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<title>slab - Rust</title>
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<div class="sidebar-menu">☰</div>
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<p class='location'>Crate slab</p><div class="sidebar-elems"><div class="block items"><ul><li><a href="#structs">Structs</a></li></ul></div><p class='location'></p><script>window.sidebarCurrent = {name: 'slab', ty: 'mod', relpath: '../'};</script></div>
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<section id='main' class="content">
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<h1 class='fqn'><span class='in-band'>Crate <a class="mod" href=''>slab</a></span><span class='out-of-band'><span id='render-detail'>
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<a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">
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[<span class='inner'>−</span>]
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</span><a class='srclink' href='../src/slab/lib.rs.html#1-911' title='goto source code'>[src]</a></span></h1>
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<div class='docblock'><p>Pre-allocated storage for a uniform data type.</p>
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<p><code>Slab</code> provides pre-allocated storage for a single data type. If many values
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of a single type are being allocated, it can be more efficient to
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pre-allocate the necessary storage. Since the size of the type is uniform,
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memory fragmentation can be avoided. Storing, clearing, and lookup
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operations become very cheap.</p>
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<p>While <code>Slab</code> may look like other Rust collections, it is not intended to be
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used as a general purpose collection. The primary difference between <code>Slab</code>
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and <code>Vec</code> is that <code>Slab</code> returns the key when storing the value.</p>
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<p>It is important to note that keys may be reused. In other words, once a
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value associated with a given key is removed from a slab, that key may be
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returned from future calls to <code>insert</code>.</p>
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<h1 id="examples" class="section-header"><a href="#examples">Examples</a></h1>
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<p>Basic storing and retrieval.</p>
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<pre class="rust rust-example-rendered">
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<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">slab</span> <span class="op">=</span> <span class="ident">Slab</span>::<span class="ident">new</span>();
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<span class="kw">let</span> <span class="ident">hello</span> <span class="op">=</span> <span class="ident">slab</span>.<span class="ident">insert</span>(<span class="string">"hello"</span>);
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<span class="kw">let</span> <span class="ident">world</span> <span class="op">=</span> <span class="ident">slab</span>.<span class="ident">insert</span>(<span class="string">"world"</span>);
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<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">slab</span>[<span class="ident">hello</span>], <span class="string">"hello"</span>);
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<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">slab</span>[<span class="ident">world</span>], <span class="string">"world"</span>);
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<span class="ident">slab</span>[<span class="ident">world</span>] <span class="op">=</span> <span class="string">"earth"</span>;
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<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">slab</span>[<span class="ident">world</span>], <span class="string">"earth"</span>);</pre>
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<p>Sometimes it is useful to be able to associate the key with the value being
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inserted in the slab. This can be done with the <code>vacant_entry</code> API as such:</p>
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<pre class="rust rust-example-rendered">
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<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">slab</span> <span class="op">=</span> <span class="ident">Slab</span>::<span class="ident">new</span>();
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<span class="kw">let</span> <span class="ident">hello</span> <span class="op">=</span> {
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<span class="kw">let</span> <span class="ident">entry</span> <span class="op">=</span> <span class="ident">slab</span>.<span class="ident">vacant_entry</span>();
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<span class="kw">let</span> <span class="ident">key</span> <span class="op">=</span> <span class="ident">entry</span>.<span class="ident">key</span>();
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<span class="ident">entry</span>.<span class="ident">insert</span>((<span class="ident">key</span>, <span class="string">"hello"</span>));
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<span class="ident">key</span>
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};
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<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">hello</span>, <span class="ident">slab</span>[<span class="ident">hello</span>].<span class="number">0</span>);
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<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="string">"hello"</span>, <span class="ident">slab</span>[<span class="ident">hello</span>].<span class="number">1</span>);</pre>
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<p>It is generally a good idea to specify the desired capacity of a slab at
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creation time. Note that <code>Slab</code> will grow the internal capacity when
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attempting to insert a new value once the existing capacity has been reached.
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To avoid this, add a check.</p>
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<pre class="rust rust-example-rendered">
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<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">slab</span> <span class="op">=</span> <span class="ident">Slab</span>::<span class="ident">with_capacity</span>(<span class="number">1024</span>);
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<span class="comment">// ... use the slab</span>
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<span class="kw">if</span> <span class="ident">slab</span>.<span class="ident">len</span>() <span class="op">==</span> <span class="ident">slab</span>.<span class="ident">capacity</span>() {
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<span class="macro">panic</span><span class="macro">!</span>(<span class="string">"slab full"</span>);
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}
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<span class="ident">slab</span>.<span class="ident">insert</span>(<span class="string">"the slab is not at capacity yet"</span>);</pre>
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<h1 id="capacity-and-reallocation" class="section-header"><a href="#capacity-and-reallocation">Capacity and reallocation</a></h1>
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<p>The capacity of a slab is the amount of space allocated for any future
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values that will be inserted in the slab. This is not to be confused with
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the <em>length</em> of the slab, which specifies the number of actual values
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currently being inserted. If a slab's length is equal to its capacity, the
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next value inserted into the slab will require growing the slab by
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reallocating.</p>
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<p>For example, a slab with capacity 10 and length 0 would be an empty slab
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with space for 10 more stored values. Storing 10 or fewer elements into the
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slab will not change its capacity or cause reallocation to occur. However,
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if the slab length is increased to 11 (due to another <code>insert</code>), it will
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have to reallocate, which can be slow. For this reason, it is recommended to
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use <a href="struct.Slab.html#with_capacity"><code>Slab::with_capacity</code></a> whenever possible to specify how many values the
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slab is expected to store.</p>
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<h1 id="implementation" class="section-header"><a href="#implementation">Implementation</a></h1>
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<p><code>Slab</code> is backed by a <code>Vec</code> of slots. Each slot is either occupied or
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vacant. <code>Slab</code> maintains a stack of vacant slots using a linked list. To
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find a vacant slot, the stack is popped. When a slot is released, it is
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pushed onto the stack.</p>
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<p>If there are no more available slots in the stack, then <code>Vec::reserve(1)</code> is
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called and a new slot is created.</p>
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</div><h2 id='structs' class='section-header'><a href="#structs">Structs</a></h2>
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<table>
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<tr class=' module-item'>
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<td><a class="struct" href="struct.Iter.html"
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title='struct slab::Iter'>Iter</a></td>
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<td class='docblock-short'>
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<p>An iterator over the values stored in the <code>Slab</code></p>
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</td>
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</tr>
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<tr class=' module-item'>
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<td><a class="struct" href="struct.IterMut.html"
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title='struct slab::IterMut'>IterMut</a></td>
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<td class='docblock-short'>
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<p>A mutable iterator over the values stored in the <code>Slab</code></p>
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</td>
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</tr>
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<tr class=' module-item'>
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<td><a class="struct" href="struct.Slab.html"
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title='struct slab::Slab'>Slab</a></td>
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<td class='docblock-short'>
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<p>Pre-allocated storage for a uniform data type</p>
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</td>
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</tr>
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<tr class=' module-item'>
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<td><a class="struct" href="struct.VacantEntry.html"
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title='struct slab::VacantEntry'>VacantEntry</a></td>
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<td class='docblock-short'>
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<p>A handle to a vacant entry in a <code>Slab</code>.</p>
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</td>
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