215 lines
21 KiB
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215 lines
21 KiB
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<title>std::variant<Types...>::emplace</title>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::variant<Types...>::</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-variant.html">variant</a></span></div>
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<td rowspan="142">(1)</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> T, <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 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> T, <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> T, <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> il, 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 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> T, <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> il, 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">(3)</td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span> <span class="sy1"><</span><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> I, <span class="kw1">class</span>... <span class="me1">Args</span><span class="sy1">></span><br />
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<a href="cpp-utility-variant-variant_alternative.html"><span class="kw1094">std::<span class="me2">variant_alternative_t</span></span></a><span class="sy1"><</span>I, variant<span class="sy1">></span><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-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><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> I, <span class="kw1">class</span>... <span class="me1">Args</span><span class="sy1">></span><br />
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<span class="kw4">constexpr</span> <a href="cpp-utility-variant-variant_alternative.html"><span class="kw1094">std::<span class="me2">variant_alternative_t</span></span></a><span class="sy1"><</span>I, variant<span class="sy1">></span><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">(4)</td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span> <span class="sy1"><</span><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> I, <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><a href="cpp-utility-variant-variant_alternative.html"><span class="kw1094">std::<span class="me2">variant_alternative_t</span></span></a><span class="sy1"><</span>I, variant<span class="sy1">></span><span class="sy3">&</span><br /></p>
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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> il, Args<span class="sy3">&&</span>... <span class="me1">args</span> <span class="br0">)</span><span class="sy4">;</span></div>
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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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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span> <span class="sy1"><</span><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> I, <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> <a href="cpp-utility-variant-variant_alternative.html"><span class="kw1094">std::<span class="me2">variant_alternative_t</span></span></a><span class="sy1"><</span>I, variant<span class="sy1">></span><span class="sy3">&</span><br /></p>
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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> il, Args<span class="sy3">&&</span>... <span class="me1">args</span> <span class="br0">)</span><span class="sy4">;</span></div>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<p>在既存的 <code>variant</code> 对象中原位构造新值。</p>
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<div class="t-li1"><span class="t-li">1)</span> 等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp">emplace<span class="sy1"><</span>I<span class="sy1">></span><span class="br0">(</span><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 class="br0">)</span></span></span> ，其中 <code>I</code> 是 <code>T</code> 在 <code>Types...</code> 中的零基下标。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="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> ，且 <code>T</code> 在 <code>Types...</code> 中准确出现一次才参与重载决议。</div>
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<div class="t-li1"><span class="t-li">2)</span> 等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp">emplace<span class="sy1"><</span>I<span class="sy1">></span><span class="br0">(</span>il, <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 class="br0">)</span></span></span> ，其中 <code>I</code> 是 <code>T</code> 在 <code>Types...</code> 中的零基下标。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="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="sy3">&</span><span class="sy1"><</span>U<span class="sy1">></span><span class="sy3">&</span>, 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> ，且 <code>T</code> 在 <code>Types...</code> 中准确出现一次才参与重载决议。</div>
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<div class="t-li1"><span class="t-li">3)</span> 首先，销毁当前所含值（若存在）。然后如同以参数 <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> 构造 <code>T_I</code> 类型值一般<a href="cpp-language-direct_initialization.html">直接初始化</a>所含值。若抛出异常，则 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 可能变为<a href="cpp-utility-variant-valueless_by_exception.html">因异常无值</a>。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="kw521">std::<span class="me2">is_constructible_v</span></span></a><span class="sy1"><</span>T_I, 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。若 <code>I</code> 不小于 <span class="t-c"><span class="mw-geshi cpp source-cpp">sizeof...<span class="br0">(</span>Types<span class="br0">)</span></span></span> 则行为未定义。</div>
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<div class="t-li1"><span class="t-li">4)</span> 首先，销毁当前所含值（若存在）。然后如同以参数 <span class="t-c"><span class="mw-geshi cpp source-cpp">il, <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> 构造 <code>T_I</code> 类型值一般<a href="cpp-language-direct_initialization.html">直接初始化</a>所含值。若抛出异常，则 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 可能变为<a href="cpp-utility-variant-valueless_by_exception.html">因异常无值</a>。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="kw521">std::<span class="me2">is_constructible_v</span></span></a><span class="sy1"><</span>T_I, <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="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。若 <code>I</code> 不小于 <span class="t-c"><span class="mw-geshi cpp source-cpp">sizeof...<span class="br0">(</span>Types<span class="br0">)</span></span></span> 则行为未定义。</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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<tr class="t-par">
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<td>args</td>
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<td>-</td>
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<td>构造新值时使用的构造函数参数</td>
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</tr>
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<tr class="t-par">
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<td>il</td>
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<td>-</td>
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<td>构造新值时使用的 <code>initializer_list</code> 参数</td>
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</tr>
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</table>
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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>到新的所含值的引用。</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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<div class="t-li1"><span class="t-li">1-4)</span> 任何初始化所含值期间可能抛出的异常。</div>
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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 <iostream></span>
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<span class="co2">#include <string></span>
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<span class="co2">#include <variant></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-variant.html"><span class="kw1085">std::<span class="me2">variant</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> v1<span class="sy4">;</span>
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v1.<span class="me1">emplace</span><span class="sy1"><</span><span class="nu0">0</span><span class="sy1">></span><span class="br0">(</span><span class="st0">"abc"</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK</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> std<span class="sy4">::</span><span class="me2">get</span><span class="sy1"><</span><span class="nu0">0</span><span class="sy1">></span><span class="br0">(</span>v1<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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v1.<span class="me1">emplace</span><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><span class="br0">(</span><span class="st0">"def"</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK</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> std<span class="sy4">::</span><span class="me2">get</span><span class="sy1"><</span><span class="nu0">0</span><span class="sy1">></span><span class="br0">(</span>v1<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<a href="cpp-utility-variant.html"><span class="kw1085">std::<span class="me2">variant</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>, <a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a><span class="sy1">></span> v2<span class="sy4">;</span>
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v2.<span class="me1">emplace</span><span class="sy1"><</span><span class="nu0">1</span><span class="sy1">></span><span class="br0">(</span><span class="st0">"ghi"</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK</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> std<span class="sy4">::</span><span class="me2">get</span><span class="sy1"><</span><span class="nu0">1</span><span class="sy1">></span><span class="br0">(</span>v2<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<span class="co1">// v2.emplace<std::string>("abc"); -> 错误</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">
|
|
<pre class="de1">abc
|
|
def
|
|
ghi</pre></div>
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</div>
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</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>
|
|
<th>DR</th>
|
|
<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-variant-operator=.html"><span class="t-lines"><span>operator=</span></span></a></div>
|
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</div>
|
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</td>
|
|
<td>赋值 <code>variant</code><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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</div>
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</div>
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</div>
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</body>
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</html>
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