191 lines
14 KiB
HTML
191 lines
14 KiB
HTML
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<title>std::tuple_element<std::pair></title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_pair_tuple_element skin-cppreference2 action-view cpp-navbar">
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>tuple_element<small><std::pair></small></h1>
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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-pair.html">pair</a></span></div>
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<div>定义于头文件 <code><a href="cpp-header-utility.html"><utility></a></code></div>
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<td></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> I, <span class="kw1">class</span> T1, <span class="kw1">class</span> T2 <span class="sy1">></span><br />
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<span class="kw1">struct</span> tuple_element<span class="sy1"><</span>I, <a href="cpp-utility-pair.html"><span class="kw1096">std::<span class="me2">pair</span></span></a><span class="sy1"><</span>T1, T2<span class="sy1">>></span><span class="sy4">;</span></span></div>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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<p><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-tuple-tuple_element.html"><span class="kw1099">std::<span class="me2">tuple_element</span></span></a></span></span> 对 pair 的部分特化使用类 tuple 语法，提供 pair 元素类型的编译时访问。若 <code>I >= 2</code> 则程序为病式。</p>
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<h3><span class="mw-headline" id=".E6.88.90.E5.91.98.E7.B1.BB.E5.9E.8B">成员类型</span></h3>
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<td>成员类型</td>
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<td>定义</td>
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<tr class="t-dsc">
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<td><code>type</code></td>
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<td><code>I == 0</code> 则为 <code>T1</code><br />
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<code>I == 1</code> 则为 <code>T2</code></td>
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<h3><span class="mw-headline" id=".E5.8F.AF.E8.83.BD.E7.9A.84.E5.AE.9E.E7.8E.B0">可能的实现</span></h3>
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<div class="cpp source-cpp">
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<pre class="de1"><span class="kw1">template</span><span class="sy1"><</span><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> I, <span class="kw1">typename</span> T<span class="sy1">></span>
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<span class="kw1">struct</span> tuple_element<span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> I, <span class="kw1">typename</span> T1, <span class="kw1">typename</span> T2<span class="sy1">></span>
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<span class="kw1">struct</span> tuple_element<span class="sy1"><</span>I, <a href="cpp-utility-pair.html"><span class="kw1096">std::<span class="me2">pair</span></span></a><span class="sy1"><</span>T1,T2<span class="sy1">></span> <span class="sy1">></span>
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<span class="br0">{</span>
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static_assert<span class="br0">(</span>I <span class="sy1"><</span> <span class="nu0">2</span>, <span class="st0">"std::pair has only 2 elements!"</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="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> T1, <span class="kw1">typename</span> T2<span class="sy1">></span>
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<span class="kw1">struct</span> tuple_element<span class="sy1"><</span><span class="nu0">0</span>, <a href="cpp-utility-pair.html"><span class="kw1096">std::<span class="me2">pair</span></span></a><span class="sy1"><</span>T1,T2<span class="sy1">></span> <span class="sy1">></span>
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<span class="br0">{</span>
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<span class="kw1">using</span> type <span class="sy1">=</span> T1<span class="sy4">;</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> T1, <span class="kw1">typename</span> T2<span class="sy1">></span>
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<span class="kw1">struct</span> tuple_element<span class="sy1"><</span><span class="nu0">1</span>, <a href="cpp-utility-pair.html"><span class="kw1096">std::<span class="me2">pair</span></span></a><span class="sy1"><</span>T1,T2<span class="sy1">></span> <span class="sy1">></span>
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<span class="br0">{</span>
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<span class="kw1">using</span> type <span class="sy1">=</span> T2<span class="sy4">;</span>
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<span class="br0">}</span><span class="sy4">;</span></pre></div>
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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 <tuple></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="kw1">namespace</span> detail <span class="br0">{</span>
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<span class="kw1">template</span> <span class="sy1"><</span><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a><span class="sy1">></span>
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<span class="kw1">struct</span> index_tag <span class="br0">{</span> <span class="kw1">explicit</span> <span class="kw4">constexpr</span> index_tag<span class="br0">(</span><span class="br0">)</span> <span class="sy1">=</span> <span class="kw1">default</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">class</span> T, <span class="kw1">class</span> U<span class="sy1">></span>
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<span class="kw4">constexpr</span> T get_val_dispatch<span class="br0">(</span><a href="cpp-utility-pair.html"><span class="kw1096">std::<span class="me2">pair</span></span></a><span class="sy1"><</span>T, U<span class="sy1">></span> <span class="kw4">const</span><span class="sy3">&</span> pair, index_tag<span class="sy1"><</span><span class="nu0">0</span><span class="sy1">></span><span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">return</span> pair.<span class="me1">first</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">class</span> T, <span class="kw1">class</span> U<span class="sy1">></span>
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<span class="kw4">constexpr</span> U get_val_dispatch<span class="br0">(</span><a href="cpp-utility-pair.html"><span class="kw1096">std::<span class="me2">pair</span></span></a><span class="sy1"><</span>T, U<span class="sy1">></span> <span class="kw4">const</span><span class="sy3">&</span> pair, index_tag<span class="sy1"><</span><span class="nu0">1</span><span class="sy1">></span><span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">return</span> pair.<span class="me1">second</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="br0">}</span> <span class="co1">// namespace detail</span>
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<span class="kw1">template</span> <span class="sy1"><</span><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> N, <span class="kw1">class</span> T, <span class="kw1">class</span> U<span class="sy1">></span>
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<span class="kw4">auto</span> <span class="kw4">constexpr</span> get_val<span class="br0">(</span><a href="cpp-utility-pair.html"><span class="kw1096">std::<span class="me2">pair</span></span></a><span class="sy1"><</span>T, U<span class="sy1">></span> <span class="kw4">const</span><span class="sy3">&</span> pair<span class="br0">)</span>
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<span class="sy2">-</span><span class="sy1">></span> <span class="kw1">typename</span> <a href="cpp-utility-tuple-tuple_element.html"><span class="kw1099">std::<span class="me2">tuple_element</span></span></a><span class="sy1"><</span>N, <a href="cpp-utility-pair.html"><span class="kw1096">std::<span class="me2">pair</span></span></a><span class="sy1"><</span>T, U<span class="sy1">>></span><span class="sy4">::</span><span class="me2">type</span>
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<span class="br0">{</span>
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<span class="kw1">return</span> detail<span class="sy4">::</span><span class="me2">get_val_dispatch</span><span class="br0">(</span>pair, detail<span class="sy4">::</span><span class="me2">index_tag</span><span class="sy1"><</span>N<span class="sy1">></span><span class="br0">{</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> main<span class="br0">(</span><span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw4">auto</span> var <span class="sy1">=</span> <a href="cpp-utility-pair-make_pair.html"><span class="kw1097">std::<span class="me2">make_pair</span></span></a><span class="br0">(</span><span class="nu0">1</span>, <a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a><span class="br0">{</span><span class="st0">"one"</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> get_val<span class="sy1"><</span><span class="nu0">0</span><span class="sy1">></span><span class="br0">(</span>var<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">" = "</span> <span class="sy1"><<</span> get_val<span class="sy1"><</span><span class="nu0">1</span><span class="sy1">></span><span class="br0">(</span>var<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>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">1 = one</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/issue2974">LWG 2974</a></td>
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<td>C++11</td>
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<td>越界下标使用不定义的初等模板</td>
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<td>使之为非良构（硬错误）</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><a href="cpp-language-structured_binding.html">结构化绑定</a> <span class="t-mark-rev t-since-cxx17">(C++17)</span></td>
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<td>绑定指定的名字到初始化器的子对象或元组元素</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-tuple-tuple_element.html"><span class="t-lines"><span>std::tuple_element<span class="t-dsc-small"><std::tuple></span></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>获得指定元素的类型<br />
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<span class="t-mark">(类模板特化)</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-array-tuple_element.html"><span class="t-lines"><span>std::tuple_element<span class="t-dsc-small"><std::array></span></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>获得 <code>array</code> 元素的类型<br />
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<span class="t-mark">(类模板特化)</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-pair-tuple_size.html"><span class="t-lines"><span>std::tuple_size<span class="t-dsc-small"><std::pair></span></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>获得 <code>pair</code> 的大小<br />
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<span class="t-mark">(类模板特化)</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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</body>
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