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<title>std::unreachable_sentinel_t, std::unreachable_sentinel</title>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>unreachable_sentinel_t, <span style="font-size:0.7em; line-height:130%">std::</span>unreachable_sentinel</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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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">struct</span> unreachable_sentinel_t<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">inline</span> <span class="kw4">constexpr</span> unreachable_sentinel_t unreachable_sentinel<span class="br0">{</span><span class="br0">}</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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<div class="t-li1"><span class="t-li">1)</span> <code>unreachable_sentinel_t</code> 是能用于表示无界区间“上界”的空类类型。</div>
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<div class="t-li1"><span class="t-li">2)</span> <code>unreachable_sentinel</code> 是 <code>unreachable_sentinel_t</code> 类型常量。</div>
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<h3><span class="mw-headline" id=".E9.9D.9E.E6.88.90.E5.91.98.E5.87.BD.E6.95.B0">非成员函数</span></h3>
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<div><strong class="selflink"><span class="t-lines"><span>operator==</span></span></strong></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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<td>比较一个 <code>unreachable_sentinel_t</code> 与任何 <a href="cpp-iterator-weakly_incrementable.html"><code>weakly_incrementable</code></a> 类型值<br />
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<span class="t-mark">(函数模板)</span></td>
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<h2><span class="mw-headline" id="operator.3D.3D.28std::unreachable_sentinel_t.29">operator==<small>(std::unreachable_sentinel_t)</small></span></h2>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span>std<span class="sy4">::</span><span class="me2">weakly_incrementable</span> I<span class="sy1">></span><br /></span>
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<p><span class="kw1">friend</span> <span class="kw4">constexpr</span> <span class="kw4">bool</span> operator<span class="sy1">==</span><span class="br0">(</span> unreachable_sentinel_t, <span class="kw4">const</span> I<span class="sy3">&</span> <span class="br0">)</span> <span class="kw1">noexcept</span><br /></p>
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<span class="br0">{</span> <span class="kw1">return</span> <span class="kw2">false</span><span class="sy4">;</span> <span class="br0">}</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>unreachable_sentinel_t</code> 能与任何 <a href="cpp-iterator-weakly_incrementable.html"><code>weakly_incrementable</code></a> 类型比较，而结果始终为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> 。</p>
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<p>此函数模板对通常<a href="cpp-language-unqualified_lookup.html">无限定</a>或<a href="cpp-language-qualified_lookup.html">有限定查找</a>不可见，而只能在 <code>std::unreachable_sentinel_t</code> 为参数的关联类时由<a href="cpp-language-adl.html">实参依赖查找</a>找到。</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 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 <cstddef></span>
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<span class="co2">#include <iterator></span>
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<span class="co2">#include <algorithm></span>
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<span class="co2">#include <iostream></span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> CharT<span class="sy1">></span>
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<a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> ntcts_len<span class="br0">(</span><span class="kw4">const</span> CharT <span class="sy2">*</span>s<span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">return</span> std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">find</span><span class="br0">(</span>s, std<span class="sy4">::</span><span class="me2">unreachable_sentinel</span>, CharT<span class="br0">{</span><span class="br0">}</span><span class="br0">)</span> <span class="sy2">-</span> s<span class="sy4">;</span>
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<span class="br0">}</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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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a>
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<span class="sy1"><<</span> ntcts_len<span class="br0">(</span><span class="st0">"The quick brown fox jumps over the lazy dog."</span><span class="br0">)</span> <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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<p>输出：</p>
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<pre class="de1">44</pre></div>
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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><a href="cpp-ranges-iota_view.html"><span class="t-lines"><span>ranges::iota_view</span><span>views::iota</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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<td>由通过重复对某个初值自增所生成的序列组成的 <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">(定制点对象)</span></td>
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