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<p class='location'>Trait DerefMut</p><div class="sidebar-elems"><div class="block items"><a class="sidebar-title" href="#required-methods">Required Methods</a><div class="sidebar-links"><a href="#tymethod.deref_mut">deref_mut</a></div><a class="sidebar-title" href="#implementors">Implementors</a></div><p class='location'><a href='../index.html'>core</a>::<wbr><a href='index.html'>ops</a></p><script>window.sidebarCurrent = {name: 'DerefMut', ty: 'trait', relpath: ''};</script><script defer src="sidebar-items.js"></script></div>
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<h1 class='fqn'><span class='in-band'>Trait <a href='../index.html'>core</a>::<wbr><a href='index.html'>ops</a>::<wbr><a class="trait" href=''>DerefMut</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/ops/deref.rs.html#166-170' title='goto source code'>[src]</a></span></h1>
<pre class='rust trait'><div class="docblock attributes">#[lang = "deref_mut"]
</div>pub trait DerefMut: <a class="trait" href="../../core/ops/trait.Deref.html" title="trait core::ops::Deref">Deref</a> {
fn <a href='#tymethod.deref_mut' class='fnname'>deref_mut</a>(&mut self) -> &mut Self::<a class="type" href="../../core/ops/trait.Deref.html#associatedtype.Target" title="type core::ops::Deref::Target">Target</a>;
}</pre><div class='docblock'><p>Used for mutable dereferencing operations, like in <code>*v = 1;</code>.</p>
<p>In addition to being used for explicit dereferencing operations with the
(unary) <code>*</code> operator in mutable contexts, <code>DerefMut</code> is also used implicitly
by the compiler in many circumstances. This mechanism is called
<a href="#more-on-deref-coercion">'<code>Deref</code> coercion'</a>. In immutable contexts, <a href="trait.Deref.html"><code>Deref</code></a> is used.</p>
<p>Implementing <code>DerefMut</code> for smart pointers makes mutating the data behind
them convenient, which is why they implement <code>DerefMut</code>. On the other hand,
the rules regarding <a href="trait.Deref.html"><code>Deref</code></a> and <code>DerefMut</code> were designed specifically to
accommodate smart pointers. Because of this, <strong><code>DerefMut</code> should only be
implemented for smart pointers</strong> to avoid confusion.</p>
<p>For similar reasons, <strong>this trait should never fail</strong>. Failure during
dereferencing can be extremely confusing when <code>DerefMut</code> is invoked
implicitly.</p>
<h1 id="more-on-deref-coercion" class="section-header"><a href="#more-on-deref-coercion">More on <code>Deref</code> coercion</a></h1>
<p>If <code>T</code> implements <code>DerefMut<Target = U></code>, and <code>x</code> is a value of type <code>T</code>,
then:</p>
<ul>
<li>In mutable contexts, <code>*x</code> on non-pointer types is equivalent to
<code>*Deref::deref(&x)</code>.</li>
<li>Values of type <code>&mut T</code> are coerced to values of type <code>&mut U</code></li>
<li><code>T</code> implicitly implements all the (mutable) methods of the type <code>U</code>.</li>
</ul>
<p>For more details, visit <a href="../../book/second-edition/ch15-02-deref.html">the chapter in <em>The Rust Programming Language</em></a> as well as the reference sections on <a href="../../reference/expressions/operator-expr.html#the-dereference-operator">the dereference operator</a>, <a href="../../reference/expressions/method-call-expr.html">method resolution</a> and <a href="../../reference/type-coercions.html">type coercions</a>.</p>
<h1 id="examples" class="section-header"><a href="#examples">Examples</a></h1>
<p>A struct with a single field which is modifiable by dereferencing the
struct.</p>
<pre class="rust rust-example-rendered">
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">ops</span>::{<span class="ident">Deref</span>, <span class="ident">DerefMut</span>};
<span class="kw">struct</span> <span class="ident">DerefMutExample</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
<span class="ident">value</span>: <span class="ident">T</span>
}
<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="ident">Deref</span> <span class="kw">for</span> <span class="ident">DerefMutExample</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
<span class="kw">type</span> <span class="ident">Target</span> <span class="op">=</span> <span class="ident">T</span>;
<span class="kw">fn</span> <span class="ident">deref</span>(<span class="kw-2">&</span><span class="self">self</span>) <span class="op">-></span> <span class="kw-2">&</span><span class="ident">T</span> {
<span class="kw-2">&</span><span class="self">self</span>.<span class="ident">value</span>
}
}
<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="ident">DerefMut</span> <span class="kw">for</span> <span class="ident">DerefMutExample</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
<span class="kw">fn</span> <span class="ident">deref_mut</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>) <span class="op">-></span> <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">T</span> {
<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>.<span class="ident">value</span>
}
}
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">x</span> <span class="op">=</span> <span class="ident">DerefMutExample</span> { <span class="ident">value</span>: <span class="string">'a'</span> };
<span class="kw-2">*</span><span class="ident">x</span> <span class="op">=</span> <span class="string">'b'</span>;
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="string">'b'</span>, <span class="kw-2">*</span><span class="ident">x</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%3Aops%3A%3A%7BDeref%2C%20DerefMut%7D%3B%0A%0Astruct%20DerefMutExample%3CT%3E%20%7B%0A%20%20%20%20value%3A%20T%0A%7D%0A%0Aimpl%3CT%3E%20Deref%20for%20DerefMutExample%3CT%3E%20%7B%0A%20%20%20%20type%20Target%20%3D%20T%3B%0A%0A%20%20%20%20fn%20deref(%26self)%20-%3E%20%26T%20%7B%0A%20%20%20%20%20%20%20%20%26self.value%0A%20%20%20%20%7D%0A%7D%0A%0Aimpl%3CT%3E%20DerefMut%20for%20DerefMutExample%3CT%3E%20%7B%0A%20%20%20%20fn%20deref_mut(%26mut%20self)%20-%3E%20%26mut%20T%20%7B%0A%20%20%20%20%20%20%20%20%26mut%20self.value%0A%20%20%20%20%7D%0A%7D%0A%0Alet%20mut%20x%20%3D%20DerefMutExample%20%7B%20value%3A%20'a'%20%7D%3B%0A*x%20%3D%20'b'%3B%0Aassert_eq!('b'%2C%20*x)%3B%0A%7D">Run</a></pre>
</div>
<h2 id='required-methods' class='small-section-header'>
Required Methods<a href='#required-methods' class='anchor'></a>
</h2>
<div class='methods'>
<h3 id='tymethod.deref_mut' class='method'><span id='deref_mut.v' class='invisible'><code>fn <a href='#tymethod.deref_mut' class='fnname'>deref_mut</a>(&mut self) -> &mut Self::<a class="type" href="../../core/ops/trait.Deref.html#associatedtype.Target" title="type core::ops::Deref::Target">Target</a></code></span></h3><div class='docblock'><p>Mutably dereferences the value.</p>
</div></div>
<h2 id='implementors' class='small-section-header'>
Implementors<a href='#implementors' class='anchor'></a>
</h2>
<ul class='item-list' id='implementors-list'>
<li><div class='out-of-band'><a class='srclink' href='../../src/core/mem.rs.html#1010-1017' title='goto source code'>[src]</a></div><code>impl<T> DerefMut for <a class="union" href="../../core/mem/union.ManuallyDrop.html" title="union core::mem::ManuallyDrop">ManuallyDrop</a><T></code></li>
<li><div class='out-of-band'><a class='srclink' href='../../src/core/ops/deref.rs.html#173-175' title='goto source code'>[src]</a></div><code>impl<'a, T: ?<a class="trait" href="../../core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a>> DerefMut for &'a mut T</code></li>
<li><div class='out-of-band'><a class='srclink' href='../../src/core/cell.rs.html#1140-1145' title='goto source code'>[src]</a></div><code>impl<'b, T: ?<a class="trait" href="../../core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a>> DerefMut for <a class="struct" href="../../core/cell/struct.RefMut.html" title="struct core::cell::RefMut">RefMut</a><'b, T></code></li>
</ul><script type="text/javascript" async
src="../../implementors/core/ops/trait.DerefMut.js">
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