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<h1 id="firstHeading" class="firstHeading">va_arg</h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-utility.html">utility</a>‎ | <a href="cpp-utility-variadic.html">variadic</a></span></div>
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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></td>
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<div><span class="mw-geshi cpp source-cpp">T va_arg<span class="br0">(</span> std<span class="sy4">::</span><span class="kw4">va_list</span> ap, T <span class="br0">)</span><span class="sy4">;</span></span></div>
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<p><code>va_arg</code> 宏展开成对应来自 <span class="t-lc">std::va_list</span> <code>ap</code> 的下个参数的 <code>T</code> 类型表达式。</p>
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<p>在调用 <code>va_arg</code> 前， <code>ap</code> 必须为到 <span class="t-lc"><a href="cpp-utility-variadic-va_start.html">va_start</a></span> 或 <span class="t-lc"><a href="cpp-utility-variadic-va_copy.html">va_copy</a></span> 的调用所初始化，中间不能夹带 <span class="t-lc"><a href="cpp-utility-variadic-va_end.html">va_end</a></span> 的调用。 <code>va_arg</code> 宏的每次调用都修改 <code>ap</code> ，令它指向下个可变参数。</p>
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<p>若 <code>ap</code> 中的下个参数类型（提升后）与 <code>T</code> 不兼容，则行为未定义，除非：</p>
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<ul>
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<li>一个类型是有符号整数类型，而另一类型是无符号整数类型，且值能用两个类型表示；或</li>
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<li>一个类型是指向 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span></span></span> 的指针，而另一类型是指向字符类型（ <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">char</span></span></span> 、 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">signed</span> <span class="kw4">char</span></span></span> 或 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">unsigned</span> <span class="kw4">char</span></span></span> ）的指针。</li>
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</ul>
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<p>若在 <code>ap</code> 中无更多参数时调用 <code>va_arg</code> ，则行为未定义。</p>
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<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
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<table class="t-par-begin">
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<tr class="t-par">
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<td>ap</td>
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<td>-</td>
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<td><span class="t-lc">std::va_list</span> 类型的实例</td>
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<tr class="t-par">
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<td>T</td>
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<td>-</td>
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<td><code>ap</code> 中下个参数的类型</td>
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<h3><span class="mw-headline" id=".E5.B1.95.E5.BC.80.E5.80.BC">展开值</span></h3>
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<p><code>ap</code> 中的下个可变参数</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 <iostream></span>
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<span class="co2">#include <cstdarg></span>
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<span class="co2">#include <cmath></span>
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<span class="kw4">double</span> stddev<span class="br0">(</span><span class="kw4">int</span> count, ...<span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw4">double</span> sum <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span>
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<span class="kw4">double</span> sum_sq <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span>
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std<span class="sy4">::</span><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, count<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="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span> i <span class="sy1"><</span> count<span class="sy4">;</span> <span class="sy2">++</span>i<span class="br0">)</span> <span class="br0">{</span>
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<span class="kw4">double</span> num <span class="sy1">=</span> va_arg<span class="br0">(</span>args, <span class="kw4">double</span><span class="br0">)</span><span class="sy4">;</span>
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sum <span class="sy2">+</span><span class="sy1">=</span> num<span class="sy4">;</span>
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sum_sq <span class="sy2">+</span><span class="sy1">=</span> num<span class="sy2">*</span>num<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="kw963">va_end</span></a><span class="br0">(</span>args<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">return</span> <a href="cpp-numeric-math-sqrt.html"><span class="kw1328">std::<span class="me2">sqrt</span></span></a><span class="br0">(</span>sum_sq<span class="sy2">/</span>count <span class="sy2">-</span> <span class="br0">(</span>sum<span class="sy2">/</span>count<span class="br0">)</span><span class="sy2">*</span><span class="br0">(</span>sum<span class="sy2">/</span>count<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-io-cout.html"><span class="kw1757">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> stddev<span class="br0">(</span><span class="nu0">4</span>, <span class="nu16">25.0</span>, <span class="nu16">27.3</span>, <span class="nu16">26.9</span>, <span class="nu16">25.7</span><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></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">0.920258</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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<table class="t-dsc-begin">
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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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<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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<div class="visualClear"></div>
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