80 lines
6.9 KiB
HTML
80 lines
6.9 KiB
HTML
<!DOCTYPE html>
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<html lang="zh" dir="ltr" class="client-nojs" xmlns="http://www.w3.org/1999/xhtml" xml:lang="zh">
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<head><meta http-equiv="x-ua-compatible" content="ie=edge">
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<title>C++ 具名要求：可默认构造 (DefaultConstructible)</title>
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<link rel="stylesheet" href="ext.css" />
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<link rel="stylesheet" href="site_modules.css" />
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_named_req_DefaultConstructible skin-cppreference2 action-view cpp-navbar">
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<div id="cpp-content-base">
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<div id="content"><a id="top"></a>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">C++ 具名要求：</span><span style="font-style:italic">可默认构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(DefaultConstructible)</span></h1>
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<div id="bodyContent">
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-named_req.html">named req</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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<p>指定该类型的对象可以默认构造 。</p>
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<h3><span class="mw-headline" id=".E8.A6.81.E6.B1.82">要求</span></h3>
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<p>以下情况下，类型 <code>T</code> 满足<span style="font-style:italic">可默认构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(DefaultConstructible)</span> ：</p>
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<p>给定</p>
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<ul>
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<li>任意标识符 <code>u</code>，</li>
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</ul>
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<p>下列表达式必须合法且拥有其指定的效果</p>
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<table class="wikitable">
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<tr>
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<th>表达式</th>
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<th>后条件</th>
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</tr>
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<tr>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp">T u</span></span></td>
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<td>对象 <code>u</code> 被<a href="cpp-language-default_initialization.html">默认初始化</a>。</td>
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</tr>
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<tr>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp">T u<span class="br0">{</span><span class="br0">}</span></span></span></td>
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<td>对象 <code>u</code> 被<a href="cpp-language-value_initialization.html">值初始化</a>或<a href="cpp-language-aggregate_initialization.html">聚合初始化</a>。</td>
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</tr>
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<tr>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp">T<span class="br0">(</span><span class="br0">)</span></span></span>
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<p><span class="t-c"><span class="mw-geshi cpp source-cpp">T<span class="br0">{</span><span class="br0">}</span></span></span></p>
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</td>
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<td>一个 <code>T</code> 类型的临时对象被<a href="cpp-language-value_initialization.html">值初始化</a>或<a href="cpp-language-aggregate_initialization.html">聚合初始化</a>。</td>
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</tr>
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</table>
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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>对于非聚合类类型的对象，必须定义公开的<a href="cpp-language-default_constructor.html">默认构造函数</a>（用户定义或隐式定义），以满足<span style="font-style:italic">可默认构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(DefaultConstructible)</span> 。</p>
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<p><a href="cpp-language-type.html">非类对象类型</a>的非 const 对象始终为<span style="font-style:italic">可默认构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(DefaultConstructible)</span> 。</p>
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<p>const 的非类类型非<span style="font-style:italic">可默认构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(DefaultConstructible)</span> 。</p>
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<p>const 的聚合类型，若其任何成员是非类类型对象，则非<span style="font-style:italic">可默认构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(DefaultConstructible)</span> 。</p>
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<p>非对象类型（函数类型，引用类型，以及（可能 cv 限定的）<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span></span></span> 类型）和 const 的非对象类型始终非<span style="font-style:italic">可默认构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(DefaultConstructible)</span> 。</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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<table class="t-dsc-begin">
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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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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</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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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-concepts-default_initializable.html"><span class="t-lines"><span>default_initializable</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>指定能默认构造一个类型的对象<br />
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<span class="t-mark">(概念)</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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</body>
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