160 lines
14 KiB
HTML
160 lines
14 KiB
HTML
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<title>std::filesystem::exists</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_filesystem_exists 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::filesystem::</span>exists</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-filesystem.html">filesystem</a></span></div>
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<table class="t-dcl-begin">
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<td>
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<div>定义于头文件 <code><a href="cpp-header-filesystem.html"><filesystem></a></code></div>
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</td>
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<td></td>
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<td></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span> exists<span class="br0">(</span> <a href="cpp-filesystem-file_status.html"><span class="kw2206">std::<span class="me2">filesystem</span><span class="sy4">::</span><span class="me2">file_status</span></span></a> s <span class="br0">)</span></span></div>
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</td>
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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="kw4">bool</span> exists<span class="br0">(</span> <span class="kw4">const</span> <a href="cpp-filesystem-path.html"><span class="kw2199">std::<span class="me2">filesystem</span><span class="sy4">::</span><span class="me2">path</span></span></a><span class="sy3">&</span> p <span class="br0">)</span><span class="sy4">;</span><br />
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<span class="kw4">bool</span> exists<span class="br0">(</span> <span class="kw4">const</span> <a href="cpp-filesystem-path.html"><span class="kw2199">std::<span class="me2">filesystem</span><span class="sy4">::</span><span class="me2">path</span></span></a><span class="sy3">&</span> p, <a href="cpp-error-error_code.html"><span class="kw877">std::<span class="me2">error_code</span></span></a><span class="sy3">&</span> ec <span class="br0">)</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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<td></td>
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<td></td>
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<td></td>
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<p>检查给定的文件状态或路径是否对应已存在的文件或目录。</p>
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<div class="t-li1"><span class="t-li">1)</span> 等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp">status_known<span class="br0">(</span>s<span class="br0">)</span> <span class="sy3">&&</span> s.<span class="me1">type</span><span class="br0">(</span><span class="br0">)</span> <span class="sy3">!</span><span class="sy1">=</span> file_type<span class="sy4">::</span><span class="me2">not_found</span></span></span>.</div>
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<div class="t-li1"><span class="t-li">2)</span> 令 <code>s</code> 分别为如同以 <span class="t-c"><span class="mw-geshi cpp source-cpp">status<span class="br0">(</span>p<span class="br0">)</span></span></span> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp">status<span class="br0">(</span>p, ec<span class="br0">)</span></span></span> （跟随符号链接）确定的 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-filesystem-file_status.html"><span class="kw2206">std::<span class="me2">filesystem</span><span class="sy4">::</span><span class="me2">file_status</span></span></a></span></span> 。返回 <span class="t-c"><span class="mw-geshi cpp source-cpp">exists<span class="br0">(</span>s<span class="br0">)</span></span></span> 。若 <span class="t-c"><span class="mw-geshi cpp source-cpp">status_known<span class="br0">(</span>s<span class="br0">)</span></span></span> 则不抛出重载调用 <span class="t-c"><span class="mw-geshi cpp source-cpp">ec.<span class="me1">clear</span><span class="br0">(</span><span class="br0">)</span></span></span> 。</div>
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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>s</td>
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<td>-</td>
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<td>要检验的文件状态</td>
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</tr>
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<tr class="t-par">
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<td>p</td>
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<td>-</td>
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<td>要检验的路径</td>
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</tr>
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<tr class="t-par">
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<td>ec</td>
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<td>-</td>
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<td>不抛出重载中报告错误的输出参数</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>若给定路径或文件状态对应存在的文件或目录，则返回 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> ，否则返回 <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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<div class="t-li1"><span class="t-li">2)</span> 不接受 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-error-error_code.html"><span class="kw877">std::<span class="me2">error_code</span></span></a><span class="sy3">&</span></span></span> 参数的重载在底层 OS API 错误时抛出 <a href="cpp-filesystem-filesystem_error.html"><tt>filesystem_error</tt></a> ，以第一 path 参数 <code>p</code> 和作为错误码参数的 OS 错误码构造。若 OS API 调用失败，则接受 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-error-error_code.html"><span class="kw877">std::<span class="me2">error_code</span></span></a><span class="sy3">&</span></span></span> 参数的重载设置该参数为 OS API 错误码，而若不出现错误则执行 <span class="t-c"><span class="mw-geshi cpp source-cpp">ec.<span class="me1">clear</span><span class="br0">(</span><span class="br0">)</span></span></span> 。若内存分配失败，则任何不标记为 <code>noexcept</code> 的重载可能抛出 <span class="t-lc"><a href="cpp-memory-new-bad_alloc.html">std::bad_alloc</a></span> 。</div>
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<h3><span class="mw-headline" id=".E6.B3.A8.E6.84.8F">注意</span></h3>
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<p>此函数提供的信息通常亦可作为目录迭代的副产物提供。在迭代中，调用 <code>exists(*iterator)</code> 的效率低于 <code>exists(iterator->status())</code></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：切换到 directory_entry::exists</td>
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</tr>
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</table>
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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 <iostream></span>
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<span class="co2">#include <fstream></span>
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<span class="co2">#include <cstdint></span>
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<span class="co2">#include <filesystem></span>
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<span class="kw1">namespace</span> fs <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">filesystem</span><span class="sy4">;</span>
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<span class="kw4">void</span> demo_exists<span class="br0">(</span><span class="kw4">const</span> fs<span class="sy4">::</span><span class="me2">path</span><span class="sy3">&</span> p, fs<span class="sy4">::</span><span class="me2">file_status</span> s <span class="sy1">=</span> fs<span class="sy4">::</span><span class="me2">file_status</span><span class="br0">{</span><span class="br0">}</span><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> p<span class="sy4">;</span>
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<span class="kw1">if</span><span class="br0">(</span>fs<span class="sy4">::</span><span class="me2">status_known</span><span class="br0">(</span>s<span class="br0">)</span> <span class="sy4">?</span> fs<span class="sy4">::</span><span class="me2">exists</span><span class="br0">(</span>s<span class="br0">)</span> <span class="sy4">:</span> fs<span class="sy4">::</span><span class="me2">exists</span><span class="br0">(</span>p<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> <span class="st0">" exists<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="kw1">else</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">" does not exist<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">int</span> main<span class="br0">(</span><span class="br0">)</span>
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<span class="br0">{</span>
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fs<span class="sy4">::</span><span class="me2">create_directory</span><span class="br0">(</span><span class="st0">"sandbox"</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-io-basic_ofstream.html"><span class="kw1733">std::<span class="me2">ofstream</span></span></a><span class="br0">(</span><span class="st0">"sandbox/file"</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 创建常规文件</span>
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fs<span class="sy4">::</span><span class="me2">create_symlink</span><span class="br0">(</span><span class="st0">"non-existing"</span>, <span class="st0">"sandbox/symlink"</span><span class="br0">)</span><span class="sy4">;</span>
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demo_exists<span class="br0">(</span><span class="st0">"sandbox"</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">for</span><span class="br0">(</span><span class="kw4">auto</span> it <span class="sy1">=</span> fs<span class="sy4">::</span><span class="me2">directory_iterator</span><span class="br0">(</span><span class="st0">"sandbox"</span><span class="br0">)</span><span class="sy4">;</span> it <span class="sy3">!</span><span class="sy1">=</span> fs<span class="sy4">::</span><span class="me2">directory_iterator</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="sy2">++</span>it<span class="br0">)</span>
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demo_exists<span class="br0">(</span><span class="sy2">*</span>it, it<span class="sy2">-</span><span class="sy1">></span>status<span class="br0">(</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 使用来自 directory_entry 的缓存状态</span>
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fs<span class="sy4">::</span><span class="me2">remove_all</span><span class="br0">(</span><span class="st0">"sandbox"</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>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">"sandbox" exists
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"sandbox/file" exists
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"sandbox/symlink" does not exist</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-filesystem-status.html"><span class="t-lines"><span>status</span><span>symlink_status</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div>
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</div>
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</td>
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<td>确定文件属性<br />
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确定文件属性，检查符号链接目标<br />
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<span class="t-mark">(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-filesystem-file_status.html"><span class="t-lines"><span>file_status</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div>
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</div>
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</td>
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<td>表示文件类型及权限<br />
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<span class="t-mark">(类)</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-filesystem-directory_entry-exists.html"><span class="t-lines"><span>exists</span></span></a></div>
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</div>
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</td>
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<td>检查 directory_entry 是否代表既存文件系统对象<br />
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<span class="t-mark">(<code>std::filesystem::directory_entry</code> 的公开成员函数)</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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</div>
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</body>
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