80 lines
6.8 KiB
HTML
80 lines
6.8 KiB
HTML
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<title>std::ranges::view_interface<D>::operator[]</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_ranges_view_interface_operator_at 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>operator[]</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>‎ | <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="kw1">template</span><span class="sy1"><</span>ranges<span class="sy4">::</span><span class="me2">random_access_range</span> R <span class="sy1">=</span> D<span class="sy1">></span><br />
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<span class="kw4">constexpr</span> decltype<span class="br0">(</span><span class="kw4">auto</span><span class="br0">)</span> operator<span class="br0">[</span><span class="br0">]</span><span class="br0">(</span> ranges<span class="sy4">::</span><span class="me2">range_difference_t</span><span class="sy1"><</span>R<span class="sy1">></span> n <span class="br0">)</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="kw1">template</span><span class="sy1"><</span>ranges<span class="sy4">::</span><span class="me2">random_access_range</span> R <span class="sy1">=</span> <span class="kw4">const</span> D<span class="sy1">></span><br />
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<span class="kw4">constexpr</span> decltype<span class="br0">(</span><span class="kw4">auto</span><span class="br0">)</span> operator<span class="br0">[</span><span class="br0">]</span><span class="br0">(</span> ranges<span class="sy4">::</span><span class="me2">range_difference_t</span><span class="sy1"><</span>R<span class="sy1">></span> n <span class="br0">)</span> <span class="kw4">const</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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<td></td>
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<td></td>
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<p><span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="br0">[</span><span class="br0">]</span></span></span> 成员函数的默认实现获得相对于迭代器在指定偏移的元素，复用迭代器类型的 <span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="br0">[</span><span class="br0">]</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> ranges<span class="sy4">::</span><span class="me2">begin</span><span class="br0">(</span>derived<span class="br0">)</span><span class="br0">[</span>n<span class="br0">]</span><span class="sy4">;</span></span></span> 。</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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<td>n</td>
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<td>-</td>
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<td>要返回的元素位置</td>
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</tr>
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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>相对于起始迭代器偏移为 <code>n</code> 的元素。</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 <span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="br0">[</span><span class="br0">]</span></span></span> 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，因为它们决不满足 <a href="cpp-ranges-random_access_range.html"><code>random_access_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::filter_view</span></li>
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<li><span class="t-lc">std::ranges::join_view</span></li>
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<li><span class="t-lc">std::ranges::split_view</span></li>
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</ul>
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<p>继承的 <span class="t-c"><span class="mw-geshi cpp source-cpp">operator<span class="br0">[</span><span class="br0">]</span></span></span> 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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