187 lines
16 KiB
HTML
187 lines
16 KiB
HTML
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<title>std::ranges::get(std::ranges::subrange)</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_ranges_subrange_get 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::ranges::</span>get<small>(std::ranges::subrange)</small></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-ranges.html">ranges</a>‎ | <a href="cpp-ranges-subrange.html">subrange</a></span></div>
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<div>定义于头文件 <code><a href="cpp-header-ranges.html"><ranges></a></code></div>
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<td></td>
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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> <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> N, <span class="kw1">class</span> I, <span class="kw1">class</span> S, ranges<span class="sy4">::</span><span class="me2">subrange_kind</span> K <span class="sy1">></span><br /></span>
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<p> requires <span class="br0">(</span>N <span class="sy1">==</span> <span class="nu0">0</span> <span class="sy3">&&</span> std<span class="sy4">::</span><span class="me2">copyable</span><span class="sy1"><</span>I<span class="sy1">></span><span class="br0">)</span> <span class="sy3">||</span> N <span class="sy1">==</span> <span class="nu0">1</span><span class="br0">)</span><br /></p>
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<span class="kw4">constexpr</span> <span class="kw4">auto</span> get<span class="br0">(</span> <span class="kw4">const</span> ranges<span class="sy4">::</span><span class="me2">subrange</span><span class="sy1"><</span>I, S, K<span class="sy1">></span><span class="sy3">&</span> r <span class="br0">)</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-cxx20">(C++20 起)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> N, <span class="kw1">class</span> I, <span class="kw1">class</span> S, ranges<span class="sy4">::</span><span class="me2">subrange_kind</span> K <span class="sy1">></span><br /></span>
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<p> requires <span class="br0">(</span>N <span class="sy1"><</span> <span class="nu0">2</span><span class="br0">)</span><br /></p>
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<span class="kw4">constexpr</span> <span class="kw4">auto</span> get<span class="br0">(</span> ranges<span class="sy4">::</span><span class="me2">subrange</span><span class="sy1"><</span>I, S, K<span class="sy1">></span><span class="sy3">&&</span> r <span class="br0">)</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-cxx20">(C++20 起)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">namespace</span> std <span class="br0">{</span> <span class="kw1">using</span> ranges<span class="sy4">::</span><span class="me2">get</span><span class="sy4">;</span> <span class="br0">}</span></span></div>
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<td>(3)</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<tr class="t-dcl-sep">
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<td></td>
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<div class="t-li1"><span class="t-li">1)</span> 分别在 <span class="t-c"><span class="mw-geshi cpp source-cpp">N <span class="sy1">==</span> <span class="nu0">0</span></span></span> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp">N <span class="sy1">==</span> <span class="nu0">1</span></span></span> 时从 <code>subrange</code> 左值（或 const 右值）获得迭代器或哨位。它主要为结构化绑定支持提供。</div>
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<div class="t-li1"><span class="t-li">2)</span> 同 <span class="t-v">(1)</span> ，除了它接收非 const <code>subrange</code> 右值。</div>
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<div class="t-li1"><span class="t-li">3)</span> 将 <span class="t-v">(1-2)</span> 导入命名空间 <code>std</code> ，这简化其使用并使得每个拥有可复制迭代器的 <code>subrange</code> 均为 pair 式类型。</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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<table class="t-par-begin">
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<tr class="t-par">
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<td>r</td>
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<td>-</td>
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<td><code>subrange</code></td>
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</tr>
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</table>
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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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<div class="t-li1"><span class="t-li">1)</span> 当 <span class="t-c"><span class="mw-geshi cpp source-cpp">N <span class="sy1">==</span> <span class="nu0">0</span></span></span> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp">N <span class="sy1">==</span> <span class="nu0">1</span></span></span> 时分别为从存储的迭代器或哨位复制构造的迭代器或哨位。</div>
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<div class="t-li1"><span class="t-li">2)</span> 同 <span class="t-v">(1)</span> ，除了若 <span class="t-c"><span class="mw-geshi cpp source-cpp">N <span class="sy1">==</span> <span class="nu0">0</span></span></span> 且 <code>I</code> 不满足 <a href="cpp-concepts-copyable.html"><code>copyable</code></a> 则移动构造迭代器。</div>
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<h3><span class="mw-headline" id=".E5.8F.AF.E8.83.BD.E7.9A.84.E5.AE.9E.E7.8E.B0">可能的实现</span></h3>
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<table class="eq-fun-cpp-table">
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<div class="cpp source-cpp">
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<pre class="de1"><span class="kw1">template</span><span class="sy1"><</span><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> N, <span class="kw1">class</span> I, <span class="kw1">class</span> S, std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">subrange_kind</span> K<span class="sy1">></span>
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requires <span class="br0">(</span>N <span class="sy1">==</span> <span class="nu0">0</span> <span class="sy3">&&</span> std<span class="sy4">::</span><span class="me2">copyable</span><span class="sy1"><</span>I<span class="sy1">></span><span class="br0">)</span> <span class="sy3">||</span> N <span class="sy1">==</span> <span class="nu0">1</span><span class="br0">)</span>
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<span class="kw4">constexpr</span> <span class="kw4">auto</span> get<span class="br0">(</span><span class="kw4">const</span> std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">subrange</span><span class="sy1"><</span>I, S, K<span class="sy1">></span><span class="sy3">&</span> r<span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">if</span> <span class="kw4">constexpr</span> <span class="br0">(</span>N <span class="sy1">==</span> <span class="nu0">0</span><span class="br0">)</span>
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<span class="kw1">return</span> r.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">else</span>
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<span class="kw1">return</span> r.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw1">template</span><span class="sy1"><</span><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> N, <span class="kw1">class</span> I, <span class="kw1">class</span> S, std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">subrange_kind</span> K<span class="sy1">></span>
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requires <span class="br0">(</span>N <span class="sy1"><</span> <span class="nu0">2</span><span class="br0">)</span>
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<span class="kw4">constexpr</span> <span class="kw4">auto</span> get<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">subrange</span><span class="sy1"><</span>I, S, K<span class="sy1">></span><span class="sy3">&&</span> r<span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">if</span> <span class="kw4">constexpr</span> <span class="br0">(</span>N <span class="sy1">==</span> <span class="nu0">0</span><span class="br0">)</span>
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<span class="kw1">return</span> r.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 可能进行移动构造</span>
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<span class="kw1">else</span>
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<span class="kw1">return</span> r.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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</div>
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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 <array></span>
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<span class="co2">#include <iostream></span>
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<span class="co2">#include <iterator></span>
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<span class="co2">#include <ranges></span>
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<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
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<span class="br0">{</span>
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<a href="cpp-container-array.html"><span class="kw1268">std::<span class="me2">array</span></span></a> a<span class="br0">{</span><span class="nu0">1</span>, <span class="sy2">-</span><span class="nu0">2</span>, <span class="nu0">3</span>, <span class="sy2">-</span><span class="nu0">4</span><span class="br0">}</span><span class="sy4">;</span>
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std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">subrange</span> sub_a<span class="br0">{</span> <a href="cpp-iterator-next.html"><span class="kw674">std::<span class="me2">next</span></span></a><span class="br0">(</span>a.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span>, <a href="cpp-iterator-prev.html"><span class="kw675">std::<span class="me2">prev</span></span></a><span class="br0">(</span>a.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span> <span class="br0">}</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="sy2">*</span>std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">get</span><span class="sy1"><</span><span class="nu0">0</span><span class="sy1">></span><span class="br0">(</span>sub_a<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">' '</span> <span class="co1">// == *(begin(a) + 1)</span>
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<span class="sy1"><<</span> <span class="sy2">*</span>std<span class="sy4">::</span><span class="me2">get</span><span class="sy1"><</span><span class="nu0">1</span><span class="sy1">></span><span class="br0">(</span>sub_a<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span> <span class="co1">// == *(end(a) - 1)</span>
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<span class="sy2">*</span>std<span class="sy4">::</span><span class="me2">get</span><span class="sy1"><</span><span class="nu0">0</span><span class="sy1">></span><span class="br0">(</span>sub_a<span class="br0">)</span> <span class="sy1">=</span> <span class="nu0">42</span><span class="sy4">;</span> <span class="co1">// OK</span>
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<span class="co1">// *std::get<2>(sub_a) = 13; // 硬错误：下标只能为 0 或 1</span>
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<span class="br0">}</span></pre></div>
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</div>
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<p>输出：</p>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="text source-text">
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<pre class="de1">-2 -4</pre></div>
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</div>
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</div>
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<h3><span class="mw-headline" id=".E7.BC.BA.E9.99.B7.E6.8A.A5.E5.91.8A">缺陷报告</span></h3>
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<p>下列更改行为的缺陷报告追溯地应用于以前出版的 C++ 标准。</p>
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<table class="dsctable" style="font-size:0.8em">
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<tr>
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<th>DR</th>
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<th>应用于</th>
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<th>出版时的行为</th>
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<th>正确行为</th>
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</tr>
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<tr>
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<td><a rel="nofollow" class="external text" href="https://cplusplus.github.io/LWG/issue3589">LWG 3589</a></td>
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<td>C++20</td>
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<td>若 <span class="t-c"><span class="mw-geshi cpp source-cpp">N <span class="sy1">==</span> <span class="nu0">0</span></span></span> 且 <code>I</code> 不实现 <a href="cpp-concepts-copyable.html"><code>copyable</code></a> 则 const 左值的重载非良构</td>
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<td>从重载集移除它</td>
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</tr>
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</table>
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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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<tr class="t-dsc">
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<td><a href="cpp-language-structured_binding.html">结构化绑定</a> <span class="t-mark-rev t-since-cxx17">(C++17)</span></td>
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<td>绑定指定的名字到初始化器的子对象或元组元素</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-tuple-get.html"><span class="t-lines"><span>std::get<span class="t-dsc-small">(std::tuple)</span></span></span></a></div>
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</div>
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</td>
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<td>元组式访问指定的元素<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-utility-pair-get.html"><span class="t-lines"><span>std::get<span class="t-dsc-small">(std::pair)</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>访问 <code>pair</code> 的一个元素<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-container-array-get.html"><span class="t-lines"><span>std::get<span class="t-dsc-small">(std::array)</span></span></span></a></div>
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</div>
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</td>
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<td>访问 <code>array</code> 的一个元素<br />
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<span class="t-mark">(函数模板)</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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</body>
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