133 lines
8.2 KiB
HTML
133 lines
8.2 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>_Alignof 运算符</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-c_language_Alignof 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">_Alignof 运算符</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-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>查询其运算数类型的对齐要求。</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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<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>_Alignof(</b></code> <span class="t-spar">类型名</span> <code><b>)</b></code></td>
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<td class="t-sdsc-nopad"></td>
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<td><span class="t-mark-rev t-since-c11">(C11 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>通常通过便利宏 <a href="c-types.html"><tt>alignof</tt></a> 使用此运算符，该宏于头文件 <code>stdalign.h</code> 提供。</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 <span class="t-spar">类型名</span> 所<a href="c-language-types.html#.E7.B1.BB.E5.9E.8B.E5.90.8D">指名</a>的类型的<a href="c-language-object.html#.E5.AF.B9.E9.BD.90">对齐要求</a>。若 <span class="t-spar">类型名</span> 为数组类型，则结果为数组元素的对齐要求。 <span class="t-spar">类型名</span> 不能为函数类型或不完整类型。</p>
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<p>结果是 <span class="t-lc"><a href="c-types-size_t.html">size_t</a></span> 类型整数常量。</p>
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<p>不求值运算数（故用作运算数的外部标识符不必有定义）。</p>
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<p>若 <span class="t-spar">类型名</span> 为 <a href="c-language-array.html">VLA</a> 类型，则不求值其大小表达式。</p>
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<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
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<p>一些 C 编译器允许把 <code>_Alignof</code> 用于表达式这种非标准扩展。</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="c-keyword-_Alignof.html"><tt>_Alignof</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="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 <stddef.h></span>
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<span class="co2">#include <stdalign.h></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">"Alignment of char = %zu<span class="es1">\n</span>"</span>, alignof<span class="br0">(</span><span class="kw4">char</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="c-io-fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"Alignment of max_align_t = %zu<span class="es1">\n</span>"</span>, alignof<span class="br0">(</span><a href="c-types-max_align_t.html"><span class="kw104">max_align_t</span></a><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="c-io-fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"alignof(float[10]) = %zu<span class="es1">\n</span>"</span>, alignof<span class="br0">(</span><span class="kw4">float</span><span class="br0">[</span><span class="nu0">10</span><span class="br0">]</span><span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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<a href="c-io-fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"alignof(struct{char c; int n;}) = %zu<span class="es1">\n</span>"</span>,
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alignof<span class="br0">(</span><span class="kw1">struct</span> <span class="br0">{</span><span class="kw4">char</span> c<span class="sy4">;</span> <span class="kw4">int</span> n<span class="sy4">;</span><span class="br0">}</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>可能的输出：</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">Alignment of char = 1
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Alignment of max_align_t = 16
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alignof(float[10]) = 4
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alignof(struct{char c; int n;}) = 4</pre></div>
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</div>
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</div>
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<h3><span class="mw-headline" id=".E7.BC.BA.E9.99.B7.E6.8A.A5.E5.91.8A">缺陷报告</span></h3>
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<p>下列更改行为的缺陷报告追溯地应用于以前出版的 C 标准。</p>
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<table class="dsctable" style="font-size:0.8em">
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<tr>
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<th>DR</th>
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<th>应用于</th>
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<th>出版时的行为</th>
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<th>正确行为</th>
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</tr>
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<tr>
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<td><a rel="nofollow" class="external text" href="http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2396.htm#dr_494">DR 494</a></td>
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<td>C11</td>
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<td>未指明是否求值 <code>_Alignof</code> 中的 VLA 中的大小表达式</td>
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<td>不求值它</td>
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</tr>
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</table>
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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-17">
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<ul>
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<li>C17 标准（ISO/IEC 9899:2018）：</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>6.5.3.4 The sizeof and _Alignof operators (p: 64-65)</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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<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>6.5.3.4 The sizeof and _Alignof operators (p: 90-91)</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-types-max_align_t.html"><span class="t-lines"><span>max_align_t</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-c11">(C11)</span></span></span></div>
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</div>
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</td>
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<td>对齐要求不小于任何其他标量类型的类型<br />
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<span class="t-mark">(typedef)</span></td>
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</tr>
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<tr class="t-dsc">
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<td><a href="c-language-_Alignas.html"><code>_Alignas</code> 说明符</a><span class="t-mark-rev t-since-c11">(C11)</span></td>
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<td>设置对象的对齐要求</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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