276 lines
38 KiB
HTML
276 lines
38 KiB
HTML
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<title>std::variant<Types...>::operator=</title>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::variant<Types...>::</span>operator=</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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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> variant<span class="sy3">&</span> operator<span class="sy1">=</span><span class="br0">(</span> <span class="kw4">const</span> variant<span class="sy3">&</span> rhs <span class="br0">)</span><span class="sy4">;</span></span></div>
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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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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> variant<span class="sy3">&</span> operator<span class="sy1">=</span><span class="br0">(</span> variant<span class="sy3">&&</span> rhs <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>(2)</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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<td></td>
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<td rowspan="142">(3)</td>
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<td></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="sy1">></span><br />
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variant<span class="sy3">&</span> operator<span class="sy1">=</span><span class="br0">(</span> T<span class="sy3">&&</span> t <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 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="sy1">></span><br />
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<span class="kw4">constexpr</span> variant<span class="sy3">&</span> operator<span class="sy1">=</span><span class="br0">(</span> T<span class="sy3">&&</span> t <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 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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<p>给既存的 <code>variant</code> 对象赋新值。</p>
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<div class="t-li1"><span class="t-li">1)</span> 复制赋值：
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<ul>
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<li>若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 与 <code>rhs</code> 均因异常无值，则不做任何事。</li>
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<li>否则，若 <code>rhs</code> 无值，但 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 非无值，则销毁 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 中所含值并使之无值。</li>
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<li>否则，若 <code>rhs</code> 保有同 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 的可选项，则赋值 <code>rhs</code> 中含有的值给 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 。若抛出异常，则 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 不变为无值：值取决于该可选项的复制赋值的异常安全保证。</li>
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<li>否则，若 <code>rhs</code> 的可选项为 nothrow 可复制构造或<i>非</i> nothrow 可移动构造（分别由 <span class="t-lc"><a href="cpp-types-is_copy_constructible.html">std::is_nothrow_copy_constructible</a></span> 和 <span class="t-lc"><a href="cpp-types-is_move_constructible.html">std::is_nothrow_move_constructible</a></span> 确定），则等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp">this<span class="sy2">-</span><span class="sy1">></span>emplace<span class="sy1"><</span>rhs.<span class="me1">index</span><span class="br0">(</span><span class="br0">)</span><span class="sy1">></span><span class="br0">(</span>get<span class="sy1"><</span>rhs.<span class="me1">index</span><span class="br0">(</span><span class="br0">)</span><span class="sy1">></span><span class="br0">(</span>rhs<span class="br0">)</span><span class="br0">)</span></span></span> 。若 <a href="cpp-utility-variant-emplace.html"><code>emplace</code></a> 内的复制构造抛异常，则 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 可能变为因异常无值（ <a href="cpp-utility-variant-valueless_by_exception.html"><code>valueless_by_exception</code></a> ）。</li>
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<li>否则，等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp">this<span class="sy2">-</span><span class="sy1">></span>operator<span class="sy1">=</span><span class="br0">(</span>variant<span class="br0">(</span>rhs<span class="br0">)</span><span class="br0">)</span></span></span> 。</li>
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</ul>
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</div>
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<div class="t-li1"><span class="t-li"></span> 定义此重载为被删除，除非 <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_i<span class="sy1">></span></span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_copy_assignable.html"><span class="kw551">std::<span class="me2">is_copy_assignable_v</span></span></a><span class="sy1"><</span>T_i<span class="sy1">></span></span></span> 对 <code>Types...</code> 中所有 <code>T_i</code> 均为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 。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_i<span class="sy1">></span></span></span> 、 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_copy_assignable.html"><span class="kw553">std::<span class="me2">is_trivially_copy_assignable_v</span></span></a><span class="sy1"><</span>T_i<span class="sy1">></span></span></span> 及 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_destructible.html"><span class="kw565">std::<span class="me2">is_trivially_destructible_v</span></span></a><span class="sy1"><</span>T_i<span class="sy1">></span></span></span> 对 <code>Types...</code> 中所有 <code>T_i</code> 均为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 则此重载为平凡。</div>
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<div class="t-li1"><span class="t-li">2)</span> 移动赋值：
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<ul>
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<li>若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 与 <code>rhs</code> 均因异常无值，则不做任何事</li>
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<li>否则，若 <code>rhs</code> 无值，但 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 非无值，则销毁 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 中所含值并使之无值</li>
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<li>否则，若 <code>rhs</code> 保有与 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 相同的可选项，则赋值 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">get</span><span class="sy1"><</span>j<span class="sy1">></span><span class="br0">(</span>std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>rhs<span class="br0">)</span><span class="br0">)</span></span></span> 给中所含值 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> ，其中 <code>j</code> 为 <code>index()</code> 。若抛出异常，则 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 不变为无值：值依赖于可该选项的移动赋值的异常安全保证。</li>
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<li>否则（若 <code>rhs</code> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 保有不同可选项），等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp">this<span class="sy2">-</span><span class="sy1">></span>emplace<span class="sy1"><</span>rhs.<span class="me1">index</span><span class="br0">(</span><span class="br0">)</span><span class="sy1">></span><span class="br0">(</span>get<span class="sy1"><</span>rhs.<span class="me1">index</span><span class="br0">(</span><span class="br0">)</span><span class="sy1">></span><span class="br0">(</span>std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>rhs<span class="br0">)</span><span class="br0">)</span><span class="br0">)</span></span></span> 。若 <code>T_i</code> 的移动构造函数抛出异常，则 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 变为<a href="cpp-utility-variant-valueless_by_exception.html">因异常无值</a>。</li>
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</ul>
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</div>
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<div class="t-li1"><span class="t-li"></span> 此重载仅若 <code>std::is_move_constructible_v<T_i></code> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_move_assignable.html"><span class="kw557">std::<span class="me2">is_move_assignable_v</span></span></a><span class="sy1"><</span>T_i<span class="sy1">></span></span></span> 对 <code>Types...</code> 中所有 <code>T_i</code> 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 <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_i<span class="sy1">></span></span></span> 、 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_move_assignable.html"><span class="kw559">std::<span class="me2">is_trivially_move_assignable_v</span></span></a><span class="sy1"><</span>T_i<span class="sy1">></span></span></span> 及 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_destructible.html"><span class="kw565">std::<span class="me2">is_trivially_destructible_v</span></span></a><span class="sy1"><</span>T_i<span class="sy1">></span></span></span> 对 <code>Types...</code> 中所有 <code>T_i</code> 均为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 则此重载为平凡。</div>
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<div class="t-li1"><span class="t-li">3)</span> 转换赋值：
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<ul>
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<li>确定若有对每个来自 <code>Types...</code> 的 <code>T_i</code> 的虚构重载函数 <span class="t-c"><span class="mw-geshi cpp source-cpp">F<span class="br0">(</span>T_i<span class="br0">)</span></span></span> 同时在作用域中，则重载决议是否为表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp">F<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>T<span class="sy1">></span><span class="br0">(</span>t<span class="br0">)</span><span class="br0">)</span></span></span> 选择可选项类型 <code>T_j</code> ，除了：</li>
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<dl>
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<dd>
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<ul>
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<li>仅若声明 <span class="t-c"><span class="mw-geshi cpp source-cpp">T_i x<span class="br0">[</span><span class="br0">]</span> <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>T<span class="sy1">></span><span class="br0">(</span>t<span class="br0">)</span> <span class="br0">}</span><span class="sy4">;</span></span></span> 对某个虚设变量 <code>x</code> 合法才考虑 <span class="t-c"><span class="mw-geshi cpp source-cpp">F<span class="br0">(</span>T_i<span class="br0">)</span></span></span> ；</li>
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<li>若 <code>T_i</code> 是（可有 cv 限定的） <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span></span></span> ，则仅若 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">:</span>remove_cvref_t<span class="sy1"><</span>T<span class="sy1">></span></span></span> 亦为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span></span></span> 才考虑 <span class="t-c"><span class="mw-geshi cpp source-cpp">F<span class="br0">(</span>T_i<span class="br0">)</span></span></span> 。</li>
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</ul>
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</dd>
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</dl>
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<ul>
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<li>若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 已保有 <code>T_j</code> ，则赋值 <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>T<span class="sy1">></span><span class="br0">(</span>t<span class="br0">)</span></span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 中所含值。若抛出异常，则 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 不变为无值：值取决于被调用赋值的异常安全保证。</li>
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<li>否则，若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_constructible.html"><span class="kw525">std::<span class="me2">is_nothrow_constructible_v</span></span></a><span class="sy1"><</span>T_j, T<span class="sy1">></span> <span class="sy3">||</span> <span class="sy3">!</span><a href="cpp-types-is_move_constructible.html"><span class="kw543">std::<span class="me2">is_nothrow_move_constructible_v</span></span></a><span class="sy1"><</span>T_j<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> ，则等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp">this<span class="sy2">-</span><span class="sy1">></span>emplace<span class="sy1"><</span>j<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>T<span class="sy1">></span><span class="br0">(</span>t<span class="br0">)</span><span class="br0">)</span></span></span> 。若 <a href="cpp-utility-variant-emplace.html"><code>emplace</code></a> 内的初始化抛异常，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>。</li>
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<li>否则，等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp">this<span class="sy2">-</span><span class="sy1">></span>emplace<span class="sy1"><</span>j<span class="sy1">></span><span class="br0">(</span>T_j<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>T<span class="sy1">></span><span class="br0">(</span>t<span class="br0">)</span><span class="br0">)</span><span class="br0">)</span></span></span> 。</li>
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</ul>
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</div>
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<div class="t-li1"><span class="t-li"></span> 此重载仅若 <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>T<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>T<span class="sy1">></span></span></span></span> <span><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></span></span> 不是与 <span class="t-c"><span class="mw-geshi cpp source-cpp">variant</span></span> 相同的类型，且 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-is_assignable.html"><span class="kw545">std::<span class="me2">is_assignable_v</span></span></a><span class="sy1"><</span>T_j<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">true</span></span></span> 且 <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_j, 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> 而且表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp">F<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>T<span class="sy1">></span><span class="br0">(</span>t<span class="br0">)</span><span class="br0">)</span></span></span> （其中 F 为上述虚构函数集）为良构才参与重载决议。
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<div class="cpp source-cpp">
|
|
<pre class="de1"><a href="cpp-utility-variant.html"><span class="kw1085">std::<span class="me2">variant</span></span></a><span class="sy1"><</span>string<span class="sy1">></span> v1<span class="sy4">;</span>
|
|
v1 <span class="sy1">=</span> <span class="st0">"abc"</span><span class="sy4">;</span> <span class="co1">// OK</span>
|
|
<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>
|
|
v2 <span class="sy1">=</span> <span class="st0">"abc"</span><span class="sy4">;</span> <span class="co1">// 错误</span>
|
|
<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="kw4">bool</span><span class="sy1">></span> v3<span class="sy4">;</span>
|
|
v3 <span class="sy1">=</span> <span class="st0">"abc"</span><span class="sy4">;</span> <span class="co1">// OK ：选择 string ； bool 不是候选</span>
|
|
<a href="cpp-utility-variant.html"><span class="kw1085">std::<span class="me2">variant</span></span></a><span class="sy1"><</span><span class="kw4">float</span>, <span class="kw4">long</span>, <span class="kw4">double</span><span class="sy1">></span> v4<span class="sy4">;</span> <span class="co1">// 保有 float</span>
|
|
v4 <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> <span class="co1">// OK ：保有 long ； float 与 double 不是候选</span></pre></div>
|
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</div>
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</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>rhs</td>
|
|
<td>-</td>
|
|
<td>另一 <code>variant</code></td>
|
|
</tr>
|
|
<tr class="t-par">
|
|
<td>t</td>
|
|
<td>-</td>
|
|
<td>可转换成 <code>variant</code> 的可选项之一的值</td>
|
|
</tr>
|
|
</table>
|
|
<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">返回值</span></h3>
|
|
<p><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span></p>
|
|
<h3><span class="mw-headline" id=".E5.BC.82.E5.B8.B8">异常</span></h3>
|
|
<div class="t-li1"><span class="t-li">1)</span> 可能抛出任何可选项的赋值及复制/移动初始化所抛的异常。</div>
|
|
<div class="t-li1"><span class="t-li">2)</span>
|
|
<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><span class="br0">(</span><span class="br0">(</span><a href="cpp-types-is_move_constructible.html"><span class="kw543">std::<span class="me2">is_nothrow_move_constructible_v</span></span></a><span class="sy1"><</span>Types<span class="sy1">></span> <span class="sy3">&&</span><br />
|
|
<a href="cpp-types-is_move_assignable.html"><span class="kw561">std::<span class="me2">is_nothrow_move_assignable_v</span></span></a><span class="sy1"><</span>Types<span class="sy1">></span><span class="br0">)</span> <span class="sy3">&&</span> ...<span class="br0">)</span><span class="br0">)</span></span></span></div>
|
|
</div>
|
|
</div>
|
|
<div class="t-li1"><span class="t-li">3)</span>
|
|
<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_assignable.html"><span class="kw549">std::<span class="me2">is_nothrow_assignable_v</span></span></a><span class="sy1"><</span>T_j<span class="sy3">&</span>, T<span class="sy1">></span> <span class="sy3">&&</span> <a href="cpp-types-is_constructible.html"><span class="kw525">std::<span class="me2">is_nothrow_constructible_v</span></span></a><span class="sy1"><</span>T_j, T<span class="sy1">></span><span class="br0">)</span></span></span></div>
|
|
</div>
|
|
</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 <iomanip></span>
|
|
<span class="co2">#include <iostream></span>
|
|
<span class="co2">#include <string></span>
|
|
<span class="co2">#include <type_traits></span>
|
|
<span class="co2">#include <variant></span>
|
|
|
|
<a href="cpp-io-basic_ostream.html"><span class="kw1729">std::<span class="me2">ostream</span></span></a><span class="sy3">&</span> operator<span class="sy1"><<</span><span class="br0">(</span><a href="cpp-io-basic_ostream.html"><span class="kw1729">std::<span class="me2">ostream</span></span></a><span class="sy3">&</span> os, <a href="cpp-utility-variant.html"><span class="kw1085">std::<span class="me2">variant</span></span></a><span class="sy1"><</span><span class="kw4">int</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="kw4">const</span><span class="sy3">&</span> va<span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
os <span class="sy1"><<</span> <span class="st0">": { "</span><span class="sy4">;</span>
|
|
|
|
<a href="cpp-utility-variant-visit.html"><span class="kw1086">std::<span class="me2">visit</span></span></a><span class="br0">(</span><span class="br0">[</span><span class="sy3">&</span><span class="br0">]</span><span class="br0">(</span><span class="kw4">auto</span><span class="sy3">&&</span> arg<span class="br0">)</span> <span class="br0">{</span>
|
|
<span class="kw1">using</span> T <span class="sy1">=</span> <a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1"><</span>decltype<span class="br0">(</span>arg<span class="br0">)</span><span class="sy1">></span><span class="sy4">;</span>
|
|
<span class="kw1">if</span> <span class="kw4">constexpr</span> <span class="br0">(</span><a href="cpp-types-is_same.html"><span class="kw589">std::<span class="me2">is_same_v</span></span></a><span class="sy1"><</span>T, <span class="kw4">int</span><span class="sy1">></span><span class="br0">)</span>
|
|
os <span class="sy1"><<</span> arg<span class="sy4">;</span>
|
|
<span class="kw1">else</span> <span class="kw1">if</span> <span class="kw4">constexpr</span> <span class="br0">(</span><a href="cpp-types-is_same.html"><span class="kw589">std::<span class="me2">is_same_v</span></span></a><span class="sy1"><</span>T, <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>
|
|
os <span class="sy1"><<</span> <a href="cpp-io-manip-quoted.html"><span class="kw1802">std::<span class="me2">quoted</span></span></a><span class="br0">(</span>arg<span class="br0">)</span><span class="sy4">;</span>
|
|
<span class="br0">}</span>, va<span class="br0">)</span><span class="sy4">;</span>
|
|
|
|
<span class="kw1">return</span> os <span class="sy1"><<</span> <span class="st0">" };<span class="es1">\n</span>"</span><span class="sy4">;</span>
|
|
<span class="br0">}</span>
|
|
|
|
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
<a href="cpp-utility-variant.html"><span class="kw1085">std::<span class="me2">variant</span></span></a><span class="sy1"><</span><span class="kw4">int</span>, <a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a><span class="sy1">></span> a<span class="br0">{</span><span class="nu0">2017</span><span class="br0">}</span>, b<span class="br0">{</span><span class="st0">"CppCon"</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="st0">"a"</span> <span class="sy1"><<</span> a <span class="sy1"><<</span> <span class="st0">"b"</span> <span class="sy1"><<</span> b <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</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="st0">"(1) operator=( const variant& rhs )<span class="es1">\n</span>"</span><span class="sy4">;</span>
|
|
a <span class="sy1">=</span> b<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="st0">"a"</span> <span class="sy1"><<</span> a <span class="sy1"><<</span> <span class="st0">"b"</span> <span class="sy1"><<</span> b <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</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="st0">"(2) operator=( variant&& rhs )<span class="es1">\n</span>"</span><span class="sy4">;</span>
|
|
a <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>b<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="st0">"a"</span> <span class="sy1"><<</span> a <span class="sy1"><<</span> <span class="st0">"b"</span> <span class="sy1"><<</span> b <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</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="st0">"(3) operator=( T&& t ), where T is int<span class="es1">\n</span>"</span><span class="sy4">;</span>
|
|
a <span class="sy1">=</span> <span class="nu0">2019</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="st0">"a"</span> <span class="sy1"><<</span> a <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</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="st0">"(3) operator=( T&& t ), where T is std::string<span class="es1">\n</span>"</span><span class="sy4">;</span>
|
|
<a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a> s<span class="br0">{</span><span class="st0">"CppNow"</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="st0">"s: "</span> <span class="sy1"><<</span> <a href="cpp-io-manip-quoted.html"><span class="kw1802">std::<span class="me2">quoted</span></span></a><span class="br0">(</span>s<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
|
|
a <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">move</span><span class="br0">(</span>s<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="st0">"a"</span> <span class="sy1"><<</span> a <span class="sy1"><<</span> <span class="st0">"s: "</span> <span class="sy1"><<</span> <a href="cpp-io-manip-quoted.html"><span class="kw1802">std::<span class="me2">quoted</span></span></a><span class="br0">(</span>s<span class="br0">)</span> <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>可能的输出：</p>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="text source-text">
|
|
<pre class="de1">a: { 2017 };
|
|
b: { "CppCon" };
|
|
|
|
(1) operator=( const variant& rhs )
|
|
a: { "CppCon" };
|
|
b: { "CppCon" };
|
|
|
|
(2) operator=( variant&& rhs )
|
|
a: { "CppCon" };
|
|
b: { "" };
|
|
|
|
(3) operator=( T&& t ), where T is int
|
|
a: { 2019 };
|
|
|
|
(3) operator=( T&& t ), where T is std::string
|
|
s: "CppNow"
|
|
a: { "CppNow" };
|
|
s: ""</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://cplusplus.github.io/LWG/issue3024">LWG 3024</a></td>
|
|
<td>C++17</td>
|
|
<td>若任何成员类型不可复制则复制赋值运算符不参与重载决议</td>
|
|
<td>改为定义为被删除</td>
|
|
</tr>
|
|
<tr>
|
|
<td><a rel="nofollow" class="external text" href="https://wg21.link/P0602R4">P0602R4</a></td>
|
|
<td>C++17</td>
|
|
<td>即使底层操作平凡，复制/移动运算符亦可能不平凡</td>
|
|
<td>要求传播平凡性</td>
|
|
</tr>
|
|
<tr>
|
|
<td><a rel="nofollow" class="external text" href="https://wg21.link/P0608R3">P0608R3</a></td>
|
|
<td>C++17</td>
|
|
<td>转换赋值盲目地组成重载集，导致不想要的转换</td>
|
|
<td>不考虑窄化和布尔转换</td>
|
|
</tr>
|
|
<tr>
|
|
<td><a rel="nofollow" class="external text" href="https://wg21.link/P2231R1">P2231R1</a></td>
|
|
<td>C++20</td>
|
|
<td>转换赋值不是 constexpr 而要求的操作在 C++20 中可以是</td>
|
|
<td>使之为 constexpr</td>
|
|
</tr>
|
|
<tr>
|
|
<td><a rel="nofollow" class="external text" href="https://cplusplus.github.io/LWG/issue3585">LWG 3585</a></td>
|
|
<td>C++17</td>
|
|
<td>转换赋值有时意外地非良构，因为无可用的移动赋值</td>
|
|
<td>使之良构</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-variant-emplace.html"><span class="t-lines"><span>emplace</span></span></a></div>
|
|
</div>
|
|
</td>
|
|
<td>原位构造 <code>variant</code> 中的值<br />
|
|
<span class="t-mark">(公开成员函数)</span></td>
|
|
</tr>
|
|
</table>
|
|
</div>
|
|
<div class="visualClear"></div>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
</body>
|
|
</html>
|