209 lines
16 KiB
HTML
209 lines
16 KiB
HTML
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<title>std::out_ptr_t</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_memory_out_ptr_t 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>out_ptr_t</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-memory.html">memory</a></span></div>
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<div>定义于头文件 <code><a href="cpp-header-memory.html"><memory></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> <span class="kw1">class</span> Smart, <span class="kw1">class</span> Pointer, <span class="kw1">class</span>... <span class="me1">Args</span> <span class="sy1">></span><br />
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<span class="kw1">class</span> out_ptr_t<span class="sy4">;</span></span></div>
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<td><span class="t-mark-rev t-since-cxx23">(C++23 起)</span></td>
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<p><code>out_ptr_t</code> 用于对通过 <span class="t-c"><span class="mw-geshi cpp source-cpp">Pointer<span class="sy2">*</span></span></span> （通常为某对象类型 <code>T</code> 的 <span class="t-c"><span class="mw-geshi cpp source-cpp">T<span class="sy2">**</span></span></span> ）或 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span><span class="sy2">**</span></span></span> 参数输出其结果的外来函数，适配智能指针之类的类型。</p>
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<p><code>out_ptr_t</code> 在构造时捕获额外的实参，为前述的这种外来函数写入的结果提供存储，而最终在销毁时以该结果和捕获的实参重设适配的 <code>Smart</code> 对象。</p>
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<p><code>out_ptr_t</code> 表现如同它拥有下列非静态数据成员：</p>
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<ul>
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<li>一个 <span class="t-c"><span class="mw-geshi cpp source-cpp">Smart<span class="sy3">&</span></span></span> 引用，在构造时绑定到被适配的对象，</li>
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<li>对 <code>Args...</code> 中的每个 <code>T</code> 有一个 <code>T</code> 类型成员，它是在构造时捕获并在析构时用于重设的实参，以及</li>
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<li>适合在其内存储 <code>Pointer</code> 以及提供 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span><span class="sy2">*</span></span></span> 对象的成员子对象，其中 <code>Pointer</code> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span><span class="sy2">*</span></span></span> 对象通常会暴露给用于重初始化的外来函数。</li>
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</ul>
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<p>用户能通过在 <code>Args...</code> 中分别指定对象类型或引用类型，控制每个参数是按复制还是按引用捕获。</p>
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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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<td>Smart</td>
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<td>-</td>
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<td>要适配的对象（常为智能指针）的类型</td>
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<td>Pointer</td>
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<td>-</td>
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<td>外来函数写入其结果所用的对象（常为裸指针）的类型</td>
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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>用于重设适配的对象的捕获实参的类型</td>
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<td colspan="3">类型要求</td>
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<td colspan="3">-
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<code>Pointer</code> 必须满足<a href="cpp-named_req-NullablePointer.html"><span style="font-style:italic">可空指针</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(NullablePointer)</span></a> 的要求。</td>
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<td colspan="3">-
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若 <code>Smart</code> 为 <span class="t-lc"><a href="cpp-memory-shared_ptr.html">std::shared_ptr</a></span> 特化且 <span class="t-c"><span class="mw-geshi cpp source-cpp">sizeof...<span class="br0">(</span>Args<span class="br0">)</span> <span class="sy1">==</span> <span class="nu0">0</span></span></span> 则程序非良构。</td>
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<h3><span class="mw-headline" id=".E7.89.B9.E5.8C.96">特化</span></h3>
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<p>不同于标准库中的大多数类模板，依赖于至少一个程序定义类型的 <code>out_ptr_t</code> 程序定义特化不需要符合对主模板的要求。</p>
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<p>此许可允许程序定义特化将存储于非标准智能指针内的裸指针暴露给外来函数。</p>
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<h3><span class="mw-headline" id=".E6.88.90.E5.91.98.E5.87.BD.E6.95.B0">成员函数</span></h3>
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<div class="t-dsc-member-div t-dsc-member-nobold-div">
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<div><a href="cpp-memory-out_ptr_t-out_ptr_t.html"><span class="t-lines"><span>(构造函数)</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx23">(C++23)</span></span></span></div>
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</td>
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<td>构造 <code>out_ptr_t</code><br />
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<span class="t-mark">(公开成员函数)</span></td>
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<div><span class="t-lines"><span>operator=</span></span></div>
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<div><span class="t-lines"><span><span class="t-cmark">[被删除]</span><span class="t-mark-rev t-since-cxx23">(C++23)</span></span></span></div>
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<td><code>out_ptr_t</code> 不可赋值<br />
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<span class="t-mark">(公开成员函数)</span></td>
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<div class="t-dsc-member-div t-dsc-member-nobold-div">
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<div><a href="cpp-memory-out_ptr_t-~out_ptr_t.html"><span class="t-lines"><span>(析构函数)</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx23">(C++23)</span></span></span></div>
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</div>
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</td>
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<td>重设适配的智能指针<br />
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<span class="t-mark">(公开成员函数)</span></td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-memory-out_ptr_t-operator_ptr.html"><span class="t-lines"><span>operator Pointer*</span><span>operator void**</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx23">(C++23)</span></span></span></div>
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</div>
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</td>
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<td>转换 <code>out_ptr_t</code> 为用于输出的存储的地址<br />
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<span class="t-mark">(公开成员函数)</span></td>
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<h3><span class="mw-headline" id=".E9.9D.9E.E6.88.90.E5.91.98.E5.87.BD.E6.95.B0">非成员函数</span></h3>
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<div><a href="cpp-memory-out_ptr_t-out_ptr.html"><span class="t-lines"><span>out_ptr</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx23">(C++23)</span></span></span></div>
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</div>
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</td>
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<td>以关联的智能指针和重设参数创建 <code>out_ptr_t</code><br />
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<span class="t-mark">(函数模板)</span></td>
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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>out_ptr_t</code> 期待外来函数不使用被指向的 <code>Pointer</code> 的值并且只重新初始化它。不使用智能指针在适配前的值。</p>
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<p><code>out_ptr_t</code> 的典型用法是由 <span class="t-lc">std::out_ptr</span> 创建其临时对象，这会立即重设智能指针。例如，给定设置器函数和智能指针，分别以 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span> foreign_setter<span class="br0">(</span>T<span class="sy2">**</span><span class="br0">)</span><span class="sy4">;</span></span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-memory-unique_ptr.html"><span class="kw733">std::<span class="me2">unique_ptr</span></span></a><span class="sy1"><</span>T, D<span class="sy1">></span> up<span class="sy4">;</span></span></span> 声明，</p>
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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="kw1">if</span> <span class="br0">(</span><span class="kw4">int</span> ec <span class="sy1">=</span> foreign_setter<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">out_ptr</span><span class="br0">(</span>up<span class="br0">)</span><span class="br0">)</span> <span class="br0">{</span>
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<span class="kw1">return</span> ec<span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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</div>
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<p>粗略地等价于</p>
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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">T <span class="sy2">*</span>raw_p<span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">int</span> ec <span class="sy1">=</span> foreign_setter<span class="br0">(</span><span class="sy3">&</span>raw_p<span class="br0">)</span><span class="sy4">;</span>
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up.<span class="me1">reset</span><span class="br0">(</span>raw_p<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">if</span> <span class="br0">(</span>ec <span class="sy3">!</span><span class="sy1">=</span> <span class="nu0">0</span><span class="br0">)</span> <span class="br0">{</span>
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<span class="kw1">return</span> ec<span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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</div>
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<p>不推荐创建拥有自动存储期以外的<a href="cpp-language-storage_duration.html">存储期</a>的 <code>out_ptr_t</code> 对象，因为这种代码较可能产生悬垂引用并在析构时导致未定义行为。</p>
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<p><code>out_ptr_t</code> 禁止会不经指定删除器就重设 <span class="t-lc"><a href="cpp-memory-shared_ptr.html">std::shared_ptr</a></span> 的用法，因为之后它会调用 <span class="t-lc"><a href="cpp-memory-shared_ptr-reset.html">std::shared_ptr::reset</a></span> 并替换定制的删除器。</p>
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<p>捕获的参数常被打包到 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-tuple.html"><span class="kw1098">std::<span class="me2">tuple</span></span></a><span class="sy1"><</span>Args...<span class="sy1">></span></span></span> 中。实现可能用不同的机制提供他们需要保有的 <code>Pointer</code> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span><span class="sy2">*</span></span></span> 对象。</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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<table class="metadata plainlinks ambox mbox-small-left ambox-notice" style="">
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<td class="mbox-empty-cell"></td>
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<td class="mbox-text" style="">本节未完成<br />
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原因：暂无示例</td>
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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-memory-inout_ptr_t.html"><span class="t-lines"><span>inout_ptr_t</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx23">(C++23)</span></span></span></div>
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</div>
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</td>
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<td>与外来指针设置器交互，从智能指针获得初始指针值，并在析构时重设它<br />
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<span class="t-mark">(类模板)</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-memory-unique_ptr.html"><span class="t-lines"><span>unique_ptr</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>拥有独有对象所有权语义的智能指针<br />
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<span class="t-mark">(类模板)</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-memory-shared_ptr.html"><span class="t-lines"><span>shared_ptr</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>拥有共享对象所有权语义的智能指针<br />
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<span class="t-mark">(类模板)</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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