93 lines
5.8 KiB
HTML
93 lines
5.8 KiB
HTML
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<html lang="zh" dir="ltr" class="client-nojs" xmlns="http://www.w3.org/1999/xhtml" xml:lang="zh">
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<title>std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::count</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_container_unordered_map_count skin-cppreference2 action-view cpp-page-template cpp-navbar">
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<div id="content"><a id="top"></a>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::</span>count</h1>
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<div id="bodyContent">
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-container.html">container</a>‎ | <a href="cpp-container-unordered_map.html">unordered map</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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<div class="t-page-template" style="">
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<p></p>
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<table class="t-dcl-begin">
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<div><span class="mw-geshi cpp source-cpp">size_type count<span class="br0">(</span> <span class="kw4">const</span> Key<span class="sy3">&</span> key <span class="br0">)</span> <span class="kw4">const</span><span class="sy4">;</span></span></div>
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</td>
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<td>(1)</td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> <span class="kw1">class</span> K <span class="sy1">></span><br />
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size_type count<span class="br0">(</span> <span class="kw4">const</span> K<span class="sy3">&</span> x <span class="br0">)</span> <span class="kw4">const</span><span class="sy4">;</span></span></div>
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</td>
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<td>(2)</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<div class="t-li1"><span class="t-li">1)</span> 返回拥有比较等于指定参数 <code>key</code> 的关键的元素数，因为此容器不允许重复故为 1 或 0 。</div>
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<div class="t-li1"><span class="t-li">2)</span> 返回键比较等价于指定参数 <code>x</code> 的元素数。此重载仅若有限定标识 <span class="t-c"><span class="mw-geshi cpp source-cpp">Hash<span class="sy4">::</span><span class="me2">is_transparent</span></span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp">KeyEqual<span class="sy4">::</span><span class="me2">is_transparent</span></span></span> 均合法并指代类型才参与重载决议。这假设能用 <code>K</code> 和 <code>Key</code> 类型一起调用这种 <code>Hash</code> ，还有 <code>KeyEqual</code> 是通透的，进而允许不用构造 <code>Key</code> 的实例就调用此函数。</div>
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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>要计量等价元素数的键值</td>
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</tr>
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<tr class="t-par">
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<td>x</td>
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<td>-</td>
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<td>能通透地与键比较的任何类型值</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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<div class="t-li1"><span class="t-li">1)</span> 拥有关键 <code>key</code> 的元素数，即 1 或 0 。</div>
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<div class="t-li1"><span class="t-li">2)</span> 键比较等价于 <code>x</code> 的元素数。</div>
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<h3><span class="mw-headline" id=".E5.A4.8D.E6.9D.82.E5.BA.A6">复杂度</span></h3>
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<p>平均为常数，最坏情况与容器大小成线性。</p>
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<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
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<table class="t-dsc-begin">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-container-unordered_map-find.html"><span class="t-lines"><span>find</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
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</div>
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</td>
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<td>寻找带有特定键的元素<br />
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<span class="t-mark">(公开成员函数)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-container-unordered_map-equal_range.html"><span class="t-lines"><span>equal_range</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
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</div>
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</td>
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<td>返回匹配特定键的元素范围<br />
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<span class="t-mark">(公开成员函数)</span></td>
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</tr>
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</table>
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</div>
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