107 lines
6.3 KiB
HTML
107 lines
6.3 KiB
HTML
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<title>std::pmr::polymorphic_allocator<T>::allocate</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_memory_polymorphic_allocator_allocate 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::pmr::polymorphic_allocator<T>::</span>allocate</h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-memory.html">memory</a>‎ | <a href="cpp-memory-polymorphic_allocator.html">polymorphic allocator</a></span></div>
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<div><span class="mw-geshi cpp source-cpp">T<span class="sy2">*</span> allocate<span class="br0">(</span> <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></span></div>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span><br />
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<span class="t-mark-rev t-until-cxx20">(C++20 前)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="br0">[</span><span class="br0">[</span>nodiscard<span class="br0">]</span><span class="br0">]</span> T<span class="sy2">*</span> allocate<span class="br0">(</span> <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></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>用底层的 memory_resource 分配 <code>n</code> 个 <code>T</code> 类型对象的存储。等价于 <code>return static_cast<T*>(resource()->allocate(n * sizeof(T), alignof(T)));</code> 。</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>n</td>
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<td>-</td>
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<td>要分配存储的对象数</td>
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</table>
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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>指向被分配存储的指针。</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>若 <span class="t-c"><span class="mw-geshi cpp source-cpp">n <span class="sy1">></span> <a href="cpp-types-numeric_limits.html"><span class="kw344">std::<span class="me2">numeric_limits</span></span></a><span class="sy1"><</span><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a><span class="sy1">></span><span class="sy4">::</span><span class="me2">max</span><span class="br0">(</span><span class="br0">)</span> <span class="sy2">/</span> sizeof<span class="br0">(</span>T<span class="br0">)</span></span></span> 则抛出 <span class="t-lc"><a href="cpp-memory-new-bad_array_new_length.html">std::bad_array_new_length</a></span> ；亦可抛出调用 <span class="t-c"><span class="mw-geshi cpp source-cpp">resource<span class="br0">(</span><span class="br0">)</span><span class="sy2">-</span><span class="sy1">></span>allocate</span></span> 所抛的任何异常。</p>
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<h3><span class="mw-headline" id=".E7.BC.BA.E9.99.B7.E6.8A.A5.E5.91.8A">缺陷报告</span></h3>
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<p>下列更改行为的缺陷报告追溯地应用于以前出版的 C++ 标准。</p>
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<table class="dsctable" style="font-size:0.8em">
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<th>DR</th>
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<th>应用于</th>
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<th>出版时的行为</th>
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<th>正确行为</th>
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<td><a rel="nofollow" class="external text" href="https://cplusplus.github.io/LWG/issue3038">LWG 3038</a></td>
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<td>C++17</td>
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<td><code>allocate</code> 可能分配大小错误的存储</td>
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<td>替而抛出 <code>length_error</code></td>
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<td><a rel="nofollow" class="external text" href="https://cplusplus.github.io/LWG/issue3237">LWG 3237</a></td>
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<td>C++17</td>
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<td><code>allocate</code> 所抛的异常与 <code>std::allocator::allocate</code> 不一致</td>
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<td>使之一致</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_traits-allocate.html"><span class="t-lines"><span>allocate</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-cmark">[静态]</span></span></span></div>
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</div>
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</td>
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<td>用分配器分配未初始化的存储<br />
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<span class="t-mark">(<code>std::allocator_traits<Alloc></code> 的公开静态成员函数)</span></td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-memory-memory_resource-allocate.html"><span class="t-lines"><span>allocate</span></span></a></div>
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<td>分配内存<br />
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<span class="t-mark">(<code>std::pmr::memory_resource</code> 的公开成员函数)</span></td>
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