='index.html'>cell</a></p><script>window.sidebarCurrent = {name: 'UnsafeCell', ty: 'struct', 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'>Struct <a href='../index.html'>core</a>::<wbr><a href='index.html'>cell</a>::<wbr><a class="struct" href=''>UnsafeCell</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/cell.rs.html#1392-1394' title='goto source code'>[src]</a></span></h1><div class="docblock type-decl"><pre class='rust struct'><div class="docblock attributes">#[lang = "unsafe_cell"]
</div>pub struct UnsafeCell<T: ?<a class="trait" href="../../core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a>> { /* fields omitted */ }</pre></div><div class='docblock'><p>The core primitive for interior mutability in Rust.</p>
<p><code>UnsafeCell<T></code> is a type that wraps some <code>T</code> and indicates unsafe interior operations on the
wrapped type. Types with an <code>UnsafeCell<T></code> field are considered to have an 'unsafe interior'.
The <code>UnsafeCell<T></code> type is the only legal way to obtain aliasable data that is considered
mutable. In general, transmuting an <code>&T</code> type into an <code>&mut T</code> is considered undefined behavior.</p>
<p>If you have a reference <code>&SomeStruct</code>, then normally in Rust all fields of <code>SomeStruct</code> are
immutable. The compiler makes optimizations based on the knowledge that <code>&T</code> is not mutably
aliased or mutated, and that <code>&mut T</code> is unique. <code>UnsafeCell<T></code> is the only core language
feature to work around this restriction. All other types that allow internal mutability, such as
<code>Cell<T></code> and <code>RefCell<T></code>, use <code>UnsafeCell</code> to wrap their internal data.</p>
<p>The <code>UnsafeCell</code> API itself is technically very simple: it gives you a raw pointer <code>*mut T</code> to
its contents. It is up to <em>you</em> as the abstraction designer to use that raw pointer correctly.</p>
<p>The precise Rust aliasing rules are somewhat in flux, but the main points are not contentious:</p>
<ul>
<li>
<p>If you create a safe reference with lifetime <code>'a</code> (either a <code>&T</code> or <code>&mut T</code>
reference) that is accessible by safe code (for example, because you returned it),
then you must not access the data in any way that contradicts that reference for the
remainder of <code>'a</code>. For example, this means that if you take the <code>*mut T</code> from an
<code>UnsafeCell<T></code> and cast it to an <code>&T</code>, then the data in <code>T</code> must remain immutable
(modulo any <code>UnsafeCell</code> data found within <code>T</code>, of course) until that reference's
lifetime expires. Similarly, if you create a <code>&mut T</code> reference that is released to
safe code, then you must not access the data within the <code>UnsafeCell</code> until that
reference expires.</p>
</li>
<li>
<p>At all times, you must avoid data races. If multiple threads have access to
the same <code>UnsafeCell</code>, then any writes must have a proper happens-before relation to all other
accesses (or use atomics).</p>
</li>
</ul>
<p>To assist with proper design, the following scenarios are explicitly declared legal
for single-threaded code:</p>
<ol>
<li>
<p>A <code>&T</code> reference can be released to safe code and there it can co-exist with other <code>&T</code>
references, but not with a <code>&mut T</code></p>
</li>
<li>
<p>A <code>&mut T</code> reference may be released to safe code provided neither other <code>&mut T</code> nor <code>&T</code>
co-exist with it. A <code>&mut T</code> must always be unique.</p>
</li>
</ol>
<p>Note that while mutating or mutably aliasing the contents of an <code>&UnsafeCell<T></code> is
okay (provided you enforce the invariants some other way), it is still undefined behavior
to have multiple <code>&mut UnsafeCell<T></code> aliases.</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">cell</span>::<span class="ident">UnsafeCell</span>;
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">marker</span>::<span class="ident">Sync</span>;
<span class="kw">struct</span> <span class="ident">NotThreadSafe</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
<span class="ident">value</span>: <span class="ident">UnsafeCell</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>,
}
<span class="kw">unsafe</span> <span class="kw">impl</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="ident">Sync</span> <span class="kw">for</span> <span class="ident">NotThreadSafe</span><span class="op"><</span><span class="ident">T</span><span class="op">></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%3AUnsafeCell%3B%0Ause%20std%3A%3Amarker%3A%3ASync%3B%0A%0A%23%5Ballow(dead_code)%5D%0Astruct%20NotThreadSafe%3CT%3E%20%7B%0A%20%20%20%20value%3A%20UnsafeCell%3CT%3E%2C%0A%7D%0A%0Aunsafe%20impl%3CT%3E%20Sync%20for%20NotThreadSafe%3CT%3E%20%7B%7D%0A%7D">Run</a></pre>
</div>
<h2 id='methods' class='small-section-header'>
Methods<a href='#methods' class='anchor'></a>
</h2>
<h3 id='impl' class='impl'><span class='in-band'><table class='table-display'><tbody><tr><td><code>impl<T> <a class="struct" href="../../core/cell/struct.UnsafeCell.html" title="struct core::cell::UnsafeCell">UnsafeCell</a><T></code><a href='#impl' class='anchor'></a></span></td><td><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../../src/core/cell.rs.html#1399-1434' title='goto source code'>[src]</a></span></td></tr></tbody></table></h3><div class='impl-items'><h4 id='method.new' class="method"><span id='new.v' class='invisible'><table class='table-display'><tbody><tr><td><code>pub const fn <a href='#method.new' class='fnname'>new</a>(value: T) -> <a class="struct" href="../../core/cell/struct.UnsafeCell.html" title="struct core::cell::UnsafeCell">UnsafeCell</a><T></code></span></td><td><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../../src/core/cell.rs.html#1414-1416' title='goto source code'>[src]</a></td></tr></tbody></table></span></h4><div class='docblock'><p>Constructs a new instance of <code>UnsafeCell</code> which will wrap the specified
value.</p>
<p>All access to the inner value through methods is <code>unsafe</code>.</p>
<h1 id="examples-1" class="section-header"><a href="#examples-1">Examples</a></h1>
<pre class="rust rust-example-rendered">
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">cell</span>::<span class="ident">UnsafeCell</span>;
<span class="kw">let</span> <span class="ident">uc</span> <span class="op">=</span> <span class="ident">UnsafeCell</span>::<span class="ident">new</span>(<span class="number">5</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%3AUnsafeCell%3B%0A%0Alet%20uc%20%3D%20UnsafeCell%3A%3Anew(5)%3B%0A%7D">Run</a></pre>
</div><h4 id='method.into_inner' class="method"><span id='into_inner.v' class='invisible'><table class='table-display'><tbody><tr><td><code>pub fn <a href='#method.into_inner' class='fnname'>into_inner</a>(self) -> T</code></span></td><td><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../../src/core/cell.rs.html#1431-1433' title='goto source code'>[src]</a></td></tr></tbody></table></span></h4><div class='docblock'><p>Unwraps the value.</p>
<h1 id="examples-2" class="section-header"><a href="#examples-2">Examples</a></h1>
<pre class="rust rust-example-rendered">
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">cell</span>::<span class="ident">UnsafeCell</span>;
<span class="kw">let</span> <span class="ident">uc</span> <span class="op">=</span> <span class="ident">UnsafeCell</span>::<span class="ident">new</span>(<span class="number">5</span>);
<span class="kw">let</span> <span class="ident">five</span> <span class="op">=</span> <span class="ident">uc</span>.<span class="ident">into_inner</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%3AUnsafeCell%3B%0A%0Alet%20uc%20%3D%20UnsafeCell%3A%3Anew(5)%3B%0A%0Alet%20five%20%3D%20uc.into_inner()%3B%0A%7D">Run</a></pre>
</div></div><h3 id='impl-1' class='impl'><span class='in-band'><table class='table-display'><tbody><tr><td><code>impl<T: ?<a class="trait" href="../../core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a>> <a class="struct" href="../../core/cell/struct.UnsafeCell.html" title="struct core::cell::UnsafeCell">UnsafeCell</a><T></code><a href='#impl-1' class='anchor'></a></span></td><td><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../../src/core/cell.rs.html#1436-1458' title='goto source code'>[src]</a></span></td></tr></tbody></table></h3><div class='impl-items'><h4 id='method.get' class="method"><span id='get.v' class='invisible'><table class='table-display'><tbody><tr><td><code>pub fn <a href='#method.get' class='fnname'>get</a>(&self) -> *mut T</code></span></td><td><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../../src/core/cell.rs.html#1455-1457' title='goto source code'>[src]</a></td></tr></tbody></table></span></h4><div class='docblock'><p>Gets a mutable pointer to the wrapped value.</p>
<p>This can be cast to a pointer of any kind.
Ensure that the access is unique (no active references, mutable or not)
when casting to <code>&mut T</code>, and ensure that there are no mutations
or mutable aliases going on when casting to <code>&T</code></p>
<h1 id="examples-3" class="section-header"><a href="#examples-3">Examples</a></h1>
<pre class="rust rust-example-rendered">
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">cell</span>::<span class="ident">UnsafeCell</span>;
<span class="kw">let</span> <span class="ident">uc</span> <span class="op">=</span> <span class="ident">UnsafeCell</span>::<span class="ident">new</span>(<span class="number">5</span>);
<span class="kw">let</span> <span class="ident">five</span> <span class="op">=</span> <span class="ident">uc</span>.<span class="ident">get</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%3AUnsafeCell%3B%0A%0Alet%20uc%20%3D%20UnsafeCell%3A%3Anew(5)%3B%0A%0Alet%20five%20%3D%20uc.get()%3B%0A%7D">Run</a></pre>
</div></div>
<h2 id='implementations' class='small-section-header'>
Trait Implementations<a href='#implementations' class='anchor'></a>
</h2>
<div id='implementations-list'><h3 id='impl-Sync' class='impl'><span class='in-band'><table class='table-display'><tbody><tr><td><code>impl<T: ?<a class="trait" href="../../core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a>> !<a class="trait" href="../../core/marker/trait.Sync.html" title="trait core::marker::Sync">Sync</a> for <a class="struct" href="../../core/cell/struct.UnsafeCell.html" title="struct core::cell::UnsafeCell">UnsafeCell</a><T></code><a href='#impl-Sync' class='anchor'></a></span></td><td><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../../src/core/cell.rs.html#1397' title='goto source code'>[src]</a></span></td></tr></tbody></table></h3><div class='impl-items'></div><h3 id='impl-Default' class='impl'><span class='in-band'><table class='table-display'><tbody><tr><td><code>impl<T: <a class="trait" href="../../core/default/trait.Default.html" title="trait core::default::Default">Default</a>> <a class="trait" href="../../core/default/trait.Default.html" title="trait core::default::Default">Default</a> for <a class="struct" href="../../core/cell/struct.UnsafeCell.html" title="struct core::cell::UnsafeCell">UnsafeCell</a><T></code><a href='#impl-Default' class='anchor'></a></span></td><td><span class='out-of-band'><div class='ghost'></div><div class='since' title='Stable since Rust version 1.10.0'>1.10.0</div><a class='srclink' href='../../src/core/cell.rs.html#1461-1466' title='goto source code'>[src]</a></span></td></tr></tbody></table></h3><div class='impl-items'><h4 id='method.default' class="method"><span id='default.v' class='invisible'><table class='table-display'><tbody><tr><td><code>fn <a href='../../core/default/trait.Default.html#tymethod.default' class='fnname'>default</a>() -> <a class="struct" href="../../core/cell/struct.UnsafeCell.html" title="struct core::cell::UnsafeCell">UnsafeCell</a><T></code></span></td><td><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../../src/core/cell.rs.html#1463-1465' title='goto source code'>[src]</a></td></tr></tbody></table></span></h4><div class='docblock'><p>Creates an <code>UnsafeCell</code>, with the <code>Default</code> value for T.</p>
</div></div><h3 id='impl-From%3CT%3E' class='impl'><span class='in-band'><table class='table-display'><tbody><tr><td><code>impl<T> <a class="trait" href="../../core/convert/trait.From.html" title="trait core::convert::From">From</a><T> for <a class="struct" href="../../core/cell/struct.UnsafeCell.html" title="struct core::cell::UnsafeCell">UnsafeCell</a><T></code><a href='#impl-From%3CT%3E' class='anchor'></a></span></td><td><span class='out-of-band'><div class='ghost'></div><div class='since' title='Stable since Rust version 1.12.0'>1.12.0</div><a class='srclink' href='../../src/core/cell.rs.html#1469-1473' title='goto source code'>[src]</a></span></td></tr></tbody></table></h3><div class='impl-items'><h4 id='method.from' class="method"><span id='from.v' class='invisible'><table class='table-display'><tbody><tr><td><code>fn <a href='../../core/convert/trait.From.html#tymethod.from' class='fnname'>from</a>(t: T) -> <a class="struct" href="../../core/cell/struct.UnsafeCell.html" title="struct core::cell::UnsafeCell">UnsafeCell</a><T></code></span></td><td><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../../src/core/cell.rs.html#1470-1472' title='goto source code'>[src]</a></td></tr></tbody></table></span></h4><div class='docblock'><p>Performs the conversion.</p>
</div></div><h3 id='impl-CoerceUnsized%3CUnsafeCell%3CU%3E%3E' class='impl'><span class='in-band'><table class='table-display'><tbody><tr><td><code>impl<T: <a class="trait" href="../../core/ops/trait.CoerceUnsized.html" title="trait core::ops::CoerceUnsized">CoerceUnsized</a><U>, U> <a class="trait" href="../../core/ops/trait.CoerceUnsized.html" title="trait core::ops::CoerceUnsized">CoerceUnsized</a><<a class="struct" href="../../core/cell/struct.UnsafeCell.html" title="struct core::cell::UnsafeCell">UnsafeCell</a><U>> for <a class="struct" href="../../core/cell/struct.UnsafeCell.html" title="struct core::cell::UnsafeCell">UnsafeCell</a><T></code><a href='#impl-CoerceUnsized%3CUnsafeCell%3CU%3E%3E' class='anchor'></a></span></td><td><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../../src/core/cell.rs.html#1476' title='goto source code'>[src]</a></span></td></tr></tbody></table></h3><div class='impl-items'></div><h3 id='impl-Debug' class='impl'><span class='in-band'><table class='table-display'><tbody><tr><td><code>impl<T: ?<a class="trait" href="../../core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a> + <a class="trait" href="../../core/fmt/trait.Debug.html" title="trait core::fmt::Debug">Debug</a>> <a class="trait" href="../../core/fmt/trait.Debug.html" title="trait core::fmt::Debug">Debug</a> for <a class="struct" href="../../core/cell/struct.UnsafeCell.html" title="struct core::cell::UnsafeCell">UnsafeCell</a><T></code><a href='#impl-Debug' class='anchor'></a></span></td><td><span class='out-of-band'><div class='ghost'></div><div class='since' title='Stable since Rust version 1.9.0'>1.9.0</div><a class='srclink' href='../../src/core/fmt/mod.rs.html#2104-2108' title='goto source code'>[src]</a></span></td></tr></tbody></table></h3><div class='impl-items'><h4 id='method.fmt' class="method"><span id='fmt.v' class='invisible'><table class='table-display'><tbody><tr><td><code>fn <a href='../../core/fmt/trait.Debug.html#tymethod.fmt' class='fnname'>fmt</a>(&self, f: &mut <a class="struct" href="../../core/fmt/struct.Formatter.html" title="struct core::fmt::Formatter">Formatter</a>) -> <a class="type" href="../../core/fmt/type.Result.html" title="type core::fmt::Result">Result</a></code></span></td><td><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../../src/core/fmt/mod.rs.html#2105-2107' title='goto source code'>[src]</a></td></tr></tbody></table></span></h4><div class='docblock'><p>Formats the value using the given formatter. <a href="../../core/fmt/trait.Debug.html#tymethod.fmt">Read more</a></p>
</div></div></div>
<h2 id='synthetic-implementations' class='small-section-header'>
Auto Trait Implementations<a href='#synthetic-implementations' class='anchor'></a>
</h2>
<div id='synthetic-implementations-list'>
<h3 id='impl-Send' class='impl'><span class='in-band'><table class='table-display'><tbody><tr><td><code>impl<T: ?<a class="trait" href="../../core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a>> <a class="trait" href="../../core/marker/trait.Send.html" title="trait core::marker::Send">Send</a> for <a class="struct" href="../../core/cell/struct.UnsafeCell.html" title="struct core::cell::UnsafeCell">UnsafeCell</a><T> <span class="where fmt-newline">where<br> T: <a class="trait" href="../../core/marker/trait.Send.html" title="trait core::marker::Send">Send</a>, </span></code><a href='#impl-Send' class='anchor'></a></span></td><td><span class='out-of-band'></span></td></tr></tbody></table></h3><div class='impl-items'></div></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><p>Search multiple things at once by splitting your query with comma (e.g. <code>str,u8</code> or <code>String,struct:Vec,test</code>)</p></div></div></aside><script>window.rootPath = "../../";window.currentCrate = "core";</script><script src="../../aliases.js"></script><script src="../../main.js"></script><script defer src="../../search-index.js"></script></body></html>
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