413 lines
47 KiB
HTML
413 lines
47 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>形参包 (C++11 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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</head>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_language_parameter_pack 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">形参包 <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>模板形参包是接受零个或更多个模板实参（非类型、类型或模板）的模板形参。函数形参包是接受零个或更多个函数实参的函数形参。</p>
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<p>至少有一个形参包的模板被称作<i>变参模板</i>。</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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<p>模板形参包（在<a href="cpp-language-type_alias.html">别名模版</a>、<a href="cpp-language-class_template.html">类模板</a>、<a href="cpp-language-variable_template.html">变量模板</a>及<a href="cpp-language-function_template.html">函数模板</a>形参列表中出现）</p>
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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><span class="t-spar">类型</span> <code><b>...</b></code> <span class="t-spar">包名</span><span class="t-mark">(可选)</span></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>
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<tr class="t-sdsc">
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<td><code><b>typename</b></code><code><b>|</b></code><code><b>class</b></code> <code><b>...</b></code> <span class="t-spar">包名</span><span class="t-mark">(可选)</span></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>
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<tr class="t-sdsc">
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<td><span class="t-spar">类型约束</span> <code><b>...</b></code> <span class="t-spar">包名</span><span class="t-mark">(可选)</span></td>
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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>
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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>template</b></code> <code><b><</b></code> <span class="t-spar">形参列表</span> <code><b>></b></code> <code><b>class</b></code> <code><b>...</b></code> <span class="t-spar">包名</span><span class="t-mark">(可选)</span></td>
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<td>(4)</td>
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<td><span class="t-mark-rev t-until-cxx17">(C++17 前)</span></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>
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<tr class="t-sdsc">
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<td><code><b>template</b></code> <code><b><</b></code> <span class="t-spar">形参列表</span> <code><b>></b></code> <code><b>typename</b></code><code><b>|</b></code><code><b>class</b></code> <code><b>...</b></code> <span class="t-spar">包名</span><span class="t-mark">(可选)</span></td>
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<td>(4)</td>
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<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></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>
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</table>
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<p>函数参数包（<a href="cpp-language-declarations.html">声明符</a>的一种形式，在变参函数模板的函数形参列表中出现）</p>
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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><span class="t-spar">包名</span> <code><b>...</b></code> <span class="t-spar">包形参名</span><span class="t-mark">(可选)</span></td>
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<td>(5)</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>
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</table>
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<p>形参包展开（在变参模板体中出现）</p>
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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><span class="t-spar">模式</span> <code><b>...</b></code></td>
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<td>(6)</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>
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</table>
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<div class="t-li1"><span class="t-li">1)</span> 可以有名字的非类型模板形参包</div>
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<div class="t-li1"><span class="t-li">2)</span> 可以有名字的类型模板形参包</div>
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<table class="t-rev-begin">
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<tr class="t-rev t-since-cxx20">
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<td>
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<div class="t-li1"><span class="t-li">3)</span> 可以有名字的受<a href="cpp-language-constraints.html">约束</a>的类型模板形参包</div>
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</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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</tr>
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</table>
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<div class="t-li1"><span class="t-li">4)</span> 可以有名字的模板模板形参包</div>
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<div class="t-li1"><span class="t-li">5)</span> 可以有名字的函数形参包</div>
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<div class="t-li1"><span class="t-li">6)</span> 形参包展开：展开成零个或更多个 <span class="t-spar">模式</span> 的逗号分隔列表。模式必须包含至少一个形参包。</div>
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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>变参类模板可以用任意数量的模板实参实例化：</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">class</span>... <span class="me1">Types</span><span class="sy1">></span> <span class="kw1">struct</span> Tuple <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
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Tuple<span class="sy1"><></span> t0<span class="sy4">;</span> <span class="co1">// Types 不包含实参</span>
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Tuple<span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> t1<span class="sy4">;</span> <span class="co1">// Types 包含一个实参：int</span>
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Tuple<span class="sy1"><</span><span class="kw4">int</span>, <span class="kw4">float</span><span class="sy1">></span> t2<span class="sy4">;</span> <span class="co1">// Types 包含两个实参：int 与 float</span>
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Tuple<span class="sy1"><</span><span class="nu0">0</span><span class="sy1">></span> error<span class="sy4">;</span> <span class="co1">// 错误：0 不是类型</span></pre></div>
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</div>
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<p>变参函数模板可以用任意数量的函数实参调用（模板实参通过<a href="cpp-language-template_argument_deduction.html">模板实参推导</a>推导）：</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">class</span>... <span class="me1">Types</span><span class="sy1">></span> <span class="kw4">void</span> f<span class="br0">(</span>Types... <span class="me1">args</span><span class="br0">)</span><span class="sy4">;</span>
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f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK：args 不包含实参</span>
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f<span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK：args 包含一个实参：int</span>
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f<span class="br0">(</span><span class="nu0">2</span>, <span class="nu16">1.0</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK：args 包含两个实参：int 与 double</span></pre></div>
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</div>
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<p>在主类模板中，模板形参包必须是模板形参列表的最后一个形参。在函数模板中，模板参数包可以在列表中更早出现，只要其后的所有形参都可以从函数实参推导或拥有默认实参即可：</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">Ts</span>, <span class="kw1">typename</span> U<span class="sy1">></span> <span class="kw1">struct</span> Invalid<span class="sy4">;</span> <span class="co1">// 错误：Ts... 不在结尾</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="kw1">typename</span> U, <span class="kw1">typename</span><span class="sy1">=</span><span class="kw4">void</span><span class="sy1">></span>
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<span class="kw4">void</span> valid<span class="br0">(</span>U, Ts...<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK：能推导出 U</span>
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<span class="co1">// void valid(Ts..., U); // 不能使用：Ts... 在此位置是不推导语境</span>
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valid<span class="br0">(</span><span class="nu16">1.0</span>, <span class="nu0">1</span>, <span class="nu0">2</span>, <span class="nu0">3</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK：推导出 U 为 double，Ts 为 {int, int, int}</span></pre></div>
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</div>
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<p>如果变参模板的每个合法的特化都要求空模板形参包，那么程序非良构，不要求诊断。</p>
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<h3><span class="mw-headline" id=".E5.8C.85.E5.B1.95.E5.BC.80">包展开</span></h3>
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<p>后随省略号且其中至少有一个形参包的名字至少出现了一次的模式会被<i>展开</i>成零个或更多个逗号分隔的模式实例，其中形参包的名字按顺序被替换成包中的各个元素：</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">class</span>... <span class="me1">Us</span><span class="sy1">></span> <span class="kw4">void</span> f<span class="br0">(</span>Us... <span class="me1">pargs</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span>... <span class="me1">Ts</span><span class="sy1">></span> <span class="kw4">void</span> g<span class="br0">(</span>Ts... <span class="me1">args</span><span class="br0">)</span>
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<span class="br0">{</span>
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f<span class="br0">(</span><span class="sy3">&</span>args...<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// “&args...” 是包展开</span>
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<span class="co1">// “&args” 是它的模式</span>
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<span class="br0">}</span>
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g<span class="br0">(</span><span class="nu0">1</span>, <span class="nu16">0.2</span>, <span class="st0">"a"</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// Ts... args 会展开成 int E1, double E2, const char* E3</span>
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<span class="co1">// &args... 会展开成 &E1, &E2, &E3</span>
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<span class="co1">// Us... 会展开成 int* E1, double* E2, const char** E3</span></pre></div>
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</div>
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<p>如果两个形参包在同一模式中出现，那么它们同时展开而且长度必须相同：</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="sy1">></span> <span class="kw1">struct</span> Tuple <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span> T1, <span class="kw1">typename</span> T2<span class="sy1">></span> <span class="kw1">struct</span> Pair <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span>... <span class="me1">Args1</span><span class="sy1">></span> <span class="kw1">struct</span> zip
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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>... <span class="me1">Args2</span><span class="sy1">></span> <span class="kw1">struct</span> with
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<span class="br0">{</span>
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<span class="kw1">typedef</span> Tuple<span class="sy1"><</span>Pair<span class="sy1"><</span>Args1, Args2<span class="sy1">></span>...<span class="sy1">></span> type<span class="sy4">;</span>
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<span class="co1">// Pair<Args1, Args2>... 是包展开</span>
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<span class="co1">// Pair<Args1, Args2> 是模式</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">typedef</span> zip<span class="sy1"><</span><span class="kw4">short</span>, <span class="kw4">int</span><span class="sy1">></span><span class="sy4">::</span><span class="me2">with</span><span class="sy1"><</span><span class="kw4">unsigned</span> <span class="kw4">short</span>, <span class="kw4">unsigned</span><span class="sy1">></span><span class="sy4">::</span><span class="me2">type</span> T1<span class="sy4">;</span>
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<span class="co1">// Pair<Args1, Args2>... 会展开成</span>
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<span class="co1">// Pair<short, unsigned short>, Pair<int, unsigned int> </span>
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<span class="co1">// T1 是 Tuple<Pair<short, unsigned short>, Pair<int, unsigned>></span>
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<span class="kw1">typedef</span> zip<span class="sy1"><</span><span class="kw4">short</span><span class="sy1">></span><span class="sy4">::</span><span class="me2">with</span><span class="sy1"><</span><span class="kw4">unsigned</span> <span class="kw4">short</span>, <span class="kw4">unsigned</span><span class="sy1">></span><span class="sy4">::</span><span class="me2">type</span> T2<span class="sy4">;</span>
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<span class="co1">// 错误：包展开中的形参包包含不同长度</span></pre></div>
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</div>
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<p>如果包展开内嵌于另一个包展开中，那么它所展开的是在最内层包展开出现的形参包，并且在外围（而非最内层）的包展开中必须提及其它形参包：</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">class</span>... <span class="me1">Args</span><span class="sy1">></span>
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<span class="kw4">void</span> g<span class="br0">(</span>Args... <span class="me1">args</span><span class="br0">)</span>
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<span class="br0">{</span>
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f<span class="br0">(</span><span class="kw1">const_cast</span><span class="sy1"><</span><span class="kw4">const</span> Args<span class="sy2">*</span><span class="sy1">></span><span class="br0">(</span><span class="sy3">&</span>args<span class="br0">)</span>...<span class="br0">)</span><span class="sy4">;</span>
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<span class="co1">// const_cast<const Args*>(&args) 是模式，它同时展开两个包（Args 与 args）</span>
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f<span class="br0">(</span>h<span class="br0">(</span>args...<span class="br0">)</span> <span class="sy2">+</span> args...<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 嵌套包展开：</span>
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<span class="co1">// 内层包展开是 “args...”，它首先展开</span>
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<span class="co1">// 外层包展开是 h(E1, E2, E3) + args 它其次被展开</span>
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<span class="co1">// （成为 h(E1, E2, E3) + E1, h(E1, E2, E3) + E2, h(E1, E2, E3) + E3）</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=".E5.B1.95.E5.BC.80.E5.9C.BA.E6.89.80">展开场所</span></h3>
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<p>展开所产生的逗号分隔列表按发生展开的各个场所可以是不同种类的列表：函数形参列表，成员初始化器列表，属性列表，等等。以下列出了所有允许的语境。</p>
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<h4><span class="mw-headline" id=".E5.87.BD.E6.95.B0.E5.AE.9E.E5.8F.82.E5.88.97.E8.A1.A8">函数实参列表</span></h4>
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<p>包展开可以在函数调用运算符的括号内出现，此时省略号左侧的最大表达式或花括号初始化器列表是被展开的模式：</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">f<span class="br0">(</span><span class="sy3">&</span>args...<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 展开成 f(&E1, &E2, &E3)</span>
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f<span class="br0">(</span>n, <span class="sy2">++</span>args...<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 展开成 f(n, ++E1, ++E2, ++E3);</span>
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f<span class="br0">(</span><span class="sy2">++</span>args..., n<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 展开成 f(++E1, ++E2, ++E3, n);</span>
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f<span class="br0">(</span><span class="kw1">const_cast</span><span class="sy1"><</span><span class="kw4">const</span> Args<span class="sy2">*</span><span class="sy1">></span><span class="br0">(</span><span class="sy3">&</span>args<span class="br0">)</span>...<span class="br0">)</span><span class="sy4">;</span>
|
|
<span class="co1">// f(const_cast<const E1*>(&X1), const_cast<const E2*>(&X2), const_cast<const E3*>(&X3))</span>
|
|
f<span class="br0">(</span>h<span class="br0">(</span>args...<span class="br0">)</span> <span class="sy2">+</span> args...<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 展开成</span>
|
|
<span class="co1">// f(h(E1, E2, E3) + E1, h(E1, E2, E3) + E2, h(E1, E2, E3) + E3)</span></pre></div>
|
|
</div>
|
|
<p>正式而言，函数调用表达式中的<i>表达式列表</i>被归类为<i>初始化器列表</i>，它的模式是<i>初始化器子句</i>，它是<i>赋值表达式</i>和<i>花括号初始化器列表</i>其中之一。</p>
|
|
<h4><span class="mw-headline" id=".E6.9C.89.E6.8B.AC.E5.8F.B7.E5.88.9D.E5.A7.8B.E5.8C.96.E5.99.A8">有括号初始化器</span></h4>
|
|
<p>包展开可以在<a href="cpp-language-direct_initialization.html">直接初始化器</a>，<a href="cpp-language-explicit_cast.html">函数式转型</a>及其他语境（<a href="cpp-language-initializer_list.html" class="mw-redirect">成员初始化器</a>，<a href="cpp-language-new.html">new 表达式</a>等）的括号内出现，这种情况下的规则与适用于上述函数调用表达式的规则相同：</p>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="cpp source-cpp">
|
|
<pre class="de1">Class c1<span class="br0">(</span><span class="sy3">&</span>args...<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 调用 Class::Class(&E1, &E2, &E3)</span>
|
|
Class c2 <span class="sy1">=</span> Class<span class="br0">(</span>n, <span class="sy2">++</span>args...<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 调用 Class::Class(n, ++E1, ++E2, ++E3);</span>
|
|
<span class="sy4">::</span><span class="me2">new</span><span class="br0">(</span><span class="br0">(</span><span class="kw4">void</span> <span class="sy2">*</span><span class="br0">)</span>p<span class="br0">)</span> U<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>Args<span class="sy1">></span><span class="br0">(</span>args<span class="br0">)</span>...<span class="br0">)</span> <span class="co1">// std::allocator::allocate</span></pre></div>
|
|
</div>
|
|
<h4><span class="mw-headline" id=".E8.8A.B1.E6.8B.AC.E5.8F.B7.E5.8C.85.E5.9B.B4.E7.9A.84.E5.88.9D.E5.A7.8B.E5.8C.96.E5.99.A8">花括号包围的初始化器</span></h4>
|
|
<p>在<i>花括号初始化器列表</i>（花括号包围的初始化器和其他<i>花括号初始化器列表</i>的列表，用于<a href="cpp-language-list_initialization.html">列表初始化</a>和其他一些语境中）中，也可以出现包展开：</p>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="cpp source-cpp">
|
|
<pre class="de1"><span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span>... <span class="me1">Ts</span><span class="sy1">></span> <span class="kw4">void</span> func<span class="br0">(</span>Ts... <span class="me1">args</span><span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
<span class="kw4">const</span> <span class="kw4">int</span> size <span class="sy1">=</span> sizeof...<span class="br0">(</span>args<span class="br0">)</span> <span class="sy2">+</span> <span class="nu0">2</span><span class="sy4">;</span>
|
|
<span class="kw4">int</span> res<span class="br0">[</span>size<span class="br0">]</span> <span class="sy1">=</span> <span class="br0">{</span><span class="nu0">1</span>, args..., <span class="nu0">2</span><span class="br0">}</span><span class="sy4">;</span>
|
|
<span class="co1">// 因为初始化器列表保证顺序，所以这可以用来对包的每个元素按顺序调用函数：</span>
|
|
<span class="kw4">int</span> dummy<span class="br0">[</span>sizeof...<span class="br0">(</span>Ts<span class="br0">)</span><span class="br0">]</span> <span class="sy1">=</span> <span class="br0">{</span><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> args, <span class="nu0">0</span><span class="br0">)</span>...<span class="br0">}</span><span class="sy4">;</span>
|
|
<span class="br0">}</span></pre></div>
|
|
</div>
|
|
<h4><span class="mw-headline" id=".E6.A8.A1.E6.9D.BF.E5.AE.9E.E5.8F.82.E5.88.97.E8.A1.A8">模板实参列表</span></h4>
|
|
<p>包展开可以在模板实参列表的任何位置使用，前提是模板拥有与该展开相匹配的形参：</p>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="cpp source-cpp">
|
|
<pre class="de1"><span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> A, <span class="kw1">class</span> B, <span class="kw1">class</span>... <span class="me1">C</span><span class="sy1">></span> <span class="kw4">void</span> func<span class="br0">(</span>A arg1, B arg2, C...<span class="me1">arg3</span><span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
container<span class="sy1"><</span>A, B, C...<span class="sy1">></span> t1<span class="sy4">;</span> <span class="co1">// 展开成 container<A, B, E1, E2, E3> </span>
|
|
container<span class="sy1"><</span>C..., A, B<span class="sy1">></span> t2<span class="sy4">;</span> <span class="co1">// 展开成 container<E1, E2, E3, A, B> </span>
|
|
container<span class="sy1"><</span>A, C..., B<span class="sy1">></span> t3<span class="sy4">;</span> <span class="co1">// 展开成 container<A, E1, E2, E3, B> </span>
|
|
<span class="br0">}</span></pre></div>
|
|
</div>
|
|
<h4><span class="mw-headline" id=".E5.87.BD.E6.95.B0.E5.BD.A2.E5.8F.82.E5.88.97.E8.A1.A8">函数形参列表</span></h4>
|
|
<p>在函数形参列表中，如果省略号在某个形参声明中（无论它是否指名函数形参包（例如在 <span class="t-spar">Args</span> <code><b>...</b></code> <span class="t-spar">args</span>中））出现，那么该形参声明是模式：</p>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="cpp source-cpp">
|
|
<pre class="de1"><span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span>... <span class="me1">Ts</span><span class="sy1">></span> <span class="kw4">void</span> f<span class="br0">(</span>Ts...<span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
|
|
f<span class="br0">(</span><span class="st0">'a'</span>, <span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// Ts... 会展开成 void f(char, int)</span>
|
|
f<span class="br0">(</span><span class="nu16">0.1</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// Ts... 会展开成 void f(double)</span>
|
|
|
|
<span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span>... <span class="me1">Ts</span>, <span class="kw4">int</span>... <span class="me1">N</span><span class="sy1">></span> <span class="kw4">void</span> g<span class="br0">(</span>Ts <span class="br0">(</span><span class="sy3">&</span>...<span class="me1">arr</span><span class="br0">)</span><span class="br0">[</span>N<span class="br0">]</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
|
|
<span class="kw4">int</span> n<span class="br0">[</span><span class="nu0">1</span><span class="br0">]</span><span class="sy4">;</span>
|
|
g<span class="sy1"><</span><span class="kw4">const</span> <span class="kw4">char</span>, <span class="kw4">int</span><span class="sy1">></span><span class="br0">(</span><span class="st0">"a"</span>, n<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// Ts (&...arr)[N] 会展开成 </span>
|
|
<span class="co1">// const char (&)[2], int(&)[1]</span></pre></div>
|
|
</div>
|
|
<p>注意：在模式 <code>Ts (&...arr)[N]</code> 中，省略号是最内层的元素，而不是像所有其他包展开中一样是最后的元素。</p>
|
|
<p>注意：不能用 <code>Ts (&...)[N]</code>，因为 C++11 语法要求带括号的省略号形参拥有名字：<a rel="nofollow" class="external text" href="http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#1488">CWG #1488</a>。</p>
|
|
<h4><span class="mw-headline" id=".E6.A8.A1.E6.9D.BF.E5.BD.A2.E5.8F.82.E5.88.97.E8.A1.A8">模板形参列表</span></h4>
|
|
<p>包展开可以在模板形参列表中出现：</p>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="cpp source-cpp">
|
|
<pre class="de1"><span class="kw1">template</span><span class="sy1"><</span><span class="kw1">typename</span>... <span class="me1">T</span><span class="sy1">></span> <span class="kw1">struct</span> value_holder
|
|
<span class="br0">{</span>
|
|
<span class="kw1">template</span><span class="sy1"><</span>T... <span class="me1">Values</span><span class="sy1">></span> <span class="co1">// 会展开成非类型模板形参列表，</span>
|
|
<span class="kw1">struct</span> apply <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// 例如 <int, char, int(&)[5]></span>
|
|
<span class="br0">}</span><span class="sy4">;</span></pre></div>
|
|
</div>
|
|
<h4><span class="mw-headline" id=".E5.9F.BA.E7.B1.BB.E8.AF.B4.E6.98.8E.E7.AC.A6.E4.B8.8E.E6.88.90.E5.91.98.E5.88.9D.E5.A7.8B.E5.8C.96.E5.99.A8.E5.88.97.E8.A1.A8">基类说明符与成员初始化器列表</span></h4>
|
|
<p>包展开可以用于指定<a href="cpp-language-class.html">类声明</a>中的基类列表。通常这也意味着它的构造函数也需要在<a href="cpp-language-initializer_list.html" class="mw-redirect">成员初始化器列表</a>中使用包展开，以调用这些基类的构造函数：</p>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="cpp source-cpp">
|
|
<pre class="de1"><span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span>... <span class="me1">Mixins</span><span class="sy1">></span>
|
|
<span class="kw1">class</span> X <span class="sy4">:</span> <span class="kw1">public</span> Mixins...
|
|
<span class="br0">{</span>
|
|
<span class="kw1">public</span><span class="sy4">:</span>
|
|
X<span class="br0">(</span><span class="kw4">const</span> Mixins<span class="sy3">&</span>... <span class="me1">mixins</span><span class="br0">)</span> <span class="sy4">:</span> Mixins<span class="br0">(</span>mixins<span class="br0">)</span>... <span class="br0">{</span><span class="br0">}</span>
|
|
<span class="br0">}</span><span class="sy4">;</span></pre></div>
|
|
</div>
|
|
<h4><span class="mw-headline" id="Lambda_.E6.8D.95.E8.8E.B7">Lambda 捕获</span></h4>
|
|
<p>包展开可以在 <a href="cpp-language-lambda.html">lambda</a> 表达式的捕获子句中出现：</p>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="cpp source-cpp">
|
|
<pre class="de1"><span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span>... <span class="me1">Args</span><span class="sy1">></span>
|
|
<span class="kw4">void</span> f<span class="br0">(</span>Args... <span class="me1">args</span><span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
<span class="kw4">auto</span> lm <span class="sy1">=</span> <span class="br0">[</span><span class="sy3">&</span>, args...<span class="br0">]</span> <span class="br0">{</span> <span class="kw1">return</span> g<span class="br0">(</span>args...<span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
|
|
lm<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
|
|
<span class="br0">}</span></pre></div>
|
|
</div>
|
|
<h4><span class="mw-headline" id="sizeof..._.E8.BF.90.E7.AE.97.E7.AC.A6">sizeof... 运算符</span></h4>
|
|
<p><a href="cpp-language-sizeof....html"><code>sizeof...</code></a> 也被归类为包展开：</p>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="cpp source-cpp">
|
|
<pre class="de1"><span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span>... <span class="me1">Types</span><span class="sy1">></span>
|
|
<span class="kw1">struct</span> count
|
|
<span class="br0">{</span>
|
|
<span class="kw4">static</span> <span class="kw4">const</span> <a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> value <span class="sy1">=</span> sizeof...<span class="br0">(</span>Types<span class="br0">)</span><span class="sy4">;</span>
|
|
<span class="br0">}</span><span class="sy4">;</span></pre></div>
|
|
</div>
|
|
<table class="t-rev-begin">
|
|
<tr class="t-rev t-until-cxx17">
|
|
<td>
|
|
<h4><span class="mw-headline" id=".E5.8A.A8.E6.80.81.E5.BC.82.E5.B8.B8.E8.AF.B4.E6.98.8E">动态异常说明</span></h4>
|
|
<p><a href="cpp-language-except_spec.html">动态异常说明</a>中的异常列表也可以是包展开：</p>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="cpp source-cpp">
|
|
<pre class="de1"><span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span>...<span class="me1">X</span><span class="sy1">></span> <span class="kw4">void</span> func<span class="br0">(</span><span class="kw4">int</span> arg<span class="br0">)</span> <span class="kw1">throw</span><span class="br0">(</span>X...<span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
<span class="co1">// ... 在不同情形下抛出不同的 X</span>
|
|
<span class="br0">}</span></pre></div>
|
|
</div>
|
|
</td>
|
|
<td><span class="t-mark-rev t-until-cxx17">(C++17 前)</span></td>
|
|
</tr>
|
|
</table>
|
|
<h4><span class="mw-headline" id=".E5.AF.B9.E9.BD.90.E8.AF.B4.E6.98.8E.E7.AC.A6">对齐说明符</span></h4>
|
|
<p>包展开可以在关键词 <a href="cpp-language-alignas.html"><code>alignas</code></a> 所用的类型列表和表达式列表中使用。</p>
|
|
<h4><span class="mw-headline" id=".E5.B1.9E.E6.80.A7.E5.88.97.E8.A1.A8">属性列表</span></h4>
|
|
<p>包展开可以在<a href="cpp-language-attributes.html">属性</a>列表中使用，如 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">[</span><span class="br0">[</span>attributes...<span class="br0">]</span><span class="br0">]</span></span></span>。例如：<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span> <span class="br0">[</span><span class="br0">[</span>attributes...<span class="br0">]</span><span class="br0">]</span> function<span class="br0">(</span><span class="br0">)</span></span></span></p>
|
|
<table class="t-rev-begin">
|
|
<tr class="t-rev t-since-cxx17">
|
|
<td>
|
|
<h4><span class="mw-headline" id=".E6.8A.98.E5.8F.A0.E8.A1.A8.E8.BE.BE.E5.BC.8F">折叠表达式</span></h4>
|
|
<p>在<a href="cpp-language-fold.html">折叠表达式</a>中，模式是不包含未展开的形参包的整个子表达式。</p>
|
|
<h4><span class="mw-headline" id="using_.E5.A3.B0.E6.98.8E">using 声明</span></h4>
|
|
<p>在 <a href="cpp-language-using_declaration.html">using 声明</a>中，省略号可以在声明器列表内出现，这对于从一个形参包进行派生时有用：</p>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="cpp source-cpp">
|
|
<pre class="de1"><span class="kw1">template</span> <span class="sy1"><</span><span class="kw1">typename</span>... <span class="me1">bases</span><span class="sy1">></span>
|
|
<span class="kw1">struct</span> X <span class="sy4">:</span> bases...
|
|
<span class="br0">{</span>
|
|
<span class="kw1">using</span> bases<span class="sy4">::</span><span class="me2">g</span>...<span class="sy4">;</span>
|
|
<span class="br0">}</span><span class="sy4">;</span>
|
|
X<span class="sy1"><</span>B, D<span class="sy1">></span> x<span class="sy4">;</span> <span class="co1">// OK：引入 B::g 与 D::g</span></pre></div>
|
|
</div>
|
|
</td>
|
|
<td><span class="t-mark-rev t-since-cxx17">(C++17 起)</span></td>
|
|
</tr>
|
|
</table>
|
|
<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
|
|
<table class="metadata plainlinks ambox mbox-small-left ambox-notice" style="">
|
|
<tr>
|
|
<td class="mbox-empty-cell"></td>
|
|
<td class="mbox-text" style="">本节未完成<br />
|
|
原因：关于部分特化和其他访问单独元素方式的一些话？提及递归 vs 对数 vs 短路，例如折叠表达式</td>
|
|
</tr>
|
|
</table>
|
|
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
|
|
<p>下面的例子定义了类似 <span class="t-lc"><a href="cpp-io-c-fprintf.html">std::printf</a></span> 的函数，并以一个值替换格式字符串中字符 % 的每次出现。</p>
|
|
<p>首个重载在仅传递格式字符串且无形参展开时调用。</p>
|
|
<p>第二个重载中分别包含针对实参头的一个模板形参和一个形参包，这样就可以在递归调用中只传递形参的尾部，直到它变为空。</p>
|
|
<p><code><b>Targs</b></code> 是模板形参包而 <code><b>Fargs</b></code> 是函数形参包。</p>
|
|
<div class="t-example">
|
|
<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="kw4">void</span> tprintf<span class="br0">(</span><span class="kw4">const</span> <span class="kw4">char</span><span class="sy2">*</span> format<span class="br0">)</span> <span class="co1">// 基础函数</span>
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<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> format<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, <span class="kw1">typename</span>... <span class="me1">Targs</span><span class="sy1">></span>
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<span class="kw4">void</span> tprintf<span class="br0">(</span><span class="kw4">const</span> <span class="kw4">char</span><span class="sy2">*</span> format, T value, Targs... <span class="me1">Fargs</span><span class="br0">)</span> <span class="co1">// 递归变参函数</span>
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<span class="br0">{</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="sy4">;</span> <span class="sy2">*</span>format <span class="sy3">!</span><span class="sy1">=</span> <span class="st0">'<span class="es5">\0</span>'</span><span class="sy4">;</span> format<span class="sy2">++</span><span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw1">if</span> <span class="br0">(</span> <span class="sy2">*</span>format <span class="sy1">==</span> <span class="st0">'%'</span> <span class="br0">)</span>
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<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> value<span class="sy4">;</span>
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tprintf<span class="br0">(</span>format <span class="sy2">+</span> <span class="nu0">1</span>, Fargs...<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 递归调用</span>
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<span class="kw1">return</span><span class="sy4">;</span>
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<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> <span class="sy2">*</span>format<span class="sy4">;</span>
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<span class="br0">}</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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tprintf<span class="br0">(</span><span class="st0">"% world% %<span class="es1">\n</span>"</span>, <span class="st0">"Hello"</span>, <span class="st0">'!'</span>, <span class="nu0">123</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">return</span> <span class="nu0">0</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">Hello world! 123</pre></div>
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</div>
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</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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<table class="t-dsc-begin">
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<tr class="t-dsc">
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<td colspan="2"><a href="cpp-language-function_template.html">函数模板</a></td>
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</tr>
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<tr class="t-dsc">
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<td colspan="2"><a href="cpp-language-class_template.html">类模板</a></td>
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</tr>
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<tr class="t-dsc">
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<td><a href="cpp-language-sizeof....html"><code>sizeof...</code></a></td>
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<td>查询形参包中的元素数量。</td>
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</tr>
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<tr class="t-dsc">
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<td colspan="2"><a href="cpp-utility-variadic.html">C 风格的变参函数</a></td>
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</tr>
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<tr class="t-dsc">
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<td><a href="cpp-preprocessor-replace.html">预处理器宏</a></td>
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<td>也可以是变参的</td>
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</tr>
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<tr class="t-dsc">
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<td colspan="2"><a href="cpp-language-fold.html">折叠表达式</a></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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</div>
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</body>
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</html>
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