148 lines
15 KiB
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148 lines
15 KiB
HTML
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<title>std::any::any</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_any_any 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::any::</span>any</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-any.html">any</a></span></div>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">constexpr</span> any<span class="br0">(</span><span class="br0">)</span> <span class="kw1">noexcept</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">any<span class="br0">(</span> <span class="kw4">const</span> any<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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<div><span class="mw-geshi cpp source-cpp">any<span class="br0">(</span> any<span class="sy3">&&</span> other <span class="br0">)</span> <span class="kw1">noexcept</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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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> <span class="kw1">class</span> ValueType <span class="sy1">></span><br />
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any<span class="br0">(</span> ValueType<span class="sy3">&&</span> value <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td>(4)</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="kw1">template</span><span class="sy1"><</span> <span class="kw1">class</span> ValueType, <span class="kw1">class</span>... <span class="me1">Args</span> <span class="sy1">></span><br />
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<span class="kw1">explicit</span> any<span class="br0">(</span> <a href="cpp-utility-in_place.html"><span class="kw1083">std::<span class="me2">in_place_type_t</span></span></a><span class="sy1"><</span>ValueType<span class="sy1">></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>(5)</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="kw1">template</span><span class="sy1"><</span> <span class="kw1">class</span> ValueType, <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="kw1">explicit</span> any<span class="br0">(</span> <a href="cpp-utility-in_place.html"><span class="kw1083">std::<span class="me2">in_place_type_t</span></span></a><span class="sy1"><</span>ValueType<span class="sy1">></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,<br /></p>
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Args<span class="sy3">&&</span>... <span class="me1">args</span> <span class="br0">)</span><span class="sy4">;</span></div>
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<td>(6)</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></td>
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<p>构造新的 <code>any</code> 对象。</p>
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<div class="t-li1"><span class="t-li">1)</span> 构造空对象。</div>
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<div class="t-li1"><span class="t-li">2-3)</span> 复制 <span class="t-v">(2)</span> 或移动 <span class="t-v">(3)</span> <code>other</code> 的内容进新实例，从而任何内容的类型和值都等于构造函数调用前的 <code>other</code> 所拥有者，或者若 <code>other</code> 为空则内容为空。正式地说，</div>
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<div class="t-li2"><span class="t-li">2)</span> 若 <code>other</code> 为空，则构造的对象为空。否则，等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp">any<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">in_place_type</span><span class="sy1"><</span>T<span class="sy1">></span>, <a href="cpp-utility-any-any_cast.html"><span class="kw1072">std::<span class="me2">any_cast</span></span></a><span class="sy1"><</span><span class="kw4">const</span> T<span class="sy3">&</span><span class="sy1">></span><span class="br0">(</span>other<span class="br0">)</span><span class="br0">)</span></span></span> ，其中 <code>T</code> 是 <code>other</code> 中含有的对象类型。</div>
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<div class="t-li2"><span class="t-li">3)</span> 若 <code>other</code> 为空，则构造的对象为空，否则，构造的对象含 <code>other</code> 中容纳的对象，或从 <code>other</code> 中的对象构造的同类型对象，考虑该对象是否为右值。</div>
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<div class="t-li1"><span class="t-li">4)</span> 构造对象，其初始内容为 <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>ValueType<span class="sy1">></span></span></span> 类型对象，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>ValueType<span class="sy1">></span><span class="br0">(</span>value<span class="br0">)</span></span></span> <a href="cpp-language-direct_initialization.html">直接初始化</a>它。
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<ul>
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<li>此重载仅若 <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>ValueType<span class="sy1">></span></span></span> 既非与 <span class="t-c"><span class="mw-geshi cpp source-cpp">any</span></span> 相同类型，亦非<span class="t-lc"><a href="cpp-utility-in_place.html">std::in_place_type_t</a></span> 的特化，且 <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><a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1"><</span>ValueType<span class="sy1">>></span></span></span> 为<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 才参与重载决议。</li>
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</ul>
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</div>
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<div class="t-li1"><span class="t-li">5)</span> 构造对象，其初始内容为 <code>std::decay_t<ValueType></code> 类型对象，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>Args<span class="sy1">></span><span class="br0">(</span>args<span class="br0">)</span>...</span></span> <a href="cpp-language-direct_initialization.html">直接非列表初始化</a>它。
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<ul>
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<li>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><a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1"><</span>ValueType<span class="sy1">></span>, Args...<span class="sy1">></span></span></span> 与 <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><a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1"><</span>ValueType<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> 才参与重载决议。</li>
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</ul>
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</div>
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<div class="t-li1"><span class="t-li">6)</span> 构造对象，其初始内容为 <code>std::decay_t<ValueType></code> 类型对象，从 <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> <a href="cpp-language-direct_initialization.html">直接非列表初始化</a>它。
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<ul>
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<li>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><a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1"><</span>ValueType<span class="sy1">></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><span class="sy3">&</span>, Args...<span class="sy1">></span></span></span> 与 <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><a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1"><</span>ValueType<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> 才参与重载决议。</li>
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</ul>
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<h3><span class="mw-headline" id=".E6.A8.A1.E6.9D.BF.E5.BD.A2.E5.8F.82">模板形参</span></h3>
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<td>ValueType</td>
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<td>-</td>
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<td>所含的值类型</td>
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<td colspan="3">类型要求</td>
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<tr class="t-par-req">
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<td colspan="3">-
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<div class="spacer"></div>
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<code>std::decay_t<ValueType></code> 必须满足<a href="cpp-named_req-CopyConstructible.html"><span style="font-style:italic">可复制构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(CopyConstructible)</span></a> 的要求。</td>
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<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
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<td>other</td>
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<td>-</td>
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<td>另一个用以复制或移动的 <code>any</code> 对象</td>
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<td>value</td>
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<td>-</td>
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<td>初始化所含的值</td>
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</tr>
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<tr class="t-par">
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<td>il, args</td>
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<td>-</td>
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<td>传递给所含对象的构造函数的参数</td>
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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">2,4,5,6)</span> 抛出任何所含类型的构造函数所抛出的异常</div>
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<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
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<p>默认构造函数为 <code>constexpr</code> ，故静态 <code>std::any</code> 在任何动态非局部初始化开始之前，作为<a href="cpp-language-initialization.html#.E9.9D.9E.E5.B1.80.E9.83.A8.E5.8F.98.E9.87.8F">静态非局部初始化</a>的一部分初始化。这使得在任何静态对象的构造函数中使用 <code>std::any</code> 对象是安全的。</p>
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<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
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<div class="t-dsc-member-div">
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<div><a href="cpp-utility-any-operator=.html"><span class="t-lines"><span>operator=</span></span></a></div>
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</div>
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</td>
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<td>赋值 <code>any</code> 对象<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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