108 lines
8.5 KiB
HTML
108 lines
8.5 KiB
HTML
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<title>std::pmr::polymorphic_allocator<T>::allocate_object</title>
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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_object</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"><span class="kw1">template</span><span class="sy1"><</span> <span class="kw1">class</span> U <span class="sy1">></span><br />
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<span class="br0">[</span><span class="br0">[</span>nodiscard<span class="br0">]</span><span class="br0">]</span> U<span class="sy2">*</span> allocate_object<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="sy1">=</span> <span class="nu0">1</span> <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>用底层内存资源分配 <code>n</code> 个 <code>U</code> 类型对象的存储。</p>
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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>U<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> ，否则等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">return</span> <span class="kw1">static_cast</span><span class="sy1"><</span>U<span class="sy2">*</span><span class="sy1">></span><span class="br0">(</span>allocate_bytes<span class="br0">(</span>n <span class="sy2">*</span> sizeof<span class="br0">(</span>U<span class="br0">)</span>, alignof<span class="br0">(</span>U<span class="br0">)</span><span class="br0">)</span> <span class="br0">)</span><span class="sy4">;</span></span></span> 。</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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<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>指向分配的存储的指针。</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>引入此函数是为了用于全特化分配器 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">polymorphic_allocator</span><span class="sy1"><></span></span></span> ，但它可能在任何特化中都有用，作为避免从 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">polymorphic_allocator</span><span class="sy1"><</span>T<span class="sy1">></span></span></span> 重绑定到 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">polymorphic_allocator</span><span class="sy1"><</span>U<span class="sy1">></span></span></span> 的快捷方式。</p>
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<p>因为不推导 <code>U</code> ，调用此函数时必须提供它为模板实参。</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>U<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> ；亦可能抛出任何调用 <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> 所抛的异常。</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 class="t-dsc-member-div">
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<div><a href="cpp-memory-polymorphic_allocator-allocate_bytes.html"><span class="t-lines"><span>allocate_bytes</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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<td>从底层资源分配未处理的对齐内存<br />
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<span class="t-mark">(公开成员函数)</span></td>
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<div><a href="cpp-memory-polymorphic_allocator-new_object.html"><span class="t-lines"><span>new_object</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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<td>分配并创建对象<br />
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<span class="t-mark">(公开成员函数)</span></td>
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<div><a href="cpp-memory-polymorphic_allocator-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">(公开成员函数)</span></td>
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<tr class="t-dsc">
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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><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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