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<p class='location'>Struct UnsafeCell</p><div class="block items"><ul><li><a href="#methods">Methods</a></li><li><a href="#implementations">Trait Implementations</a></li></ul></div><p class='location'><a href='../index.html'>std</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>
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<h1 class='fqn'><span class='in-band'>Struct <a href='../index.html'>std</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#1075-1077' title='goto source code'>[src]</a></span></h1>
<pre class='rust struct'><div class="docblock attributes">#[lang = "unsafe_cell"]
</div>pub struct UnsafeCell<T> <span class="where fmt-newline">where<br> T: ?<a class="trait" href="../../std/marker/trait.Sized.html" title="trait std::marker::Sized">Sized</a>, </span> { /* fields omitted */ }</pre><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>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. When building abstractions like <code>Cell</code>, <code>RefCell</code>,
<code>Mutex</code>, etc, you need to turn these optimizations off. <code>UnsafeCell</code> is the only legal way
to do this. When <code>UnsafeCell<T></code> is immutably aliased, it is still safe to obtain a mutable
reference to its interior and/or to mutate it. However, it is up to the abstraction designer
to ensure that no two mutable references obtained this way are active at the same time, and
that there are no active mutable references or mutations when an immutable reference is obtained
from the cell. This is often done via runtime checks.</p>
<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>
<p>Types like <code>Cell<T></code> and <code>RefCell<T></code> use this type to wrap their internal data.</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=fn%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'>Methods</h2><h3 class='impl'><span class='in-band'><code>impl<T> <a class="struct" href="../../std/cell/struct.UnsafeCell.html" title="struct std::cell::UnsafeCell">UnsafeCell</a><T></code></span><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../../src/core/cell.rs.html#1082-1122' title='goto source code'>[src]</a></span></h3>
<div class='impl-items'><h4 id='method.new' class="method"><span id='new.v' class='invisible'><code>const fn <a href='#method.new' class='fnname'>new</a>(value: T) -> <a class="struct" href="../../std/cell/struct.UnsafeCell.html" title="struct std::cell::UnsafeCell">UnsafeCell</a><T></code></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=fn%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'><code>unsafe fn <a href='#method.into_inner' class='fnname'>into_inner</a>(self) -> T</code></span></h4>
<div class='docblock'><p>Unwraps the value.</p>
<h1 id='safety' class='section-header'><a href='#safety'>Safety</a></h1>
<p>This function is unsafe because this thread or another thread may currently be
inspecting the inner 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="kw">unsafe</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=fn%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%20unsafe%20%7B%20uc.into_inner()%20%7D%3B%0A%7D">Run</a></pre>
</div></div><h3 class='impl'><span class='in-band'><code>impl<T> <a class="struct" href="../../std/cell/struct.UnsafeCell.html" title="struct std::cell::UnsafeCell">UnsafeCell</a><T> <span class="where fmt-newline">where<br> T: ?<a class="trait" href="../../std/marker/trait.Sized.html" title="trait std::marker::Sized">Sized</a>, </span></code></span><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../../src/core/cell.rs.html#1124-1146' title='goto source code'>[src]</a></span></h3>
<div class='impl-items'><h4 id='method.get' class="method"><span id='get.v' class='invisible'><code>fn <a href='#method.get' class='fnname'>get</a>(&self) -> <a class="primitive" href="../primitive.pointer.html">*mut T</a></code></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 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=fn%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'>Trait Implementations</h2><h3 class='impl'><span class='in-band'><code>impl<T, U> <a class="trait" href="../../std/ops/trait.CoerceUnsized.html" title="trait std::ops::CoerceUnsized">CoerceUnsized</a><<a class="struct" href="../../std/cell/struct.UnsafeCell.html" title="struct std::cell::UnsafeCell">UnsafeCell</a><U>> for <a class="struct" href="../../std/cell/struct.UnsafeCell.html" title="struct std::cell::UnsafeCell">UnsafeCell</a><T> <span class="where fmt-newline">where<br> T: <a class="trait" href="../../std/ops/trait.CoerceUnsized.html" title="trait std::ops::CoerceUnsized">CoerceUnsized</a><U>, </span></code></span><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../../src/core/cell.rs.html#1164' title='goto source code'>[src]</a></span></h3>
<div class='impl-items'></div><h3 class='impl'><span class='in-band'><code>impl<T> !<a class="trait" href="../../std/marker/trait.Sync.html" title="trait std::marker::Sync">Sync</a> for <a class="struct" href="../../std/cell/struct.UnsafeCell.html" title="struct std::cell::UnsafeCell">UnsafeCell</a><T> <span class="where fmt-newline">where<br> T: ?<a class="trait" href="../../std/marker/trait.Sized.html" title="trait std::marker::Sized">Sized</a>, </span></code></span><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../../src/core/cell.rs.html#1080' title='goto source code'>[src]</a></span></h3>
<div class='impl-items'></div><h3 class='impl'><span class='in-band'><code>impl<T> <a class="trait" href="../../std/convert/trait.From.html" title="trait std::convert::From">From</a><T> for <a class="struct" href="../../std/cell/struct.UnsafeCell.html" title="struct std::cell::UnsafeCell">UnsafeCell</a><T></code></span><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#1157-1161' title='goto source code'>[src]</a></span></h3>
<div class='impl-items'><h4 id='method.from' class="method"><span id='from.v' class='invisible'><code>fn <a href='../../std/convert/trait.From.html#tymethod.from' class='fnname'>from</a>(t: T) -> <a class="struct" href="../../std/cell/struct.UnsafeCell.html" title="struct std::cell::UnsafeCell">UnsafeCell</a><T></code></span></h4>
<div class='docblock'><p>Performs the conversion.</p>
</div></div><h3 class='impl'><span class='in-band'><code>impl<T> <a class="trait" href="../../std/default/trait.Default.html" title="trait std::default::Default">Default</a> for <a class="struct" href="../../std/cell/struct.UnsafeCell.html" title="struct std::cell::UnsafeCell">UnsafeCell</a><T> <span class="where fmt-newline">where<br> T: <a class="trait" href="../../std/default/trait.Default.html" title="trait std::default::Default">Default</a>, </span></code></span><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#1149-1154' title='goto source code'>[src]</a></span></h3>
<div class='impl-items'><h4 id='method.default' class="method"><span id='default.v' class='invisible'><code>fn <a href='../../std/default/trait.Default.html#tymethod.default' class='fnname'>default</a>() -> <a class="struct" href="../../std/cell/struct.UnsafeCell.html" title="struct std::cell::UnsafeCell">UnsafeCell</a><T></code></span></h4>
<div class='docblock'><p>Creates an <code>UnsafeCell</code>, with the <code>Default</code> value for T.</p>
</div></div><h3 class='impl'><span class='in-band'><code>impl<T> <a class="trait" href="../../std/fmt/trait.Debug.html" title="trait std::fmt::Debug">Debug</a> for <a class="struct" href="../../std/cell/struct.UnsafeCell.html" title="struct std::cell::UnsafeCell">UnsafeCell</a><T> <span class="where fmt-newline">where<br> T: <a class="trait" href="../../std/fmt/trait.Debug.html" title="trait std::fmt::Debug">Debug</a> + ?<a class="trait" href="../../std/marker/trait.Sized.html" title="trait std::marker::Sized">Sized</a>, </span></code></span><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#1713-1717' title='goto source code'>[src]</a></span></h3>
<div class='impl-items'><h4 id='method.fmt' class="method"><span id='fmt.v' class='invisible'><code>fn <a href='../../std/fmt/trait.Debug.html#tymethod.fmt' class='fnname'>fmt</a>(&self, f: &mut <a class="struct" href="../../std/fmt/struct.Formatter.html" title="struct std::fmt::Formatter">Formatter</a>) -> <a class="enum" href="../../std/result/enum.Result.html" title="enum std::result::Result">Result</a><<a class="primitive" href="../primitive.tuple.html">()</a>, <a class="struct" href="../../std/fmt/struct.Error.html" title="struct std::fmt::Error">Error</a>></code></span></h4>
<div class='docblock'><p>Formats the value using the given formatter.</p>
</div></div><h3 class='impl'><span class='in-band'><code>impl<T: ?<a class="trait" href="../../std/marker/trait.Sized.html" title="trait std::marker::Sized">Sized</a>> !<a class="trait" href="../../std/panic/trait.RefUnwindSafe.html" title="trait std::panic::RefUnwindSafe">RefUnwindSafe</a> for <a class="struct" href="../../std/cell/struct.UnsafeCell.html" title="struct std::cell::UnsafeCell">UnsafeCell</a><T></code></span><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/std/panic.rs.html#226' title='goto source code'>[src]</a></span></h3>
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