180 lines
9.0 KiB
HTML
180 lines
9.0 KiB
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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<title>va_arg</title>
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-c_variadic_va_arg 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">va_arg</h1>
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<div id="bodyContent">
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<div id="contentSub"><span class="subpages">< <a href="c.html">c</a>‎ | <a href="c-variadic.html">variadic</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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<div>定义于头文件 <code><stdarg.h></code></div>
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<td></td>
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<div><span class="mw-geshi c source-c">T va_arg<span class="br0">(</span> va_list 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 <code>T</code> 类型的表达式，表达式对应来自 <span class="t-lc">va_list</span> <code>ap</code> 的下个参数。</p>
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<p>调用 <code>va_arg</code> 前，必须调用 <span class="t-lc">va_start</span> 或 <span class="t-lc">va_copy</span> 初始化 <code>ap</code> ，中间不能有 <span class="t-lc">va_end</span> 调用。每次调用 <code>va_arg</code> 宏都会修改 <code>ap</code> ，令它指向下一个可变参数。</p>
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<p>若在 <code>ap</code> 中无更多参数时调用 <code>va_arg</code> ，或若 <code>ap</code> 中的下个参数（提升后）与 <code>T</code> 的类型不兼容，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，另一类型是对应的无符号整数类型，而且值可以被两类型一同表示；或</li>
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<li>一个类型是指向 <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">void</span></span></span> 的指针，而另一个是指向字符类型的指针。</li>
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</ul>
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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">va_list</span> 类型实例</td>
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</tr>
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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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</tr>
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</table>
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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="c source-c">
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<pre class="de1"><span class="co2">#include <stdio.h></span>
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<span class="co2">#include <stdarg.h></span>
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<span class="co2">#include <math.h></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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va_list args<span class="sy4">;</span>
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va_start<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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va_end<span class="br0">(</span>args<span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">return</span> <a href="c-numeric-math-sqrt.html"><span class="kw665">sqrt</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="kw4">void</span><span class="br0">)</span>
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<span class="br0">{</span>
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<a href="c-io-fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"%f<span class="es1">\n</span>"</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="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">0.920258</pre></div>
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</div>
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</div>
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<h3><span class="mw-headline" id=".E5.BC.95.E7.94.A8">引用</span></h3>
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<div class="t-ref-std-11">
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<ul>
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<li>C11 标准（ISO/IEC 9899:2011）：</li>
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</ul>
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<dl>
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<dd>
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<ul>
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<li>7.16.1.1 The va_arg macro (p: 269-270)</li>
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</ul>
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</dd>
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</dl>
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<div class="t-ref-std-c99">
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<ul>
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<li>C99 标准（ISO/IEC 9899:1999）：</li>
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</ul>
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<dl>
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<dd>
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<ul>
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<li>7.15.1.1 The va_arg macro (p: 249-250)</li>
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</ul>
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</dd>
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</dl>
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<div class="t-ref-std-c89">
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<ul>
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<li>C89/C90 标准（ISO/IEC 9899:1990）：</li>
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</ul>
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<dl>
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<dd>
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<ul>
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<li>4.8.1.2 The va_arg macro</li>
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</ul>
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</dd>
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</dl>
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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>
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<div class="t-dsc-member-div">
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<div><a href="c-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-c99">(C99)</span></span></span></div>
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</div>
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</td>
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<td>创造函数可变参数的副本<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="c-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>结束函数可变参数的行程<br />
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<span class="t-mark">(宏函数)</span></td>
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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="c-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 及 va_copy 所需信息<br />
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<span class="t-mark">(typedef)</span></td>
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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="c-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>令函数得以访问可变参数<br />
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<span class="t-mark">(宏函数)</span></td>
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</tr>
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</table>
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</div>
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</div>
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</div>
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<div class="visualClear"></div>
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