288 lines
20 KiB
HTML
288 lines
20 KiB
HTML
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<html lang="zh" dir="ltr" class="client-nojs" xmlns="http://www.w3.org/1999/xhtml" xml:lang="zh">
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<title>std::uses_allocator</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_memory_uses_allocator 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>uses_allocator</h1>
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<div id="bodyContent">
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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 id="mw-content-text" lang="zh" dir="ltr" class="mw-content-ltr" xml:lang="zh">
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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> T, <span class="kw1">class</span> Alloc <span class="sy1">></span> <span class="kw1">struct</span> uses_allocator<span class="sy4">;</span></span></div>
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<td><span class="t-mark-rev t-since-cxx11">(C++11 起)</span></td>
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<p>若 <code>T</code> 拥有：1）内嵌定义类型 <code>allocator_type</code> ，并且该类型能从 <code>Alloc</code> 转换<span class="t-rev-inl t-since-libfund_ts"><span>，2）或者该类型是 <span class="t-lc"><a href="cpp-experimental-erased_type.html">std::experimental::erased_type</a></span> 的别名</span> <span><span class="t-mark-rev t-since-libfund-ts t-mark-ts">(库基础 TS)</span></span></span>，则成员常量 <code>value</code> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 。否则 <code>value</code> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> 。</p>
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<h3><span class="mw-headline" id=".E8.BE.85.E5.8A.A9.E5.8F.98.E9.87.8F.E6.A8.A1.E6.9D.BF">辅助变量模板</span></h3>
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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> T, <span class="kw1">class</span> Alloc <span class="sy1">></span><br />
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<span class="kw1">inline</span> <span class="kw4">constexpr</span> <span class="kw4">bool</span> uses_allocator_v <span class="sy1">=</span> uses_allocator<span class="sy1"><</span>T, Alloc<span class="sy1">></span><span class="sy4">::</span><span class="me2">value</span><span class="sy4">;</span></span></div>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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<td></td>
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<div class="t-template-editlink" style=""></div>
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<div class="t-inherited">
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<h2><span class="mw-headline" id=".E7.BB.A7.E6.89.BF.E8.87.AA_std::integral_constant">继承自 <a href="cpp-types-integral_constant.html"><span style="font-size:0.7em; line-height:130%">std::</span>integral_constant</a></span></h2>
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<h3><span class="mw-headline" id=".E6.88.90.E5.91.98.E5.B8.B8.E9.87.8F">成员常量</span></h3>
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<div><span class="t-lines"><span>value</span></span></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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</td>
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<td>若 <code>T</code> 使用分配器 <code>Alloc</code>则为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> ，否则为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span><br />
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<span class="t-mark">(公开静态成员常量)</span></td>
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<h3><span class="mw-headline" id=".E6.88.90.E5.91.98.E5.87.BD.E6.95.B0">成员函数</span></h3>
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<div><span class="t-lines"><span>operator bool</span></span></div>
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</td>
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<td>转换对象为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">bool</span></span></span> ，返回 <code>value</code><br />
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<span class="t-mark">(公开成员函数)</span></td>
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<div><span class="t-lines"><span>operator()</span></span></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx14">(C++14)</span></span></span></div>
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<td>返回 <code>value</code><br />
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<span class="t-mark">(公开成员函数)</span></td>
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</table>
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<h3><span class="mw-headline" id=".E6.88.90.E5.91.98.E7.B1.BB.E5.9E.8B">成员类型</span></h3>
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<td>类型</td>
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<td>定义</td>
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<td><code>value_type</code></td>
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<td><code>bool</code></td>
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<td><code>type</code></td>
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<td><span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-integral_constant.html"><span class="kw647">std::<span class="me2">integral_constant</span></span></a><span class="sy1"><</span><span class="kw4">bool</span>, value<span class="sy1">></span></span></span></td>
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<h3><span class="mw-headline" id=".E4.BD.BF.E7.94.A8.E5.88.86.E9.85.8D.E5.99.A8.E6.9E.84.E9.80.A0">使用分配器构造</span></h3>
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<p>传递分配器 <code>alloc</code> 给某类型 <code>T</code> 的构造函数有三种约定：</p>
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<ul>
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<li>若 <code>T</code> 不使用兼容的分配器（ <span class="t-lc">std::uses_allocator_v<T, Alloc></span> 为 <code>false</code> ），则忽略 <code>alloc</code> 。</li>
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<li>否则 <span class="t-lc">std::uses_allocator_v<T, Alloc></span> 为 <code>true</code> ，而</li>
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</ul>
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<dl>
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<dd>
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<ul>
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<li>若 <code>T</code> 使用<i>前导分配器约定</i>（可用 <span class="t-c"><span class="mw-geshi cpp source-cpp">T<span class="br0">(</span><a href="cpp-memory-allocator_arg.html"><span class="kw703">std::<span class="me2">allocator_arg</span></span></a>, alloc, args...<span class="br0">)</span></span></span> 调用），则使用分配器构造使用此形式</li>
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<li>若 <code>T</code> 使用<i>尾随分配器约定</i>（可用 <span class="t-c"><span class="mw-geshi cpp source-cpp">T<span class="br0">(</span>args..., alloc<span class="br0">)</span></span></span> 调用），则使用分配器构造使用此形式</li>
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<li>否则，程序为谬构（这表示 <span class="t-lc">std::uses_allocator_v<T, Alloc></span> 为 <code>true</code> ，但类型不遵守二种允许的约定之一）</li>
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</ul>
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</dd>
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</dl>
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<ul>
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<li>作为特例， <span class="t-lc"><a href="cpp-utility-pair.html">std::pair</a></span> 被当做使用分配器类型，即使 <code>std::uses_allocator</code> 对 <code>pair</code> 为 <code>false</code> （不同于 <span class="t-lc"><a href="cpp-utility-tuple.html">std::tuple</a></span> 等）；见 <span class="t-rev-inl t-until-cxx20"><span><span class="t-lc">std::polymoprhic_allocator::construct</span> 与 <span class="t-lc"><a href="cpp-memory-scoped_allocator_adaptor-construct.html">std::scoped_allocator_adaptor::construct</a></span></span> <span><span class="t-mark-rev t-until-cxx20">(C++20 前)</span></span></span><span class="t-rev-inl t-since-cxx20"><span><span class="t-lc">std::uses_allocator_construction_args</span></span> <span><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></span></span> 的 <code>pair</code> 特定重载。</li>
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</ul>
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<table class="t-rev-begin">
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<p>工具函数 <span class="t-lc">std::make_obj_using_allocator</span> 及 <span class="t-lc">std::uninitialized_construct_using_allocator</span> 可用于显式创建遵循上述协议的对象，而 <span class="t-lc">std::uses_allocator_construction_args</span> 能用于准备匹配类型所期待的使用分配器构造的口味的实参列表。</p>
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</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<h3><span class="mw-headline" id=".E7.89.B9.E5.8C.96">特化</span></h3>
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<p>类型特性 <span class="t-lc"><strong class="selflink">std::uses_allocator</strong></span> 的自定义特化对无成员 typedef <code>allocator_type</code> 的类型允许，但必须满足下列二个要求之一：</p>
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<div class="t-li1"><span class="t-li">1)</span> <code>T</code> 拥有接收 <span class="t-lc"><a href="cpp-memory-allocator_arg_t.html">std::allocator_arg_t</a></span> 为首参数， <code>Alloc</code> 为第二参数的构造函数。</div>
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<div class="t-li1"><span class="t-li">2)</span> <code>T</code> 拥有接收 <code>Alloc</code> 为最后参数的构造函数。</div>
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<p>上述情况中， <code>Alloc</code> 是满足<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> 要求的类型<span class="t-rev-inl t-since-libfund_ts"><span>，或是可转换到 <span class="t-lc">std::experimental::pmr::memory_resource*</span> 的指针类型</span> <span><span class="t-mark-rev t-since-libfund-ts t-mark-ts">(库基础 TS)</span></span></span>。</p>
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<p>标准库已提供下列特化：</p>
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<div><a href="cpp-utility-tuple-uses_allocator.html"><span class="t-lines"><span>std::uses_allocator<span class="t-dsc-small"><std::tuple></span></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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</div>
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</td>
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<td>特化 <span class="t-lc"><strong class="selflink">std::uses_allocator</strong></span> 类型特征<br />
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<span class="t-mark">(类模板特化)</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-container-queue-uses_allocator.html"><span class="t-lines"><span>std::uses_allocator<span class="t-dsc-small"><std::queue></span></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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</div>
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</td>
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<td>特化 <span class="t-lc"><strong class="selflink">std::uses_allocator</strong></span> 类型特征<br />
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<span class="t-mark">(类模板特化)</span></td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-container-priority_queue-uses_allocator.html"><span class="t-lines"><span>std::uses_allocator<span class="t-dsc-small"><std::priority_queue></span></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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</div>
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</td>
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<td>特化 <span class="t-lc"><strong class="selflink">std::uses_allocator</strong></span> 类型特征<br />
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<span class="t-mark">(类模板特化)</span></td>
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</tr>
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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-container-stack-uses_allocator.html"><span class="t-lines"><span>std::uses_allocator<span class="t-dsc-small"><std::stack></span></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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</div>
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</td>
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<td>特化 <span class="t-lc"><strong class="selflink">std::uses_allocator</strong></span> 类型特征<br />
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<span class="t-mark">(类模板特化)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-utility-functional-function-uses_allocator.html"><span class="t-lines"><span>std::uses_allocator<span class="t-dsc-small"><std::function></span></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 class="t-mark-rev t-until-cxx17">(C++17 前)</span></span></span></div>
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</div>
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</td>
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<td>特化 <span class="t-lc"><strong class="selflink">std::uses_allocator</strong></span> 类型特性<br />
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<span class="t-mark">(类模板特化)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-thread-promise-uses_allocator.html"><span class="t-lines"><span>std::uses_allocator<span class="t-dsc-small"><std::promise></span></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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</div>
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</td>
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<td>特化 <span class="t-lc"><strong class="selflink">std::uses_allocator</strong></span> 类型特征<br />
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<span class="t-mark">(类模板特化)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-thread-packaged_task-uses_allocator.html"><span class="t-lines"><span>std::uses_allocator<span class="t-dsc-small"><std::packaged_task></span></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 class="t-mark-rev t-until-cxx17">(C++17 前)</span></span></span></div>
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</div>
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</td>
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<td>特化 <span class="t-lc"><strong class="selflink">std::uses_allocator</strong></span> 类型特征<br />
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<span class="t-mark">(类模板特化)</span></td>
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</tr>
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</table>
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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><span class="t-lc"><a href="cpp-memory-scoped_allocator_adaptor.html">std::scoped_allocator_adaptor</a></span> 、 <span class="t-lc"><a href="cpp-utility-tuple.html">std::tuple</a></span> 及 <span class="t-lc"><a href="cpp-memory-polymorphic_allocator.html">std::pmr::polymorphic_allocator</a></span> 使用此类型特性。自定义分配器或封装类型亦可使用之，以确定要构造的对象或成员是否足以使用分配器（例如是容器的情况），该情况下应传递分配器给其构造函数。</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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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-memory-allocator_arg.html"><span class="t-lines"><span>allocator_arg</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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</div>
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</td>
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<td>用于选择具分配器的构造函数的 <span class="t-lc"><a href="cpp-memory-allocator_arg_t.html">std::allocator_arg_t</a></span> 对象<br />
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<span class="t-mark">(常量)</span></td>
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<tr class="t-dsc">
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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_arg_t.html"><span class="t-lines"><span>allocator_arg_t</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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</div>
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</td>
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<td>标签类型，用于选择具分配器的构造函数重载<br />
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<span class="t-mark">(类)</span></td>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-memory-uses_allocator_construction_args.html"><span class="t-lines"><span>uses_allocator_construction_args</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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</div>
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<td>准备匹配给定类型所要求的使用分配器构造的口味的参数列表<br />
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<span class="t-mark">(函数模板)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-memory-make_obj_using_allocator.html"><span class="t-lines"><span>make_obj_using_allocator</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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</div>
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</td>
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<td>以使用分配器构造的手段创建给类型的对象<br />
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<span class="t-mark">(函数模板)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-memory-uninitialized_construct_using_allocator.html"><span class="t-lines"><span>uninitialized_construct_using_allocator</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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</div>
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</td>
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<td>以使用分配器构造的手段在指定的内存位置创建给定类型的对象<br />
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<span class="t-mark">(函数模板)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-memory-scoped_allocator_adaptor.html"><span class="t-lines"><span>scoped_allocator_adaptor</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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</div>
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</td>
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<td>为多级容器实现的多级分配器<br />
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<span class="t-mark">(类模板)</span></td>
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</tr>
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</table>
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</div>
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<div class="visualClear"></div>
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</div>
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</div>
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</div>
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</body>
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</html>
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