126 lines
8.0 KiB
HTML
126 lines
8.0 KiB
HTML
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<title>std::allocator_traits<Alloc>::allocate</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_memory_allocator_traits_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::allocator_traits<Alloc>::</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-allocator_traits.html">allocator traits</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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<td rowspan="142">(1)</td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">static</span> pointer allocate<span class="br0">(</span> Alloc<span class="sy3">&</span> a, size_type n <span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 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> <span class="kw4">static</span> <span class="kw4">constexpr</span> pointer allocate<span class="br0">(</span> Alloc<span class="sy3">&</span> a, size_type 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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<td rowspan="142">(2)</td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">static</span> pointer allocate<span class="br0">(</span> Alloc<span class="sy3">&</span> a, size_type n,<br />
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const_void_pointer hint <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 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> <span class="kw4">static</span> <span class="kw4">constexpr</span> pointer allocate<span class="br0">(</span> Alloc<span class="sy3">&</span> a, size_type n,<br />
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const_void_pointer hint <span class="br0">)</span><span class="sy4">;</span></span></div>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<p>用分配器 <code>a</code> 分配 <span class="t-c"><span class="mw-geshi cpp source-cpp">n<span class="sy2">*</span>sizeof<span class="br0">(</span>Alloc<span class="sy4">::</span><span class="me2">value_type</span><span class="br0">)</span></span></span> 字节的未初始化存储。于该存储创建 <span class="t-c"><span class="mw-geshi cpp source-cpp">Alloc<span class="sy4">::</span><span class="me2">value_type</span><span class="br0">[</span>n<span class="br0">]</span></span></span> 类型数组，但不构造其任何元素。</p>
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<div class="t-li1"><span class="t-li">1)</span> 调用 <span class="t-c"><span class="mw-geshi cpp source-cpp">a.<span class="me1">allocate</span><span class="br0">(</span>n<span class="br0">)</span></span></span></div>
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<div class="t-li1"><span class="t-li">2)</span> 附带地传递内存位置提示 <code>hint</code> 。若可能则调用 <span class="t-c"><span class="mw-geshi cpp source-cpp">a.<span class="me1">allocate</span><span class="br0">(</span>n, hint<span class="br0">)</span></span></span> 。若不可能（例如无双参数成员函数 allocate() ），则调用 <span class="t-c"><span class="mw-geshi cpp source-cpp">a.<span class="me1">allocate</span><span class="br0">(</span>n<span class="br0">)</span></span></span></div>
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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>使用的分配器</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</td>
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<td>hint</td>
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<td>-</td>
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<td>指向临近内存位置的指针</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>调用 <code>a.allocate(n)</code> 所返回的指针。</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>在 <a rel="nofollow" class="external text" href="https://wg21.link/P0593R6">P0593R6</a> 前不保证 <code>Alloc::allocate</code> 创建数组对象，这使得按照核心语言规范，对 <span class="t-lc"><a href="cpp-container-vector.html">std::vector</a></span> 及某些其他容器使用非默认分配器并非实际良定义。</p>
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<p>在调用 <code>allocate</code> 后、构造元素前， <span class="t-c"><span class="mw-geshi cpp source-cpp">Alloc<span class="sy4">::</span><span class="me2">value_type</span><span class="sy2">*</span></span></span> 的指针算术在分配的数组内是良定义的，但若访问元素则行为未定义。</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>
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<div class="t-dsc-member-div">
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<div><a href="cpp-memory-allocator-allocate.html"><span class="t-lines"><span>allocate</span></span></a></div>
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</div>
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</td>
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<td>分配未初始化的存储<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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