182 lines
16 KiB
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182 lines
16 KiB
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<title>std::unordered_map<Key,T,Hash,KeyEqual,Allocator>::insert_or_assign</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_container_unordered_map_insert_or_assign skin-cppreference2 action-view cpp-page-template cpp-navbar">
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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>insert_or_assign</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 class="t-page-template" style="">
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<p></p>
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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> M<span class="sy1">></span><br />
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<a href="cpp-utility-pair.html"><span class="kw1096">std::<span class="me2">pair</span></span></a><span class="sy1"><</span>iterator, <span class="kw4">bool</span><span class="sy1">></span> insert_or_assign<span class="br0">(</span><span class="kw4">const</span> key_type<span class="sy3">&</span> k, M<span class="sy3">&&</span> obj<span class="br0">)</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-cxx17">(C++17 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> M<span class="sy1">></span><br />
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<a href="cpp-utility-pair.html"><span class="kw1096">std::<span class="me2">pair</span></span></a><span class="sy1"><</span>iterator, <span class="kw4">bool</span><span class="sy1">></span> insert_or_assign<span class="br0">(</span>key_type<span class="sy3">&&</span> k, M<span class="sy3">&&</span> obj<span class="br0">)</span><span class="sy4">;</span></span></div>
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<td>(2)</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 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> M<span class="sy1">></span><br />
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iterator insert_or_assign<span class="br0">(</span>const_iterator hint, <span class="kw4">const</span> key_type<span class="sy3">&</span> k, M<span class="sy3">&&</span> obj<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-cxx17">(C++17 起)</span></td>
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</tr>
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<tr class="t-dcl t-since-cxx17">
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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> M<span class="sy1">></span><br />
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iterator insert_or_assign<span class="br0">(</span>const_iterator hint, key_type<span class="sy3">&&</span> k, M<span class="sy3">&&</span> obj<span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td>(4)</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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<tr class="t-dcl-sep">
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<div class="t-li1"><span class="t-li">1,3)</span> 若等价于 <code>k</code> 的键已存在于容器中，则赋值 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-forward.html"><span class="kw1117">std::<span class="me2">forward</span></span></a><span class="sy1"><</span>M<span class="sy1">></span><span class="br0">(</span>obj<span class="br0">)</span></span></span> 给对应于键 <code>k</code> 的 <code>mapped_type</code> 。若键不存在，则如同用 <a href="cpp-container-unordered_map-insert.html"><code>insert</code></a> 插入从 <span class="t-c"><span class="mw-geshi cpp source-cpp">value_type<span class="br0">(</span>k, <a href="cpp-utility-forward.html"><span class="kw1117">std::<span class="me2">forward</span></span></a><span class="sy1"><</span>M<span class="sy1">></span><span class="br0">(</span>obj<span class="br0">)</span><span class="br0">)</span></span></span> 构造的新值。</div>
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<div class="t-li1"><span class="t-li">2,4)</span> 同 <span class="t-v">(1,3)</span> ，除了从 <span class="t-c"><span class="mw-geshi cpp source-cpp">value_type<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>k<span class="br0">)</span>, <a href="cpp-utility-forward.html"><span class="kw1117">std::<span class="me2">forward</span></span></a><span class="sy1"><</span>M<span class="sy1">></span><span class="br0">(</span>obj<span class="br0">)</span><span class="br0">)</span></span></span> 构造被映射值。</div>
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<p>若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_assignable.html"><span class="kw545">std::<span class="me2">is_assignable_v</span></span></a><span class="sy1"><</span>mapped_type<span class="sy3">&</span>, M<span class="sy3">&&</span><span class="sy1">></span></span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> 则<span class="t-rev-inl t-until-cxx20"><span>行为未定义</span> <span><span class="t-mark-rev t-until-cxx20">(C++20 前)</span></span></span><span class="t-rev-inl t-since-cxx20"><span>程序非良构</span> <span><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></span></span>。</p>
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<p>若插入发生且导致容器的重哈希，则所有迭代器被非法化。否则迭代器不受影响。重哈希仅若新元素数量大于 <span class="t-c"><span class="mw-geshi cpp source-cpp">max_load_factor<span class="br0">(</span><span class="br0">)</span><span class="sy2">*</span>bucket_count<span class="br0">(</span><span class="br0">)</span></span></span> 才发生。</p>
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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>k</td>
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<td>-</td>
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<td>用于查找和若找不到则插入的键</td>
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</tr>
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<tr class="t-par">
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<td>hint</td>
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<td>-</td>
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<td>指向新元素将要插入位置之前位置的迭代器</td>
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</tr>
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<tr class="t-par">
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<td>obj</td>
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<td>-</td>
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<td>要插入或赋值的值</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">1,2)</span> 若插入发生则 bool 组分为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> ，若赋值发生则 bool 组分为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> 。迭代器组分指向插入或更新的元素。</div>
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<div class="t-li1"><span class="t-li">3,4)</span> 指向被插入或更新的元素的迭代器。</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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<div class="t-li1"><span class="t-li">1,2)</span> 与 <a href="cpp-container-unordered_map-emplace.html"><code>emplace</code></a> 的相同</div>
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<div class="t-li1"><span class="t-li">3,4)</span> 与 <a href="cpp-container-unordered_map-emplace_hint.html"><code>emplace_hint</code></a> 的相同</div>
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<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
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<p><code>insert_or_assign</code> 返回多于 <code>operator[]</code> 的信息，而且不要求被映射类型的可默认构造性。</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="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 <iostream></span>
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<span class="co2">#include <unordered_map></span>
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<span class="co2">#include <string></span>
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<span class="kw4">auto</span> print_node <span class="sy1">=</span> <span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="kw4">const</span> <span class="kw4">auto</span> <span class="sy3">&</span>node<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">"["</span> <span class="sy1"><<</span> node.<span class="me1">first</span> <span class="sy1"><<</span> <span class="st0">"] = "</span> <span class="sy1"><<</span> node.<span class="me1">second</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><span class="sy4">;</span>
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<span class="kw4">auto</span> print_result <span class="sy1">=</span> <span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="kw4">auto</span> <span class="kw4">const</span> <span class="sy3">&</span>pair<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="br0">(</span>pair.<span class="me1">second</span> <span class="sy4">?</span> <span class="st0">"inserted: "</span> <span class="sy4">:</span> <span class="st0">"assigned: "</span><span class="br0">)</span><span class="sy4">;</span>
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print_node<span class="br0">(</span><span class="sy2">*</span>pair.<span class="me1">first</span><span class="br0">)</span><span class="sy4">;</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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<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>, <a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a><span class="sy1">></span> myMap<span class="sy4">;</span>
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print_result<span class="br0">(</span> myMap.<span class="me1">insert_or_assign</span><span class="br0">(</span><span class="st0">"a"</span>, <span class="st0">"apple"</span> <span class="br0">)</span> <span class="br0">)</span><span class="sy4">;</span>
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print_result<span class="br0">(</span> myMap.<span class="me1">insert_or_assign</span><span class="br0">(</span><span class="st0">"b"</span>, <span class="st0">"banana"</span> <span class="br0">)</span> <span class="br0">)</span><span class="sy4">;</span>
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print_result<span class="br0">(</span> myMap.<span class="me1">insert_or_assign</span><span class="br0">(</span><span class="st0">"c"</span>, <span class="st0">"cherry"</span> <span class="br0">)</span> <span class="br0">)</span><span class="sy4">;</span>
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print_result<span class="br0">(</span> myMap.<span class="me1">insert_or_assign</span><span class="br0">(</span><span class="st0">"c"</span>, <span class="st0">"clementine"</span><span class="br0">)</span> <span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">const</span> <span class="kw4">auto</span> <span class="sy3">&</span>node <span class="sy4">:</span> myMap<span class="br0">)</span> <span class="br0">{</span> print_node<span class="br0">(</span>node<span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span>
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<span class="br0">}</span></pre></div>
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</div>
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<p>可能的输出：</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">inserted: [a] = apple
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inserted: [b] = banana
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inserted: [c] = cherry
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assigned: [c] = clementine
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[c] = clementine
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[a] = apple
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[b] = banana</pre></div>
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</div>
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</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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<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-operator_at.html"><span class="t-lines"><span>operator[]</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>访问或插入指定的元素<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-at.html"><span class="t-lines"><span>at</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>访问指定的元素，同时进行越界检查<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-insert.html"><span class="t-lines"><span>insert</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>插入元素<span class="t-rev-inl t-since-cxx17"><span>或结点</span> <span><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></span></span><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-emplace.html"><span class="t-lines"><span>emplace</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>原位构造元素<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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</table>
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</div>
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<div class="visualClear"></div>
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</div>
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</div>
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</div>
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</body>
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