260 lines
22 KiB
HTML
260 lines
22 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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<head><meta http-equiv="x-ua-compatible" content="ie=edge">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
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<title>std::ranges::views::join, std::ranges::join_view</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_ranges_join_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>join, <span style="font-size:0.7em; line-height:130%">std::ranges::</span>join_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 id="mw-content-text" lang="zh" dir="ltr" class="mw-content-ltr" xml:lang="zh">
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<table class="t-dcl-begin">
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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>
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<td></td>
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<td></td>
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<tr class="t-dcl t-since-cxx20">
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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">input_range</span> V <span class="sy1">></span><br /></span>
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<p> requires ranges<span class="sy4">::</span><span class="me2">view</span><span class="sy1"><</span>V<span class="sy1">></span> <span class="sy3">&&</span> ranges<span class="sy4">::</span><span class="me2">input_range</span><span class="sy1"><</span>ranges<span class="sy4">::</span><span class="me2">range_reference_t</span><span class="sy1"><</span>V<span class="sy1">>></span><br /></p>
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<span class="kw1">class</span> join_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>join_view<span class="sy1"><</span>V<span class="sy1">>></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-cxx20">(C++20 起)</span></td>
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<tr class="t-dcl t-since-cxx20">
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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> join <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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</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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<div class="t-li1"><span class="t-li">1)</span> 表示由从拉平范围的视图获得的序列组成的 <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> <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>。对于任何适合的子表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp">e</span></span> 表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp">views<span class="sy4">::</span><span class="me2">join</span><span class="br0">(</span>e<span class="br0">)</span></span></span> <i>表达式等价</i>于 <span class="t-c"><span class="mw-geshi cpp source-cpp">join_view<span class="sy1"><</span>views<span class="sy4">::</span><span class="me2">all_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 class="br0">{</span>e<span class="br0">}</span></span></span> 。</div>
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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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<tr class="t-dsc">
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<div class="t-dsc-member-div t-dsc-member-nobold-div">
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<div><a href="cpp-ranges-join_view-join_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>join_view</code><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="https://zh.cppreference.com/mwiki/index.php?title=cpp/ranges/join_view/base&action=edit&redlink=1" class="new" title="cpp/ranges/join view/base（页面不存在）"><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-join_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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</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="https://zh.cppreference.com/mwiki/index.php?title=cpp/ranges/join_view/end&action=edit&redlink=1" class="new" title="cpp/ranges/join view/end（页面不存在）"><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>
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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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<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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<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-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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</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="https://zh.cppreference.com/mwiki/index.php?title=cpp/ranges/join_view/deduction_guides&action=edit&redlink=1" class="new" title="cpp/ranges/join view/deduction guides（页面不存在）">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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<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="https://zh.cppreference.com/mwiki/index.php?title=cpp/ranges/join_view/iterator&action=edit&redlink=1" class="new" title="cpp/ranges/join view/iterator（页面不存在）"><i><span class="t-lines"><span>iterator</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>迭代器类型<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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<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="https://zh.cppreference.com/mwiki/index.php?title=cpp/ranges/join_view/sentinel&action=edit&redlink=1" class="new" title="cpp/ranges/join view/sentinel（页面不存在）"><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=".E6.B3.A8.E8.A7.A3">注解</span></h3>
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<p>在接纳 <a rel="nofollow" class="external text" href="https://wg21.link/P2328R1">P2328R1</a> 前，内层范围类型（ <span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">range_reference_t</span><span class="sy1"><</span>V<span class="sy1">></span></span></span> ）不能为容器类型（但能为容器的引用）。例如不允许接合 <span class="t-lc"><a href="cpp-string-basic_string.html">std::string</a></span> 纯右值的 <a href="cpp-ranges-transform_view.html"><code>transform_view</code></a> 。</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">struct</span> Person <span class="br0">{</span> <span class="kw4">int</span> age<span class="sy4">;</span> <a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> name<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">auto</span> f<span class="br0">(</span><a href="cpp-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1"><</span>Person<span class="sy1">></span><span class="sy3">&</span> v<span class="br0">)</span> <span class="br0">{</span>
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<span class="co1">// return v | std::views::transform([](auto& p) { return p.name; })</span>
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<span class="co1">// | std::views::join; // P2328R1 前错误</span>
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<span class="kw1">return</span> v <span class="sy3">|</span> std<span class="sy4">::</span><span class="me2">views</span><span class="sy4">::</span><span class="me2">transform</span><span class="br0">(</span><span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="kw4">auto</span><span class="sy3">&</span> p<span class="br0">)</span> <span class="sy2">-</span><span class="sy1">></span> <a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a><span class="sy3">&</span> <span class="br0">{</span> <span class="kw1">return</span> p.<span class="me1">name</span><span class="sy4">;</span> <span class="br0">}</span><span class="br0">)</span>
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<span class="sy3">|</span> std<span class="sy4">::</span><span class="me2">views</span><span class="sy4">::</span><span class="me2">join</span><span class="sy4">;</span> <span class="co1">// OK</span>
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<span class="br0">}</span></pre></div>
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</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 <iostream></span>
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<span class="co2">#include <ranges></span>
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<span class="co2">#include <string_view></span>
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<span class="co2">#include <vector></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="kw1">using</span> <span class="kw1">namespace</span> std<span class="sy4">::</span><span class="me2">literals</span><span class="sy4">;</span>
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<span class="kw4">const</span> <span class="kw4">auto</span> bits <span class="sy1">=</span> <span class="br0">{</span> <span class="st0">"https:"</span>sv, <span class="st0">"//"</span>sv, <span class="st0">"cppreference"</span>sv, <span class="st0">"."</span>sv, <span class="st0">"com"</span>sv <span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">char</span> <span class="kw4">const</span> c <span class="sy4">:</span> bits <span class="sy3">|</span> std<span class="sy4">::</span><span class="me2">views</span><span class="sy4">::</span><span class="me2">join</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> c<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="kw4">const</span> <a href="cpp-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1"><</span><a href="cpp-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">>></span> v<span class="br0">{</span> <span class="br0">{</span><span class="nu0">1</span>,<span class="nu0">2</span><span class="br0">}</span>, <span class="br0">{</span><span class="nu0">3</span>,<span class="nu0">4</span>,<span class="nu0">5</span><span class="br0">}</span>, <span class="br0">{</span><span class="nu0">6</span><span class="br0">}</span>, <span class="br0">{</span><span class="nu0">7</span>,<span class="nu0">8</span>,<span class="nu0">9</span><span class="br0">}</span> <span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">auto</span> jv <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">join_view</span><span class="br0">(</span>v<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">int</span> <span class="kw4">const</span> e <span class="sy4">:</span> jv<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> e <span class="sy1"><<</span> <span class="st0">' '</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">https://cppreference.com
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1 2 3 4 5 6 7 8 9</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/issue3474">LWG 3474</a></td>
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<td>C++20</td>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp">views<span class="sy4">::</span><span class="me2">join</span><span class="br0">(</span>e<span class="br0">)</span></span></span> 在 <span class="t-c"><span class="mw-geshi cpp source-cpp">e</span></span> 为 <code>join_view</code> 时返回 <span class="t-c"><span class="mw-geshi cpp source-cpp">e</span></span> 的副本</td>
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<td>返回嵌套的 <code>join_view</code></td>
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</tr>
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<tr>
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<td><a rel="nofollow" class="external text" href="https://wg21.link/P2328R1">P2328R1</a></td>
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<td>C++20</td>
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<td><code>join_view</code> 无法接合非视图 <a href="cpp-ranges-range.html"><code>range</code></a> 纯右值</td>
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<td>使得可接合</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="https://zh.cppreference.com/mwiki/index.php?title=cpp/ranges/split_view&action=edit&redlink=1" class="new" title="cpp/ranges/split view（页面不存在）"><span class="t-lines"><span>ranges::split_view</span><span>views::split</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>用某个分隔符切割另一 <a href="cpp-ranges-view.html"><code>view</code></a> 所获得的子范围的 <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="https://zh.cppreference.com/mwiki/index.php?title=cpp/ranges/lazy_split_view&action=edit&redlink=1" class="new" title="cpp/ranges/lazy split view（页面不存在）"><span class="t-lines"><span>ranges::lazy_split_view</span><span>views::lazy_split</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>
|
|
</td>
|
|
<td>用某个分隔符切割另一 <a href="cpp-ranges-view.html"><code>view</code></a> 所获得的子范围的 <a href="cpp-ranges-view.html"><code>view</code></a><br />
|
|
<span class="t-mark">(类模板)</span> <span class="t-mark">(范围适配器对象)</span></td>
|
|
</tr>
|
|
</table>
|
|
</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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