113 lines
8.5 KiB
HTML
113 lines
8.5 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(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_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<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>查询类型的对齐要求。</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 class="t-sdsc-nopad"><code><b>alignof(</b></code> <span class="t-spar">类型标识</span> <code><b>)</b></code></td>
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<td class="t-sdsc-nopad"></td>
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<td class="t-sdsc-nopad"></td>
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<tr>
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<td colspan="10" class="t-sdsc-sep"></td>
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</table>
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<p>返回 <span class="t-lc"><a href="cpp-types-size_t.html">std::size_t</a></span> 类型的值。</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>返回由<a href="cpp-language-type.html#.E7.B1.BB.E5.9E.8B.E7.9A.84.E5.91.BD.E5.90.8D">类型标识</a>所指示的类型的任何实例所要求的<a href="cpp-language-object.html#.E5.AF.B9.E9.BD.90">对齐</a>字节数，该类型可以为完整对象类型、元素类型完整的数组类型或者到这些类型之一的引用类型。</p>
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<p>若类型为引用类型，则运算符返回<i>被引用</i>类型的对齐；若类型为数组类型，则返回元素类型的对齐要求。</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="cpp-keyword-alignof.html"><tt>alignof</tt></a></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>有关 <code>alignof</code> 的返回值的含义和性质，参见<a href="cpp-language-object.html#.E5.AF.B9.E9.BD.90">对齐</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="cpp source-cpp">
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<pre class="de1"><span class="co2">#include <iostream></span>
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<span class="kw1">struct</span> Foo <span class="br0">{</span>
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<span class="kw4">int</span> i<span class="sy4">;</span>
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<span class="kw4">float</span> f<span class="sy4">;</span>
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<span class="kw4">char</span> c<span class="sy4">;</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<span class="co1">// 注：下面的 `alignas(alignof(long double))` 如果需要可以简化为 </span>
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<span class="co1">// `alignas(long double)`</span>
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<span class="kw1">struct</span> alignas<span class="br0">(</span>alignof<span class="br0">(</span><span class="kw4">long</span> <span class="kw4">double</span><span class="br0">)</span><span class="br0">)</span> Foo2 <span class="br0">{</span>
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<span class="co1">// Foo2 成员的定义...</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">struct</span> Empty <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">struct</span> alignas<span class="br0">(</span><span class="nu0">64</span><span class="br0">)</span> Empty64 <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</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="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"对齐字节数"</span> <span class="st0">"<span class="es1">\n</span>"</span>
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<span class="st0">"- char ："</span> <span class="sy1"><<</span> alignof<span class="br0">(</span><span class="kw4">char</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">"<span class="es1">\n</span>"</span>
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<span class="st0">"- 指针 ："</span> <span class="sy1"><<</span> alignof<span class="br0">(</span><span class="kw4">int</span><span class="sy2">*</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">"<span class="es1">\n</span>"</span>
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<span class="st0">"- Foo 类 ："</span> <span class="sy1"><<</span> alignof<span class="br0">(</span>Foo<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">"<span class="es1">\n</span>"</span>
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<span class="st0">"- Foo2 类 ："</span> <span class="sy1"><<</span> alignof<span class="br0">(</span>Foo2<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">"<span class="es1">\n</span>"</span>
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<span class="st0">"- 空类 ："</span> <span class="sy1"><<</span> alignof<span class="br0">(</span>Empty<span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">"<span class="es1">\n</span>"</span>
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<span class="st0">"- alignas(64) Empty："</span> <span class="sy1"><<</span> alignof<span class="br0">(</span>Empty64<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>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">对齐字节数
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- char ：1
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- 指针 ：8
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- Foo 类 ：4
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- Foo2 类 ：16
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- 空类 ：1
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- alignas(64) Empty：64</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><a href="cpp-language-object.html#.E5.AF.B9.E9.BD.90">对齐要求</a></td>
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<td>限制对象所能分配于其中的地址</td>
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</tr>
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<tr class="t-dsc">
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<td><a href="cpp-language-alignas.html"><code>alignas</code> 说明符</a><span class="t-mark-rev t-since-cxx11">(C++11)</span></td>
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<td>指定该变量的存储应该按指定量对齐</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-types-alignment_of.html"><span class="t-lines"><span>alignment_of</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>获取类型的对齐要求<br />
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<span class="t-mark">(类模板)</span></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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