103 lines
9.6 KiB
HTML
103 lines
9.6 KiB
HTML
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<title>std::allocate_at_least</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_memory_allocate_at_least 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>allocate_at_least</h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-memory.html">memory</a></span></div>
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<div>定义于头文件 <code><a href="cpp-header-memory.html"><memory></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> Alloc <span class="sy1">></span><br /></span>
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<p><span class="br0">[</span><span class="br0">[</span>nodiscard<span class="br0">]</span><span class="br0">]</span> <span class="kw4">constexpr</span> std<span class="sy4">::</span><span class="me2">allocation_result</span><span class="sy1"><</span><br />
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<span class="kw1">typename</span> <a href="cpp-memory-allocator_traits.html"><span class="kw701">std::<span class="me2">allocator_traits</span></span></a><span class="sy1"><</span>Alloc<span class="sy1">></span><span class="sy4">::</span><span class="me2">pointer</span><span class="sy1">></span><br /></p>
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allocate_at_least<span class="br0">(</span> Alloc<span class="sy3">&</span> a, <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> n <span class="br0">)</span><span class="sy4">;</span></div>
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<td><span class="t-mark-rev t-since-cxx23">(C++23 起)</span></td>
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<p><code>std::allocate_at_least</code> 调用 <span class="t-c"><span class="mw-geshi cpp source-cpp">a.<span class="me1">allocate_at_least</span><span class="br0">(</span>n<span class="br0">)</span></span></span> 并返回其结果，若该调用为良构，否则它等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">return</span> <span class="br0">{</span>a.<span class="me1">allocate</span><span class="br0">(</span>n<span class="br0">)</span>, n<span class="br0">}</span><span class="sy4">;</span></span></span> 。</p>
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<p>对于<a href="cpp-named_req-Allocator.html"><span style="font-style:italic">分配器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(Allocator)</span></a> <code>a</code> ， <code>std::allocator_at_least</code> 尝试分配至少 <code>n</code> 个 <span class="t-c"><span class="mw-geshi cpp source-cpp">Alloc<span class="sy4">::</span><span class="me2">value_type</span></span></span> 对象的存储，并提供准确 <code>n</code> 个对象的存储的后备机制（类似 <span class="t-lc"><a href="cpp-memory-allocator_traits.html">std::allocator_traits</a></span> 所提供者）。</p>
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<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
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<td>a</td>
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<td>-</td>
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<td>用于分配存储的分配器</td>
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<td>n</td>
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<td>-</td>
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<td>要分配存储的对象数的下界</td>
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<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">返回值</span></h3>
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<p><span class="t-c"><span class="mw-geshi cpp source-cpp">a.<span class="me1">allocate_at_least</span><span class="br0">(</span>n<span class="br0">)</span></span></span> 若它为良构，否则为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">{</span>a.<span class="me1">allocate</span><span class="br0">(</span>n<span class="br0">)</span>, n<span class="br0">}</span></span></span> 。</p>
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<h3><span class="mw-headline" id=".E5.BC.82.E5.B8.B8">异常</span></h3>
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<p>在选择的分配函数抛出时抛出其所抛的异常。</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><a href="cpp-named_req-Allocator.html"><span style="font-style:italic">分配器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(Allocator)</span></a> 类型的 <code>allocate_at_least</code> 成员函数主要为连续容器，例如 <span class="t-lc"><a href="cpp-container-vector.html">std::vector</a></span> 与 <span class="t-lc"><a href="cpp-string-basic_string.html">std::basic_string</a></span> 提供，以通过使得其容量在可能时匹配实际分配的大小减少重分配。由于 <code>std::allocate_at_least</code> 提供后备机制，故能在适合处直接使用它。</p>
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<p>给定 <code>Alloc</code> 类型的分配器对象 <code>a</code> ，令 <code>result</code> 代表从 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">allocate_at_least</span><span class="br0">(</span>a, n<span class="br0">)</span></span></span> 返回的值，应该由 <span class="t-c"><span class="mw-geshi cpp source-cpp">a.<span class="me1">deallocate</span><span class="br0">(</span>result.<span class="me1">ptr</span>, m<span class="br0">)</span></span></span> （常经由 <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>Alloc<span class="sy1">></span><span class="sy4">::</span><span class="me2">deallocate</span><span class="br0">(</span>a, result.<span class="me1">ptr</span>, m<span class="br0">)</span></span></span> 调用）解分配存储以避免内存泄漏。</p>
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<p>用于解分配的参数 <code>m</code> 必须不小于 <code>n</code> 且不大于 <span class="t-c"><span class="mw-geshi cpp source-cpp">result.<span class="me1">count</span></span></span> ，否则行为未定义。注意若分配器不支持 <code>allocate_at_least</code> 则 <code>n</code> 始终等于 <span class="t-c"><span class="mw-geshi cpp source-cpp">result.<span class="me1">count</span></span></span> ，这表示要求 <code>m</code> 等于 <code>n</code> 。</p>
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<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
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<td class="mbox-text" style="">本节未完成<br />
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原因：暂无示例</td>
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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-memory-allocator-allocate_at_least.html"><span class="t-lines"><span>allocate_at_least</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx23">(C++23)</span></span></span></div>
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</td>
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<td>分配与请求的大小至少一样大的未初始化存储<br />
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<span class="t-mark">(<code>std::allocator<T></code> 的公开成员函数)</span></td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-memory-allocator_traits.html"><span class="t-lines"><span>allocator_traits</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></div>
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<td>提供关于分配器类型的信息<br />
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<span class="t-mark">(类模板)</span></td>
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