186 lines
17 KiB
HTML
186 lines
17 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>属性说明符序列(C23 起)</title>
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<link rel="stylesheet" href="ext.css" />
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-c_language_attributes 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">属性说明符序列<span class="t-mark-rev t-since-c23">(C23 起)</span></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>对类型、对象、表达式等引入实现定义的属性。</p>
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<dl>
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<dd><code><b>[[</b></code><span class="t-spar">attr</span><code><b>]]</b></code> <code><b>[[</b></code><span class="t-spar">attr1</span>, <span class="t-spar">attr2</span>, <span class="t-spar">attr3</span><code><b>(</b></code><span class="t-spar">args</span><code><b>)</b></code><code><b>]]</b></code> <code><b>[[</b></code><span class="t-spar">属性前缀</span><code><b>::</b></code><span class="t-spar">attr</span><code><b>(</b></code><span class="t-spar">arg</span><code><b>)</b></code><code><b>]]</b></code></dd>
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</dl>
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<p>正式而言，语法是</p>
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<table class="t-sdsc-begin">
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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>[[</b></code> <span class="t-spar">属性列表</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-c23">(C23 起)</span></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>其中 <span class="t-spar">属性列表</span> 是零或多个 <span class="t-spar">属性记号</span> 的逗号分隔列表</p>
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<table class="t-sdsc-begin">
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<td><span class="t-spar">标准属性</span></td>
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<td>(1)</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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<td><span class="t-spar">属性前缀</span> <code><b>::</b></code> <span class="t-spar">标识符</span></td>
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<td>(2)</td>
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<td class="t-sdsc-nopad"></td>
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<td colspan="10" class="t-sdsc-sep"></td>
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<td><span class="t-spar">标准属性</span> <code><b>(</b></code> <span class="t-spar">实参列表</span> <code><b>)</b></code></td>
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<td>(3)</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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<tr class="t-sdsc">
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<td><span class="t-spar">属性前缀</span> <code><b>::</b></code> <span class="t-spar">标识符</span> <code><b>(</b></code> <span class="t-spar">实参列表</span> <code><b>)</b></code></td>
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<td>(4)</td>
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<td class="t-sdsc-nopad"></td>
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<div class="t-li1"><span class="t-li">1)</span> 标准属性，例如 <span class="t-c"><span class="mw-geshi c source-c"><span class="br0">[</span><span class="br0">[</span>fallthrough<span class="br0">]</span><span class="br0">]</span></span></span></div>
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<div class="t-li1"><span class="t-li">2)</span> 有命名空间的属性，例如 <span class="t-c"><span class="mw-geshi c source-c"><span class="br0">[</span><span class="br0">[</span>gnu<span class="sy4">::</span><span class="me2">unused</span><span class="br0">]</span><span class="br0">]</span></span></span></div>
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<div class="t-li1"><span class="t-li">3)</span> 有实参的标准属性，例如 <span class="t-c"><span class="mw-geshi c source-c"><span class="br0">[</span><span class="br0">[</span>deprecated<span class="br0">(</span><span class="st0">"reason"</span><span class="br0">)</span><span class="br0">]</span><span class="br0">]</span></span></span></div>
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<div class="t-li1"><span class="t-li">4)</span> 有命名空间和实参列表的属性</div>
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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>属性为各种由实现定义的语言扩展（例如 GNU 与 IBM 的语言扩展 <code>__attribute__((...))</code>，微软的语言扩展 <code>__declspec()</code> 等）提供了统一化的语法。</p>
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<p>属性可用在 C 程序中的几乎所有位置，而且可应用于几乎所有事物：类型、变量、函数、名字、代码块、整个翻译单元，不过每个特定的属性都仅在实现所容许之处有效：<code>[[expect_true]]</code> 可能是只能与 <span class="t-c"><span class="mw-geshi c source-c"><span class="kw1">if</span></span></span>，而非与类声明一同使用的属性，<code>[[omp::parallel()]]</code> 可能是应用到代码块或 <span class="t-c"><span class="mw-geshi c source-c"><span class="kw1">for</span></span></span> 循环，而非到类型 <code>int</code> 等的属性。（请注意这两个属性只是虚构的例子，有关标准与一些非标准属性，见下文）</p>
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<p>在声明中，属性可出现在整个声明之前，或直接跟在被声明实体的名字之后，这些情况下它们被组合起来。大多数其他情形中，属性应用于直接位于其之前的实体。</p>
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<p>不同于 C++ 中的 <a href="cpp-language-alignas.html"><tt>alignas</tt></a> 与 <code>[[<a href="cpp-language-attributes-noreturn.html">noreturn</a>]]</code> ， <a href="c-language-_Alignas.html"><code>_Alignas</code></a> 与 <a href="c-language-_Noreturn.html"><code>_Noreturn</code></a> 说明符不是属性说明符序列的一部分。</p>
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<p>两个连续的方括号记号（<code>[[</code>）只能出现于引入属性说明符之处，或在属性实参之内。</p>
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<p>除了以下所列出的标准属性之外，实现还可能支持任意拥有由实现定义的行为的非标准属性。所有实现所未知的属性均被忽略，且不产生错误。</p>
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<p>每个 <span class="t-spar">标准属性</span> 均为标准化保留。即每个非标准属性均由实现提供的 <span class="t-spar">属性前缀</span> 冠以前缀，例如 <code>[[gnu::may_alias]]</code> 与 <code>[[clang::no_sanitize]]</code> 。</p>
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<h3><span class="mw-headline" id=".E6.A0.87.E5.87.86.E5.B1.9E.E6.80.A7">标准属性</span></h3>
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<p>C++ 标准仅定义下列属性。每个名字形如 <code>attr</code> 的标准属性亦能拼写成 <code>__attr__</code> 而其含义不变。</p>
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<table class="t-dsc-begin">
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<tr class="t-dsc">
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<td><code>[[<a href="c-language-attributes-deprecated.html">deprecated</a>]]</code><span class="t-mark-rev t-since-c23">(C23)</span><br />
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<code>[[deprecated("reason")]]</code><span class="t-mark-rev t-since-c23">(C23)</span></td>
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<td>指示允许使用声明有此属性的名称或实体，但因 <span class="t-spar">reason</span> 而不鼓励使用。</td>
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</tr>
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<tr class="t-dsc">
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<td><code>[[<a href="c-language-attributes-fallthrough.html">fallthrough</a>]]</code><span class="t-mark-rev t-since-c23">(C23)</span></td>
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<td>指示从前一 case 标号直落是有意的，而在发生直落时给出警告的编译器不应该为此诊断。</td>
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</tr>
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<tr class="t-dsc">
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<td><code>[[<a href="c-language-attributes-nodiscard.html">nodiscard</a>]]</code><span class="t-mark-rev t-since-c23">(C23)</span><br />
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<code>[[nodiscard("reason")]]</code><span class="t-mark-rev t-since-c23">(C23)</span></td>
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<td>若返回值被舍弃，则鼓励编译器发布警告。</td>
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</tr>
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<tr class="t-dsc">
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<td><code>[[<a href="c-language-attributes-maybe_unused.html">maybe_unused</a>]]</code><span class="t-mark-rev t-since-c23">(C23)</span></td>
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<td>抑制编译器在未使用实体上的警告，若存在。</td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E5.B1.9E.E6.80.A7.E6.B5.8B.E8.AF.95">属性测试</span></h3>
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<table class="t-sdsc-begin">
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<td colspan="10" class="t-sdsc-sep"></td>
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<td class="t-sdsc-nopad"><code><b>__has_c_attribute(</b></code> <span class="t-spar">属性记号</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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<tr>
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<td colspan="10" class="t-sdsc-sep"></td>
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</table>
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<p>检查 <span class="t-spar">属性记号</span> 所指名的属性记号的存在。</p>
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<p>对于标准属性，它将展开成该属性被添加到工作草案中时的年份和月份（见下表），特定于厂商的属性则以某个非零整数常量确定。</p>
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<p>能在 <a href="c-preprocessor-conditional.html"><tt>#if</tt></a> 与 <a href="c-preprocessor-conditional.html"><tt>#elif</tt></a> 的表达式中展开 <code>__has_c_attribute</code> 。 <a href="c-preprocessor-conditional.html"><tt>#ifdef</tt></a> 、 <a href="c-preprocessor-conditional.html"><tt>#ifndef</tt></a> 和 <a href="c-preprocessor-conditional.html"><tt>defined</tt></a> 把它当做已定义的宏，但不能在别处使用它。</p>
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<table class="wikitable sortable">
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<tr>
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<th><span class="t-spar">属性记号</span></th>
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<th>属性</th>
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<th>值</th>
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<th><span title="引入该特性的标准">标准</span></th>
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</tr>
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<tr>
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<td><code>deprecated</code></td>
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<td><code>[[<a href="c-language-attributes-deprecated.html">deprecated</a>]]</code></td>
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<td><span class="t-c"><span class="mw-geshi c source-c"><span class="nu0">201904L</span></span></span></td>
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<td><span class="t-mark-rev t-since-c23">(C23)</span></td>
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</tr>
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<tr>
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<td><code>fallthrough</code></td>
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<td><code>[[<a href="c-language-attributes-fallthrough.html">fallthrough</a>]]</code></td>
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<td><span class="t-c"><span class="mw-geshi c source-c"><span class="nu0">201904L</span></span></span></td>
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<td><span class="t-mark-rev t-since-c23">(C23)</span></td>
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</tr>
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<tr>
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<td><code>maybe_unused</code></td>
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<td><code>[[<a href="c-language-attributes-maybe_unused.html">maybe_unused</a>]]</code></td>
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<td><span class="t-c"><span class="mw-geshi c source-c"><span class="nu0">201904L</span></span></span></td>
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<td><span class="t-mark-rev t-since-c23">(C23)</span></td>
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</tr>
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<tr>
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<td><code>nodiscard</code></td>
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<td><code>[[<a href="c-language-attributes-nodiscard.html">nodiscard</a>]]</code></td>
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<td><span class="t-c"><span class="mw-geshi c source-c"><span class="nu0">202003L</span></span></span></td>
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<td><span class="t-mark-rev t-since-c23">(C23)</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="c source-c">
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<pre class="de1"><span class="br0">[</span><span class="br0">[</span>gnu<span class="sy4">::</span><span class="me2">always_inline</span><span class="br0">]</span><span class="br0">]</span> <span class="br0">[</span><span class="br0">[</span>gnu<span class="sy4">::</span><span class="me2">hot</span><span class="br0">]</span><span class="br0">]</span> <span class="br0">[</span><span class="br0">[</span>gnu<span class="sy4">::</span><span class="kw4">const</span><span class="br0">]</span><span class="br0">]</span> <span class="br0">[</span><span class="br0">[</span>nodiscard<span class="br0">]</span><span class="br0">]</span>
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<span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 声明 f 带四个属性</span>
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<span class="br0">[</span><span class="br0">[</span>gnu<span class="sy4">::</span><span class="me2">always_inline</span>, gnu<span class="sy4">::</span><span class="kw4">const</span>, gnu<span class="sy4">::</span><span class="me2">hot</span>, nodiscard<span class="br0">]</span><span class="br0">]</span>
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<span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 同上，但用含四个属性的单个属性说明符</span>
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<span class="kw4">int</span> f<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="nu0">0</span><span class="sy4">;</span> <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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<span class="br0">}</span></pre></div>
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</div>
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</div>
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<h3><span class="mw-headline" id=".E5.A4.96.E9.83.A8.E9.93.BE.E6.8E.A5">外部链接</span></h3>
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<ul>
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<li><a rel="nofollow" class="external text" href="https://clang.llvm.org/docs/AttributeReference.html">Clang 中的属性</a></li>
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</ul>
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<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
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<div class="visualClear"></div>
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