175 lines
22 KiB
HTML
175 lines
22 KiB
HTML
<!DOCTYPE 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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<head><meta http-equiv="x-ua-compatible" content="ie=edge">
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<title>折叠表达式(C++17 起)</title>
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<link rel="stylesheet" href="ext.css" />
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<link rel="stylesheet" href="site_modules.css" />
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_language_fold skin-cppreference2 action-view cpp-navbar">
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<div id="cpp-content-base">
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<h1 id="firstHeading" class="firstHeading">折叠表达式<span class="t-mark-rev t-since-cxx17">(C++17 起)</span></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-language.html">language</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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<p>以二元运算符对<a href="cpp-language-parameter_pack.html">形参包</a>进行规约（<a href="https://en.wikipedia.org/wiki/Fold_(higher-order_function)" class="extiw" title="enwiki:Fold (higher-order function)">折叠</a>）。</p>
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<h3><span class="mw-headline" id=".E8.AF.AD.E6.B3.95">语法</span></h3>
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<table class="t-sdsc-begin">
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<tr>
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<td colspan="10" class="t-sdsc-sep"></td>
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</tr>
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<tr class="t-sdsc">
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<td><code><b>(</b></code> <span class="t-spar">形参包</span> <span class="t-spar">运算符</span> <code><b>...</b></code> <code><b>)</b></code></td>
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<td>(1)</td>
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<td class="t-sdsc-nopad"></td>
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</tr>
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<tr>
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<td colspan="10" class="t-sdsc-sep"></td>
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<tr class="t-sdsc">
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<td><code><b>(</b></code> <code><b>...</b></code> <span class="t-spar">运算符</span> <span class="t-spar">形参包</span> <code><b>)</b></code></td>
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<td>(2)</td>
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<td class="t-sdsc-nopad"></td>
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</tr>
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<tr>
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<td colspan="10" class="t-sdsc-sep"></td>
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<tr class="t-sdsc">
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<td><code><b>(</b></code> <span class="t-spar">形参包</span> <span class="t-spar">运算符</span> <code><b>...</b></code> <span class="t-spar">运算符</span> <span class="t-spar">初值</span> <code><b>)</b></code></td>
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<td>(3)</td>
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<td class="t-sdsc-nopad"></td>
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</tr>
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<tr>
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<td colspan="10" class="t-sdsc-sep"></td>
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<tr class="t-sdsc">
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<td><code><b>(</b></code> <span class="t-spar">初值</span> <span class="t-spar">运算符</span> <code><b>...</b></code> <span class="t-spar">运算符</span> <span class="t-spar">形参包</span> <code><b>)</b></code></td>
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<td>(4)</td>
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<td class="t-sdsc-nopad"></td>
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</tr>
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<tr>
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<td colspan="10" class="t-sdsc-sep"></td>
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<div class="t-li1"><span class="t-li">1)</span> 一元右折叠</div>
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<div class="t-li1"><span class="t-li">2)</span> 一元左折叠</div>
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<div class="t-li1"><span class="t-li">3)</span> 二元右折叠</div>
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<div class="t-li1"><span class="t-li">4)</span> 二元左折叠</div>
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<table class="t-par-begin">
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<tr class="t-par">
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<td><span class="t-spar">运算符</span></td>
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<td>-</td>
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<td>任何下列 32 个<i>二元</i>运算符之一：<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">+</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">-</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">/</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">%</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">^</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">&</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">|</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy1">=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy1"><</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy1">></span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy1"><<</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy1">>></span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">+</span><span class="sy1">=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">-</span><span class="sy1">=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">*</span><span class="sy1">=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">/</span><span class="sy1">=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">%</span><span class="sy1">=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">^</span><span class="sy1">=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">&</span><span class="sy1">=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">|</span><span class="sy1">=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy1"><<=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy1">>>=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy1">==</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">!</span><span class="sy1">=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy1"><=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy1">>=</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">&&</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">||</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp">,</span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp">.<span class="sy2">*</span></span></span> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">-</span><span class="sy1">></span><span class="sy2">*</span></span></span>。在二元折叠中，两个 <span class="t-spar">运算符</span> 必须相同。</td>
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</tr>
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<tr class="t-par">
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<td><span class="t-spar">形参包</span></td>
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<td>-</td>
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<td>含有未展开的<a href="cpp-language-parameter_pack.html">形参包</a>且在顶层不含<a href="cpp-language-operator_precedence.html">优先级</a>低于转型（正式而言，是 <span class="t-spar">转型表达式</span>）的运算符的表达式</td>
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</tr>
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<tr class="t-par">
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<td><span class="t-spar">初值</span></td>
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<td>-</td>
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<td>不含未展开的<a href="cpp-language-parameter_pack.html">形参包</a>且在顶层不含<a href="cpp-language-operator_precedence.html">优先级</a>低于转型（正式而言，是 <span class="t-spar">转型表达式</span>）的运算符的表达式</td>
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</tr>
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</table>
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<p>注意开闭括号也是折叠表达式的一部分。</p>
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<h3><span class="mw-headline" id=".E8.A7.A3.E9.87.8A">解释</span></h3>
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<p><i>折叠表达式</i>的实例化按以下方式展开成表达式 <code>e</code>：</p>
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<div class="t-li1"><span class="t-li">1)</span> 一元右折叠 <code>(E <span class="t-spar">运算符</span> ...)</code> 成为 <code>(E<sub>1</sub> <span class="t-spar">运算符</span> (... <span class="t-spar">运算符</span> (E<sub>N-1</sub> <span class="t-spar">运算符</span> E<sub>N</sub>)))</code></div>
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<div class="t-li1"><span class="t-li">2)</span> 一元左折叠 <code>(... <span class="t-spar">运算符</span> E)</code> 成为 <code>(((E<sub>1</sub> <span class="t-spar">运算符</span> E<sub>2</sub>) <span class="t-spar">运算符</span> ...) <span class="t-spar">运算符</span> E<sub>N</sub>)</code></div>
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<div class="t-li1"><span class="t-li">3)</span> 二元右折叠 <code>(E <span class="t-spar">运算符</span> ... <span class="t-spar">运算符</span> I)</code> 成为 <code>(E<sub>1</sub> <span class="t-spar">运算符</span> (... <span class="t-spar">运算符</span> (E<sub>N−1</sub> <span class="t-spar">运算符</span> (E<sub>N</sub> <span class="t-spar">运算符</span> I))))</code></div>
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<div class="t-li1"><span class="t-li">4)</span> 二元左折叠 <code>(I <span class="t-spar">运算符</span> ... <span class="t-spar">运算符</span> E)</code> 成为 <code>((((I <span class="t-spar">运算符</span> E<sub>1</sub>) <span class="t-spar">运算符</span> E<sub>2</sub>) <span class="t-spar">运算符</span> ...) <span class="t-spar">运算符</span> E<sub>N</sub>)</code></div>
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<p>（其中 <code>N</code> 是包展开中的元素数量）</p>
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<p>例如，</p>
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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="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span>... <span class="me1">Args</span><span class="sy1">></span>
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<span class="kw4">bool</span> all<span class="br0">(</span>Args... <span class="me1">args</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="br0">(</span>... <span class="sy3">&&</span> args<span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span>
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<span class="kw4">bool</span> b <span class="sy1">=</span> all<span class="br0">(</span><span class="kw2">true</span>, <span class="kw2">true</span>, <span class="kw2">true</span>, <span class="kw2">false</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// 在 all() 中，一元左折叠展开成</span>
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<span class="co1">// return ((true && true) && true) && false;</span>
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<span class="co1">// b 是 false</span></pre></div>
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</div>
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<p>将一元折叠用于长度为零的包展开时，只能使用下列运算符：</p>
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<div class="t-li1"><span class="t-li">1)</span> 逻辑与（<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">&&</span></span></span>）。空包的值为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span></div>
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<div class="t-li1"><span class="t-li">2)</span> 逻辑或（<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy3">||</span></span></span>）。空包的值为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span></div>
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<div class="t-li1"><span class="t-li">3)</span> 逗号运算符（<span class="t-c"><span class="mw-geshi cpp source-cpp">,</span></span>）。空包的值为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span><span class="br0">(</span><span class="br0">)</span></span></span></div>
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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>如果用作 <span class="t-spar">初值</span> 或 <span class="t-spar">形参包</span> 的表达式在顶层具有优先级低于转型的运算符，那么它可以加括号：</p>
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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="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> ...<span class="me1">Args</span><span class="sy1">></span>
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<span class="kw4">int</span> sum<span class="br0">(</span>Args<span class="sy3">&&</span>... <span class="me1">args</span><span class="br0">)</span>
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<span class="br0">{</span>
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<span class="co1">// return (args + ... + 1 * 2); // 错误：优先级低于转型的运算符</span>
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<span class="kw1">return</span> <span class="br0">(</span>args <span class="sy2">+</span> ... <span class="sy2">+</span> <span class="br0">(</span><span class="nu0">1</span> <span class="sy2">*</span> <span class="nu0">2</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK</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 <iostream></span>
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<span class="co2">#include <vector></span>
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<span class="co2">#include <climits></span>
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<span class="co2">#include <cstdint></span>
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<span class="co2">#include <type_traits></span>
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<span class="co2">#include <utility></span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> ...<span class="me1">Args</span><span class="sy1">></span>
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<span class="kw4">void</span> printer<span class="br0">(</span>Args<span class="sy3">&&</span>... <span class="me1">args</span><span class="br0">)</span>
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<span class="br0">{</span>
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<span class="br0">(</span><a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> ... <span class="sy1"><<</span> args<span class="br0">)</span> <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="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T, <span class="kw1">class</span>... <span class="me1">Args</span><span class="sy1">></span>
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<span class="kw4">void</span> push_back_vec<span class="br0">(</span><a href="cpp-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1"><</span>T<span class="sy1">></span><span class="sy3">&</span> v, Args<span class="sy3">&&</span>... <span class="me1">args</span><span class="br0">)</span>
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<span class="br0">{</span>
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static_assert<span class="br0">(</span><span class="br0">(</span><a href="cpp-types-is_constructible.html"><span class="kw521">std::<span class="me2">is_constructible_v</span></span></a><span class="sy1"><</span>T, Args<span class="sy3">&</span><span class="sy1">></span> <span class="sy3">&&</span> ...<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">(</span>v.<span class="me1">push_back</span><span class="br0">(</span>args<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">// 基于 http://stackoverflow.com/a/36937049 的编译期端序交换 </span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> T, <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a>... <span class="me1">N</span><span class="sy1">></span>
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<span class="kw4">constexpr</span> T bswap_impl<span class="br0">(</span>T i, <a href="cpp-utility-integer_sequence.html"><span class="kw1108">std::<span class="me2">index_sequence</span></span></a><span class="sy1"><</span>N...<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> <span class="br0">(</span><span class="br0">(</span><span class="br0">(</span>i <span class="sy1">>></span> N<span class="sy2">*</span><a href="cpp-types-climits.html"><span class="kw387">CHAR_BIT</span></a> <span class="sy3">&</span> <a href="cpp-types-integer.html"><span class="kw120">std::<span class="me2">uint8_t</span></span></a><span class="br0">(</span><span class="sy2">-</span><span class="nu0">1</span><span class="br0">)</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="br0">(</span>sizeof<span class="br0">(</span>T<span class="br0">)</span><span class="sy2">-</span><span class="nu0">1</span><span class="sy2">-</span>N<span class="br0">)</span><span class="sy2">*</span><a href="cpp-types-climits.html"><span class="kw387">CHAR_BIT</span></a><span class="br0">)</span> <span class="sy3">|</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> T, <span class="kw1">class</span> U <span class="sy1">=</span> <a href="cpp-types-make_unsigned.html"><span class="kw619">std::<span class="me2">make_unsigned_t</span></span></a><span class="sy1"><</span>T<span class="sy1">>></span>
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<span class="kw4">constexpr</span> U bswap<span class="br0">(</span>T i<span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">return</span> bswap_impl<span class="sy1"><</span>U<span class="sy1">></span><span class="br0">(</span>i, <a href="cpp-utility-integer_sequence.html"><span class="kw1110">std::<span class="me2">make_index_sequence</span></span></a><span class="sy1"><</span>sizeof<span class="br0">(</span>T<span class="br0">)</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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<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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printer<span class="br0">(</span><span class="nu0">1</span>, <span class="nu0">2</span>, <span class="nu0">3</span>, <span class="st0">"abc"</span><span class="br0">)</span><span class="sy4">;</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="sy4">;</span>
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push_back_vec<span class="br0">(</span>v, <span class="nu0">6</span>, <span class="nu0">2</span>, <span class="nu0">45</span>, <span class="nu0">12</span><span class="br0">)</span><span class="sy4">;</span>
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push_back_vec<span class="br0">(</span>v, <span class="nu0">1</span>, <span class="nu0">2</span>, <span class="nu0">9</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">int</span> i <span class="sy4">:</span> v<span class="br0">)</span> <a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> i <span class="sy1"><<</span> <span class="st0">' '</span><span class="sy4">;</span>
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static_assert<span class="br0">(</span>bswap<span class="sy1"><</span><a href="cpp-types-integer.html"><span class="kw121">std::<span class="me2">uint16_t</span></span></a><span class="sy1">></span><span class="br0">(</span>0x1234u<span class="br0">)</span><span class="sy1">==</span>0x3412u<span class="br0">)</span><span class="sy4">;</span>
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static_assert<span class="br0">(</span>bswap<span class="sy1"><</span><a href="cpp-types-integer.html"><span class="kw123">std::<span class="me2">uint64_t</span></span></a><span class="sy1">></span><span class="br0">(</span>0x0123456789abcdefULL<span class="br0">)</span><span class="sy1">==</span>0xefcdab8967452301ULL<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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<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">123abc
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6 2 45 12 1 2 9</pre></div>
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