69 lines
8.0 KiB
HTML
69 lines
8.0 KiB
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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<title>C++ 具名要求：可默认插入 (DefaultInsertable)</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_named_req_DefaultInsertable skin-cppreference2 action-view cpp-navbar">
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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">可默认插入</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(DefaultInsertable)</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>指定该类型的实例能以给定的分配器原位默认构造。</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><b>T</b></code> <strong class="selflink"><span style="font-style:italic">可默认插入</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(DefaultInsertable)</span></strong> 进<a href="cpp-named_req-Container.html"><span style="font-style:italic">容器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(Container)</span></a> <code><b>X</b></code>（其 <code>value_type</code> 等同于 <code>T</code>）：给定</p>
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<table class="t-dsc-begin">
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<td><code><b>A</b></code></td>
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<td>分配器类型</td>
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</tr>
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<tr class="t-dsc">
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<td><code><b>m</b></code></td>
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<td><code><b>A</b></code> 类型的左值</td>
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</tr>
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<tr class="t-dsc">
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<td><code><b>p</b></code></td>
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<td>容器所准备的 <code><b>T*</b></code> 类型的指针</td>
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</tr>
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<p>其中 <code><b>X::allocator_type</b></code> 等同于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-memory-allocator_traits.html"><span class="kw701">std::<span class="me2">allocator_traits</span></span></a><span class="sy1"><</span>A<span class="sy1">></span><span class="sy4">::</span><span class="me2">rebind_alloc</span><span class="sy1"><</span>T<span class="sy1">></span></span></span>，</p>
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<p>下列表达式良构：</p>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="cpp source-cpp">
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<pre class="de1"><a href="cpp-memory-allocator_traits.html"><span class="kw701">std::<span class="me2">allocator_traits</span></span></a><span class="sy1"><</span>A<span class="sy1">></span><span class="sy4">::</span><span class="me2">construct</span><span class="br0">(</span>m, p<span class="br0">)</span><span class="sy4">;</span></pre></div>
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</div>
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<p><br />
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若 <code><b>X</b></code> 非知分配器或为 <span class="t-lc"><a href="cpp-string-basic_string.html">std::basic_string</a></span> 特化，则如同 <code><b>A</b></code> 就是 <span class="t-lc">std::allocator<T></span> 一样进行定义，但并不需要创建分配器对象，且不会实例化用户定义的 <span class="t-lc"><a href="cpp-memory-allocator.html">std::allocator</a></span> 的特化。</p>
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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>默认情况下，这会调用布置 new，如用 <span class="t-rev-inl t-until-cxx20"><span><span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy4">::</span><span class="me2">new</span><span class="br0">(</span><span class="br0">(</span><span class="kw4">void</span><span class="sy2">*</span><span class="br0">)</span>p<span class="br0">)</span> T<span class="br0">(</span><span class="br0">)</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">construct_at</span><span class="br0">(</span>p<span class="br0">)</span></span></span></span> <span><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></span></span> 一样（即<a href="cpp-language-value_initialization.html">值初始化</a> <code>p</code> 所指向的对象）。若不欲值初始化，例如当对象是非类类型且不需要清零时，则可通过提供<a rel="nofollow" class="external text" href="http://stackoverflow.com/a/21028912/273767">定制的 <code>Allocator::construct</code></a> 予以避免。</p>
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<p>尽管 C++23 前要求在构造 <span class="t-lc"><a href="cpp-string-basic_string.html">std::basic_string</a></span> 的元素时使用定制的 <code>construct</code>，所有实现都使用默认机制。<a rel="nofollow" class="external text" href="https://wg21.link/P1072R10">P1072R10</a> 更正了该要求以匹配既存实现。</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 colspan="2"><a href="cpp-named_req-DefaultConstructible.html"><span style="font-style:italic">可默认构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(DefaultConstructible)</span></a></td>
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</tr>
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<tr class="t-dsc">
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<td colspan="2"><a href="cpp-named_req-CopyInsertable.html"><span style="font-style:italic">可复制插入</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(CopyInsertable)</span></a></td>
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<tr class="t-dsc">
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<td colspan="2"><a href="cpp-named_req-MoveInsertable.html"><span style="font-style:italic">可移动插入</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(MoveInsertable)</span></a></td>
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<tr class="t-dsc">
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<td colspan="2"><a href="cpp-named_req-EmplaceConstructible.html"><span style="font-style:italic">可就位构造</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(EmplaceConstructible)</span></a></td>
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</tr>
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<tr class="t-dsc">
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<td colspan="2"><a href="cpp-named_req-Erasable.html"><span style="font-style:italic">可擦除</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(Erasable)</span></a></td>
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</tr>
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</table>
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