78 lines
6.7 KiB
HTML
78 lines
6.7 KiB
HTML
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<title>std::sentinel_for</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_iterator_sentinel_for 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::</span>sentinel_for</h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-iterator.html">iterator</a></span></div>
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<div>定义于头文件 <code><a href="cpp-header-iterator.html"><iterator></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><span class="kw1">class</span> S, <span class="kw1">class</span> I<span class="sy1">></span><br /></span>
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<p> concept sentinel_for <span class="sy1">=</span><br />
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std<span class="sy4">::</span><span class="me2">semiregular</span><span class="sy1"><</span>S<span class="sy1">></span> <span class="sy3">&&</span><br />
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std<span class="sy4">::</span><span class="me2">input_or_output_iterator</span><span class="sy1"><</span>I<span class="sy1">></span> <span class="sy3">&&</span><br /></p>
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__WeaklyEqualityComparableWith<span class="sy1"><</span>S, I<span class="sy1">></span><span class="sy4">;</span></div>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<p><code>sentinel_for</code> 概念描述的 <a href="cpp-iterator-input_or_output_iterator.html"><code>input_or_output_iterator</code></a> 与 <a href="cpp-concepts-semiregular.html"><code>semiregular</code></a> 类型间的关系，二者的值代表范围。仅用于阐释的概念 <code><i>__WeaklyEqualityComparableWith</i></code> 描述于 <a href="cpp-concepts-equality_comparable.html"><code>equality_comparable</code></a> 。</p>
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<h3><span class="mw-headline" id=".E8.AF.AD.E4.B9.89.E8.A6.81.E6.B1.82">语义要求</span></h3>
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<p>令 <code>s</code> 与 <code>i</code> 分别表示 <code>S</code> 与 <code>I</code> 类型的值，使得 <code>[i, s)</code> 代表范围。 <code>sentinel_for<S, I></code> 仅若下列条件成立才得到实现：</p>
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<ul>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp">i <span class="sy1">==</span> s</span></span> 为良定义。</li>
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<li>若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span><span class="br0">(</span>i <span class="sy3">!</span><span class="sy1">=</span> s<span class="br0">)</span></span></span> 则 <code>i</code> 可解引用且 <code>[++i, s)</code> 代表范围。</li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">assignable_from</span><span class="sy1"><</span>I<span class="sy3">&</span>, S<span class="sy1">></span></span></span> 要么得到实现，要么不被满足。</li>
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</ul>
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<p><code>==</code> 能随时间而更改。给定迭代器 <code>i</code> 和哨位 <code>s</code> 并使得 <code>[i, s)</code> 代表范围且 <span class="t-c"><span class="mw-geshi cpp source-cpp">i <span class="sy3">!</span><span class="sy1">=</span> s</span></span> ，不要求自增任何等于 <code>i</code> 的迭代器后 <code>[i, s)</code> 继续代表范围（从而这种自增后不再要求 <span class="t-c"><span class="mw-geshi cpp source-cpp">i <span class="sy1">==</span> s</span></span> 是良定义的）。</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>不要求哨位类型与其对应的迭代器类型实现 <a href="cpp-concepts-equality_comparable.html"><code>equality_comparable_with</code></a> ，因为哨位类型可以不可与自身比较，而且不要求它们拥有共用引用类型。</p>
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<p>C++17 起已容许在<a href="cpp-language-range-for.html">基于范围的 <code>for</code> 循环</a>中使用异于迭代器类型的哨位类型。</p>
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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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<td><a rel="nofollow" class="external text" href="https://cplusplus.github.io/LWG/issue3453">LWG 3453</a></td>
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<td>C++20</td>
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<td><code>sentinel_for</code> 的语义要求对 <code>ranges::advance</code> 过于宽松</td>
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<td>已强化</td>
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