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<section id='main' class="content struct">
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<h1 class='fqn'><span class='in-band'>Struct <a href='../../index.html'>bitflags</a>::<wbr><a href='../index.html'>__core</a>::<wbr><a href='index.html'>collections</a>::<wbr><a class='struct' href=''>HashSet</a></span><span class='out-of-band'><span id='render-detail'>
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[<span class='inner'>−</span>]
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</span><a id='src-2393' class='srclink' href='https://doc.rust-lang.org/nightly/std/collections/hash/set/struct.HashSet.html?gotosrc=2393' title='goto source code'>[src]</a></span></h1>
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<pre class='rust struct'>pub struct HashSet<T, S = <a class='struct' href='../../../bitflags/__core/collections/hash_map/struct.RandomState.html' title='bitflags::__core::collections::hash_map::RandomState'>RandomState</a>> {
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// some fields omitted
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}</pre><span class="since">1.0.0</span><div class='docblock'><p>An implementation of a hash set using the underlying representation of a
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HashMap where the value is ().</p>
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<p>As with the <code>HashMap</code> type, a <code>HashSet</code> requires that the elements
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implement the <code>Eq</code> and <code>Hash</code> traits. This can frequently be achieved by
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using <code>#[derive(PartialEq, Eq, Hash)]</code>. If you implement these yourself,
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it is important that the following property holds:</p>
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<pre><code class="language-text">k1 == k2 -> hash(k1) == hash(k2)
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</code></pre>
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<p>In other words, if two keys are equal, their hashes must be equal.</p>
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<p>It is a logic error for an item to be modified in such a way that the
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item's hash, as determined by the <code>Hash</code> trait, or its equality, as
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determined by the <code>Eq</code> trait, changes while it is in the set. This is
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normally only possible through <code>Cell</code>, <code>RefCell</code>, global state, I/O, or
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unsafe code.</p>
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<h1 id='examples' class='section-header'><a href='#examples'>Examples</a></h1>
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<pre class='rust rust-example-rendered'>
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<span class='kw'>use</span> <span class='ident'>std</span>::<span class='ident'>collections</span>::<span class='ident'>HashSet</span>;
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<span class='comment'>// Type inference lets us omit an explicit type signature (which</span>
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<span class='comment'>// would be `HashSet<&str>` in this example).</span>
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<span class='kw'>let</span> <span class='kw-2'>mut</span> <span class='ident'>books</span> <span class='op'>=</span> <span class='ident'>HashSet</span>::<span class='ident'>new</span>();
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<span class='comment'>// Add some books.</span>
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<span class='ident'>books</span>.<span class='ident'>insert</span>(<span class='string'>"A Dance With Dragons"</span>);
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<span class='ident'>books</span>.<span class='ident'>insert</span>(<span class='string'>"To Kill a Mockingbird"</span>);
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<span class='ident'>books</span>.<span class='ident'>insert</span>(<span class='string'>"The Odyssey"</span>);
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<span class='ident'>books</span>.<span class='ident'>insert</span>(<span class='string'>"The Great Gatsby"</span>);
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<span class='comment'>// Check for a specific one.</span>
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<span class='kw'>if</span> <span class='op'>!</span><span class='ident'>books</span>.<span class='ident'>contains</span>(<span class='string'>"The Winds of Winter"</span>) {
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<span class='macro'>println</span><span class='macro'>!</span>(<span class='string'>"We have {} books, but The Winds of Winter ain't one."</span>,
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<span class='ident'>books</span>.<span class='ident'>len</span>());
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}
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<span class='comment'>// Remove a book.</span>
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<span class='ident'>books</span>.<span class='ident'>remove</span>(<span class='string'>"The Odyssey"</span>);
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<span class='comment'>// Iterate over everything.</span>
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<span class='kw'>for</span> <span class='ident'>book</span> <span class='kw'>in</span> <span class='kw-2'>&</span><span class='ident'>books</span> {
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<span class='macro'>println</span><span class='macro'>!</span>(<span class='string'>"{}"</span>, <span class='ident'>book</span>);
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}</pre>
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<p>The easiest way to use <code>HashSet</code> with a custom type is to derive
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<code>Eq</code> and <code>Hash</code>. We must also derive <code>PartialEq</code>, this will in the
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future be implied by <code>Eq</code>.</p>
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<pre class='rust rust-example-rendered'>
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<span class='kw'>use</span> <span class='ident'>std</span>::<span class='ident'>collections</span>::<span class='ident'>HashSet</span>;
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<span class='attribute'>#[<span class='ident'>derive</span>(<span class='ident'>Hash</span>, <span class='ident'>Eq</span>, <span class='ident'>PartialEq</span>, <span class='ident'>Debug</span>)]</span>
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<span class='kw'>struct</span> <span class='ident'>Viking</span><span class='op'><</span><span class='lifetime'>'a</span><span class='op'>></span> {
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<span class='ident'>name</span>: <span class='kw-2'>&</span><span class='lifetime'>'a</span> <span class='ident'>str</span>,
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<span class='ident'>power</span>: <span class='ident'>usize</span>,
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}
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<span class='kw'>let</span> <span class='kw-2'>mut</span> <span class='ident'>vikings</span> <span class='op'>=</span> <span class='ident'>HashSet</span>::<span class='ident'>new</span>();
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<span class='ident'>vikings</span>.<span class='ident'>insert</span>(<span class='ident'>Viking</span> { <span class='ident'>name</span>: <span class='string'>"Einar"</span>, <span class='ident'>power</span>: <span class='number'>9</span> });
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<span class='ident'>vikings</span>.<span class='ident'>insert</span>(<span class='ident'>Viking</span> { <span class='ident'>name</span>: <span class='string'>"Einar"</span>, <span class='ident'>power</span>: <span class='number'>9</span> });
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<span class='ident'>vikings</span>.<span class='ident'>insert</span>(<span class='ident'>Viking</span> { <span class='ident'>name</span>: <span class='string'>"Olaf"</span>, <span class='ident'>power</span>: <span class='number'>4</span> });
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<span class='ident'>vikings</span>.<span class='ident'>insert</span>(<span class='ident'>Viking</span> { <span class='ident'>name</span>: <span class='string'>"Harald"</span>, <span class='ident'>power</span>: <span class='number'>8</span> });
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<span class='comment'>// Use derived implementation to print the vikings.</span>
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<span class='kw'>for</span> <span class='ident'>x</span> <span class='kw'>in</span> <span class='kw-2'>&</span><span class='ident'>vikings</span> {
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<span class='macro'>println</span><span class='macro'>!</span>(<span class='string'>"{:?}"</span>, <span class='ident'>x</span>);
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}</pre>
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