93 lines
6.3 KiB
HTML
93 lines
6.3 KiB
HTML
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<title>std::default_initializable</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_concepts_default_initializable 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>default_initializable</h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-concepts.html">concepts</a></span></div>
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<div>定义于头文件 <code><a href="cpp-header-concepts.html"><concepts></a></code></div>
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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 /></span>
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<p>concept default_initializable <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">constructible_from</span><span class="sy1"><</span>T<span class="sy1">></span> <span class="sy3">&&</span> requires <span class="br0">{</span> T<span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span> <span class="br0">}</span> <span class="sy3">&&</span><br /></p>
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<span class="coMULTI">/* T t; 为良构，见后述 */</span><span class="sy4">;</span></div>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<p><code>default_initializable</code> 概念检查 <code>T</code> 类型变量能否</p>
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<ul>
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<li><a href="cpp-language-value_initialization.html">值初始化</a>（ <span class="t-c"><span class="mw-geshi cpp source-cpp">T<span class="br0">(</span><span class="br0">)</span></span></span> 为良构）</li>
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<li>从空初始化器列表<a href="cpp-language-list_initialization.html">直接列表初始化</a>（ <span class="t-c"><span class="mw-geshi cpp source-cpp">T<span class="br0">{</span><span class="br0">}</span></span></span> 为良构）；以及</li>
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<li><a href="cpp-language-default_initialization.html">默认初始化</a>（ <span class="t-c"><span class="mw-geshi cpp source-cpp">T t<span class="sy4">;</span></span></span> 为良构）。</li>
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</ul>
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<p>如同在与 T 无关的语境中进行访问检查。仅考虑变量初始化的立即语境的合法性。</p>
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<h3><span class="mw-headline" id=".E5.8F.AF.E8.83.BD.E7.9A.84.E5.AE.9E.E7.8E.B0">可能的实现</span></h3>
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<div class="cpp source-cpp">
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<pre class="de1"><span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span>
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concept default_initializable <span class="sy1">=</span>
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std<span class="sy4">::</span><span class="me2">constructible_from</span><span class="sy1"><</span>T<span class="sy1">></span> <span class="sy3">&&</span>
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requires <span class="br0">{</span> T<span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span> <span class="br0">}</span> <span class="sy3">&&</span>
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requires <span class="br0">{</span> <span class="sy4">::</span><span class="me2">new</span> <span class="br0">(</span><span class="kw1">static_cast</span><span class="sy1"><</span><span class="kw4">void</span><span class="sy2">*</span><span class="sy1">></span><span class="br0">(</span>nullptr<span class="br0">)</span><span class="br0">)</span> T<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span></pre></div>
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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><a href="cpp-concepts-constructible_from.html"><span class="t-lines"><span>constructible_from</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>指定该类型的变量能从一组实参类型进行构造，或绑定到一组实参类型<br />
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<span class="t-mark">(概念)</span></td>
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<div><a href="cpp-types-is_default_constructible.html"><span class="t-lines"><span>is_default_constructible</span><span>is_trivially_default_constructible</span><span>is_nothrow_default_constructible</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
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</td>
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<td>检查类型是否有默认构造函数<br />
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<span class="t-mark">(类模板)</span></td>
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