167 lines
16 KiB
HTML
167 lines
16 KiB
HTML
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<title>std::set<Key,Compare,Allocator>::emplace</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_container_set_emplace 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::set<Key,Compare,Allocator>::</span>emplace</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-set.html">set</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>... <span class="me1">Args</span> <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> emplace<span class="br0">(</span> Args<span class="sy3">&&</span>... <span class="me1">args</span> <span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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<tr class="t-dcl-sep">
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<td></td>
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<td></td>
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<td></td>
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<p>若容器中无拥有该关键的元素，则插入以给定的 <code>args</code> 原位构造的新元素到容器。</p>
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<p>细心地使用 <code>emplace</code> 允许在构造新元素的同时避免不必要的复制或移动操作。 准确地以与提供给 <code>emplace</code> 者相同的参数，通过 <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>Args<span class="sy1">></span><span class="br0">(</span>args<span class="br0">)</span>...</span></span> 转发调用新元素的构造函数。 即使容器中已有拥有该关键的元素，也可能构造元素，该情况下新构造的元素将被立即销毁。</p>
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<p>没有迭代器或引用被非法化。</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>args</td>
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<td>-</td>
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<td>要转发给元素构造函数的参数</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>返回由指向被插入元素，或若不发生插入则为既存元素的迭代器，和指代插入是否发生的 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span></span></span> （若发生插入则为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> ，否则为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></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>若任何操作抛出异常，则此函数无效果。</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>与容器大小成对数。</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 <chrono></span>
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<span class="co2">#include <functional></span>
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<span class="co2">#include <iomanip></span>
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<span class="co2">#include <iostream></span>
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<span class="co2">#include <set></span>
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<span class="co2">#include <string></span>
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<span class="kw1">class</span> Dew
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<span class="br0">{</span>
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<span class="kw1">private</span><span class="sy4">:</span>
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<span class="kw4">int</span> a<span class="sy4">;</span>
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<span class="kw4">int</span> b<span class="sy4">;</span>
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<span class="kw4">int</span> c<span class="sy4">;</span>
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<span class="kw1">public</span><span class="sy4">:</span>
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Dew<span class="br0">(</span><span class="kw4">int</span> _a, <span class="kw4">int</span> _b, <span class="kw4">int</span> _c<span class="br0">)</span>
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<span class="sy4">:</span> a<span class="br0">(</span>_a<span class="br0">)</span>, b<span class="br0">(</span>_b<span class="br0">)</span>, c<span class="br0">(</span>_c<span class="br0">)</span>
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<span class="br0">{</span><span class="br0">}</span>
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<span class="kw4">bool</span> operator<span class="sy1"><</span><span class="br0">(</span><span class="kw4">const</span> Dew <span class="sy3">&</span>other<span class="br0">)</span> <span class="kw4">const</span>
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<span class="br0">{</span>
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<span class="kw1">if</span> <span class="br0">(</span>a <span class="sy1"><</span> other.<span class="me1">a</span><span class="br0">)</span>
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<span class="kw1">return</span> <span class="kw2">true</span><span class="sy4">;</span>
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<span class="kw1">if</span> <span class="br0">(</span>a <span class="sy1">==</span> other.<span class="me1">a</span> <span class="sy3">&&</span> b <span class="sy1"><</span> other.<span class="me1">b</span><span class="br0">)</span>
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<span class="kw1">return</span> <span class="kw2">true</span><span class="sy4">;</span>
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<span class="kw1">return</span> <span class="br0">(</span>a <span class="sy1">==</span> other.<span class="me1">a</span> <span class="sy3">&&</span> b <span class="sy1">==</span> other.<span class="me1">b</span> <span class="sy3">&&</span> c <span class="sy1"><</span> other.<span class="me1">c</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="kw4">const</span> <span class="kw4">int</span> nof_operations <span class="sy1">=</span> <span class="nu0">120</span><span class="sy4">;</span>
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<span class="kw4">int</span> set_emplace<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
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<a href="cpp-container-set.html"><span class="kw1277">std::<span class="me2">set</span></span></a><span class="sy1"><</span>Dew<span class="sy1">></span> set<span class="sy4">;</span>
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<span class="kw1">for</span><span class="br0">(</span><span class="kw4">int</span> i <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> i <span class="sy1"><</span> nof_operations<span class="sy4">;</span> <span class="sy2">++</span>i<span class="br0">)</span>
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<span class="kw1">for</span><span class="br0">(</span><span class="kw4">int</span> j <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> j <span class="sy1"><</span> nof_operations<span class="sy4">;</span> <span class="sy2">++</span>j<span class="br0">)</span>
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<span class="kw1">for</span><span class="br0">(</span><span class="kw4">int</span> k <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> k <span class="sy1"><</span> nof_operations<span class="sy4">;</span> <span class="sy2">++</span>k<span class="br0">)</span>
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set.<span class="me1">emplace</span><span class="br0">(</span>i, j, k<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">return</span> set.<span class="me1">size</span><span class="br0">(</span><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> set_insert<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
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<a href="cpp-container-set.html"><span class="kw1277">std::<span class="me2">set</span></span></a><span class="sy1"><</span>Dew<span class="sy1">></span> set<span class="sy4">;</span>
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<span class="kw1">for</span><span class="br0">(</span><span class="kw4">int</span> i <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> i <span class="sy1"><</span> nof_operations<span class="sy4">;</span> <span class="sy2">++</span>i<span class="br0">)</span>
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<span class="kw1">for</span><span class="br0">(</span><span class="kw4">int</span> j <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> j <span class="sy1"><</span> nof_operations<span class="sy4">;</span> <span class="sy2">++</span>j<span class="br0">)</span>
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<span class="kw1">for</span><span class="br0">(</span><span class="kw4">int</span> k <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> k <span class="sy1"><</span> nof_operations<span class="sy4">;</span> <span class="sy2">++</span>k<span class="br0">)</span>
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set.<span class="me1">insert</span><span class="br0">(</span>Dew<span class="br0">(</span>i, j, k<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">return</span> set.<span class="me1">size</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw4">void</span> timeit<span class="br0">(</span><a href="cpp-utility-functional-function.html"><span class="kw1059">std::<span class="me2">function</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="br0">(</span><span class="br0">)</span><span class="sy1">></span> set_test, <a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> what <span class="sy1">=</span> <span class="st0">""</span><span class="br0">)</span> <span class="br0">{</span>
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<span class="kw4">auto</span> start <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">system_clock</span><span class="sy4">::</span><span class="me2">now</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw4">int</span> setsize <span class="sy1">=</span> set_test<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw4">auto</span> stop <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">chrono</span><span class="sy4">::</span><span class="me2">system_clock</span><span class="sy4">::</span><span class="me2">now</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-chrono-duration.html"><span class="kw993">std::<span class="me2">chrono</span><span class="sy4">::</span><span class="me2">duration</span></span></a><span class="sy1"><</span><span class="kw4">double</span>, <a href="cpp-numeric-ratio-ratio.html"><span class="kw1528">std::<span class="me2">milli</span></span></a><span class="sy1">></span> time <span class="sy1">=</span> stop <span class="sy2">-</span> start<span class="sy4">;</span>
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<span class="kw1">if</span> <span class="br0">(</span>what.<span class="me1">size</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1">></span> <span class="nu0">0</span> <span class="sy3">&&</span> setsize <span class="sy1">></span> <span class="nu0">0</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> <a href="cpp-io-manip-fixed.html"><span class="kw1784">std::<span class="me2">fixed</span></span></a> <span class="sy1"><<</span> <a href="cpp-io-manip-setprecision.html"><span class="kw1796">std::<span class="me2">setprecision</span></span></a><span class="br0">(</span><span class="nu0">2</span><span class="br0">)</span>
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<span class="sy1"><<</span> time.<span class="me1">count</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">" ms for "</span> <span class="sy1"><<</span> what <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>
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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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set_insert<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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timeit<span class="br0">(</span>set_insert, <span class="st0">"insert"</span><span class="br0">)</span><span class="sy4">;</span>
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timeit<span class="br0">(</span>set_emplace, <span class="st0">"emplace"</span><span class="br0">)</span><span class="sy4">;</span>
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timeit<span class="br0">(</span>set_insert, <span class="st0">"insert"</span><span class="br0">)</span><span class="sy4">;</span>
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timeit<span class="br0">(</span>set_emplace, <span class="st0">"emplace"</span><span class="br0">)</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>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">638.45 ms for insert
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619.44 ms for emplace
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609.43 ms for insert
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652.55 ms for emplace</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-set-emplace_hint.html"><span class="t-lines"><span>emplace_hint</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>使用提示原位构造元素<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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<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-set-insert.html"><span class="t-lines"><span>insert</span></span></a></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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</table>
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</div>
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<div class="visualClear"></div>
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</div>
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