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73 lines
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<title>C 属性： maybe_unused (C23 起)</title>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">C 属性：</span> maybe_unused <span class="t-mark-rev t-since-c23">(C23 起)</span></h1>
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<div id="contentSub"><span class="subpages">< <a href="c.html">c</a>‎ | <a href="c-language.html">language</a>‎ | <a href="c-language-attributes.html">attributes</a></span></div>
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<p>抑制对未使用实体的警告。</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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<td class="t-sdsc-nopad"><code><b>[[</b></code> <code><b>maybe_unused</b></code> <code><b>]]</b></code><br />
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<code><b>[[</b></code> <code><b>__maybe_unused__</b></code> <code><b>]]</b></code></td>
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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>此属性能出现在下列实体的声明中：</p>
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<ul>
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<li><a href="c-language-struct.html">结构体</a>/<a href="c-language-union.html">联合体</a>： <span class="t-c"><span class="mw-geshi c source-c"><span class="kw1">struct</span> <span class="br0">[</span><span class="br0">[</span>maybe_unused<span class="br0">]</span><span class="br0">]</span> S<span class="sy4">;</span></span></span> ，</li>
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<li><a href="c-language-typedef.html">typedef 名</a>： <span class="t-c"><span class="mw-geshi c source-c"><span class="br0">[</span><span class="br0">[</span>maybe_unused<span class="br0">]</span><span class="br0">]</span> <span class="kw1">typedef</span> S<span class="sy2">*</span> PS<span class="sy4">;</span></span></span> ，</li>
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<li>对象： <span class="t-c"><span class="mw-geshi c source-c"><span class="br0">[</span><span class="br0">[</span>maybe_unused<span class="br0">]</span><span class="br0">]</span> <span class="kw4">int</span> x<span class="sy4">;</span></span></span> ，</li>
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<li>结构体/联合体成员： <span class="t-c"><span class="mw-geshi c source-c"><span class="kw1">union</span> U <span class="br0">{</span> <span class="br0">[</span><span class="br0">[</span>maybe_unused<span class="br0">]</span><span class="br0">]</span> <span class="kw4">int</span> n<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span></span></span> ，</li>
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<li><a href="c-language-function_definition.html">函数</a>： <span class="t-c"><span class="mw-geshi c source-c"><span class="br0">[</span><span class="br0">[</span>maybe_unused<span class="br0">]</span><span class="br0">]</span> <span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span><span class="sy4">;</span></span></span> ，</li>
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<li><a href="c-language-enum.html">枚举</a>： <span class="t-c"><span class="mw-geshi c source-c"><span class="kw2">enum</span> <span class="br0">[</span><span class="br0">[</span>maybe_unused<span class="br0">]</span><span class="br0">]</span> E <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span></span></span> ，</li>
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<li>枚举项： <span class="t-c"><span class="mw-geshi c source-c"><span class="kw2">enum</span> <span class="br0">{</span> A <span class="br0">[</span><span class="br0">[</span>maybe_unused<span class="br0">]</span><span class="br0">]</span>, B <span class="br0">[</span><span class="br0">[</span>maybe_unused<span class="br0">]</span><span class="br0">]</span> <span class="sy1">=</span> <span class="nu0">42</span> <span class="br0">}</span><span class="sy4">;</span></span></span> 。</li>
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</ul>
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<p>若编译器对未使用实体发布警告，则对任何声明为 <code>maybe_unused</code> 的实体抑制该警告。</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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<pre class="de1"><span class="co2">#include <assert.h></span>
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<span class="br0">[</span><span class="br0">[</span>maybe_unused<span class="br0">]</span><span class="br0">]</span> <span class="kw4">void</span> f<span class="br0">(</span><span class="br0">[</span><span class="br0">[</span>maybe_unused<span class="br0">]</span><span class="br0">]</span> _Bool cond1, <span class="br0">[</span><span class="br0">[</span>maybe_unused<span class="br0">]</span><span class="br0">]</span> _Bool cond2<span class="br0">)</span>
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<span class="br0">{</span>
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<span class="br0">[</span><span class="br0">[</span>maybe_unused<span class="br0">]</span><span class="br0">]</span> _Bool b <span class="sy1">=</span> cond1 <span class="sy3">&&</span> cond2<span class="sy4">;</span>
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<a href="c-error-assert.html"><span class="kw407">assert</span></a><span class="br0">(</span>b<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 发布模式中， assert 被编译掉，而 b 不被使用</span>
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<span class="co1">// 无警告，因为它声明为 [[maybe_unused]]</span>
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<span class="br0">}</span> <span class="co1">// 参数 cond1 与 cond2 不被使用，无警告</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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f<span class="br0">(</span><span class="nu0">1</span>, <span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span></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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