60 lines
2.6 KiB
HTML
60 lines
2.6 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::nullopt</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_optional_nullopt skin-cppreference2 action-view cpp-navbar">
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<div id="cpp-content-base">
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<div id="content"><a id="top"></a>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::</span>nullopt</h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-utility.html">utility</a>‎ | <a href="cpp-utility-optional.html">optional</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-optional.html"><optional></a></code></div>
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<td></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">inline</span> <span class="kw4">constexpr</span> nullopt_t nullopt<span class="br0">{</span><span class="coMULTI">/*unspecified*/</span><span class="br0">}</span><span class="sy4">;</span></span></div>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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<p><code>std::nullopt</code> 是 <span class="t-lc"><a href="cpp-utility-optional-nullopt_t.html">std::nullopt_t</a></span> 类型常量，用于指示 <code>optional</code> 类型有未初始化状态。</p>
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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-utility-optional-nullopt_t.html"><span class="t-lines"><span>nullopt_t</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div>
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</div>
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</td>
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<td>带未初始化状态的 <code>optional</code> 类型的指示器<br />
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<span class="t-mark">(类)</span></td>
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</table>
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</body>
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</html>
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