78 lines
5.9 KiB
HTML
78 lines
5.9 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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<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
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<title>sizeof... 运算符 (C++11 起)</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_language_sizeof skin-cppreference2 action-view cpp-navbar">
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<div id="cpp-content-base">
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<div id="content"><a id="top"></a>
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<h1 id="firstHeading" class="firstHeading">sizeof... 运算符 <span class="t-mark-rev t-since-cxx11">(C++11 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>查询<a href="cpp-language-parameter_pack.html">形参包</a>中的元素数量。</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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<td colspan="10" class="t-sdsc-sep"></td>
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<tr class="t-sdsc">
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<td class="t-sdsc-nopad"><code><b>sizeof...(</b></code> <span class="t-spar">形参包</span> <code><b>)</b></code></td>
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<td class="t-sdsc-nopad"></td>
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<td class="t-sdsc-nopad"></td>
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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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</table>
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<p>返回 <span class="t-lc"><a href="cpp-types-size_t.html">std::size_t</a></span> 类型的常量。</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>返回<a href="cpp-language-parameter_pack.html">形参包</a>中的元素数量。</p>
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<h3><span class="mw-headline" id=".E5.85.B3.E9.94.AE.E8.AF.8D">关键词</span></h3>
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<p><a href="cpp-keyword-sizeof.html"><tt>sizeof</tt></a></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 <array></span>
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<span class="co2">#include <iostream></span>
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<span class="co2">#include <type_traits></span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span>... <span class="me1">Ts</span><span class="sy1">></span>
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<span class="kw4">constexpr</span> <span class="kw4">auto</span> make_array<span class="br0">(</span>Ts<span class="sy3">&&</span>... <span class="me1">ts</span><span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">using</span> CT <span class="sy1">=</span> <a href="cpp-types-common_type.html"><span class="kw636">std::<span class="me2">common_type_t</span></span></a><span class="sy1"><</span>Ts...<span class="sy1">></span><span class="sy4">;</span>
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<span class="kw1">return</span> <a href="cpp-container-array.html"><span class="kw1268">std::<span class="me2">array</span></span></a><span class="sy1"><</span>CT, sizeof...<span class="br0">(</span>Ts<span class="br0">)</span><span class="sy1">></span><span class="br0">{</span><a href="cpp-utility-forward.html"><span class="kw1117">std::<span class="me2">forward</span></span></a><span class="sy1"><</span>CT<span class="sy1">></span><span class="br0">(</span>ts<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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<a href="cpp-container-array.html"><span class="kw1268">std::<span class="me2">array</span></span></a><span class="sy1"><</span><span class="kw4">double</span>, 4ul<span class="sy1">></span> arr <span class="sy1">=</span> make_array<span class="br0">(</span><span class="nu0">1</span>, <span class="nu17">2.71f</span>, <span class="nu16">3.14</span>, <span class="st0">'*'</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="st0">"arr = { "</span><span class="sy4">;</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">auto</span> s<span class="br0">{</span>arr.<span class="me1">size</span><span class="br0">(</span><span class="br0">)</span><span class="br0">}</span><span class="sy4">;</span> <span class="kw4">double</span> elem <span class="sy4">:</span> arr<span class="br0">)</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> elem <span class="sy1"><<</span> <span class="br0">(</span><span class="sy2">--</span>s <span class="sy4">?</span> <span class="st0">", "</span> <span class="sy4">:</span> <span class="st0">" "</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="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">arr = { 1, 2.71, 3.14, 42 }</pre></div>
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<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
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<ul>
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<li><a href="cpp-language-sizeof.html">sizeof</a></li>
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</ul>
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</div>
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<div class="visualClear"></div>
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