170 lines
15 KiB
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170 lines
15 KiB
HTML
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<title>std::deque<T,Allocator>::emplace</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_container_deque_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::deque<T,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-deque.html">deque</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>... <span class="me1">Args</span> <span class="sy1">></span><br />
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iterator emplace<span class="br0">(</span> const_iterator pos, 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 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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<td></td>
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<p>直接于 <code>pos</code> 前插入元素到容器中。</p>
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<p>通过 <span class="t-lc"><a href="cpp-memory-allocator_traits-construct.html">std::allocator_traits::construct</a></span> 构造元素，常用布置 new 在容器提供的位置原位构造元素。然而若要求的位置已被既存的元素占据，则首先在另一位置构造被插入的元素，然后再将他移动赋值到要求的位置中。</p>
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<p>将参数 <code>args...</code> 作为 <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> 转发给构造函数。 <code>args...</code> 可以直接或间接地指代容器中的值。</p>
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<p>所有迭代器，含尾后迭代器，都被非法化。引用亦被非法化，除非 <span class="t-c"><span class="mw-geshi cpp source-cpp">pos <span class="sy1">==</span> begin<span class="br0">(</span><span class="br0">)</span></span></span> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp">pos <span class="sy1">==</span> end<span class="br0">(</span><span class="br0">)</span></span></span> ，该情况下它们不被非法化。</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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<td>pos</td>
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<td>-</td>
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<td>将构造新元素到其前的迭代器</td>
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</tr>
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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>转发给元素构造函数的参数</td>
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</tr>
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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>T （容器元素类型）</code> 必须满足<a href="cpp-named_req-MoveAssignable.html"><span style="font-style:italic">可移动赋值</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(MoveAssignable)</span></a> 、 <a href="cpp-named_req-MoveInsertable.html"><span style="font-style:italic">可移动插入</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(MoveInsertable)</span></a> 和 <a href="cpp-named_req-EmplaceConstructible.html"><span style="font-style:italic">可就位构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(EmplaceConstructible)</span></a> 的要求。</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>指向被安置的元素的迭代器。</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>与 <code>pos</code> 和容器两端距离的较小者成线性。</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 value_type 的复制构造函数、移动构造函数、赋值运算符或移动赋值运算符以外的操作抛异常，或若在用 <code>emplace</code> 在任一端插入单个元素时抛异常，则无效果（强异常保证）。</p>
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<p>否则，效果未指定。</p>
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<p><br /></p>
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<p><br /></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 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 <string></span>
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<span class="co2">#include <deque></span>
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<span class="kw1">struct</span> A <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> s<span class="sy4">;</span>
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A<span class="br0">(</span><a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> str<span class="br0">)</span> <span class="sy4">:</span> s<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>str<span class="br0">)</span><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> <span class="st0">" constructed<span class="es1">\n</span>"</span><span class="sy4">;</span> <span class="br0">}</span>
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A<span class="br0">(</span><span class="kw4">const</span> A<span class="sy3">&</span> o<span class="br0">)</span> <span class="sy4">:</span> s<span class="br0">(</span>o.<span class="me1">s</span><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> <span class="st0">" copy constructed<span class="es1">\n</span>"</span><span class="sy4">;</span> <span class="br0">}</span>
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A<span class="br0">(</span>A<span class="sy3">&&</span> o<span class="br0">)</span> <span class="sy4">:</span> s<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>o.<span class="me1">s</span><span class="br0">)</span><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> <span class="st0">" move constructed<span class="es1">\n</span>"</span><span class="sy4">;</span> <span class="br0">}</span>
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A<span class="sy3">&</span> operator<span class="sy1">=</span><span class="br0">(</span><span class="kw4">const</span> A<span class="sy3">&</span> other<span class="br0">)</span> <span class="br0">{</span>
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s <span class="sy1">=</span> other.<span class="me1">s</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">" copy assigned<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="kw1">return</span> <span class="sy2">*</span>this<span class="sy4">;</span>
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<span class="br0">}</span>
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A<span class="sy3">&</span> operator<span class="sy1">=</span><span class="br0">(</span>A<span class="sy3">&&</span> other<span class="br0">)</span> <span class="br0">{</span>
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s <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>other.<span class="me1">s</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">" move assigned<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="kw1">return</span> <span class="sy2">*</span>this<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">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-deque.html"><span class="kw1271">std::<span class="me2">deque</span></span></a><span class="sy1"><</span>A<span class="sy1">></span> container<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">"construct 2 times A:<span class="es1">\n</span>"</span><span class="sy4">;</span>
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A two <span class="br0">{</span> <span class="st0">"two"</span> <span class="br0">}</span><span class="sy4">;</span>
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A three <span class="br0">{</span> <span class="st0">"three"</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">"emplace:<span class="es1">\n</span>"</span><span class="sy4">;</span>
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container.<span class="me1">emplace</span><span class="br0">(</span>container.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span>, <span class="st0">"one"</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">"emplace with A&:<span class="es1">\n</span>"</span><span class="sy4">;</span>
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container.<span class="me1">emplace</span><span class="br0">(</span>container.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span>, two<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">"emplace with A&&:<span class="es1">\n</span>"</span><span class="sy4">;</span>
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container.<span class="me1">emplace</span><span class="br0">(</span>container.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span>, std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>three<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">"content:<span class="es1">\n</span>"</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> obj <span class="sy4">:</span> container<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> obj.<span class="me1">s</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>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">construct 2 times A:
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constructed
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constructed
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emplace:
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constructed
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emplace with A&:
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copy constructed
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emplace with A&&:
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move constructed
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content vec:
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one two three</pre></div>
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</div>
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</div>
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<h3><span class="mw-headline" id=".E7.BC.BA.E9.99.B7.E6.8A.A5.E5.91.8A">缺陷报告</span></h3>
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<p>下列更改行为的缺陷报告追溯地应用于以前出版的 C++ 标准。</p>
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<table class="dsctable" style="font-size:0.8em">
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<tr>
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<th>DR</th>
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<th>应用于</th>
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<th>出版时的行为</th>
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<th>正确行为</th>
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</tr>
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<tr>
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<td><a rel="nofollow" class="external text" href="https://cplusplus.github.io/LWG/issue2164">LWG 2164</a></td>
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<td>C++11</td>
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<td>不清楚参数是否能指代容器</td>
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<td>已澄清</td>
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</tr>
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</table>
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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-deque-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<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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