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<title>变参数函数</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_utility_variadic skin-cppreference2 action-view cpp-navbar">
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<h1 id="firstHeading" class="firstHeading">变参数函数</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-utility.html">utility</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>变参数函数是接收<a href="cpp-language-variadic_arguments.html">可变数量参数</a>的函数(例如 <span class="t-lc"><a href="cpp-io-c-fprintf.html">std::printf</a></span> )。</p>
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<p>为声明变参数函数,省略号要出现在参数列表后,例如 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">int</span> printf<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="sy4">;</span></span></span> ,可以前附可选的逗号。语法上的额外细节、自动参数转换及替用项见<a href="cpp-language-variadic_arguments.html">变参数</a>。</p>
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<p>为从函数体内访问可变参数,标准库提供下列库设施:</p>
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<table class="t-dsc-begin">
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<tr class="t-dsc-header">
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<td colspan="2">
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<div>定义于头文件 <code><a href="cpp-header-cstdarg.html"><cstdarg></a></code></div>
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</td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-utility-variadic-va_start.html"><span class="t-lines"><span>va_start</span></span></a></div>
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</div>
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</td>
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<td>启用对可变函数实参的访问<br />
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<span class="t-mark">(宏函数)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-utility-variadic-va_arg.html"><span class="t-lines"><span>va_arg</span></span></a></div>
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</div>
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</td>
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<td>访问下一个可变函数实参<br />
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<span class="t-mark">(宏函数)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-utility-variadic-va_copy.html"><span class="t-lines"><span>va_copy</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx11">(C++11)</span></span></span></div>
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</div>
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</td>
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<td>制造可变函数实参的副本<br />
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<span class="t-mark">(宏函数)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-utility-variadic-va_end.html"><span class="t-lines"><span>va_end</span></span></a></div>
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</div>
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</td>
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<td>结束对可变函数实参的遍历<br />
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<span class="t-mark">(宏函数)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-utility-variadic-va_list.html"><span class="t-lines"><span>va_list</span></span></a></div>
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</div>
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</td>
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<td>保有 va_start、va_arg、va_end 和 va_copy 所需的信息<br />
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<span class="t-mark">(typedef)</span></td>
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</tr>
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</table>
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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 <cstdarg></span>
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<span class="kw4">void</span> simple_printf<span class="br0">(</span><span class="kw4">const</span> <span class="kw4">char</span><span class="sy2">*</span> fmt...<span class="br0">)</span> <span class="co1">// C 风格 "const char* fmt, ..." 亦合法</span>
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<span class="br0">{</span>
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<span class="kw4">va_list</span> args<span class="sy4">;</span>
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<a href="cpp-utility-variadic-va_start.html"><span class="kw961">va_start</span></a><span class="br0">(</span>args, fmt<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">while</span> <span class="br0">(</span><span class="sy2">*</span>fmt <span class="sy3">!</span><span class="sy1">=</span> <span class="st0">'<span class="es5">\0</span>'</span><span class="br0">)</span> <span class="br0">{</span>
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<span class="kw1">if</span> <span class="br0">(</span><span class="sy2">*</span>fmt <span class="sy1">==</span> <span class="st0">'d'</span><span class="br0">)</span> <span class="br0">{</span>
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<span class="kw4">int</span> i <span class="sy1">=</span> <a href="cpp-utility-variadic-va_arg.html"><span class="kw963">va_arg</span></a><span class="br0">(</span>args, <span class="kw4">int</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> i <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> <span class="kw1">else</span> <span class="kw1">if</span> <span class="br0">(</span><span class="sy2">*</span>fmt <span class="sy1">==</span> <span class="st0">'c'</span><span class="br0">)</span> <span class="br0">{</span>
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<span class="co1">// 注意自动转换到整数类型</span>
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<span class="kw4">int</span> c <span class="sy1">=</span> <a href="cpp-utility-variadic-va_arg.html"><span class="kw963">va_arg</span></a><span class="br0">(</span>args, <span class="kw4">int</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="kw1">static_cast</span><span class="sy1"><</span><span class="kw4">char</span><span class="sy1">></span><span class="br0">(</span>c<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> <span class="kw1">else</span> <span class="kw1">if</span> <span class="br0">(</span><span class="sy2">*</span>fmt <span class="sy1">==</span> <span class="st0">'f'</span><span class="br0">)</span> <span class="br0">{</span>
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<span class="kw4">double</span> d <span class="sy1">=</span> <a href="cpp-utility-variadic-va_arg.html"><span class="kw963">va_arg</span></a><span class="br0">(</span>args, <span class="kw4">double</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> d <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="sy2">++</span>fmt<span class="sy4">;</span>
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<span class="br0">}</span>
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<a href="cpp-utility-variadic-va_end.html"><span class="kw964">va_end</span></a><span class="br0">(</span>args<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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simple_printf<span class="br0">(</span><span class="st0">"dcff"</span>, <span class="nu0">3</span>, <span class="st0">'a'</span>, <span class="nu16">1.999</span>, <span class="nu16">42.5</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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</div>
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<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">3
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a
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1.999
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42.5</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"></table>
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</div>
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<div class="visualClear"></div>
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