106 lines
8.4 KiB
HTML
106 lines
8.4 KiB
HTML
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<title>std::ranges::view_interface<D>::back</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_ranges_view_interface_back 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::view_interface<D>::</span>back</h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-ranges.html">ranges</a>‎ | <a href="cpp-ranges-view_interface.html">view interface</a></span></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> decltype<span class="br0">(</span><span class="kw4">auto</span><span class="br0">)</span> back<span class="br0">(</span><span class="br0">)</span> requires ranges<span class="sy4">::</span><span class="me2">bidirectional_range</span><span class="sy1"><</span>D<span class="sy1">></span> <span class="sy3">&&</span><br />
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ranges<span class="sy4">::</span><span class="me2">common_range</span><span class="sy1"><</span>D<span class="sy1">></span><span class="sy4">;</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="kw4">constexpr</span> decltype<span class="br0">(</span><span class="kw4">auto</span><span class="br0">)</span> back<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span><br />
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requires ranges<span class="sy4">::</span><span class="me2">bidirectional_range</span><span class="sy1"><</span><span class="kw4">const</span> D<span class="sy1">></span> <span class="sy3">&&</span> ranges<span class="sy4">::</span><span class="me2">common_range</span><span class="sy1"><</span><span class="kw4">const</span> D<span class="sy1">></span><span class="sy4">;</span></span></div>
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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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<p><code>back</code> 成员函数的默认实现返回派生类型视图中的末元素。元素是按值还是按引用返回取决于迭代器类型的 <span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="sy2">*</span></span></span> 。</p>
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<div class="t-li1"><span class="t-li">1)</span> 令 <code>derived</code> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">static_cast</span><span class="sy1"><</span>D<span class="sy3">&</span><span class="sy1">></span><span class="br0">(</span><span class="sy2">*</span>this<span class="br0">)</span></span></span> 。等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">return</span> <span class="sy2">*</span>ranges<span class="sy4">::</span><span class="me2">prev</span><span class="br0">(</span>ranges<span class="sy4">::</span><span class="me2">end</span><span class="br0">(</span>derived<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span></span></span> 。若 <a href="cpp-ranges-view_interface-empty.html"><code>empty()</code></a> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> （即起始迭代器与哨位比较相等）则行为未定义，即使以相同方式获得的迭代器可解引用。</div>
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<div class="t-li1"><span class="t-li">2)</span> 同 <span class="t-v">(1)</span> ，除了 <code>derived</code> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">static_cast</span><span class="sy1"><</span><span class="kw4">const</span> D<span class="sy3">&</span><span class="sy1">></span><span class="br0">(</span><span class="sy2">*</span>this<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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<p>（无）</p>
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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>视图中的末元素。</p>
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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>C++20 中，标准库无派生自 <span class="t-lc">std::ranges::view_interface</span> 的类型提供其自身的 <code>back</code> 成员函数。</p>
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<p>然而，下列派生类型不能使用默认实现，因为它们决不一同满足 <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> ：</p>
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<ul>
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<li><span class="t-lc">std::ranges::basic_istream_view</span></li>
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<li><span class="t-lc">std::ranges::common_view</span></li>
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<li><span class="t-lc">std::ranges::split_view</span></li>
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<li><span class="t-lc">std::ranges::take_while_view</span></li>
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</ul>
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<p>继承的 <code>back</code> 成员函数对 <span class="t-lc">std::ranges::empty_view</span> 可用，但对它的调用始终导致未定义行为。</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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<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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<div class="t-dsc-member-div">
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<div><a href="cpp-iterator-rbegin.html"><span class="t-lines"><span>rbegin</span><span>crbegin</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx14">(C++14)</span></span></span></div>
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</div>
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</td>
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<td>返回指向一个容器或数组的逆向迭代器<br />
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<span class="t-mark">(函数模板)</span></td>
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<div><a href="cpp-ranges-rbegin.html"><span class="t-lines"><span>ranges::rbegin</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>返回指向范围的逆向迭代器<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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<div class="t-dsc-member-div">
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<div><a href="cpp-ranges-crbegin.html"><span class="t-lines"><span>ranges::crbegin</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>返回指向只读范围的逆向迭代器<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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</table>
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</div>
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