91 lines
8.5 KiB
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91 lines
8.5 KiB
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<title>std::hash<Key>::operator()</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_hash_operator skin-cppreference2 action-view cpp-navbar">
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span><span class="t-dsc-small">hash<Key>::</span>operator()</h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-utility.html">utility</a>‎ | <a href="cpp-utility-hash.html">hash</a></span></div>
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<div id="mw-content-text" lang="zh" dir="ltr" class="mw-content-ltr" xml:lang="zh">
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<p><span class="t-lc"><a href="cpp-utility-hash.html">std::hash</a></span> 的特化应定义满足下列条件的 <code>operator()</code> ：</p>
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<ul>
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<li>接收单个 <code>key</code> 类型的参数 <code>Key</code> 。</li>
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<li>返回表示 <code>key</code> 哈希值的 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a></span></span> 类型值。</li>
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<li>对于二个相等的参数 <code>k1</code> 与 <code>k2</code> ， <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-hash.html"><span class="kw1124">std::<span class="me2">hash</span></span></a><span class="sy1"><</span>Key<span class="sy1">></span><span class="br0">(</span><span class="br0">)</span><span class="br0">(</span>k1<span class="br0">)</span> <span class="sy1">==</span> <a href="cpp-utility-hash.html"><span class="kw1124">std::<span class="me2">hash</span></span></a><span class="sy1"><</span>Key<span class="sy1">></span><span class="br0">(</span><span class="br0">)</span><span class="br0">(</span>k2<span class="br0">)</span></span></span> 。</li>
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<li>对于二个相异而不等的参数 <code>k1</code> 与 <code>k2</code> ， <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-hash.html"><span class="kw1124">std::<span class="me2">hash</span></span></a><span class="sy1"><</span>Key<span class="sy1">></span><span class="br0">(</span><span class="br0">)</span><span class="br0">(</span>k1<span class="br0">)</span> <span class="sy1">==</span> <a href="cpp-utility-hash.html"><span class="kw1124">std::<span class="me2">hash</span></span></a><span class="sy1"><</span>Key<span class="sy1">></span><span class="br0">(</span><span class="br0">)</span><span class="br0">(</span>k2<span class="br0">)</span></span></span> 的概率应该非常小，接近 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="nu16">1.0</span><span class="sy2">/</span><a href="cpp-types-numeric_limits.html"><span class="kw344">std::<span class="me2">numeric_limits</span></span></a><span class="sy1"><</span>size_t<span class="sy1">></span><span class="sy4">::</span><span class="me2">max</span><span class="br0">(</span><span class="br0">)</span></span></span> 。</li>
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</ul>
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<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
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<table class="t-par-begin">
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<tr class="t-par">
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<td>key</td>
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<td>-</td>
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<td>要被哈希的对象</td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">返回值</span></h3>
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<p>表示哈希值的 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a></span></span></p>
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<h3><span class="mw-headline" id=".E5.BC.82.E5.B8.B8">异常</span></h3>
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<p>哈希函数不应抛异常。</p>
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<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
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<div class="t-example">
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<p>下列代码演示如何为自定义类特化 <span class="t-lc"><a href="cpp-utility-hash.html">std::hash</a></span> 模板。</p>
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<div class="t-example-live-link"></div>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="cpp source-cpp">
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<pre class="de1"><span class="co2">#include <functional></span>
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<span class="co2">#include <iostream></span>
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<span class="co2">#include <string></span>
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<span class="kw1">struct</span> Employee <span class="br0">{</span>
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<a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> name<span class="sy4">;</span>
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<span class="kw4">unsigned</span> <span class="kw4">int</span> ID<span class="sy4">;</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">namespace</span> std <span class="br0">{</span>
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<span class="kw1">template</span> <span class="sy1"><></span>
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<span class="kw1">class</span> hash<span class="sy1"><</span>Employee<span class="sy1">></span> <span class="br0">{</span>
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<span class="kw1">public</span><span class="sy4">:</span>
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size_t operator<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span><span class="kw4">const</span> Employee <span class="sy3">&</span>employee<span class="br0">)</span> <span class="kw4">const</span>
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<span class="br0">{</span>
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<span class="co1">// 用 Fowler-Noll-Vo hash 哈希函数的变体计算 employee 的哈希</span>
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size_t result <span class="sy1">=</span> <span class="nu0">2166136261</span><span class="sy4">;</span>
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<span class="kw1">for</span> <span class="br0">(</span>size_t i <span class="sy1">=</span> <span class="nu0">0</span>, ie <span class="sy1">=</span> employee.<span class="me1">name</span>.<span class="me1">size</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> i <span class="sy3">!</span><span class="sy1">=</span> ie<span class="sy4">;</span> <span class="sy2">++</span>i<span class="br0">)</span> <span class="br0">{</span>
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result <span class="sy1">=</span> <span class="br0">(</span>result <span class="sy2">*</span> <span class="nu0">16777619</span><span class="br0">)</span> <span class="sy3">^</span> employee.<span class="me1">name</span><span class="br0">[</span>i<span class="br0">]</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw1">return</span> result <span class="sy3">^</span> <span class="br0">(</span>employee.<span class="me1">ID</span> <span class="sy1"><<</span> <span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
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<span class="br0">{</span>
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Employee employee<span class="sy4">;</span>
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employee.<span class="me1">name</span> <span class="sy1">=</span> <span class="st0">"Zaphod Beeblebrox"</span><span class="sy4">;</span>
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employee.<span class="me1">ID</span> <span class="sy1">=</span> <span class="nu0">42</span><span class="sy4">;</span>
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<a href="cpp-utility-hash.html"><span class="kw1124">std::<span class="me2">hash</span></span></a><span class="sy1"><</span>Employee<span class="sy1">></span> hash_fn<span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> hash_fn<span class="br0">(</span>employee<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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</div>
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<p>输出：</p>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="text source-text">
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<pre class="de1">177237019</pre></div>
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