252 lines
20 KiB
HTML
252 lines
20 KiB
HTML
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<title>std::integer_sequence</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_integer_sequence 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>integer_sequence</h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-utility.html">utility</a></span></div>
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<div>定义于头文件 <code><a href="cpp-header-utility.html"><utility></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, T... <span class="me1">Ints</span> <span class="sy1">></span><br />
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<span class="kw1">struct</span> integer_sequence<span class="sy4">;</span></span></div>
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<td><span class="t-mark-rev t-since-cxx14">(C++14 起)</span></td>
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<p>类模板 <code>std::integer_sequence</code> 表示一个编译时的整数序列。在用作<a href="cpp-language-function_template.html">函数模板</a>的实参时，能推导<a href="cpp-language-parameter_pack.html">参数包</a> <code>Ints</code> 并将它用于包展开。</p>
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<h3><span class="mw-headline" id=".E6.A8.A1.E6.9D.BF.E5.BD.A2.E5.8F.82">模板形参</span></h3>
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<td>T</td>
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<td>-</td>
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<td>用于序列元素的整数类型</td>
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<td>...Ints</td>
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<td>-</td>
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<td>表示序列的非类型形参包</td>
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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>成员类型</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>T</code></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><strong class="selflink"><span class="t-lines"><span>size</span></span></strong></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>返回 <code>Ints</code> 中的元素数<br />
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<span class="t-mark">(公开静态成员函数)</span></td>
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<h2><span class="mw-headline" id="std::integer_sequence::size"><span style="font-size:0.7em; line-height:130%">std::integer_sequence::</span>size</span></h2>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw4">static</span> <span class="kw4">constexpr</span> <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> size<span class="br0">(</span><span class="br0">)</span> <span class="kw1">noexcept</span><span class="sy4">;</span></span></div>
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<p>返回 <code>Ints</code> 中的元素数。等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp">sizeof...<span class="br0">(</span>Ints<span class="br0">)</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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<p>（无）</p>
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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><code>Ints</code> 中的元素数。</p>
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<h3><span class="mw-headline" id=".E8.BE.85.E5.8A.A9.E6.A8.A1.E6.9D.BF">辅助模板</span></h3>
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<p>为帮助 <code>T</code> 为 <span class="t-lc"><a href="cpp-types-size_t.html">std::size_t</a></span> 的常用情况，定义辅助别名模板 <code><b>std::index_sequence</b></code> 。</p>
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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>... <span class="me1">Ints</span><span class="sy1">></span><br />
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<span class="kw1">using</span> index_sequence <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">integer_sequence</span><span class="sy1"><</span><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a>, Ints...<span class="sy1">></span><span class="sy4">;</span></span></div>
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<p>分别定义辅助别名模板 <code><b>std::make_integer_sequence</b></code> 与 <code><b>std::make_index_sequence</b></code> 以简化以 0, 1, 2, ..., <code>N-1</code> 为 <code>Ints</code> 创建 <code>std::integer_sequence</code> 与 <code>std::index_sequence</code> 类型：</p>
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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, T N<span class="sy1">></span><br />
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<span class="kw1">using</span> make_integer_sequence <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">integer_sequence</span><span class="sy1"><</span>T, <span class="coMULTI">/* a sequence 0, 1, 2, ..., N-1 */</span> <span class="sy1">></span><span class="sy4">;</span></span></div>
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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="sy1">></span><br />
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<span class="kw1">using</span> make_index_sequence <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">make_integer_sequence</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="sy1">></span><span class="sy4">;</span></span></div>
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<p>若 <code>N</code> 为负则程序为谬构。若 <code>N</code> 为零，则指示类型为 <code>integer_sequence<T></code> 。</p>
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<p>定义辅助类模板 <code><b>std::index_sequence_for</b></code> ，以转换任何类型参数包为同长度的下标序列：</p>
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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>... <span class="me1">T</span><span class="sy1">></span><br />
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<span class="kw1">using</span> index_sequence_for <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">make_index_sequence</span><span class="sy1"><</span>sizeof...<span class="br0">(</span>T<span class="br0">)</span><span class="sy1">></span><span class="sy4">;</span></span></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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<p>注意：另一示例见 <a href="cpp-utility-apply.html#.E5.8F.AF.E8.83.BD.E7.9A.84.E5.AE.9E.E7.8E.B0"><code>std::apply</code> 中可能的实现</a>。</p>
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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 <tuple></span>
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<span class="co2">#include <iostream></span>
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<span class="co2">#include <array></span>
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<span class="co2">#include <utility></span>
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<span class="co1">// 调试辅助</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> T, T... <span class="me1">ints</span><span class="sy1">></span>
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<span class="kw4">void</span> print_sequence<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">integer_sequence</span><span class="sy1"><</span>T, ints...<span class="sy1">></span> int_seq<span class="br0">)</span>
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<span class="br0">{</span>
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<a href="cpp-io-cout.html"><span class="kw1753">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"The sequence of size "</span> <span class="sy1"><<</span> int_seq.<span class="me1">size</span><span class="br0">(</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">": "</span><span class="sy4">;</span>
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<span class="br0">(</span><span class="br0">(</span><a href="cpp-io-cout.html"><span class="kw1753">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> ints <span class="sy1"><<</span> <span class="st0">' '</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="kw1753">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="co1">// 转换数组为 tuple</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> Array, <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a>... <span class="me1">I</span><span class="sy1">></span>
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<span class="kw4">auto</span> a2t_impl<span class="br0">(</span><span class="kw4">const</span> Array<span class="sy3">&</span> a, std<span class="sy4">::</span><span class="me2">index_sequence</span><span class="sy1"><</span>I...<span class="sy1">></span><span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">return</span> <a href="cpp-utility-tuple-make_tuple.html"><span class="kw1107">std::<span class="me2">make_tuple</span></span></a><span class="br0">(</span>a<span class="br0">[</span>I<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><span class="kw1">typename</span> T, <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> N, <span class="kw1">typename</span> Indices <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">make_index_sequence</span><span class="sy1"><</span>N<span class="sy1">>></span>
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<span class="kw4">auto</span> a2t<span class="br0">(</span><span class="kw4">const</span> <a href="cpp-container-array.html"><span class="kw1263">std::<span class="me2">array</span></span></a><span class="sy1"><</span>T, N<span class="sy1">></span><span class="sy3">&</span> a<span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">return</span> a2t_impl<span class="br0">(</span>a, Indices<span class="br0">{</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="co1">// 漂亮地打印 tuple</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> Ch, <span class="kw1">class</span> Tr, <span class="kw1">class</span> Tuple, <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a>... <span class="me1">Is</span><span class="sy1">></span>
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<span class="kw4">void</span> print_tuple_impl<span class="br0">(</span><a href="cpp-io-basic_ostream.html"><span class="kw1710">std::<span class="me2">basic_ostream</span></span></a><span class="sy1"><</span>Ch,Tr<span class="sy1">></span><span class="sy3">&</span> os,
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<span class="kw4">const</span> Tuple<span class="sy3">&</span> t,
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std<span class="sy4">::</span><span class="me2">index_sequence</span><span class="sy1"><</span>Is...<span class="sy1">></span><span class="br0">)</span>
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<span class="br0">{</span>
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<span class="br0">(</span><span class="br0">(</span>os <span class="sy1"><<</span> <span class="br0">(</span>Is <span class="sy1">==</span> <span class="nu0">0</span><span class="sy4">?</span> <span class="st0">""</span> <span class="sy4">:</span> <span class="st0">", "</span><span class="br0">)</span> <span class="sy1"><<</span> std<span class="sy4">::</span><span class="me2">get</span><span class="sy1"><</span>Is<span class="sy1">></span><span class="br0">(</span>t<span class="br0">)</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><span class="kw1">class</span> Ch, <span class="kw1">class</span> Tr, <span class="kw1">class</span>... <span class="me1">Args</span><span class="sy1">></span>
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<span class="kw4">auto</span><span class="sy3">&</span> operator<span class="sy1"><<</span><span class="br0">(</span><a href="cpp-io-basic_ostream.html"><span class="kw1710">std::<span class="me2">basic_ostream</span></span></a><span class="sy1"><</span>Ch, Tr<span class="sy1">></span><span class="sy3">&</span> os, <span class="kw4">const</span> <a href="cpp-utility-tuple.html"><span class="kw1098">std::<span class="me2">tuple</span></span></a><span class="sy1"><</span>Args...<span class="sy1">></span><span class="sy3">&</span> t<span class="br0">)</span>
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<span class="br0">{</span>
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os <span class="sy1"><<</span> <span class="st0">"("</span><span class="sy4">;</span>
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print_tuple_impl<span class="br0">(</span>os, t, std<span class="sy4">::</span><span class="me2">index_sequence_for</span><span class="sy1"><</span>Args...<span class="sy1">></span><span class="br0">{</span><span class="br0">}</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">return</span> os <span class="sy1"><<</span> <span class="st0">")"</span><span class="sy4">;</span>
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<span class="br0">}</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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print_sequence<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">integer_sequence</span><span class="sy1"><</span><span class="kw4">unsigned</span>, <span class="nu0">9</span>, <span class="nu0">2</span>, <span class="nu0">5</span>, <span class="nu0">1</span>, <span class="nu0">9</span>, <span class="nu0">1</span>, <span class="nu0">6</span><span class="sy1">></span><span class="br0">{</span><span class="br0">}</span><span class="br0">)</span><span class="sy4">;</span>
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print_sequence<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">make_integer_sequence</span><span class="sy1"><</span><span class="kw4">int</span>, <span class="nu0">20</span><span class="sy1">></span><span class="br0">{</span><span class="br0">}</span><span class="br0">)</span><span class="sy4">;</span>
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print_sequence<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">make_index_sequence</span><span class="sy1"><</span><span class="nu0">10</span><span class="sy1">></span><span class="br0">{</span><span class="br0">}</span><span class="br0">)</span><span class="sy4">;</span>
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print_sequence<span class="br0">(</span>std<span class="sy4">::</span><span class="me2">index_sequence_for</span><span class="sy1"><</span><span class="kw4">float</span>, <a href="cpp-io-basic_iostream.html"><span class="kw1726">std::<span class="me2">iostream</span></span></a>, <span class="kw4">char</span><span class="sy1">></span><span class="br0">{</span><span class="br0">}</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="cpp-container-array.html"><span class="kw1263">std::<span class="me2">array</span></span></a><span class="sy1"><</span><span class="kw4">int</span>, <span class="nu0">4</span><span class="sy1">></span> array <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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<span class="co1">// 转换 array 为 tuple</span>
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<span class="kw4">auto</span> tuple <span class="sy1">=</span> a2t<span class="br0">(</span>array<span class="br0">)</span><span class="sy4">;</span>
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static_assert<span class="br0">(</span><a href="cpp-types-is_same.html"><span class="kw588">std::<span class="me2">is_same</span></span></a><span class="sy1"><</span>decltype<span class="br0">(</span>tuple<span class="br0">)</span>,
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<a href="cpp-utility-tuple.html"><span class="kw1098">std::<span class="me2">tuple</span></span></a><span class="sy1"><</span><span class="kw4">int</span>, <span class="kw4">int</span>, <span class="kw4">int</span>, <span class="kw4">int</span><span class="sy1">>></span><span class="sy4">::</span><span class="me2">value</span>, <span class="st0">""</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 打印到 cout</span>
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<a href="cpp-io-cout.html"><span class="kw1753">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> tuple <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</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">The sequence of size 7: 9 2 5 1 9 1 6
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The sequence of size 20: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
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The sequence of size 10: 0 1 2 3 4 5 6 7 8 9
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The sequence of size 3: 0 1 2
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(1, 2, 3, 4)</pre></div>
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</body>
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