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<!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::optional<T>::optional</title>
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<link rel="stylesheet" href="site_modules.css" />
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_optional_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%">std::optional<T>::</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-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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<tr class="t-dcl t-since-cxx17">
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> optional<span class="br0">(</span><span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span><br />
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<span class="kw4">constexpr</span> optional<span class="br0">(</span> <a href="cpp-utility-optional-nullopt_t.html"><span class="kw1078">std::<span class="me2">nullopt_t</span></span></a> <span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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</td>
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<td>(1)</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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</tr>
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<tr class="t-dcl t-since-cxx17">
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<td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> optional<span class="br0">(</span> <span class="kw4">const</span> optional<span class="sy3">&</span> other <span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td>(2)</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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</tr>
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<tr class="t-dcl t-since-cxx17">
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<td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> optional<span class="br0">(</span> optional<span class="sy3">&&</span> other <span class="br0">)</span> <span class="kw1">noexcept</span><span class="br0">(</span><span class="coMULTI">/* see below */</span><span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td>(3)</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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</tr>
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</tbody>
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<tbody class="t-dcl-rev t-dcl-rev-num">
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<tr class="t-dcl-rev-aux">
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<td></td>
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<td rowspan="142">(4)</td>
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<td></td>
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</tr>
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<tr class="t-dcl t-since-cxx17 t-until-cxx20">
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<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> U <span class="sy1">></span><br />
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optional<span class="br0">(</span> <span class="kw4">const</span> optional<span class="sy1"><</span>U<span class="sy1">></span><span class="sy3">&</span> other <span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span><br />
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<span class="t-mark-rev t-until-cxx20">(C++20 前)</span><br />
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<span class="t-mark">(条件性 <code>explicit</code>)</span></td>
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</tr>
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<tr class="t-dcl t-since-cxx20">
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<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> U <span class="sy1">></span><br />
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<span class="kw4">constexpr</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> other <span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span><br />
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<span class="t-mark">(条件性 <code>explicit</code>)</span></td>
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</tr>
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</tbody>
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<tbody></tbody>
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<tbody class="t-dcl-rev t-dcl-rev-num">
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<tr class="t-dcl-rev-aux">
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<td></td>
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<td rowspan="142">(5)</td>
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<td></td>
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</tr>
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<tr class="t-dcl t-since-cxx17 t-until-cxx20">
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<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> U <span class="sy1">></span><br />
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optional<span class="br0">(</span> optional<span class="sy1"><</span>U<span class="sy1">></span><span class="sy3">&&</span> other <span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span><br />
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<span class="t-mark-rev t-until-cxx20">(C++20 前)</span><br />
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<span class="t-mark">(条件性 <code>explicit</code>)</span></td>
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</tr>
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<tr class="t-dcl t-since-cxx20">
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<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> U <span class="sy1">></span><br />
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<span class="kw4">constexpr</span> optional<span class="br0">(</span> optional<span class="sy1"><</span>U<span class="sy1">></span><span class="sy3">&&</span> other <span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span><br />
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<span class="t-mark">(条件性 <code>explicit</code>)</span></td>
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</tr>
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</tbody>
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<tbody>
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<tr class="t-dcl t-since-cxx17">
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<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>... <span class="me1">Args</span> <span class="sy1">></span><br />
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<span class="kw4">constexpr</span> <span class="kw1">explicit</span> optional<span class="br0">(</span> <a href="cpp-utility-in_place.html"><span class="kw1082">std::<span class="me2">in_place_t</span></span></a>, Args<span class="sy3">&&</span>... <span class="me1">args</span> <span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td>(6)</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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</tr>
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<tr class="t-dcl t-since-cxx17">
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<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> U, <span class="kw1">class</span>... <span class="me1">Args</span> <span class="sy1">></span><br /></span>
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<p><span class="kw4">constexpr</span> <span class="kw1">explicit</span> optional<span class="br0">(</span> <a href="cpp-utility-in_place.html"><span class="kw1082">std::<span class="me2">in_place_t</span></span></a>,<br />
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<a href="cpp-utility-initializer_list.html"><span class="kw960">std::<span class="me2">initializer_list</span></span></a><span class="sy1"><</span>U<span class="sy1">></span> ilist,<br /></p>
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Args<span class="sy3">&&</span>... <span class="me1">args</span> <span class="br0">)</span><span class="sy4">;</span></div>
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</td>
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<td>(7)</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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</tr>
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<tr class="t-dcl t-since-cxx17">
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<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> U <span class="sy1">=</span> T <span class="sy1">></span><br />
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<span class="kw4">constexpr</span> optional<span class="br0">(</span> U<span class="sy3">&&</span> value <span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td>(8)</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span><br />
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<span class="t-mark">(条件性 <code>explicit</code>)</span></td>
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</tr>
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<tr class="t-dcl-sep">
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<td></td>
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<td></td>
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<td></td>
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</tr>
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</tbody>
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</table>
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<p>构造新的 <code>optional</code> 对象。</p>
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<div class="t-li1"><span class="t-li">1)</span> 构造<i>不含值</i>的对象。</div>
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<div class="t-li1"><span class="t-li">2)</span> 复制构造函数：若 <code>other</code> 含值，则初始化所含值，如同以表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>other</span></span> <a href="cpp-language-direct_initialization.html">直接初始化</a>（但不是直接列表初始化） <code>T</code> 类型对象。若 <code>other</code> 不含值，则构造一个<i>不含值</i>的对象。
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<ul>
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<li>若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_copy_constructible.html"><span class="kw533">std::<span class="me2">is_copy_constructible_v</span></span></a><span class="sy1"><</span>T<span class="sy1">></span></span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> 则定义此构造函数为被删除。</li>
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<li>若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_copy_constructible.html"><span class="kw535">std::<span class="me2">is_trivially_copy_constructible_v</span></span></a><span class="sy1"><</span>T<span class="sy1">></span></span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> ，则它是平凡构造函数。</li>
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</ul>
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</div>
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<div class="t-li1"><span class="t-li">3)</span> 移动构造函数：若 <code>other</code> 含值，则初始化所含值，如同以表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span><span class="sy2">*</span>other<span class="br0">)</span></span></span> <a href="cpp-language-direct_initialization.html">直接初始化</a>（但不是直接列表初始化） <code>T</code> 类型对象，且<i>不</i>令 <code>other</code> 为空：被移动源的 <code>optional</code> 仍然<i>含值</i>，但该值自身是被移动的。
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<ul>
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<li>若 <code>other</code> 不含值，则构造一个<i>不含值</i>的对象。此构造函数不参与重载决议，除非 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_move_constructible.html"><span class="kw539">std::<span class="me2">is_move_constructible_v</span></span></a><span class="sy1"><</span>T<span class="sy1">></span></span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span></li>
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<li>。若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_move_constructible.html"><span class="kw541">std::<span class="me2">is_trivially_move_constructible_v</span></span></a><span class="sy1"><</span>T<span class="sy1">></span></span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> ，则它是平凡构造函数。</li>
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</ul>
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</div>
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<div class="t-li1"><span class="t-li">4)</span> 转换复制构造函数：若 <code>other</code> 不含值，则构造不含值的 <code>optional</code> 对象。否则，构造含值的 <code>optional</code> 对象，如同以表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>other</span></span> <a href="cpp-language-direct_initialization.html">直接初始化</a>（但不是直接列表初始化） <code>T</code> 类型对象一般初始化。
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<ul>
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<li>此构造函数不参与重载决议，除非满足下列条件：
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<ul>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_constructible.html"><span class="kw521">std::<span class="me2">is_constructible_v</span></span></a><span class="sy1"><</span>T, <span class="kw4">const</span> U<span class="sy3">&</span><span class="sy1">></span></span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 。</li>
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<li><code>T</code> 不可从任何 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1"><</span>U<span class="sy1">></span></span></span> （可为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span></span></span> ）类型表达式构造或转换，即下列 8 个类型特征均为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> ：
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<ul>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_constructible.html"><span class="kw521">std::<span class="me2">is_constructible_v</span></span></a><span class="sy1"><</span>T, <a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1"><</span>U<span class="sy1">></span><span class="sy3">&</span><span class="sy1">></span></span></span></li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_constructible.html"><span class="kw521">std::<span class="me2">is_constructible_v</span></span></a><span class="sy1"><</span>T, <span class="kw4">const</span> <a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1"><</span>U<span class="sy1">></span><span class="sy3">&</span><span class="sy1">></span></span></span></li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_constructible.html"><span class="kw521">std::<span class="me2">is_constructible_v</span></span></a><span class="sy1"><</span>T, <a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1"><</span>U<span class="sy1">></span><span class="sy3">&&</span><span class="sy1">></span></span></span></li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_constructible.html"><span class="kw521">std::<span class="me2">is_constructible_v</span></span></a><span class="sy1"><</span>T, <span class="kw4">const</span> <a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1"><</span>U<span class="sy1">></span><span class="sy3">&&</span><span class="sy1">></span></span></span></li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_convertible.html"><span class="kw593">std::<span class="me2">is_convertible_v</span></span></a><span class="sy1"><</span><a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1"><</span>U<span class="sy1">></span><span class="sy3">&</span>, T<span class="sy1">></span></span></span></li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_convertible.html"><span class="kw593">std::<span class="me2">is_convertible_v</span></span></a><span class="sy1"><</span><span class="kw4">const</span> <a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1"><</span>U<span class="sy1">></span><span class="sy3">&</span>, T<span class="sy1">></span></span></span></li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_convertible.html"><span class="kw593">std::<span class="me2">is_convertible_v</span></span></a><span class="sy1"><</span><a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1"><</span>U<span class="sy1">></span><span class="sy3">&&</span>, T<span class="sy1">></span></span></span></li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_convertible.html"><span class="kw593">std::<span class="me2">is_convertible_v</span></span></a><span class="sy1"><</span><span class="kw4">const</span> <a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1"><</span>U<span class="sy1">></span><span class="sy3">&&</span>, T<span class="sy1">></span></span></span></li>
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</ul>
|
||||
</li>
|
||||
</ul>
|
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</li>
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<li>此构造函数为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">explicit</span></span></span> 若且唯若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_convertible.html"><span class="kw593">std::<span class="me2">is_convertible_v</span></span></a><span class="sy1"><</span><span class="kw4">const</span> U<span class="sy3">&</span>, T<span class="sy1">></span></span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> 。</li>
|
||||
</ul>
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</div>
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<div class="t-li1"><span class="t-li">5)</span> 转换移动构造函数：若 <code>other</code> 不含值，则构造不含值的 <code>optional</code> 对象。否则，构造含值的 <code>optional</code> 对象，如同以表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span><span class="sy2">*</span>other<span class="br0">)</span></span></span> <a href="cpp-language-direct_initialization.html">直接初始化</a>（但不是直接列表初始化） <code>T</code> 类型对象一般初始化。
|
||||
<ul>
|
||||
<li>此构造函数不参与重载决议，除非满足下列条件：
|
||||
<ul>
|
||||
<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_constructible.html"><span class="kw521">std::<span class="me2">is_constructible_v</span></span></a><span class="sy1"><</span>T, U<span class="sy3">&&</span><span class="sy1">></span></span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 。</li>
|
||||
<li><code>T</code> 不可从任何<span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1"><</span>U<span class="sy1">></span></span></span> （可为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span></span></span> ）类型表达式构造或转换，即下列 8 个类型特征均为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> ：
|
||||
<ul>
|
||||
<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_constructible.html"><span class="kw521">std::<span class="me2">is_constructible_v</span></span></a><span class="sy1"><</span>T, <a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1"><</span>U<span class="sy1">></span><span class="sy3">&</span><span class="sy1">></span></span></span></li>
|
||||
<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_constructible.html"><span class="kw521">std::<span class="me2">is_constructible_v</span></span></a><span class="sy1"><</span>T, <span class="kw4">const</span> <a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1"><</span>U<span class="sy1">></span><span class="sy3">&</span><span class="sy1">></span></span></span></li>
|
||||
<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_constructible.html"><span class="kw521">std::<span class="me2">is_constructible_v</span></span></a><span class="sy1"><</span>T, <a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1"><</span>U<span class="sy1">></span><span class="sy3">&&</span><span class="sy1">></span></span></span></li>
|
||||
<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_constructible.html"><span class="kw521">std::<span class="me2">is_constructible_v</span></span></a><span class="sy1"><</span>T, <span class="kw4">const</span> <a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1"><</span>U<span class="sy1">></span><span class="sy3">&&</span><span class="sy1">></span></span></span></li>
|
||||
<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_convertible.html"><span class="kw593">std::<span class="me2">is_convertible_v</span></span></a><span class="sy1"><</span><a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1"><</span>U<span class="sy1">></span><span class="sy3">&</span>, T<span class="sy1">></span></span></span></li>
|
||||
<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_convertible.html"><span class="kw593">std::<span class="me2">is_convertible_v</span></span></a><span class="sy1"><</span><span class="kw4">const</span> <a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1"><</span>U<span class="sy1">></span><span class="sy3">&</span>, T<span class="sy1">></span></span></span></li>
|
||||
<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_convertible.html"><span class="kw593">std::<span class="me2">is_convertible_v</span></span></a><span class="sy1"><</span><a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1"><</span>U<span class="sy1">></span><span class="sy3">&&</span>, T<span class="sy1">></span></span></span></li>
|
||||
<li><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_convertible.html"><span class="kw593">std::<span class="me2">is_convertible_v</span></span></a><span class="sy1"><</span><span class="kw4">const</span> <a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1"><</span>U<span class="sy1">></span><span class="sy3">&&</span>, T<span class="sy1">></span></span></span></li>
|
||||
</ul>
|
||||
</li>
|
||||
</ul>
|
||||
</li>
|
||||
<li>此构造函数为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">explicit</span></span></span> ，若且唯若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_convertible.html"><span class="kw593">std::<span class="me2">is_convertible_v</span></span></a><span class="sy1"><</span>U<span class="sy3">&&</span>, T<span class="sy1">></span></span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> 。</li>
|
||||
</ul>
|
||||
</div>
|
||||
<div class="t-li1"><span class="t-li">6)</span> 构造一个<i>含值</i>的对象，如同从参数 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-forward.html"><span class="kw1117">std::<span class="me2">forward</span></span></a><span class="sy1"><</span>Args<span class="sy1">></span><span class="br0">(</span>args<span class="br0">)</span>...</span></span> <a href="cpp-language-direct_initialization.html">直接初始化</a>（但不是直接列表初始化） <code>T</code> 类型对象一般初始化。
|
||||
<ul>
|
||||
<li>若选择的 <code>T</code> 构造函数为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span></span></span> 构造函数，则此构造函数为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span></span></span> 构造函数。</li>
|
||||
<li>该函数仅若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_constructible.html"><span class="kw521">std::<span class="me2">is_constructible_v</span></span></a><span class="sy1"><</span>T, Args...<span class="sy1">></span></span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 才参与重载决议。</li>
|
||||
</ul>
|
||||
</div>
|
||||
<div class="t-li1"><span class="t-li">7)</span> 构造一个<i>含值</i>的对象，如同从参数 <span class="t-c"><span class="mw-geshi cpp source-cpp">ilist, <a href="cpp-utility-forward.html"><span class="kw1117">std::<span class="me2">forward</span></span></a><span class="sy1"><</span>Args<span class="sy1">></span><span class="br0">(</span>args<span class="br0">)</span>...</span></span> <a href="cpp-language-direct_initialization.html">直接初始化</a>（但不是直接列表初始化） <code>T</code> 类型对象一般初始化。
|
||||
<ul>
|
||||
<li>若选择的 <code>T</code> 构造函数为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span></span></span> 构造函数，则此构造函数为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span></span></span> 构造函数。</li>
|
||||
<li>该函数仅若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_constructible.html"><span class="kw521">std::<span class="me2">is_constructible_v</span></span></a><span class="sy1"><</span>T, <a href="cpp-utility-initializer_list.html"><span class="kw960">std::<span class="me2">initializer_list</span></span></a><span class="sy1"><</span>U<span class="sy1">></span><span class="sy3">&</span>, Args<span class="sy3">&&</span>...<span class="sy1">></span></span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 才参与重载决议。</li>
|
||||
</ul>
|
||||
</div>
|
||||
<div class="t-li1"><span class="t-li">8)</span> 构造一个<i>含值</i>的对象，如同从参数 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-forward.html"><span class="kw1117">std::<span class="me2">forward</span></span></a><span class="sy1"><</span>U<span class="sy1">></span><span class="br0">(</span>value<span class="br0">)</span></span></span> <a href="cpp-language-direct_initialization.html">直接初始化</a>（但不是直接列表初始化） <code>T</code> 类型对象一般初始化。
|
||||
<ul>
|
||||
<li>若选择的 <code>T</code> 构造函数为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span></span></span> 构造函数，则此构造函数为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span></span></span> 构造函数。</li>
|
||||
<li>此构造函数仅若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_constructible.html"><span class="kw521">std::<span class="me2">is_constructible_v</span></span></a><span class="sy1"><</span>T, U<span class="sy3">&&</span><span class="sy1">></span></span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 且 <span class="t-rev-inl t-until-cxx20"><span><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1"><</span>U<span class="sy1">></span></span></span></span> <span><span class="t-mark-rev t-until-cxx20">(C++20 前)</span></span></span><span class="t-rev-inl t-since-cxx20"><span><span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">remove_cvref_t</span><span class="sy1"><</span>U<span class="sy1">></span></span></span></span> <span><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></span></span> 既非 <span class="t-lc"><a href="cpp-utility-in_place.html">std::in_place_t</a></span> 亦非 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1"><</span>T<span class="sy1">></span></span></span> 才参与重载决议。</li>
|
||||
<li>此构造函数为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">explicit</span></span></span> 若且唯若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_convertible.html"><span class="kw593">std::<span class="me2">is_convertible_v</span></span></a><span class="sy1"><</span>U<span class="sy3">&&</span>, T<span class="sy1">></span></span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> 。</li>
|
||||
</ul>
|
||||
</div>
|
||||
<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
|
||||
<table class="t-par-begin">
|
||||
<tr class="t-par">
|
||||
<td>other</td>
|
||||
<td>-</td>
|
||||
<td>要复制其所含值的另一 <code>optional</code> 对象</td>
|
||||
</tr>
|
||||
<tr class="t-par">
|
||||
<td>value</td>
|
||||
<td>-</td>
|
||||
<td>初始化所含值所用的值</td>
|
||||
</tr>
|
||||
<tr class="t-par">
|
||||
<td>args...</td>
|
||||
<td>-</td>
|
||||
<td>初始化所含值所用的参数</td>
|
||||
</tr>
|
||||
<tr class="t-par">
|
||||
<td>ilist</td>
|
||||
<td>-</td>
|
||||
<td>初始化所含值所用的 initializer_list</td>
|
||||
</tr>
|
||||
</table>
|
||||
<h3><span class="mw-headline" id=".E5.BC.82.E5.B8.B8">异常</span></h3>
|
||||
<div class="t-li1"><span class="t-li">2)</span> 抛出任何 <code>T</code> 的构造函数所抛的异常。</div>
|
||||
<div class="t-li1"><span class="t-li">3)</span> 抛出任何 <code>T</code> 的构造函数所抛的异常。拥有下列
|
||||
<div class="t-noexcept-full"><a href="cpp-language-noexcept_spec.html"><tt>noexcept</tt></a> 说明：
|
||||
<div class="t-noexcept-box"><span class="t-cc"><span class="mw-geshi cpp source-cpp"><span class="kw1">noexcept</span><span class="br0">(</span><a href="cpp-types-is_move_constructible.html"><span class="kw542">std::<span class="me2">is_nothrow_move_constructible</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">::</span><span class="me2">value</span><span class="br0">)</span></span></span></div>
|
||||
</div>
|
||||
</div>
|
||||
<div class="t-li1"><span class="t-li">4-8)</span> 抛出任何 <code>T</code> 的构造函数所抛的异常。</div>
|
||||
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
|
||||
<div class="t-example">
|
||||
<div class="t-example-live-link"></div>
|
||||
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
||||
<div class="cpp source-cpp">
|
||||
<pre class="de1"><span class="co2">#include <optional></span>
|
||||
<span class="co2">#include <iostream></span>
|
||||
<span class="co2">#include <string></span>
|
||||
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
|
||||
<span class="br0">{</span>
|
||||
<a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> o1, <span class="co1">// 空</span>
|
||||
o2 <span class="sy1">=</span> <span class="nu0">1</span>, <span class="co1">// 从右值初始化</span>
|
||||
o3 <span class="sy1">=</span> o2<span class="sy4">;</span> <span class="co1">// 复制构造函数</span>
|
||||
|
||||
<span class="co1">// 调用 std::string( initializer_list<CharT> ) 构造函数</span>
|
||||
<a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1"><</span><a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a><span class="sy1">></span> o4<span class="br0">(</span><a href="cpp-utility-in_place.html"><span class="kw1081">std::<span class="me2">in_place</span></span></a>, <span class="br0">{</span><span class="st0">'a'</span>, <span class="st0">'b'</span>, <span class="st0">'c'</span><span class="br0">}</span><span class="br0">)</span><span class="sy4">;</span>
|
||||
|
||||
<span class="co1">// 调用 std::string( size_type count, CharT ch ) 构造函数</span>
|
||||
<a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1"><</span><a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a><span class="sy1">></span> o5<span class="br0">(</span><a href="cpp-utility-in_place.html"><span class="kw1081">std::<span class="me2">in_place</span></span></a>, <span class="nu0">3</span>, <span class="st0">'A'</span><span class="br0">)</span><span class="sy4">;</span>
|
||||
|
||||
<span class="co1">// 从 std::string 移动构造，用推导指引拾取类型</span>
|
||||
|
||||
<a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a> o6<span class="br0">(</span><a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a><span class="br0">{</span><span class="st0">"deduction"</span><span class="br0">}</span><span class="br0">)</span><span class="sy4">;</span>
|
||||
|
||||
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="sy2">*</span>o2 <span class="sy1"><<</span> <span class="st0">' '</span> <span class="sy1"><<</span> <span class="sy2">*</span>o3 <span class="sy1"><<</span> <span class="st0">' '</span> <span class="sy1"><<</span> <span class="sy2">*</span>o4 <span class="sy1"><<</span> <span class="st0">' '</span> <span class="sy1"><<</span> <span class="sy2">*</span>o5 <span class="sy1"><<</span> <span class="st0">' '</span> <span class="sy1"><<</span> <span class="sy2">*</span>o6 <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
|
||||
<span class="br0">}</span></pre></div>
|
||||
</div>
|
||||
<p>输出：</p>
|
||||
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
||||
<div class="text source-text">
|
||||
<pre class="de1">1 1 abc AAA deduction</pre></div>
|
||||
</div>
|
||||
</div>
|
||||
<h3><span class="mw-headline" id=".E7.BC.BA.E9.99.B7.E6.8A.A5.E5.91.8A">缺陷报告</span></h3>
|
||||
<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>
|
||||
<table class="dsctable" style="font-size:0.8em">
|
||||
<tr>
|
||||
<th>DR</th>
|
||||
<th>应用于</th>
|
||||
<th>出版时的行为</th>
|
||||
<th>正确行为</th>
|
||||
</tr>
|
||||
<tr>
|
||||
<td><a rel="nofollow" class="external text" href="https://wg21.link/P0602R4">P0602R4</a></td>
|
||||
<td>C++17</td>
|
||||
<td>即使底层构造函数平凡，复制/移动构造函数亦可能不平凡</td>
|
||||
<td>要求传播平凡性</td>
|
||||
</tr>
|
||||
<tr>
|
||||
<td><a rel="nofollow" class="external text" href="https://wg21.link/P2231R1">P2231R1</a></td>
|
||||
<td>C++20</td>
|
||||
<td>源自另一 <code>optional</code> 的转换构造函数不是 constexpr 而要求的操作在 C++20 中能是</td>
|
||||
<td>使之为 constexpr</td>
|
||||
</tr>
|
||||
</table>
|
||||
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
|
||||
<table class="t-dsc-begin">
|
||||
<tr class="t-dsc">
|
||||
<td>
|
||||
<div class="t-dsc-member-div">
|
||||
<div><a href="cpp-utility-optional-make_optional.html"><span class="t-lines"><span>make_optional</span></span></a></div>
|
||||
<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div>
|
||||
</div>
|
||||
</td>
|
||||
<td>创建一个 <code>optional</code> 对象<br />
|
||||
<span class="t-mark">(函数模板)</span></td>
|
||||
</tr>
|
||||
</table>
|
||||
</div>
|
||||
<div class="visualClear"></div>
|
||||
</div>
|
||||
</div>
|
||||
</div>
|
||||
</body>
|
||||
</html>
|
||||
Reference in New Issue
Block a user