62 lines
8.3 KiB
HTML
62 lines
8.3 KiB
HTML
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<title>C++ 具名要求：可移动插入 (MoveInsertable) (C++11 起)</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_named_req_MoveInsertable 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">可移动插入</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(MoveInsertable)</span> <span class="t-mark-rev t-since-cxx11">(C++11 起)</span></h1>
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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>指定该类型的对象能以给定的分配器，从一个该类型的右值构造到未初始化存储之中。</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">可移动插入</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(MoveInsertable)</span></strong> 进容器 <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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<tr class="t-dsc">
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<td><code><b>rv</b></code></td>
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<td><code><b>T</b></code> 类型的右值表达式</td>
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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>下列表达式为良构：</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, rv<span class="br0">)</span><span class="sy4">;</span></pre></div>
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</div>
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<p>并且求值后，<code>*p</code> 的值等价于 <code>rv</code> 先前保有的值（<code>rv</code> 保持有效，但处于未指明的状态）。</p>
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<p>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>若 <code>A</code> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-memory-allocator.html"><span class="kw700">std::<span class="me2">allocator</span></span></a><span class="sy1"><</span>T<span class="sy1">></span></span></span>，则这将调用布置 new，如同用 <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>rv<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, rv<span class="br0">)</span></span></span></span> <span><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></span></span>。</p>
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<p>若使用了 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-memory-allocator.html"><span class="kw700">std::<span class="me2">allocator</span></span></a><span class="sy1"><</span>T<span class="sy1">></span></span></span> 或类似的分配器，则该类不必为满足此类型要求而实现<a href="cpp-language-move_constructor.html">移动构造函数</a>：接收 <code>const T&</code> 实参的<a href="cpp-language-copy_constructor.html">复制构造函数</a>能绑定右值表达式。若<span style="font-style:italic">可移动插入</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(MoveInsertable)</span> 的类实现了移动构造函数，则它可能亦实现<a href="cpp-utility-move.html">移动语义</a>以利用“构造后 <code>rv</code> 的值未指明”的事实。</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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<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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