155 lines
16 KiB
HTML
155 lines
16 KiB
HTML
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<title>std::any::emplace</title>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::any::</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-utility.html">utility</a>‎ | <a href="cpp-utility-any.html">any</a></span></div>
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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> ValueType, <span class="kw1">class</span>... <span class="me1">Args</span> <span class="sy1">></span><br />
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<a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1"><</span>ValueType<span class="sy1">></span><span class="sy3">&</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>(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> ValueType, <span class="kw1">class</span> U, <span class="kw1">class</span>... <span class="me1">Args</span> <span class="sy1">></span><br />
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<a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1"><</span>ValueType<span class="sy1">></span><span class="sy3">&</span> emplace<span class="br0">(</span> <a href="cpp-utility-initializer_list.html"><span class="kw960">std::<span class="me2">initializer_list</span></span></a><span class="sy1"><</span>U<span class="sy1">></span> il, 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>(2)</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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<p>更改所含对象为从参数构造的 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1"><</span>ValueType<span class="sy1">></span></span></span> 类型对象。</p>
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<p>首先用 <span class="t-lc"><a href="cpp-utility-any-reset.html">reset()</a></span> 销毁当前所含对象（若存在），然后：</p>
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<div class="t-li1"><span class="t-li">1)</span> 构造从 <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> <a href="cpp-language-direct_initialization.html">直接非列表初始化</a>的 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1"><</span>ValueType<span class="sy1">></span></span></span> 类型对象为所含对象。
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<ul>
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<li>此重载仅若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_constructible.html"><span class="kw521">std::<span class="me2">is_constructible_v</span></span></a><span class="sy1"><</span><a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1"><</span>ValueType<span class="sy1">></span>, Args...<span class="sy1">></span></span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_copy_constructible.html"><span class="kw533">std::<span class="me2">is_copy_constructible_v</span></span></a><span class="sy1"><</span><a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1"><</span>ValueType<span class="sy1">>></span></span></span> 皆为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 才参与重载决议。</li>
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</ul>
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<div class="t-li1"><span class="t-li">2)</span> 构造从 <span class="t-c"><span class="mw-geshi cpp source-cpp">il, <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> <a href="cpp-language-direct_initialization.html">直接非列表初始化</a>的 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1"><</span>ValueType<span class="sy1">></span></span></span> 类型对象所含对象。
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<ul>
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<li>此重载仅若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_constructible.html"><span class="kw521">std::<span class="me2">is_constructible_v</span></span></a><span class="sy1"><</span><a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1"><</span>ValueType<span class="sy1">></span>, <a href="cpp-utility-initializer_list.html"><span class="kw960">std::<span class="me2">initializer_list</span></span></a><span class="sy1"><</span>U<span class="sy1">></span><span class="sy3">&</span>, Args...<span class="sy1">></span></span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_copy_constructible.html"><span class="kw533">std::<span class="me2">is_copy_constructible_v</span></span></a><span class="sy1"><</span><a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1"><</span>ValueType<span class="sy1">>></span></span></span> 皆为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 才参与重载决议。</li>
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<h3><span class="mw-headline" id=".E6.A8.A1.E6.9D.BF.E5.BD.A2.E5.8F.82">模板形参</span></h3>
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<table class="t-par-begin">
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<tr class="t-par">
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<td>ValueType</td>
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<td>-</td>
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<td>所含的值类型</td>
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<tr class="t-par-hitem">
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<td colspan="3">类型要求</td>
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<tr class="t-par-req">
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<td colspan="3">-
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<div class="spacer"></div>
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<code>std::decay_t<ValueType></code> 必须满足<a href="cpp-named_req-CopyConstructible.html"><span style="font-style:italic">可复制构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(CopyConstructible)</span></a> 的要求。</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>到新的所含对象的引用。</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>抛出 <code>T</code> 构造函数所抛的任何异常。若抛出异常，则销毁先前所含对象（若存在），而 <code>*this</code> 不含值。</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 <algorithm></span>
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<span class="co2">#include <any></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="co2">#include <vector></span>
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<span class="kw1">class</span> Star
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<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">int</span> id<span class="sy4">;</span>
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<span class="kw1">public</span><span class="sy4">:</span>
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Star<span class="br0">(</span><a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> name, <span class="kw4">int</span> id<span class="br0">)</span> <span class="sy4">:</span> name <span class="br0">{</span> name <span class="br0">}</span>, id <span class="br0">{</span> id <span class="br0">}</span>
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<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">"Star::Star(string, int)<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw4">void</span> print<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span>
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<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">"Star{ <span class="es1">\"</span>"</span> <span class="sy1"><<</span> name <span class="sy1"><<</span> <span class="st0">"<span class="es1">\"</span> : "</span> <span class="sy1"><<</span> id <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><span class="sy4">;</span>
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<span class="kw4">auto</span> main<span class="br0">(</span><span class="br0">)</span> <span class="sy2">-</span><span class="sy1">></span> <span class="kw4">int</span>
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<span class="br0">{</span>
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<a href="cpp-utility-any.html"><span class="kw1071">std::<span class="me2">any</span></span></a> celestial<span class="sy4">;</span>
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<span class="co1">// (1) emplace( Args&&... args );</span>
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celestial.<span class="me1">emplace</span><span class="sy1"><</span>Star<span class="sy1">></span><span class="br0">(</span><span class="st0">"Procyon"</span>, <span class="nu0">2943</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw4">const</span> <span class="kw4">auto</span><span class="sy2">*</span> star <span class="sy1">=</span> <a href="cpp-utility-any-any_cast.html"><span class="kw1072">std::<span class="me2">any_cast</span></span></a><span class="sy1"><</span>Star<span class="sy1">></span><span class="br0">(</span><span class="sy3">&</span>celestial<span class="br0">)</span><span class="sy4">;</span>
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star<span class="sy2">-</span><span class="sy1">></span>print<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-utility-any.html"><span class="kw1071">std::<span class="me2">any</span></span></a> av<span class="sy4">;</span>
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<span class="co1">// (2) emplace( std::initializer_list<U> il, Args&&... args );</span>
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av.<span class="me1">emplace</span><span class="sy1"><</span><a href="cpp-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1"><</span><span class="kw4">char</span><span class="sy1">>></span><span class="br0">(</span><span class="br0">{</span> <span class="st0">'C'</span>, <span class="st0">'+'</span>, <span class="st0">'+'</span>, <span class="st0">'1'</span>, <span class="st0">'7'</span> <span class="br0">}</span> <span class="coMULTI">/* 无参数 */</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> av.<span class="me1">type</span><span class="br0">(</span><span class="br0">)</span>.<span class="me1">name</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="kw4">const</span> <span class="kw4">auto</span><span class="sy2">*</span> va <span class="sy1">=</span> <a href="cpp-utility-any-any_cast.html"><span class="kw1072">std::<span class="me2">any_cast</span></span></a><span class="sy1"><</span><a href="cpp-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1"><</span><span class="kw4">char</span><span class="sy1">>></span><span class="br0">(</span><span class="sy3">&</span>av<span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-algorithm-for_each.html"><span class="kw1568">std::<span class="me2">for_each</span></span></a><span class="br0">(</span>va<span class="sy2">-</span><span class="sy1">></span>cbegin<span class="br0">(</span><span class="br0">)</span>, va<span class="sy2">-</span><span class="sy1">></span>cend<span class="br0">(</span><span class="br0">)</span>, <span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="kw4">char</span> <span class="kw4">const</span><span class="sy3">&</span> c<span class="br0">)</span> <span class="br0">{</span> <a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> c<span class="sy4">;</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> <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>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">Star::Star(string, int)
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Star{ "Procyon" : 2943 };
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St6vectorIcSaIcEE
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C++17</pre></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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<div class="t-dsc-member-div t-dsc-member-nobold-div">
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<div><a href="cpp-utility-any-any.html"><span class="t-lines"><span>(构造函数)</span></span></a></div>
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</div>
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</td>
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<td>构造 <code>any</code> 对象<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-utility-any-reset.html"><span class="t-lines"><span>reset</span></span></a></div>
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</div>
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</td>
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<td>销毁所含对象<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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</body>
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