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<p class='location'>Struct Weak</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.upgrade">upgrade</a></div><a class="sidebar-title" href="#implementations">Trait Implementations</a><div class="sidebar-links"><a href="#impl-CoerceUnsized%3CWeak%3CU%3E%3E">CoerceUnsized<Weak<U>></a><a href="#impl-Sync">Sync</a><a href="#impl-Clone">Clone</a><a href="#impl-Default">Default</a><a href="#impl-Debug">Debug</a><a href="#impl-Drop">Drop</a><a href="#impl-Send">Send</a></div></div><p class='location'><a href='../index.html'>std</a>::<wbr><a href='index.html'>sync</a></p><script>window.sidebarCurrent = {name: 'Weak', ty: 'struct', relpath: ''};</script><script defer src="sidebar-items.js"></script></div>
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<h1 class='fqn'><span class='in-band'>Struct <a href='../index.html'>std</a>::<wbr><a href='index.html'>sync</a>::<wbr><a class="struct" href=''>Weak</a></span><span class='out-of-band'><span class='since' title='Stable since Rust version 1.4.0'>1.4.0</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/alloc/arc.rs.html#236-238' title='goto source code'>[src]</a></span></h1>
<pre class='rust struct'>pub struct Weak<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><code>Weak</code> is a version of <a href="struct.Arc.html"><code>Arc</code></a> that holds a non-owning reference to the
managed value. The value is accessed by calling <a href="struct.Weak.html#method.upgrade"><code>upgrade</code></a> on the <code>Weak</code>
pointer, which returns an <a href="../../std/option/enum.Option.html"><code>Option</code></a><code><</code><a href="struct.Arc.html"><code>Arc</code></a><code><T>></code>.</p>
<p>Since a <code>Weak</code> reference does not count towards ownership, it will not
prevent the inner value from being dropped, and <code>Weak</code> itself makes no
guarantees about the value still being present and may return <a href="../../std/option/enum.Option.html#variant.None"><code>None</code></a>
when <a href="struct.Weak.html#method.upgrade"><code>upgrade</code></a>d.</p>
<p>A <code>Weak</code> pointer is useful for keeping a temporary reference to the value
within <a href="struct.Arc.html"><code>Arc</code></a> without extending its lifetime. It is also used to prevent
circular references between <a href="struct.Arc.html"><code>Arc</code></a> pointers, since mutual owning references
would never allow either <a href="struct.Arc.html"><code>Arc</code></a> to be dropped. For example, a tree could
have strong <a href="struct.Arc.html"><code>Arc</code></a> pointers from parent nodes to children, and <code>Weak</code>
pointers from children back to their parents.</p>
<p>The typical way to obtain a <code>Weak</code> pointer is to call <a href="struct.Arc.html#method.downgrade"><code>Arc::downgrade</code></a>.</p>
</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'><code>impl<T> <a class="struct" href="../../std/sync/struct.Weak.html" title="struct std::sync::Weak">Weak</a><T></code><a href='#impl' class='anchor'></a></span><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../../src/alloc/arc.rs.html#975-1002' 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>pub fn <a href='#method.new' class='fnname'>new</a>() -> <a class="struct" href="../../std/sync/struct.Weak.html" title="struct std::sync::Weak">Weak</a><T></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/alloc/arc.rs.html#991-1001' title='goto source code'>[src]</a></span></h4>
<div class='docblock'><p>Constructs a new <code>Weak<T></code>, allocating memory for <code>T</code> without initializing
it. Calling <a href="struct.Weak.html#method.upgrade"><code>upgrade</code></a> on the return value always gives <a href="../../std/option/enum.Option.html#variant.None"><code>None</code></a>.</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">sync</span>::<span class="ident">Weak</span>;
<span class="kw">let</span> <span class="ident">empty</span>: <span class="ident">Weak</span><span class="op"><</span><span class="ident">i64</span><span class="op">></span> <span class="op">=</span> <span class="ident">Weak</span>::<span class="ident">new</span>();
<span class="macro">assert</span><span class="macro">!</span>(<span class="ident">empty</span>.<span class="ident">upgrade</span>().<span class="ident">is_none</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%3Async%3A%3AWeak%3B%0A%0Alet%20empty%3A%20Weak%3Ci64%3E%20%3D%20Weak%3A%3Anew()%3B%0Aassert!(empty.upgrade().is_none())%3B%0A%7D">Run</a></pre>
</div></div><h3 id='impl-1' class='impl'><span class='in-band'><code>impl<T> <a class="struct" href="../../std/sync/struct.Weak.html" title="struct std::sync::Weak">Weak</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><a href='#impl-1' class='anchor'></a></span><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../../src/alloc/arc.rs.html#1004-1068' title='goto source code'>[src]</a></span></h3>
<div class='impl-items'><h4 id='method.upgrade' class="method"><span id='upgrade.v' class='invisible'><code>pub fn <a href='#method.upgrade' class='fnname'>upgrade</a>(&self) -> <a class="enum" href="../../std/option/enum.Option.html" title="enum std::option::Option">Option</a><<a class="struct" href="../../std/sync/struct.Arc.html" title="struct std::sync::Arc">Arc</a><T>></code></span><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../../src/alloc/arc.rs.html#1032-1061' title='goto source code'>[src]</a></span></h4>
<div class='docblock'><p>Attempts to upgrade the <code>Weak</code> pointer to an <a href="struct.Arc.html"><code>Arc</code></a>, extending
the lifetime of the value if successful.</p>
<p>Returns <a href="../../std/option/enum.Option.html#variant.None"><code>None</code></a> if the value has since been dropped.</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">sync</span>::<span class="ident">Arc</span>;
<span class="kw">let</span> <span class="ident">five</span> <span class="op">=</span> <span class="ident">Arc</span>::<span class="ident">new</span>(<span class="number">5</span>);
<span class="kw">let</span> <span class="ident">weak_five</span> <span class="op">=</span> <span class="ident">Arc</span>::<span class="ident">downgrade</span>(<span class="kw-2">&</span><span class="ident">five</span>);
<span class="kw">let</span> <span class="ident">strong_five</span>: <span class="prelude-ty">Option</span><span class="op"><</span><span class="ident">Arc</span><span class="op"><</span>_<span class="op">>></span> <span class="op">=</span> <span class="ident">weak_five</span>.<span class="ident">upgrade</span>();
<span class="macro">assert</span><span class="macro">!</span>(<span class="ident">strong_five</span>.<span class="ident">is_some</span>());
<span class="comment">// Destroy all strong pointers.</span>
<span class="ident">drop</span>(<span class="ident">strong_five</span>);
<span class="ident">drop</span>(<span class="ident">five</span>);
<span class="macro">assert</span><span class="macro">!</span>(<span class="ident">weak_five</span>.<span class="ident">upgrade</span>().<span class="ident">is_none</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%3Async%3A%3AArc%3B%0A%0Alet%20five%20%3D%20Arc%3A%3Anew(5)%3B%0A%0Alet%20weak_five%20%3D%20Arc%3A%3Adowngrade(%26five)%3B%0A%0Alet%20strong_five%3A%20Option%3CArc%3C_%3E%3E%20%3D%20weak_five.upgrade()%3B%0Aassert!(strong_five.is_some())%3B%0A%0A%2F%2F%20Destroy%20all%20strong%20pointers.%0Adrop(strong_five)%3B%0Adrop(five)%3B%0A%0Aassert!(weak_five.upgrade().is_none())%3B%0A%7D">Run</a></pre>
</div></div>
<h2 id='implementations' class='small-section-header'>
Trait Implementations<a href='#implementations' class='anchor'></a>
</h2>
<h3 id='impl-CoerceUnsized%3CWeak%3CU%3E%3E' 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/sync/struct.Weak.html" title="struct std::sync::Weak">Weak</a><U>> for <a class="struct" href="../../std/sync/struct.Weak.html" title="struct std::sync::Weak">Weak</a><T> <span class="where fmt-newline">where<br> T: <a class="trait" href="../../std/marker/trait.Unsize.html" title="trait std::marker::Unsize">Unsize</a><U> + ?<a class="trait" href="../../std/marker/trait.Sized.html" title="trait std::marker::Sized">Sized</a>,<br> U: ?<a class="trait" href="../../std/marker/trait.Sized.html" title="trait std::marker::Sized">Sized</a>, </span></code><a href='#impl-CoerceUnsized%3CWeak%3CU%3E%3E' class='anchor'></a></span><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../../src/alloc/arc.rs.html#246' title='goto source code'>[src]</a></span></h3>
<div class='impl-items'></div><h3 id='impl-Sync' 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/sync/struct.Weak.html" title="struct std::sync::Weak">Weak</a><T> <span class="where fmt-newline">where<br> T: <a class="trait" href="../../std/marker/trait.Send.html" title="trait std::marker::Send">Send</a> + <a class="trait" href="../../std/marker/trait.Sync.html" title="trait std::marker::Sync">Sync</a> + ?<a class="trait" href="../../std/marker/trait.Sized.html" title="trait std::marker::Sized">Sized</a>, </span></code><a href='#impl-Sync' class='anchor'></a></span><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../../src/alloc/arc.rs.html#243' title='goto source code'>[src]</a></span></h3>
<div class='impl-items'></div><h3 id='impl-Clone' class='impl'><span class='in-band'><code>impl<T> <a class="trait" href="../../std/clone/trait.Clone.html" title="trait std::clone::Clone">Clone</a> for <a class="struct" href="../../std/sync/struct.Weak.html" title="struct std::sync::Weak">Weak</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><a href='#impl-Clone' class='anchor'></a></span><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../../src/alloc/arc.rs.html#1071-1100' title='goto source code'>[src]</a></span></h3>
<div class='impl-items'><h4 id='method.clone' class="method"><span id='clone.v' class='invisible'><code>fn <a href='../../std/clone/trait.Clone.html#tymethod.clone' class='fnname'>clone</a>(&self) -> <a class="struct" href="../../std/sync/struct.Weak.html" title="struct std::sync::Weak">Weak</a><T></code></span><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../../src/alloc/arc.rs.html#1084-1099' title='goto source code'>[src]</a></span></h4>
<div class='docblock'><p>Makes a clone of the <code>Weak</code> pointer that points to the same 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">sync</span>::{<span class="ident">Arc</span>, <span class="ident">Weak</span>};
<span class="kw">let</span> <span class="ident">weak_five</span> <span class="op">=</span> <span class="ident">Arc</span>::<span class="ident">downgrade</span>(<span class="kw-2">&</span><span class="ident">Arc</span>::<span class="ident">new</span>(<span class="number">5</span>));
<span class="ident">Weak</span>::<span class="ident">clone</span>(<span class="kw-2">&</span><span class="ident">weak_five</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%3Async%3A%3A%7BArc%2C%20Weak%7D%3B%0A%0Alet%20weak_five%20%3D%20Arc%3A%3Adowngrade(%26Arc%3A%3Anew(5))%3B%0A%0AWeak%3A%3Aclone(%26weak_five)%3B%0A%7D">Run</a></pre>
</div><h4 id='method.clone_from' class="method"><span id='clone_from.v' class='invisible'><code>fn <a href='../../std/clone/trait.Clone.html#method.clone_from' class='fnname'>clone_from</a>(&mut self, source: <a class="primitive" href="../primitive.reference.html">&</a>Self)</code></span><span class='out-of-band'><div class='ghost'></div><div class='since' title='Stable since Rust version 1.0.0'>1.0.0</div><a class='srclink' href='../../src/core/clone.rs.html#112-114' title='goto source code'>[src]</a></span></h4>
<div class='docblock'><p>Performs copy-assignment from <code>source</code>. <a href="../../std/clone/trait.Clone.html#method.clone_from">Read more</a></p>
</div></div><h3 id='impl-Default' 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/sync/struct.Weak.html" title="struct std::sync::Weak">Weak</a><T></code><a href='#impl-Default' class='anchor'></a></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/alloc/arc.rs.html#1103-1121' 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/sync/struct.Weak.html" title="struct std::sync::Weak">Weak</a><T></code></span><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../../src/alloc/arc.rs.html#1118-1120' title='goto source code'>[src]</a></span></h4>
<div class='docblock'><p>Constructs a new <code>Weak<T></code>, allocating memory for <code>T</code> without initializing
it. Calling <a href="struct.Weak.html#method.upgrade"><code>upgrade</code></a> on the return value always gives <a href="../../std/option/enum.Option.html#variant.None"><code>None</code></a>.</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">sync</span>::<span class="ident">Weak</span>;
<span class="kw">let</span> <span class="ident">empty</span>: <span class="ident">Weak</span><span class="op"><</span><span class="ident">i64</span><span class="op">></span> <span class="op">=</span> <span class="ident">Default</span>::<span class="ident">default</span>();
<span class="macro">assert</span><span class="macro">!</span>(<span class="ident">empty</span>.<span class="ident">upgrade</span>().<span class="ident">is_none</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%3Async%3A%3AWeak%3B%0A%0Alet%20empty%3A%20Weak%3Ci64%3E%20%3D%20Default%3A%3Adefault()%3B%0Aassert!(empty.upgrade().is_none())%3B%0A%7D">Run</a></pre>
</div></div><h3 id='impl-Debug' 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/sync/struct.Weak.html" title="struct std::sync::Weak">Weak</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><a href='#impl-Debug' class='anchor'></a></span><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../../src/alloc/arc.rs.html#249-253' 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.unit.html">()</a>, <a class="struct" href="../../std/fmt/struct.Error.html" title="struct std::fmt::Error">Error</a>></code></span><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../../src/alloc/arc.rs.html#250-252' title='goto source code'>[src]</a></span></h4>
<div class='docblock'><p>Formats the value using the given formatter. <a href="../../std/fmt/trait.Debug.html#tymethod.fmt">Read more</a></p>
</div></div><h3 id='impl-Drop' class='impl'><span class='in-band'><code>impl<T> <a class="trait" href="../../std/ops/trait.Drop.html" title="trait std::ops::Drop">Drop</a> for <a class="struct" href="../../std/sync/struct.Weak.html" title="struct std::sync::Weak">Weak</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><a href='#impl-Drop' class='anchor'></a></span><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../../src/alloc/arc.rs.html#1124-1167' title='goto source code'>[src]</a></span></h3>
<div class='impl-items'><h4 id='method.drop' class="method"><span id='drop.v' class='invisible'><code>fn <a href='../../std/ops/trait.Drop.html#tymethod.drop' class='fnname'>drop</a>(&mut self)</code></span><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../../src/alloc/arc.rs.html#1149-1166' title='goto source code'>[src]</a></span></h4>
<div class='docblock'><p>Drops the <code>Weak</code> pointer.</p>
<h1 id="examples-4" class="section-header"><a href="#examples-4">Examples</a></h1>
<pre class="rust rust-example-rendered">
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">sync</span>::{<span class="ident">Arc</span>, <span class="ident">Weak</span>};
<span class="kw">struct</span> <span class="ident">Foo</span>;
<span class="kw">impl</span> <span class="ident">Drop</span> <span class="kw">for</span> <span class="ident">Foo</span> {
<span class="kw">fn</span> <span class="ident">drop</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>) {
<span class="macro">println</span><span class="macro">!</span>(<span class="string">"dropped!"</span>);
}
}
<span class="kw">let</span> <span class="ident">foo</span> <span class="op">=</span> <span class="ident">Arc</span>::<span class="ident">new</span>(<span class="ident">Foo</span>);
<span class="kw">let</span> <span class="ident">weak_foo</span> <span class="op">=</span> <span class="ident">Arc</span>::<span class="ident">downgrade</span>(<span class="kw-2">&</span><span class="ident">foo</span>);
<span class="kw">let</span> <span class="ident">other_weak_foo</span> <span class="op">=</span> <span class="ident">Weak</span>::<span class="ident">clone</span>(<span class="kw-2">&</span><span class="ident">weak_foo</span>);
<span class="ident">drop</span>(<span class="ident">weak_foo</span>); <span class="comment">// Doesn't print anything</span>
<span class="ident">drop</span>(<span class="ident">foo</span>); <span class="comment">// Prints "dropped!"</span>
<span class="macro">assert</span><span class="macro">!</span>(<span class="ident">other_weak_foo</span>.<span class="ident">upgrade</span>().<span class="ident">is_none</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%3Async%3A%3A%7BArc%2C%20Weak%7D%3B%0A%0Astruct%20Foo%3B%0A%0Aimpl%20Drop%20for%20Foo%20%7B%0A%20%20%20%20fn%20drop(%26mut%20self)%20%7B%0A%20%20%20%20%20%20%20%20println!(%22dropped!%22)%3B%0A%20%20%20%20%7D%0A%7D%0A%0Alet%20foo%20%3D%20Arc%3A%3Anew(Foo)%3B%0Alet%20weak_foo%20%3D%20Arc%3A%3Adowngrade(%26foo)%3B%0Alet%20other_weak_foo%20%3D%20Weak%3A%3Aclone(%26weak_foo)%3B%0A%0Adrop(weak_foo)%3B%20%20%20%2F%2F%20Doesn't%20print%20anything%0Adrop(foo)%3B%20%20%20%20%20%20%20%20%2F%2F%20Prints%20%22dropped!%22%0A%0Aassert!(other_weak_foo.upgrade().is_none())%3B%0A%7D">Run</a></pre>
</div></div><h3 id='impl-Send' class='impl'><span class='in-band'><code>impl<T> <a class="trait" href="../../std/marker/trait.Send.html" title="trait std::marker::Send">Send</a> for <a class="struct" href="../../std/sync/struct.Weak.html" title="struct std::sync::Weak">Weak</a><T> <span class="where fmt-newline">where<br> T: <a class="trait" href="../../std/marker/trait.Send.html" title="trait std::marker::Send">Send</a> + <a class="trait" href="../../std/marker/trait.Sync.html" title="trait std::marker::Sync">Sync</a> + ?<a class="trait" href="../../std/marker/trait.Sized.html" title="trait std::marker::Sized">Sized</a>, </span></code><a href='#impl-Send' class='anchor'></a></span><span class='out-of-band'><div class='ghost'></div><a class='srclink' href='../../src/alloc/arc.rs.html#241' title='goto source code'>[src]</a></span></h3>
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