106 lines
7.4 KiB
HTML
106 lines
7.4 KiB
HTML
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<title>std::in_place, std::in_place_type, std::in_place_index, std::in_place_t, std::in_place_type_t, std::in_place_index_t</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_in_place 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::</span>in_place, <span style="font-size:0.7em; line-height:130%">std::</span>in_place_type, <span style="font-size:0.7em; line-height:130%">std::</span>in_place_index, <span style="font-size:0.7em; line-height:130%">std::</span>in_place_t, <span style="font-size:0.7em; line-height:130%">std::</span>in_place_type_t, <span style="font-size:0.7em; line-height:130%">std::</span>in_place_index_t</h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-utility.html">utility</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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<div>定义于头文件 <code><a href="cpp-header-utility.html"><utility></a></code></div>
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<td></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">struct</span> in_place_t <span class="br0">{</span><br /></span>
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<p> <span class="kw1">explicit</span> in_place_t<span class="br0">(</span><span class="br0">)</span> <span class="sy1">=</span> <span class="kw1">default</span><span class="sy4">;</span><br />
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<span class="br0">}</span><span class="sy4">;</span><br /></p>
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<span class="kw1">inline</span> <span class="kw4">constexpr</span> in_place_t in_place<span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span></div>
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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> T<span class="sy1">></span> <span class="kw1">struct</span> in_place_type_t <span class="br0">{</span><br /></span>
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<p> <span class="kw1">explicit</span> in_place_type_t<span class="br0">(</span><span class="br0">)</span> <span class="sy1">=</span> <span class="kw1">default</span><span class="sy4">;</span><br />
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<span class="br0">}</span><span class="sy4">;</span><br />
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<span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span><br /></p>
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<span class="kw1">inline</span> <span class="kw4">constexpr</span> in_place_type_t<span class="sy1"><</span>T<span class="sy1">></span> in_place_type<span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span></div>
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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><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> I<span class="sy1">></span> <span class="kw1">struct</span> in_place_index_t <span class="br0">{</span><br /></span>
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<p> <span class="kw1">explicit</span> in_place_index_t<span class="br0">(</span><span class="br0">)</span> <span class="sy1">=</span> <span class="kw1">default</span><span class="sy4">;</span><br />
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<span class="br0">}</span><span class="sy4">;</span><br />
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<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="sy1">></span><br /></p>
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<span class="kw1">inline</span> <span class="kw4">constexpr</span> in_place_index_t<span class="sy1"><</span>I<span class="sy1">></span> in_place_index<span class="br0">{</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></td>
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<p><code>std::in_place</code> 、 <code>std::in_place_type</code> 和 <code>std::in_place_index</code> 是消歧义标签，能传递给 <span class="t-lc"><a href="cpp-utility-optional.html">std::optional</a></span> 、 <span class="t-lc"><a href="cpp-utility-variant.html">std::variant</a></span> 和 <span class="t-lc"><a href="cpp-utility-any.html">std::any</a></span> 的构造函数，以指示应该原位构造对象，以及（对于后二者）要构造的对象的类型。</p>
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<p>对应的类型/类型模板 <code>std::in_place_t</code> 、 <code>std::in_place_type_t</code> 和 <code>std::in_place_index_t</code> 能用于构造函数的参数列表中，以匹配有意的标签。</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-optional.html"><span class="t-lines"><span>optional</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div>
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</div>
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</td>
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<td>可能或可能不保有一个对象的包装器<br />
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<span class="t-mark">(类模板)</span></td>
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<div><a href="cpp-utility-variant.html"><span class="t-lines"><span>variant</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div>
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</div>
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</td>
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<td>类型安全的可辨识联合体<br />
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<span class="t-mark">(类模板)</span></td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-utility-any.html"><span class="t-lines"><span>any</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx17">(C++17)</span></span></span></div>
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</div>
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<td>可保有任何<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> 类型的实例的对象。<br />
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<span class="t-mark">(类)</span></td>
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</div>
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<div class="visualClear"></div>
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