105 lines
11 KiB
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105 lines
11 KiB
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<title>std::queue 的推导指引</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_container_queue_deduction_guides skin-cppreference2 action-view cpp-page-template cpp-navbar">
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<h1 id="firstHeading" class="firstHeading"><code>std::queue</code> 的推导指引</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-container.html">container</a>‎ | <a href="cpp-container-queue.html">queue</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 class="t-page-template" style="">
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<p></p>
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<table class="t-dcl-begin">
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<div>定义于头文件 <code><a href="cpp-header-queue.html"><queue></a></code></div>
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<td></td>
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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> Container<span class="sy1">></span><br /></span>
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<p>queue<span class="br0">(</span>Container<span class="br0">)</span><br /></p>
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<span class="sy2">-</span><span class="sy1">></span> queue<span class="sy1"><</span><span class="kw1">typename</span> Container<span class="sy4">::</span><span class="me2">value_type</span>, Container<span class="sy1">></span><span class="sy4">;</span></div>
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</td>
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<td>(1)</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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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> InputIt<span class="sy1">></span><br /></span>
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<p>queue<span class="br0">(</span>InputIt, InputIt<span class="br0">)</span><br /></p>
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<span class="sy2">-</span><span class="sy1">></span> queue<span class="sy1"><</span><span class="coMULTI">/*iter-value-t*/</span><span class="sy1"><</span>InputIt<span class="sy1">>></span><span class="sy4">;</span></div>
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</td>
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<td>(2)</td>
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<td><span class="t-mark-rev t-since-cxx23">(C++23 起)</span></td>
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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> Container, <span class="kw1">class</span> Alloc<span class="sy1">></span><br /></span>
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<p>queue<span class="br0">(</span>Container, Alloc<span class="br0">)</span><br /></p>
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<span class="sy2">-</span><span class="sy1">></span> queue<span class="sy1"><</span><span class="kw1">typename</span> Container<span class="sy4">::</span><span class="me2">value_type</span>, Container<span class="sy1">></span><span class="sy4">;</span></div>
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</td>
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<td>(3)</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
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<tr class="t-dcl t-since-cxx23">
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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> InputIt, <span class="kw1">class</span> Alloc<span class="sy1">></span><br /></span>
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<p>queue<span class="br0">(</span>InputIt, InputIt, Alloc<span class="br0">)</span><br />
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<span class="sy2">-</span><span class="sy1">></span> queue<span class="sy1"><</span><span class="coMULTI">/*iter-value-t*/</span><span class="sy1"><</span>InputIt<span class="sy1">></span>,<br /></p>
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<a href="cpp-container-deque.html"><span class="kw1271">std::<span class="me2">deque</span></span></a><span class="sy1"><</span><span class="coMULTI">/*iter-value-t*/</span><span class="sy1"><</span>InputIt<span class="sy1">></span>, Alloc<span class="sy1">>></span><span class="sy4">;</span></div>
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</td>
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<td>(4)</td>
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<td><span class="t-mark-rev t-since-cxx23">(C++23 起)</span></td>
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<tr class="t-dcl-sep">
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<td></td>
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<td></td>
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<p>为 <code>queue</code> 提供<a href="cpp-language-class_template_argument_deduction.html">推导指引</a>以允许从底层容器类型推导。</p>
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<div class="t-li1"><span class="t-li">1)</span> 从参数推导底层容器类型。</div>
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<div class="t-li1"><span class="t-li">2)</span> 从迭代器推导元素类型，以 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-container-deque.html"><span class="kw1271">std::<span class="me2">deque</span></span></a><span class="sy1"><</span><span class="coMULTI">/*iter-value-t*/</span><span class="sy1"><</span>InputIt<span class="sy1">>></span></span></span> 为底层容器类型，其中 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="coMULTI">/*iter-value-t*/</span><span class="sy1"><</span>InputIt<span class="sy1">></span></span></span> 代表 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">typename</span> <a href="cpp-iterator-iterator_traits.html"><span class="kw651">std::<span class="me2">iterator_traits</span></span></a><span class="sy1"><</span>InputIt<span class="sy1">></span><span class="sy4">::</span><span class="me2">value_type</span></span></span> 。</div>
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<div class="t-li1"><span class="t-li">3-4)</span> 同 <span class="t-v">(1-2)</span> ，除了提供分配器。</div>
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<p>这些重载仅若</p>
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<ul>
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<li><code>InputIt</code> （若存在）满足<a href="cpp-named_req-InputIterator.html"><span style="font-style:italic">老式输入迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyInputIterator)</span></a> ，</li>
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<li><code>Container</code> （若存在）不满足<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> ，</li>
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<li>对于 <span class="t-rev-inl t-until-cxx23"><span><span class="t-v">(3)</span></span> <span><span class="t-mark-rev t-until-cxx23">(C++23 前)</span></span></span><span class="t-rev-inl t-since-cxx23"><span><span class="t-v">(4)</span></span> <span><span class="t-mark-rev t-since-cxx23">(C++23 起)</span></span></span> ，<code>Alloc</code> 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> ，以及</li>
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<li>若 <code>Container</code> 与 <code>Alloc</code> 均存在则 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-memory-uses_allocator.html"><span class="kw705">std::<span class="me2">uses_allocator_v</span></span></a><span class="sy1"><</span>Container, Alloc<span class="sy1">></span></span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span></li>
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</ul>
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<p>才参与重载决议。</p>
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<p>注意：库确定类型是否满足<a href="cpp-named_req-InputIterator.html"><span style="font-style:italic">老式输入迭代器</span> <span style="font-family: Georgia, 'DejaVu Serif', serif; font-style:italic">(LegacyInputIterator)</span></a> 的程度是未指定的，除了最低要求是整数类型不具备输入迭代器的条件。类似地，确定类型是否满足<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> 是未指定的，除了最低要求是成员类型 <code>Alloc::value_type</code> 必须存在，且表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-declval.html"><span class="kw1120">std::<span class="me2">declval</span></span></a><span class="sy1"><</span>Alloc<span class="sy3">&</span><span class="sy1">></span><span class="br0">(</span><span class="br0">)</span>.<span class="me1">allocate</span><span class="br0">(</span><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a><span class="br0">{</span><span class="br0">}</span><span class="br0">)</span></span></span> 在作为不求值操作数时必须为良构。</p>
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<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
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<div class="t-example">
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<div class="t-example-live-link"></div>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="cpp source-cpp">
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<pre class="de1"><span class="co2">#include <vector></span>
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<span class="co2">#include <queue></span>
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<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
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<a href="cpp-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> v <span class="sy1">=</span> <span class="br0">{</span><span class="nu0">1</span>,<span class="nu0">2</span>,<span class="nu0">3</span>,<span class="nu0">4</span><span class="br0">}</span><span class="sy4">;</span>
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<a href="cpp-container-queue.html"><span class="kw1298">std::<span class="me2">queue</span></span></a> s<span class="br0">{</span>v<span class="br0">}</span><span class="sy4">;</span> <span class="co1">// 指引 #1 推导 std::queue<int, vector<int>></span>
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<span class="br0">}</span></pre></div>
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