159 lines
14 KiB
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159 lines
14 KiB
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<title>std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::find</title>
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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>find</h1>
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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><span class="mw-geshi cpp source-cpp">iterator find<span class="br0">(</span> <span class="kw4">const</span> Key<span class="sy3">&</span> key <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td>(1)</td>
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<div><span class="mw-geshi cpp source-cpp">const_iterator find<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>(2)</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> iterator find<span class="br0">(</span> <span class="kw4">const</span> K<span class="sy3">&</span> x <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td>(3)</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 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> const_iterator find<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>(4)</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">1,2)</span> 寻找键等于 <code>key</code> 的的元素。</div>
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<div class="t-li1"><span class="t-li">3,4)</span> 寻找键比较<i>等价</i>于值 <code>x</code> 的元素。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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<td>key</td>
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<td>-</td>
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<td>要搜索的元素键值</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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<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>指向键等于 <code>key</code> 的元素的迭代器。若找不到这种元素，则返回尾后（见 <span class="t-lc"><a href="cpp-container-unordered_map-end.html">end()</a></span> ）迭代器。</p>
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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=".E7.A4.BA.E4.BE.8B">示例</span></h3>
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<div class="t-example">
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<div class="cpp source-cpp">
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<pre class="de1"><span class="co2">#include <cstddef></span>
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<span class="co2">#include <iostream></span>
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<span class="co2">#include <functional></span>
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<span class="co2">#include <string></span>
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<span class="co2">#include <string_view></span>
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<span class="co2">#include <unordered_map></span>
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<span class="kw1">using</span> <span class="kw1">namespace</span> std<span class="sy4">::</span><span class="me2">literals</span><span class="sy4">;</span>
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<span class="kw1">using</span> <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a><span class="sy4">;</span>
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<span class="kw1">struct</span> string_hash
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<span class="br0">{</span>
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<span class="kw1">using</span> hash_type <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><a href="cpp-string-basic_string_view.html"><span class="kw1248">std::<span class="me2">string_view</span></span></a><span class="sy1">></span><span class="sy4">;</span>
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<span class="kw1">using</span> is_transparent <span class="sy1">=</span> <span class="kw4">void</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> <span class="kw4">char</span><span class="sy2">*</span> str<span class="br0">)</span> <span class="kw4">const</span> <span class="br0">{</span> <span class="kw1">return</span> hash_type<span class="br0">{</span><span class="br0">}</span><span class="br0">(</span>str<span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span>
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size_t operator<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span><a href="cpp-string-basic_string_view.html"><span class="kw1248">std::<span class="me2">string_view</span></span></a> str<span class="br0">)</span> <span class="kw4">const</span> <span class="br0">{</span> <span class="kw1">return</span> hash_type<span class="br0">{</span><span class="br0">}</span><span class="br0">(</span>str<span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span>
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size_t operator<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span><a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> <span class="kw4">const</span><span class="sy3">&</span> str<span class="br0">)</span> <span class="kw4">const</span> <span class="br0">{</span> <span class="kw1">return</span> hash_type<span class="br0">{</span><span class="br0">}</span><span class="br0">(</span>str<span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span>
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<span class="br0">}</span><span class="sy4">;</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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<span class="co1">// 简单比较演示</span>
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<a href="cpp-container-unordered_map.html"><span class="kw1293">std::<span class="me2">unordered_map</span></span></a><span class="sy1"><</span><span class="kw4">int</span>,<span class="kw4">char</span><span class="sy1">></span> example <span class="sy1">=</span> <span class="br0">{</span><span class="br0">{</span><span class="nu0">1</span>,<span class="st0">'a'</span><span class="br0">}</span>,<span class="br0">{</span><span class="nu0">2</span>,<span class="st0">'b'</span><span class="br0">}</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">auto</span> search <span class="sy1">=</span> example.<span class="me1">find</span><span class="br0">(</span><span class="nu0">2</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">if</span> <span class="br0">(</span>search <span class="sy3">!</span><span class="sy1">=</span> example.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span> <span class="br0">{</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> <span class="st0">"Found "</span> <span class="sy1"><<</span> search<span class="sy2">-</span><span class="sy1">></span>first <span class="sy1"><<</span> <span class="st0">" "</span> <span class="sy1"><<</span> search<span class="sy2">-</span><span class="sy1">></span>second <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> <span class="kw1">else</span> <span class="br0">{</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> <span class="st0">"Not found<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="co1">// C++20 演示：无序容器的异质查找（通透哈希）</span>
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<a href="cpp-container-unordered_map.html"><span class="kw1293">std::<span class="me2">unordered_map</span></span></a><span class="sy1"><</span><a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a>, size_t, string_hash, <a href="cpp-utility-functional-equal_to.html"><span class="kw1035">std::<span class="me2">equal_to</span></span></a><span class="sy1"><>></span> map<span class="br0">{</span> <span class="br0">{</span><span class="st0">"one"</span>s, <span class="nu0">1</span><span class="br0">}</span> <span class="br0">}</span><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> <a href="cpp-io-manip-boolalpha.html"><span class="kw1764">std::<span class="me2">boolalpha</span></span></a>
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<span class="sy1"><<</span> <span class="br0">(</span>map.<span class="me1">find</span><span class="br0">(</span><span class="st0">"one"</span><span class="br0">)</span> <span class="sy3">!</span><span class="sy1">=</span> map.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
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<span class="sy1"><<</span> <span class="br0">(</span>map.<span class="me1">find</span><span class="br0">(</span><span class="st0">"one"</span>s<span class="br0">)</span> <span class="sy3">!</span><span class="sy1">=</span> map.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
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<span class="sy1"><<</span> <span class="br0">(</span>map.<span class="me1">find</span><span class="br0">(</span><span class="st0">"one"</span>sv<span class="br0">)</span> <span class="sy3">!</span><span class="sy1">=</span> map.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span><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">Found 2 b
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true
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true
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true</pre></div>
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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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<div class="t-dsc-member-div">
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<div><a href="cpp-container-unordered_map-count.html"><span class="t-lines"><span>count</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数量<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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<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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</div>
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<div class="visualClear"></div>
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