197 lines
16 KiB
HTML
197 lines
16 KiB
HTML
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<title>std::optional<T>::emplace</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_optional_emplace skin-cppreference2 action-view cpp-navbar">
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::optional<T>::</span>emplace</h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-utility.html">utility</a>‎ | <a href="cpp-utility-optional.html">optional</a></span></div>
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<td></td>
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<td rowspan="142">(1)</td>
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<td></td>
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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>... <span class="me1">Args</span> <span class="sy1">></span><br />
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T<span class="sy3">&</span> emplace<span class="br0">(</span> 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 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></td>
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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>... <span class="me1">Args</span> <span class="sy1">></span><br />
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<span class="kw4">constexpr</span> T<span class="sy3">&</span> emplace<span class="br0">(</span> 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><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<td rowspan="142">(2)</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 />
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T<span class="sy3">&</span> emplace<span class="br0">(</span> <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, 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 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></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 />
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<span class="kw4">constexpr</span> T<span class="sy3">&</span> emplace<span class="br0">(</span> <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, 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 class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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</tbody>
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<tbody>
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<p>原位构造所含值。若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 已在此调用前含值，则调用其析构函数销毁所含值。</p>
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<div class="t-li1"><span class="t-li">1)</span> 以 <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> 为参数<a href="cpp-language-direct_initialization.html">直接初始化</a>（但不是直接列表初始化）所含值。</div>
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<div class="t-li1"><span class="t-li">2)</span> 以 <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> 为参数<a href="cpp-language-direct_initialization.html">直接初始化</a>（但不是直接列表初始化）所含值。此重载仅若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_constructible.html"><span class="kw520">std::<span class="me2">is_constructible</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 class="sy4">::</span><span class="me2">value</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。</div>
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<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
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<table class="t-par-begin">
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<td>args...</td>
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<td>-</td>
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<td>传递给构造函数的参数</td>
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<tr class="t-par">
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<td>ilist</td>
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<td>-</td>
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<td>传递给构造函数的初始化器列表</td>
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<tr class="t-par-hitem">
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<td colspan="3">类型要求</td>
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<tr class="t-par-req">
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<td colspan="3">-
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<div class="spacer"></div>
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对于重载 <span class="t-v">(1)</span> ， <code>T</code> 必须可从 <code>Args...</code> 构造</td>
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<td colspan="3">-
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<div class="spacer"></div>
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对于重载 <span class="t-v">(2)</span> ， <code>T</code> 必须可从 <span class="t-lc"><a href="cpp-utility-initializer_list.html">std::initializer_list</a></span> 及 <code>Args...</code> 构造</td>
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<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">返回值</span></h3>
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<p>到新的被含有值的引用。</p>
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<h3><span class="mw-headline" id=".E5.BC.82.E5.B8.B8">异常</span></h3>
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<p>选择的 <code>T</code> 构造函数所抛出的任何异常。若抛出异常，则 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 在此调用后不含值（若先前存在所含值，则销毁它）。</p>
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<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
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<div class="t-example">
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<div class="t-example-live-link"></div>
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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 <optional></span>
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<span class="co2">#include <iostream></span>
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<span class="kw1">struct</span> A <span class="br0">{</span>
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<a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> s<span class="sy4">;</span>
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A<span class="br0">(</span><a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> str<span class="br0">)</span> <span class="sy4">:</span> s<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>str<span class="br0">)</span><span class="br0">)</span> <span class="br0">{</span> <a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">" constructed<span class="es1">\n</span>"</span><span class="sy4">;</span> <span class="br0">}</span>
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~A<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span> <a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">" destructed<span class="es1">\n</span>"</span><span class="sy4">;</span> <span class="br0">}</span>
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A<span class="br0">(</span><span class="kw4">const</span> A<span class="sy3">&</span> o<span class="br0">)</span> <span class="sy4">:</span> s<span class="br0">(</span>o.<span class="me1">s</span><span class="br0">)</span> <span class="br0">{</span> <a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">" copy constructed<span class="es1">\n</span>"</span><span class="sy4">;</span> <span class="br0">}</span>
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A<span class="br0">(</span>A<span class="sy3">&&</span> o<span class="br0">)</span> <span class="sy4">:</span> s<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>o.<span class="me1">s</span><span class="br0">)</span><span class="br0">)</span> <span class="br0">{</span> <a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">" move constructed<span class="es1">\n</span>"</span><span class="sy4">;</span> <span class="br0">}</span>
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A<span class="sy3">&</span> operator<span class="sy1">=</span><span class="br0">(</span><span class="kw4">const</span> A<span class="sy3">&</span> other<span class="br0">)</span> <span class="br0">{</span>
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s <span class="sy1">=</span> other.<span class="me1">s</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">" copy assigned<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="kw1">return</span> <span class="sy2">*</span>this<span class="sy4">;</span>
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<span class="br0">}</span>
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A<span class="sy3">&</span> operator<span class="sy1">=</span><span class="br0">(</span>A<span class="sy3">&&</span> other<span class="br0">)</span> <span class="br0">{</span>
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s <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>other.<span class="me1">s</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">" move assigned<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="kw1">return</span> <span class="sy2">*</span>this<span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
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<span class="br0">{</span>
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<a href="cpp-utility-optional.html"><span class="kw1075">std::<span class="me2">optional</span></span></a><span class="sy1"><</span>A<span class="sy1">></span> opt<span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"Assign:<span class="es1">\n</span>"</span><span class="sy4">;</span>
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opt <span class="sy1">=</span> A<span class="br0">(</span><span class="st0">"Lorem ipsum dolor sit amet, consectetur adipiscing elit nec."</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"Emplace:<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="co1">// 由于 opt 含值，这亦将销毁该值</span>
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opt.<span class="me1">emplace</span><span class="br0">(</span><span class="st0">"Lorem ipsum dolor sit amet, consectetur efficitur. "</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"End example<span class="es1">\n</span>"</span><span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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</div>
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<p>输出：</p>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="text source-text">
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<pre class="de1">Assign:
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constructed
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move constructed
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destructed
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Emplace:
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destructed
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constructed
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End example
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destructed</pre></div>
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<h3><span class="mw-headline" id=".E7.BC.BA.E9.99.B7.E6.8A.A5.E5.91.8A">缺陷报告</span></h3>
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<p>下列更改行为的缺陷报告追溯地应用于以前出版的 C++ 标准。</p>
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<table class="dsctable" style="font-size:0.8em">
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<tr>
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<th>DR</th>
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<th>应用于</th>
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<th>出版时的行为</th>
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<th>正确行为</th>
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</tr>
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<tr>
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<td><a rel="nofollow" class="external text" href="https://wg21.link/P2231R1">P2231R1</a></td>
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<td>C++20</td>
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<td><code>emplace</code> 不是 constexpr ，而要求的操作在 C++20 中能为 constexpr</td>
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<td>使之为 constexpr</td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
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<table class="t-dsc-begin">
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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=.html"><span class="t-lines"><span>operator=</span></span></a></div>
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</div>
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</td>
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<td>对内容赋值<br />
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<span class="t-mark">(公开成员函数)</span></td>
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</tr>
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</table>
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</div>
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<div class="visualClear"></div>
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