316 lines
26 KiB
HTML
316 lines
26 KiB
HTML
<!DOCTYPE html>
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<html lang="zh" dir="ltr" class="client-nojs" xmlns="http://www.w3.org/1999/xhtml" xml:lang="zh">
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<title>std::ranges::views::take, std::ranges::take_view</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_ranges_take_view 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::ranges::</span><span style="font-size:0.7em; line-height:130%">views::</span>take, <span style="font-size:0.7em; line-height:130%">std::ranges::</span>take_view</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-ranges.html">ranges</a></span></div>
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<div>定义于头文件 <code><a href="cpp-header-ranges.html"><ranges></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> ranges<span class="sy4">::</span><span class="me2">view</span> V <span class="sy1">></span><br />
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<span class="kw1">class</span> take_view <span class="sy4">:</span> <span class="kw1">public</span> ranges<span class="sy4">::</span><span class="me2">view_interface</span><span class="sy1"><</span>take_view<span class="sy1"><</span>V<span class="sy1">>></span></span></div>
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<td>(1)</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">namespace</span> views <span class="br0">{</span><br /></span>
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<p> <span class="kw1">inline</span> <span class="kw4">constexpr</span> <span class="coMULTI">/*unspecified*/</span> take <span class="sy1">=</span> <span class="coMULTI">/*unspecified*/</span><span class="sy4">;</span><br /></p>
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<span class="br0">}</span></div>
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</td>
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<td>(2)</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<td></td>
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<div class="t-li1"><span class="t-li">1)</span> 范围适配器，表示来自底层序列的始于开端终于给定边界的元素的 <a href="cpp-ranges-view.html"><code>view</code></a> 。</div>
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<div class="t-li1"><span class="t-li">2)</span> <code>views::take</code> 是一个<a href="cpp-ranges.html#.E8.8C.83.E5.9B.B4.E9.80.82.E9.85.8D.E5.99.A8.E5.AF.B9.E8.B1.A1">范围适配器对象</a>。表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp">views<span class="sy4">::</span><span class="me2">take</span><span class="br0">(</span>e, f<span class="br0">)</span></span></span> 产生表示来自 <span class="t-c"><span class="mw-geshi cpp source-cpp">e</span></span> 的首 <span class="t-c"><span class="mw-geshi cpp source-cpp">f</span></span> 个元素的视图。结果不一定是 <code>take_view</code> 。
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<p>它<i>表达式等价</i>于（其中 <code>T</code> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">remove_cvref_t</span><span class="sy1"><</span>decltype<span class="br0">(</span><span class="br0">(</span>e<span class="br0">)</span><span class="br0">)</span><span class="sy1">></span></span></span> 而 <code>D</code> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">range_difference_t</span><span class="sy1"><</span>decltype<span class="br0">(</span><span class="br0">(</span>e<span class="br0">)</span><span class="br0">)</span><span class="sy1">></span></span></span> ）：</p>
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<ul>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">(</span><span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span>f, <span class="kw1">static_cast</span><span class="sy1"><</span>T<span class="sy1">></span><span class="br0">(</span>e<span class="br0">)</span><span class="br0">)</span></span></span> ，若 <code>T</code> 为 <span class="t-lc">ranges::empty_view</span> ，除了 <span class="t-c"><span class="mw-geshi cpp source-cpp">e</span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp">f</span></span> 的求值具有不确定顺序；</li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp">U<span class="br0">(</span>ranges<span class="sy4">::</span><span class="me2">begin</span><span class="br0">(</span>e<span class="br0">)</span>, ranges<span class="sy4">::</span><span class="me2">begin</span><span class="br0">(</span>e<span class="br0">)</span> <span class="sy2">+</span> <a href="cpp-algorithm-min.html"><span class="kw1642">std::<span class="me2">min</span></span></a><span class="sy1"><</span>D<span class="sy1">></span><span class="br0">(</span>ranges<span class="sy4">::</span><span class="me2">distance</span><span class="br0">(</span>e<span class="br0">)</span>, f<span class="br0">)</span><span class="br0">)</span></span></span> ，若 <code>T</code> 是 <span class="t-lc">std::span</span>、 <span class="t-lc"><a href="cpp-string-basic_string_view.html">std::basic_string_view</a></span> 或 <span class="t-lc">ranges::subrange</span> 的实现 <a href="cpp-ranges-random_access_range.html"><code>random_access_range</code></a> 及 <a href="cpp-ranges-sized_range.html"><code>sized_range</code></a> 的特化，其中 <code>U</code> 为</li>
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<dl>
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<dd>
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<ul>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">span</span><span class="sy1"><</span><span class="kw1">typename</span> T<span class="sy4">::</span><span class="me2">element_type</span><span class="sy1">></span></span></span> ，若 <code>T</code> 为 <span class="t-lc">std::span</span> 的特化；</li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp">T</span></span> ，若 <code>T</code> 为 <span class="t-lc"><a href="cpp-string-basic_string_view.html">std::basic_string_view</a></span> 的特化；</li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">subrange</span><span class="sy1"><</span>ranges<span class="sy4">::</span><span class="me2">iterator_t</span><span class="sy1"><</span>T<span class="sy1">>></span></span></span> ，若 <code>T</code> 为 <span class="t-lc">ranges::subrange</span> 的特化；</li>
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</ul>
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</dd>
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</dl>
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<ul>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">iota_view</span><span class="br0">(</span><span class="sy2">*</span>ranges<span class="sy4">::</span><span class="me2">begin</span><span class="br0">(</span>e<span class="br0">)</span>, <span class="sy2">*</span><span class="br0">(</span>ranges<span class="sy4">::</span><span class="me2">begin</span><span class="br0">(</span>e<span class="br0">)</span> <span class="sy2">+</span> <a href="cpp-algorithm-min.html"><span class="kw1642">std::<span class="me2">min</span></span></a><span class="sy1"><</span>D<span class="sy1">></span><span class="br0">(</span>ranges<span class="sy4">::</span><span class="me2">distance</span><span class="br0">(</span>e<span class="br0">)</span>, f<span class="br0">)</span><span class="br0">)</span><span class="br0">)</span></span></span> ，若 <code>T</code> 为 <span class="t-lc">ranges::iota_view</span> 实现 <a href="cpp-ranges-random_access_range.html"><code>random_access_range</code></a> 及 <a href="cpp-ranges-sized_range.html"><code>sized_range</code></a> 的特化；</li>
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<li>否则为 <span class="t-c"><span class="mw-geshi cpp source-cpp">take_view<span class="br0">(</span>e, f<span class="br0">)</span></span></span> 。</li>
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</ul>
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所有情况下 <span class="t-c"><span class="mw-geshi cpp source-cpp">decltype<span class="br0">(</span><span class="br0">(</span>f<span class="br0">)</span><span class="br0">)</span></span></span> 必须实现 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">convertible_to</span><span class="sy1"><</span>D<span class="sy1">></span></span></span> 。</div>
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<p><br /></p>
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<h3><span class="mw-headline" id=".E8.A1.A8.E8.BE.BE.E5.BC.8F.E7.AD.89.E4.BB.B7">表达式等价</span></h3>
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<p>表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp">e</span></span> <i>表达式等价</i>于表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp">f</span></span> ，若 <span class="t-c"><span class="mw-geshi cpp source-cpp">e</span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp">f</span></span> 拥有相同效果，均为<a href="cpp-language-noexcept_spec.html">潜在抛出</a>或均非潜在抛出（即 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">noexcept</span><span class="br0">(</span>e<span class="br0">)</span> <span class="sy1">==</span> <span class="kw1">noexcept</span><span class="br0">(</span>f<span class="br0">)</span></span></span> ），且均为<a href="cpp-language-constant_expression.html">常量子表达式</a>或均非常量子表达式。</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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<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-ranges-take_view-take_view.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-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>构造 <code>take_view</code><br />
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<span class="t-mark">(公开成员函数)</span></td>
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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-ranges-take_view-base.html"><span class="t-lines"><span>base</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>返回底层（适配的）视图的副本<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-ranges-take_view-begin.html"><span class="t-lines"><span>begin</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>返回指向起始的迭代器<br />
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<span class="t-mark">(公开成员函数)</span></td>
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-ranges-take_view-end.html"><span class="t-lines"><span>end</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>返回指向末尾的迭代器或哨位<br />
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<span class="t-mark">(公开成员函数)</span></td>
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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-ranges-take_view-size.html"><span class="t-lines"><span>size</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>返回元素数，只要底层（适配的）范围满足 <a href="cpp-ranges-sized_range.html"><code>sized_range</code></a><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>
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<td colspan="2">
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<h5><span class="mw-headline" id=".E7.BB.A7.E6.89.BF.E8.87.AA_std::ranges::view_interface">继承自 <span class="t-lc">std::ranges::view_interface</span></span></h5>
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</td>
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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-ranges-view_interface-empty.html"><span class="t-lines"><span>empty</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>返回视图是否为空。若视图满足 <a href="cpp-ranges-forward_range.html"><code>forward_range</code></a> 则提供。<br />
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<span class="t-mark">(<code>std::ranges::view_interface<D></code> 的公开成员函数)</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-ranges-view_interface-operator_bool.html"><span class="t-lines"><span>operator bool</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>返回派生视图是否为非空。若 <span class="t-lc">ranges::empty</span> 可应用于它则提供。<br />
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<span class="t-mark">(<code>std::ranges::view_interface<D></code> 的公开成员函数)</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-ranges-view_interface-data.html"><span class="t-lines"><span>data</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>返回派生视图的数据的地址。若视图的迭代器类型满足 <a href="cpp-iterator-contiguous_iterator.html"><code>contiguous_iterator</code></a> 则提供。<br />
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<span class="t-mark">(<code>std::ranges::view_interface<D></code> 的公开成员函数)</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-ranges-view_interface-front.html"><span class="t-lines"><span>front</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>返回派生视图中的首元素。若视图满足 <a href="cpp-ranges-forward_range.html"><code>forward_range</code></a> 则提供。<br />
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<span class="t-mark">(<code>std::ranges::view_interface<D></code> 的公开成员函数)</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-ranges-view_interface-back.html"><span class="t-lines"><span>back</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>返回派生视图中的末元素。若视图满足 <a href="cpp-ranges-bidirectional_range.html"><code>bidirectional_range</code></a> 与 <a href="cpp-ranges-common_range.html"><code>common_range</code></a> 则提供。<br />
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<span class="t-mark">(<code>std::ranges::view_interface<D></code> 的公开成员函数)</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-ranges-view_interface-operator_at.html"><span class="t-lines"><span>operator[]</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>返回派生视图中的第 n 个元素。若视图满足 <a href="cpp-ranges-random_access_range.html"><code>random_access_range</code></a> 则提供。<br />
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<span class="t-mark">(<code>std::ranges::view_interface<D></code> 的公开成员函数)</span></td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E6.8E.A8.E5.AF.BC.E6.8C.87.E5.BC.95"><a href="cpp-ranges-take_view-deduction_guides.html">推导指引</a></span></h3>
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<h3><span class="mw-headline" id=".E5.B5.8C.E5.A5.97.E7.B1.BB">嵌套类</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-ranges-take_view-sentinel.html"><i><span class="t-lines"><span>sentinel</span></span></i></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>哨位类型<br />
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<span class="t-mark">(仅用于阐释的成员类模板)</span></td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E8.BE.85.E5.8A.A9.E6.A8.A1.E6.9D.BF">辅助模板</span></h3>
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<table class="t-dcl-begin">
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<tbody>
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<tr class="t-dcl t-since-cxx20">
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<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> T<span class="sy1">></span><br /></span>
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<p><span class="kw1">inline</span> <span class="kw4">constexpr</span> <span class="kw4">bool</span> enable_borrowed_range<span class="sy1"><</span>std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">take_view</span><span class="sy1"><</span>T<span class="sy1">>></span> <span class="sy1">=</span><br /></p>
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std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">enable_borrowed_range</span><span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">;</span></div>
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</td>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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</tr>
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<tr class="t-dcl-sep">
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<td></td>
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<td></td>
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<td></td>
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</tr>
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</tbody>
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</table>
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<p><a href="cpp-ranges-borrowed_range.html"><tt>std::ranges::enable_borrowed_range</tt></a> 的此特化使得 <code>take_view</code> 在底层类型实现 <a href="cpp-ranges-borrowed_range.html"><code>borrowed_range</code></a> 时实现它。</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 <iostream></span>
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<span class="co2">#include <ranges></span>
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<span class="kw4">auto</span> print <span class="sy1">=</span> <span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="kw4">char</span> x<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> x<span class="sy4">;</span> <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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<span class="kw4">constexpr</span> <span class="kw4">char</span> pi<span class="br0">[</span><span class="br0">]</span> <span class="br0">{</span> <span class="st0">'3'</span>, <span class="st0">'.'</span>, <span class="st0">'1'</span>, <span class="st0">'4'</span>, <span class="st0">'1'</span>, <span class="st0">'5'</span>, <span class="st0">'9'</span>, <span class="st0">'2'</span> <span class="br0">}</span><span class="sy4">;</span>
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std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">for_each</span><span class="br0">(</span>pi <span class="sy3">|</span> std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">views</span><span class="sy4">::</span><span class="me2">take</span><span class="br0">(</span><span class="nu0">6</span><span class="br0">)</span>, print<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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std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">for_each</span><span class="br0">(</span>std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">take_view</span><span class="br0">{</span>pi, <span class="nu0">42</span><span class="br0">}</span>, print<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// safely takes only 8 chars</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">3.1415
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3.141592</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/issue3494">LWG 3494</a></td>
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<td>C++20</td>
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<td><code>take_view</code> 决不是 <code>borrowed_range</code></td>
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<td>若底层视图是 <code>borrowed_range</code> 则它是</td>
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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/issue3407">LWG 3407</a></td>
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<td>C++20</td>
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<td><code>views::take</code> 有时无法构造具大小随机访问范围</td>
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<td>调整结果类型使构造始终合法</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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<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-ranges-view_counted.html"><span class="t-lines"><span>views::counted</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>从迭代器和计数创建子范围<br />
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<span class="t-mark">(定制点对象)</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-ranges-take_while_view.html"><span class="t-lines"><span>ranges::take_while_view</span><span>views::take_while</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>由另一 <a href="cpp-ranges-view.html"><code>view</code></a> 的到首个谓词返回 <code>false</code> 为止的起始元素组成的 <a href="cpp-ranges-view.html"><code>view</code></a><br />
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<span class="t-mark">(类模板)</span> <span class="t-mark">(范围适配器对象)</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-algorithm-ranges-copy_n.html"><span class="t-lines"><span>ranges::copy_n</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>将一定数目的元素复制到一个新的位置<br />
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<span class="t-mark">(niebloid)</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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</html>
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