269 lines
34 KiB
HTML
269 lines
34 KiB
HTML
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<title>std::optional<T>::operator=</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_optional_operator 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>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-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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<td rowspan="142">(1)</td>
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<div><span class="mw-geshi cpp source-cpp">optional<span class="sy3">&</span> operator<span class="sy1">=</span><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 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="kw4">constexpr</span> optional<span class="sy3">&</span> operator<span class="sy1">=</span><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 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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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> optional<span class="sy3">&</span> operator<span class="sy1">=</span><span class="br0">(</span> <span class="kw4">const</span> optional<span class="sy3">&</span> other <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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<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="sy3">&</span> operator<span class="sy1">=</span><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>(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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<td rowspan="142">(4)</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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<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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optional<span class="sy3">&</span> operator<span class="sy1">=</span><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 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> <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="sy3">&</span> operator<span class="sy1">=</span><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 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></tbody>
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<tbody class="t-dcl-rev t-dcl-rev-num">
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<td rowspan="142">(5)</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> U <span class="sy1">></span><br />
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optional<span class="sy3">&</span> operator<span class="sy1">=</span><span class="br0">(</span> <span class="kw4">const</span> optional<span class="sy1"><</span>U<span class="sy1">></span><span class="sy3">&</span> 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></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="sy3">&</span> operator<span class="sy1">=</span><span class="br0">(</span> <span class="kw4">const</span> optional<span class="sy1"><</span>U<span class="sy1">></span><span class="sy3">&</span> other <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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</tbody>
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<tbody></tbody>
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<tbody class="t-dcl-rev t-dcl-rev-num">
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<td rowspan="142">(6)</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> U <span class="sy1">></span><br />
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optional<span class="sy3">&</span> operator<span class="sy1">=</span><span class="br0">(</span> optional<span class="sy1"><</span>U<span class="sy1">></span><span class="sy3">&&</span> 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></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="sy3">&</span> operator<span class="sy1">=</span><span class="br0">(</span> optional<span class="sy1"><</span>U<span class="sy1">></span><span class="sy3">&&</span> 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></td>
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</tr>
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</tbody>
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<tbody>
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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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<p>以 <code>other</code> 的内容替换 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 的内容。</p>
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<div class="t-li1"><span class="t-li">1)</span> 若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 在调用前含值，则通过调用其析构函数销毁所含值，如同用 <span class="t-c"><span class="mw-geshi cpp source-cpp">value<span class="br0">(</span><span class="br0">)</span>.<span class="me1">T</span><span class="sy4">::</span>~T<span class="br0">(</span><span class="br0">)</span></span></span> 。此调用后 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 不含值。</div>
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<div class="t-li1"><span class="t-li">2-3)</span> 赋值 <code>other</code> 的状态。<br />
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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>other</code> 均不含值，则函数无效果。</li>
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<li>若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 含值，但 <code>other</code> 不含值，则通过调用其析构函数销毁被含有值。 <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。</li>
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<li>若 <code>other</code> 含值，则取决于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 是否含值，以 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>other</span></span><span class="t-v">(2)</span> 或 <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><span class="t-v">(3)</span> 直接初始化<a href="cpp-language-direct_initialization.html">直接初始化</a>或赋值被含有值。注意被移动的 <code>optional</code> 仍然<i>含值</i>。</li>
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<li>重载 <span class="t-v">(2)</span> 定义为被删除，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<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<span class="sy1">></span></span></span> 皆为 <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<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<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<span class="sy1">></span></span></span> 皆为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 则它为平凡。</li>
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<li>重载 <span class="t-v">(3)</span> 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> 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<span class="sy1">></span></span></span> 皆为 <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_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> 、 <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<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<span class="sy1">></span></span></span> 皆为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 则它为平凡。</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> 完美转发赋值：取决于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 在调用前是否含值，从 <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> 直接初始化，或从 <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> 赋值被含有值。该函数不参与重载决议，除非 <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> 不是 <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> ， <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="sy1">></span></span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> ， <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<span class="sy3">&</span>, U<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> ，且至少下列一项为真：<br />
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<ul>
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<li><code>T</code> 不是<a href="cpp-language-type.html">标量类型</a>；</li>
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<li><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> 不是 <code>T</code> 。</li>
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</ul>
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</div>
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<div class="t-li1"><span class="t-li">5-6)</span> 赋值 <code>other</code> 的状态。<br />
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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>other</code> 皆不含值，则函数无效果。</li>
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<li>若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 容纳值，但 <code>other</code> 不含值，则通过调用其析构函数销毁被含有值。 <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。</li>
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<li>若 <code>other</code> 含值，则取决于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 是否含值，以 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>other</span></span><span class="t-v">(5)</span> 或 <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><span class="t-v">(6)</span> <a href="cpp-language-direct_initialization.html">直接初始化</a>或赋值被含有值。注意被移动的 <code>optional</code> 仍然<i>含值</i>。</li>
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<li>这些重载不参与重载决议，除非满足下列条件：
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<ul>
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<li><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> 类型（可以为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span></span></span> ）的表达式构造、转换、或赋值，即下列 12 个类型特性全为 <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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<li><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<span class="sy3">&</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_assignable.html"><span class="kw545">std::<span class="me2">is_assignable_v</span></span></a><span class="sy1"><</span>T<span class="sy3">&</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><span class="sy1">></span></span></span></li>
|
|
<li><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<span class="sy3">&</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_assignable.html"><span class="kw545">std::<span class="me2">is_assignable_v</span></span></a><span class="sy1"><</span>T<span class="sy3">&</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><span class="sy1">></span></span></span>.</li>
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|
</ul>
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|
</li>
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|
<li>对于重载 <span class="t-v">(5)</span> ， <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> 与 <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<span class="sy3">&</span>, <span class="kw4">const</span> U<span class="sy3">&</span><span class="sy1">></span></span></span> 皆为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 。</li>
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|
<li>对于重载 <span class="t-v">(6)</span> ， <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="sy1">></span></span></span> 与 <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<span class="sy3">&</span>, U<span class="sy1">></span></span></span> 皆为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 。</li>
|
|
</ul>
|
|
</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>要赋值给被含有值的 <code>optional</code> 对象</td>
|
|
</tr>
|
|
<tr class="t-par">
|
|
<td>value</td>
|
|
<td>-</td>
|
|
<td>要赋值给被含有值的值</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">2-6)</span> 抛出任何 <code>T</code> 的赋值运算符所抛的异常。若抛出异常，则 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> （还有情况 <span class="t-v">(2-3)</span> 和 <span class="t-v">(5-6)</span> 的 <code>other</code> ）的初始化状态不改变，即若对象含值，则它仍然含值，反之亦然。 <code>value</code> 和 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span>this</span></span> 及 <code>other</code> 所含有的值的内容依赖于异常来源操作（复制构造函数、移动构造函数等）的异常安全保证。</div>
|
|
<span class="t-v">(3)</span> 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_assignable.html"><span class="kw561">std::<span class="me2">is_nothrow_move_assignable_v</span></span></a><span class="sy1"><</span>T<span class="sy1">></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<span class="sy1">></span><span class="br0">)</span></span></span></div>
|
|
</div>
|
|
<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
|
|
<p><code>optional</code> 对象 <code>op</code> 可以通过 <span class="t-c"><span class="mw-geshi cpp source-cpp">op <span class="sy1">=</span> <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span></span></span> 和 <span class="t-c"><span class="mw-geshi cpp source-cpp">op <span class="sy1">=</span> nullopt<span class="sy4">;</span></span></span> 变成空 <code>optional</code> 。第一个表达式以 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">{</span><span class="br0">}</span></span></span> 构造空的 <span class="t-c"><span class="mw-geshi cpp source-cpp">optional</span></span> 对象并将它赋值给 <code>op</code> 。</p>
|
|
<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="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">const</span> <span class="kw4">char</span><span class="sy2">*</span><span class="sy1">></span> s1 <span class="sy1">=</span> <span class="st0">"abc"</span>, s2<span class="sy4">;</span> <span class="co1">// 构造函数</span>
|
|
s2 <span class="sy1">=</span> s1<span class="sy4">;</span> <span class="co1">// 赋值</span>
|
|
s1 <span class="sy1">=</span> <span class="st0">"def"</span><span class="sy4">;</span> <span class="co1">// 衰变赋值（ U = char[4], T = const char* ）</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>s2 <span class="sy1"><<</span> <span class="st0">' '</span> <span class="sy1"><<</span> <span class="sy2">*</span>s1 <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">abc def</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>即使底层操作平凡，复制/移动赋值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/P2231R1">P2231R1</a></td>
|
|
<td>C++20</td>
|
|
<td>转换赋值运算符不是 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-emplace.html"><span class="t-lines"><span>emplace</span></span></a></div>
|
|
</div>
|
|
</td>
|
|
<td>原位构造所含值<br />
|
|
<span class="t-mark">(公开成员函数)</span></td>
|
|
</tr>
|
|
</table>
|
|
</div>
|
|
<div class="visualClear"></div>
|
|
</div>
|
|
</div>
|
|
</div>
|
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</body>
|
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</html>
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