73 lines
8.2 KiB
HTML
73 lines
8.2 KiB
HTML
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<title>std::allocator<T>::allocate_at_least</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_memory_allocator_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::allocator<T>::</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>‎ | <a href="cpp-memory-allocator.html">allocator</a></span></div>
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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">constexpr</span> std<span class="sy4">::</span><span class="me2">allocation_result</span><span class="sy1"><</span>T<span class="sy2">*</span><span class="sy1">></span><br />
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allocate_at_least<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-cxx23">(C++23 起)</span></td>
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<p>调用 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy4">::</span><a href="cpp-memory-new-operator_new.html"><span class="kw687">operator new</span></a></span></span> （可能提供额外的 <span class="t-lc"><a href="cpp-memory-new-align_val_t.html">std::align_val_t</a></span> 参数）分配 <span class="t-c"><span class="mw-geshi cpp source-cpp">count <span class="sy2">*</span> sizeof<span class="br0">(</span>T<span class="br0">)</span></span></span> 字节的未初始化存储，其中 <code>count</code> 是未指定的不小于 <code>n</code> 的整数值，但未指定何时及如何调用此函数。</p>
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<p>然后，此函数在该存储中创建一个 <span class="t-c"><span class="mw-geshi cpp source-cpp">T<span class="br0">[</span>count<span class="br0">]</span></span></span> 数组并开始其生存期，但不开始其任何元素的生存期。</p>
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<p>为在常量表达式中使用此函数，必须在同一表达式的求值内解分配其所分配的存储。</p>
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<p>若 <code>T</code> 为<a href="cpp-language-type.html#.E4.B8.8D.E5.AE.8C.E6.95.B4.E7.B1.BB.E5.9E.8B">不完整类型</a>则此函数的使用为非良构。</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的下界</td>
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</tr>
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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">std<span class="sy4">::</span><span class="me2">allocation_result</span><span class="sy1"><</span>T<span class="sy2">*</span><span class="sy1">></span><span class="br0">{</span>p, count<span class="br0">}</span></span></span> ，其中 <code>p</code> 指向 <code>T</code> 类型的 <code>n</code> 个对象的数组首元素的指针，数组元素尚未构造。</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"><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 class="sy1"><</span> n</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> ，或若分配失败则抛出 <span class="t-lc"><a href="cpp-memory-new-bad_alloc.html">std::bad_alloc</a></span> 。</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><code>allocate_at_least</code> 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。</p>
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<p>遣词“未指定何时及如何”令标准库容器可以<a href="cpp-language-new.html#.E5.88.86.E9.85.8D">组合或优化掉堆分配</a>，即使对直接调用 <code>::operator new</code> 禁止这种优化。例如 libc++ 实现了它（ <a rel="nofollow" class="external autonumber" href="https://github.com/llvm-mirror/libcxx/blob/master@%7B2017-02-09%7D/include/memory#L1766-L1772">[1]</a> 与 <a rel="nofollow" class="external autonumber" href="https://github.com/llvm-mirror/libcxx/blob/master@%7B2017-02-09%7D/include/new#L211-L217">[2]</a> ）</p>
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<p>在调用 <code>allocate_at_least</code> 后、构造元素前， <span class="t-c"><span class="mw-geshi cpp source-cpp">T<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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<div><a href="cpp-memory-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>经由分配器分配至少与请求的大小一样大的存储<br />
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<span class="t-mark">(函数模板)</span></td>
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