316 lines
39 KiB
HTML
316 lines
39 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>标准库头文件 <optional></title>
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<link rel="stylesheet" href="ext.css" />
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<link rel="stylesheet" href="site_modules.css" />
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</head>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_header_optional 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%">标准库头文件</span> <optional></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-header.html">header</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-dsc-begin">
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<tr>
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<td colspan="2">
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<h3><span class="mw-headline" id=".E5.8C.85.E5.90.AB">包含</span></h3>
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</td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-header-compare.html"><span class="t-lines"><span><compare></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><a href="cpp-language-operator_comparison.html#.E4.B8.89.E8.B7.AF.E6.AF.94.E8.BE.83">三路比较运算符</a>支持</td>
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</tr>
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<tr>
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<td colspan="2">
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<h3><span class="mw-headline" id=".E7.B1.BB">类</span></h3>
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</td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-utility-optional.html"><span class="t-lines"><span>optional</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>可能或可能不保有一个对象的包装器<br />
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<span class="t-mark">(类模板)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-utility-optional-bad_optional_access.html"><span class="t-lines"><span>bad_optional_access</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>指示进行了到不含值的 <code>optional</code> 的有检查访问的异常<br />
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<span class="t-mark">(类)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-utility-optional-hash.html"><span class="t-lines"><span>std::hash<span class="t-dsc-small"><std::optional></span></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>特化 <span class="t-lc"><a href="cpp-utility-hash.html">std::hash</a></span> 算法<br />
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<span class="t-mark">(类模板特化)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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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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</tr>
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<tr>
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<td colspan="2">
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<h5><span class="mw-headline" id=".E5.89.8D.E7.BD.AE.E5.A3.B0.E6.98.8E">前置声明</span></h5>
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</td>
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</tr>
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<tr class="t-dsc-header">
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<td colspan="2">
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<div>定义于头文件 <code><a href="cpp-header-functional.html"><functional></a></code></div>
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</td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-utility-hash.html"><span class="t-lines"><span>hash</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
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</div>
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</td>
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<td>散列函数对象<br />
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<span class="t-mark">(类模板)</span></td>
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</tr>
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<tr>
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<td colspan="2">
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<h3><span class="mw-headline" id=".E5.B8.B8.E9.87.8F">常量</span></h3>
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</td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-utility-optional-nullopt.html"><span class="t-lines"><span>nullopt</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><code>nullopt_t</code> 类型对象<br />
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<span class="t-mark">(常量)</span></td>
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</tr>
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<tr>
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<td colspan="2">
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<h3><span class="mw-headline" id=".E5.87.BD.E6.95.B0">函数</span></h3>
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</td>
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</tr>
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<tr>
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<td colspan="2">
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<h5><span class="mw-headline" id=".E6.AF.94.E8.BE.83">比较</span></h5>
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</td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-utility-optional-operator_cmp.html"><span class="t-lines"><span>operator==</span><span>operator!=</span><span>operator<</span><span>operator<=</span><span>operator></span><span>operator>=</span><span>operator<=></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><span class="t-mark-rev t-since-cxx17">(C++17)</span></span><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span><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>比较 <code>optional</code> 对象<br />
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<span class="t-mark">(函数模板)</span></td>
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</tr>
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<tr>
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<td colspan="2">
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<h5><span class="mw-headline" id=".E7.89.B9.E5.8C.96.E7.9A.84.E7.AE.97.E6.B3.95">特化的算法</span></h5>
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</td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-utility-optional-swap2.html"><span class="t-lines"><span>std::swap<span class="t-dsc-small">(std::optional)</span></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>特化 <span class="t-lc"><a href="cpp-algorithm-swap.html">std::swap</a></span> 算法<br />
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<span class="t-mark">(函数模板)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-utility-optional-make_optional.html"><span class="t-lines"><span>make_optional</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>创建一个 <code>optional</code> 对象<br />
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<span class="t-mark">(函数模板)</span></td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E6.A6.82.E8.A6.81">概要</span></h3>
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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 <compare></span>
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<span class="kw1">namespace</span> std <span class="br0">{</span>
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<span class="co1">// 类模板 optional</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span>
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<span class="kw1">class</span> optional<span class="sy4">;</span>
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<span class="co1">// 无值状态指示器</span>
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<span class="kw1">struct</span> nullopt_t<span class="br0">{</span><span class="coMULTI">/* 见描述 */</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">inline</span> <span class="kw4">constexpr</span> nullopt_t nullopt<span class="br0">(</span><span class="coMULTI">/* 未指明 */</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 类 bad_optional_access</span>
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<span class="kw1">class</span> bad_optional_access<span class="sy4">;</span>
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<span class="co1">// 关系运算符</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T, <span class="kw1">class</span> U<span class="sy1">></span>
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<span class="kw4">constexpr</span> <span class="kw4">bool</span> operator<span class="sy1">==</span><span class="br0">(</span><span class="kw4">const</span> optional<span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&</span>, <span class="kw4">const</span> optional<span class="sy1"><</span>U<span class="sy1">></span><span class="sy3">&</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T, <span class="kw1">class</span> U<span class="sy1">></span>
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<span class="kw4">constexpr</span> <span class="kw4">bool</span> operator<span class="sy3">!</span><span class="sy1">=</span><span class="br0">(</span><span class="kw4">const</span> optional<span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&</span>, <span class="kw4">const</span> optional<span class="sy1"><</span>U<span class="sy1">></span><span class="sy3">&</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T, <span class="kw1">class</span> U<span class="sy1">></span>
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<span class="kw4">constexpr</span> <span class="kw4">bool</span> operator<span class="sy1"><</span><span class="br0">(</span><span class="kw4">const</span> optional<span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&</span>, <span class="kw4">const</span> optional<span class="sy1"><</span>U<span class="sy1">></span><span class="sy3">&</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T, <span class="kw1">class</span> U<span class="sy1">></span>
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<span class="kw4">constexpr</span> <span class="kw4">bool</span> operator<span class="sy1">></span><span class="br0">(</span><span class="kw4">const</span> optional<span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&</span>, <span class="kw4">const</span> optional<span class="sy1"><</span>U<span class="sy1">></span><span class="sy3">&</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T, <span class="kw1">class</span> U<span class="sy1">></span>
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<span class="kw4">constexpr</span> <span class="kw4">bool</span> operator<span class="sy1"><=</span><span class="br0">(</span><span class="kw4">const</span> optional<span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&</span>, <span class="kw4">const</span> optional<span class="sy1"><</span>U<span class="sy1">></span><span class="sy3">&</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T, <span class="kw1">class</span> U<span class="sy1">></span>
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<span class="kw4">constexpr</span> <span class="kw4">bool</span> operator<span class="sy1">>=</span><span class="br0">(</span><span class="kw4">const</span> optional<span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&</span>, <span class="kw4">const</span> optional<span class="sy1"><</span>U<span class="sy1">></span><span class="sy3">&</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T, three_way_comparable_with<span class="sy1"><</span>T<span class="sy1">></span> U<span class="sy1">></span>
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<span class="kw4">constexpr</span> compare_three_way_result_t<span class="sy1"><</span>T,U<span class="sy1">></span>
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operator<span class="sy1"><=></span><span class="br0">(</span><span class="kw4">const</span> optional<span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&</span>, <span class="kw4">const</span> optional<span class="sy1"><</span>U<span class="sy1">></span><span class="sy3">&</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 与 nullopt 比较</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span> <span class="kw4">constexpr</span> <span class="kw4">bool</span> operator<span class="sy1">==</span><span class="br0">(</span><span class="kw4">const</span> optional<span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&</span>, nullopt_t<span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span>
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<span class="kw4">constexpr</span> strong_ordering operator<span class="sy1"><=></span><span class="br0">(</span><span class="kw4">const</span> optional<span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&</span>, nullopt_t<span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span>
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<span class="co1">// 与 T 比较</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T, <span class="kw1">class</span> U<span class="sy1">></span> <span class="kw4">constexpr</span> <span class="kw4">bool</span> operator<span class="sy1">==</span><span class="br0">(</span><span class="kw4">const</span> optional<span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&</span>, <span class="kw4">const</span> U<span class="sy3">&</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T, <span class="kw1">class</span> U<span class="sy1">></span> <span class="kw4">constexpr</span> <span class="kw4">bool</span> operator<span class="sy1">==</span><span class="br0">(</span><span class="kw4">const</span> T<span class="sy3">&</span>, <span class="kw4">const</span> optional<span class="sy1"><</span>U<span class="sy1">></span><span class="sy3">&</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T, <span class="kw1">class</span> U<span class="sy1">></span> <span class="kw4">constexpr</span> <span class="kw4">bool</span> operator<span class="sy3">!</span><span class="sy1">=</span><span class="br0">(</span><span class="kw4">const</span> optional<span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&</span>, <span class="kw4">const</span> U<span class="sy3">&</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T, <span class="kw1">class</span> U<span class="sy1">></span> <span class="kw4">constexpr</span> <span class="kw4">bool</span> operator<span class="sy3">!</span><span class="sy1">=</span><span class="br0">(</span><span class="kw4">const</span> T<span class="sy3">&</span>, <span class="kw4">const</span> optional<span class="sy1"><</span>U<span class="sy1">></span><span class="sy3">&</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T, <span class="kw1">class</span> U<span class="sy1">></span> <span class="kw4">constexpr</span> <span class="kw4">bool</span> operator<span class="sy1"><</span><span class="br0">(</span><span class="kw4">const</span> optional<span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&</span>, <span class="kw4">const</span> U<span class="sy3">&</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T, <span class="kw1">class</span> U<span class="sy1">></span> <span class="kw4">constexpr</span> <span class="kw4">bool</span> operator<span class="sy1"><</span><span class="br0">(</span><span class="kw4">const</span> T<span class="sy3">&</span>, <span class="kw4">const</span> optional<span class="sy1"><</span>U<span class="sy1">></span><span class="sy3">&</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T, <span class="kw1">class</span> U<span class="sy1">></span> <span class="kw4">constexpr</span> <span class="kw4">bool</span> operator<span class="sy1">></span><span class="br0">(</span><span class="kw4">const</span> optional<span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&</span>, <span class="kw4">const</span> U<span class="sy3">&</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T, <span class="kw1">class</span> U<span class="sy1">></span> <span class="kw4">constexpr</span> <span class="kw4">bool</span> operator<span class="sy1">></span><span class="br0">(</span><span class="kw4">const</span> T<span class="sy3">&</span>, <span class="kw4">const</span> optional<span class="sy1"><</span>U<span class="sy1">></span><span class="sy3">&</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T, <span class="kw1">class</span> U<span class="sy1">></span> <span class="kw4">constexpr</span> <span class="kw4">bool</span> operator<span class="sy1"><=</span><span class="br0">(</span><span class="kw4">const</span> optional<span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&</span>, <span class="kw4">const</span> U<span class="sy3">&</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T, <span class="kw1">class</span> U<span class="sy1">></span> <span class="kw4">constexpr</span> <span class="kw4">bool</span> operator<span class="sy1"><=</span><span class="br0">(</span><span class="kw4">const</span> T<span class="sy3">&</span>, <span class="kw4">const</span> optional<span class="sy1"><</span>U<span class="sy1">></span><span class="sy3">&</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T, <span class="kw1">class</span> U<span class="sy1">></span> <span class="kw4">constexpr</span> <span class="kw4">bool</span> operator<span class="sy1">>=</span><span class="br0">(</span><span class="kw4">const</span> optional<span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&</span>, <span class="kw4">const</span> U<span class="sy3">&</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T, <span class="kw1">class</span> U<span class="sy1">></span> <span class="kw4">constexpr</span> <span class="kw4">bool</span> operator<span class="sy1">>=</span><span class="br0">(</span><span class="kw4">const</span> T<span class="sy3">&</span>, <span class="kw4">const</span> optional<span class="sy1"><</span>U<span class="sy1">></span><span class="sy3">&</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T, three_way_comparable_with<span class="sy1"><</span>T<span class="sy1">></span> U<span class="sy1">></span>
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<span class="kw4">constexpr</span> compare_three_way_result_t<span class="sy1"><</span>T,U<span class="sy1">></span>
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operator<span class="sy1"><=></span><span class="br0">(</span><span class="kw4">const</span> optional<span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&</span>, <span class="kw4">const</span> U<span class="sy3">&</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 特化的算法</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span>
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<span class="kw4">constexpr</span> <span class="kw4">void</span> swap<span class="br0">(</span>optional<span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&</span>, optional<span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&</span><span class="br0">)</span> <span class="kw1">noexcept</span><span class="br0">(</span><span class="coMULTI">/* 见描述 */</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span>
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<span class="kw4">constexpr</span> optional<span class="sy1"><</span><span class="coMULTI">/* 见描述 */</span><span class="sy1">></span> make_optional<span class="br0">(</span>T<span class="sy3">&&</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T, <span class="kw1">class</span>... <span class="me1">Args</span><span class="sy1">></span>
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<span class="kw4">constexpr</span> optional<span class="sy1"><</span>T<span class="sy1">></span> make_optional<span class="br0">(</span>Args<span class="sy3">&&</span>... <span class="me1">args</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T, <span class="kw1">class</span> U, <span class="kw1">class</span>... <span class="me1">Args</span><span class="sy1">></span>
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<span class="kw4">constexpr</span> optional<span class="sy1"><</span>T<span class="sy1">></span> make_optional<span class="br0">(</span>initializer_list<span class="sy1"><</span>U<span class="sy1">></span> il, Args<span class="sy3">&&</span>... <span class="me1">args</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 散列支持</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span> <span class="kw1">struct</span> hash<span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span> <span class="kw1">struct</span> hash<span class="sy1"><</span>optional<span class="sy1"><</span>T<span class="sy1">>></span><span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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</div>
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<h4><span class="mw-headline" id=".E7.B1.BB.E6.A8.A1.E6.9D.BF_std::optional">类模板 <span class="t-lc"><a href="cpp-utility-optional.html">std::optional</a></span></span></h4>
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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="kw1">namespace</span> std <span class="br0">{</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span>
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<span class="kw1">class</span> optional <span class="br0">{</span>
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<span class="kw1">public</span><span class="sy4">:</span>
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<span class="kw1">using</span> value_type <span class="sy1">=</span> T<span class="sy4">;</span>
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<span class="co1">// 构造函数</span>
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<span class="kw4">constexpr</span> optional<span class="br0">(</span><span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span>
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<span class="kw4">constexpr</span> optional<span class="br0">(</span>nullopt_t<span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span>
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<span class="kw4">constexpr</span> optional<span class="br0">(</span><span class="kw4">const</span> optional<span class="sy3">&</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw4">constexpr</span> optional<span class="br0">(</span>optional<span class="sy3">&&</span><span class="br0">)</span> <span class="kw1">noexcept</span><span class="br0">(</span><span class="coMULTI">/* 见描述 */</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span>... <span class="me1">Args</span><span class="sy1">></span>
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<span class="kw4">constexpr</span> <span class="kw1">explicit</span> optional<span class="br0">(</span>in_place_t, Args<span class="sy3">&&</span>...<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> U, <span class="kw1">class</span>... <span class="me1">Args</span><span class="sy1">></span>
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<span class="kw4">constexpr</span> <span class="kw1">explicit</span> optional<span class="br0">(</span>in_place_t, initializer_list<span class="sy1"><</span>U<span class="sy1">></span>, Args<span class="sy3">&&</span>...<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> U <span class="sy1">=</span> T<span class="sy1">></span>
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<span class="kw4">constexpr</span> <span class="kw1">explicit</span><span class="br0">(</span><span class="coMULTI">/* 见描述 */</span><span class="br0">)</span> optional<span class="br0">(</span>U<span class="sy3">&&</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> U<span class="sy1">></span>
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<span class="kw4">constexpr</span> <span class="kw1">explicit</span><span class="br0">(</span><span class="coMULTI">/* 见描述 */</span><span class="br0">)</span> optional<span class="br0">(</span><span class="kw4">const</span> optional<span class="sy1"><</span>U<span class="sy1">></span><span class="sy3">&</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> U<span class="sy1">></span>
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<span class="kw4">constexpr</span> <span class="kw1">explicit</span><span class="br0">(</span><span class="coMULTI">/* 见描述 */</span><span class="br0">)</span> optional<span class="br0">(</span>optional<span class="sy1"><</span>U<span class="sy1">></span><span class="sy3">&&</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 析构函数</span>
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<span class="kw4">constexpr</span> ~optional<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 赋值</span>
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<span class="kw4">constexpr</span> optional<span class="sy3">&</span> operator<span class="sy1">=</span><span class="br0">(</span>nullopt_t<span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span>
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<span class="kw4">constexpr</span> optional<span class="sy3">&</span> operator<span class="sy1">=</span><span class="br0">(</span><span class="kw4">const</span> optional<span class="sy3">&</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw4">constexpr</span> optional<span class="sy3">&</span> operator<span class="sy1">=</span><span class="br0">(</span>optional<span class="sy3">&&</span><span class="br0">)</span> <span class="kw1">noexcept</span><span class="br0">(</span><span class="coMULTI">/* 见描述 */</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> U <span class="sy1">=</span> T<span class="sy1">></span> <span class="kw4">constexpr</span> optional<span class="sy3">&</span> operator<span class="sy1">=</span><span class="br0">(</span>U<span class="sy3">&&</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> U<span class="sy1">></span> <span class="kw4">constexpr</span> optional<span class="sy3">&</span> operator<span class="sy1">=</span><span class="br0">(</span><span class="kw4">const</span> optional<span class="sy1"><</span>U<span class="sy1">></span><span class="sy3">&</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> U<span class="sy1">></span> <span class="kw4">constexpr</span> optional<span class="sy3">&</span> operator<span class="sy1">=</span><span class="br0">(</span>optional<span class="sy1"><</span>U<span class="sy1">></span><span class="sy3">&&</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span>... <span class="me1">Args</span><span class="sy1">></span> <span class="kw4">constexpr</span> T<span class="sy3">&</span> emplace<span class="br0">(</span>Args<span class="sy3">&&</span>...<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> U, <span class="kw1">class</span>... <span class="me1">Args</span><span class="sy1">></span> <span class="kw4">constexpr</span> T<span class="sy3">&</span> emplace<span class="br0">(</span>initializer_list<span class="sy1"><</span>U<span class="sy1">></span>, Args<span class="sy3">&&</span>...<span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 交换</span>
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<span class="kw4">constexpr</span> <span class="kw4">void</span> swap<span class="br0">(</span>optional<span class="sy3">&</span><span class="br0">)</span> <span class="kw1">noexcept</span><span class="br0">(</span><span class="coMULTI">/* 见描述 */</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 观察器</span>
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<span class="kw4">constexpr</span> <span class="kw4">const</span> T<span class="sy2">*</span> operator<span class="sy2">-</span><span class="sy1">></span><span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span> <span class="kw1">noexcept</span><span class="sy4">;</span>
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<span class="kw4">constexpr</span> T<span class="sy2">*</span> operator<span class="sy2">-</span><span class="sy1">></span><span class="br0">(</span><span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span>
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<span class="kw4">constexpr</span> <span class="kw4">const</span> T<span class="sy3">&</span> operator<span class="sy2">*</span><span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span><span class="sy3">&</span> <span class="kw1">noexcept</span><span class="sy4">;</span>
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<span class="kw4">constexpr</span> T<span class="sy3">&</span> operator<span class="sy2">*</span><span class="br0">(</span><span class="br0">)</span> <span class="sy3">&</span> <span class="kw1">noexcept</span><span class="sy4">;</span>
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<span class="kw4">constexpr</span> T<span class="sy3">&&</span> operator<span class="sy2">*</span><span class="br0">(</span><span class="br0">)</span> <span class="sy3">&&</span> <span class="kw1">noexcept</span><span class="sy4">;</span>
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<span class="kw4">constexpr</span> <span class="kw4">const</span> T<span class="sy3">&&</span> operator<span class="sy2">*</span><span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span><span class="sy3">&&</span> <span class="kw1">noexcept</span><span class="sy4">;</span>
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<span class="kw4">constexpr</span> <span class="kw1">explicit</span> operator <span class="kw4">bool</span><span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span> <span class="kw1">noexcept</span><span class="sy4">;</span>
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<span class="kw4">constexpr</span> <span class="kw4">bool</span> has_value<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span> <span class="kw1">noexcept</span><span class="sy4">;</span>
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<span class="kw4">constexpr</span> <span class="kw4">const</span> T<span class="sy3">&</span> value<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span><span class="sy3">&</span><span class="sy4">;</span>
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<span class="kw4">constexpr</span> T<span class="sy3">&</span> value<span class="br0">(</span><span class="br0">)</span> <span class="sy3">&</span><span class="sy4">;</span>
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<span class="kw4">constexpr</span> T<span class="sy3">&&</span> value<span class="br0">(</span><span class="br0">)</span> <span class="sy3">&&</span><span class="sy4">;</span>
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<span class="kw4">constexpr</span> <span class="kw4">const</span> T<span class="sy3">&&</span> value<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span><span class="sy3">&&</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> U<span class="sy1">></span> <span class="kw4">constexpr</span> T value_or<span class="br0">(</span>U<span class="sy3">&&</span><span class="br0">)</span> <span class="kw4">const</span><span class="sy3">&</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> U<span class="sy1">></span> <span class="kw4">constexpr</span> T value_or<span class="br0">(</span>U<span class="sy3">&&</span><span class="br0">)</span> <span class="sy3">&&</span><span class="sy4">;</span>
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<span class="co1">// 单子操作</span>
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<span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">class</span> F<span class="sy1">></span> <span class="kw4">constexpr</span> <span class="kw4">auto</span> and_then<span class="br0">(</span>F<span class="sy3">&&</span> f<span class="br0">)</span> <span class="sy3">&</span><span class="sy4">;</span>
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<span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">class</span> F<span class="sy1">></span> <span class="kw4">constexpr</span> <span class="kw4">auto</span> and_then<span class="br0">(</span>F<span class="sy3">&&</span> f<span class="br0">)</span> <span class="sy3">&&</span><span class="sy4">;</span>
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<span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">class</span> F<span class="sy1">></span> <span class="kw4">constexpr</span> <span class="kw4">auto</span> and_then<span class="br0">(</span>F<span class="sy3">&&</span> f<span class="br0">)</span> <span class="kw4">const</span><span class="sy3">&</span><span class="sy4">;</span>
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<span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">class</span> F<span class="sy1">></span> <span class="kw4">constexpr</span> <span class="kw4">auto</span> and_then<span class="br0">(</span>F<span class="sy3">&&</span> f<span class="br0">)</span> <span class="kw4">const</span><span class="sy3">&&</span><span class="sy4">;</span>
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<span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">class</span> F<span class="sy1">></span> <span class="kw4">constexpr</span> <span class="kw4">auto</span> transform<span class="br0">(</span>F<span class="sy3">&&</span> f<span class="br0">)</span> <span class="sy3">&</span><span class="sy4">;</span>
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<span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">class</span> F<span class="sy1">></span> <span class="kw4">constexpr</span> <span class="kw4">auto</span> transform<span class="br0">(</span>F<span class="sy3">&&</span> f<span class="br0">)</span> <span class="sy3">&&</span><span class="sy4">;</span>
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<span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">class</span> F<span class="sy1">></span> <span class="kw4">constexpr</span> <span class="kw4">auto</span> transform<span class="br0">(</span>F<span class="sy3">&&</span> f<span class="br0">)</span> <span class="kw4">const</span><span class="sy3">&</span><span class="sy4">;</span>
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<span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">class</span> F<span class="sy1">></span> <span class="kw4">constexpr</span> <span class="kw4">auto</span> transform<span class="br0">(</span>F<span class="sy3">&&</span> f<span class="br0">)</span> <span class="kw4">const</span><span class="sy3">&&</span><span class="sy4">;</span>
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<span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">class</span> F<span class="sy1">></span> <span class="kw4">constexpr</span> optional or_else<span class="br0">(</span>F<span class="sy3">&&</span> f<span class="br0">)</span> <span class="sy3">&&</span><span class="sy4">;</span>
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<span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">class</span> F<span class="sy1">></span> <span class="kw4">constexpr</span> optional or_else<span class="br0">(</span>F<span class="sy3">&&</span> f<span class="br0">)</span> <span class="kw4">const</span><span class="sy3">&</span><span class="sy4">;</span>
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<span class="co1">// 修改器</span>
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|
<span class="kw4">constexpr</span> <span class="kw4">void</span> reset<span class="br0">(</span><span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span>
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<span class="kw1">private</span><span class="sy4">:</span>
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T <span class="sy2">*</span>val<span class="sy4">;</span> <span class="co1">// 仅用于阐释</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span>
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optional<span class="br0">(</span>T<span class="br0">)</span> <span class="sy2">-</span><span class="sy1">></span> optional<span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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