70 lines
7.3 KiB
HTML
70 lines
7.3 KiB
HTML
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<title>C++ 属性： maybe_unused (C++17 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-cxx17">(C++17 起)</span></h1>
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-language.html">language</a>‎ | <a href="cpp-language-attributes.html">attributes</a></span></div>
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<p>抑制针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>[[maybe_unused]]</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>此属性可出现在下列实体的声明中：</p>
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<ul>
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<li><a href="cpp-language-classes.html">class/struct/union</a>：<span class="t-c"><span class="mw-geshi cpp source-cpp"><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="cpp-language-typedef.html">typedef</a>，包括<a href="cpp-language-type_alias.html">别名声明</a>：<span class="t-c"><span class="mw-geshi cpp source-cpp"><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>，<span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">using</span> PS <span class="br0">[</span><span class="br0">[</span>maybe_unused<span class="br0">]</span><span class="br0">]</span> <span class="sy1">=</span> S<span class="sy2">*</span><span class="sy4">;</span></span></span>，</li>
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<li>变量，包括<a href="cpp-language-static.html">静态数据成员</a>：<span class="t-c"><span class="mw-geshi cpp source-cpp"><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><a href="cpp-language-data_members.html">非静态数据成员</a>：<span class="t-c"><span class="mw-geshi cpp source-cpp"><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="cpp-language-function.html">函数</a>：<span class="t-c"><span class="mw-geshi cpp source-cpp"><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="sy4">;</span></span></span>，</li>
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<li><a href="cpp-language-enum.html">枚举</a>：<span class="t-c"><span class="mw-geshi cpp source-cpp"><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 cpp source-cpp"><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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<p>若编译器针对未使用实体发布警告，则对于任何声明为 <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 <cassert></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> <span class="kw4">bool</span> thing1,
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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">bool</span> thing2<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> <span class="kw4">bool</span> b <span class="sy1">=</span> thing1 <span class="sy3">&&</span> thing2<span class="sy4">;</span>
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<a href="cpp-error-assert.html"><span class="kw774">assert</span></a><span class="br0">(</span>b<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// 发行模式中，assert 在编译中被去掉，因而未使用 b</span>
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<span class="co1">// 无警告`，因为它被声明为 [[maybe_unused]]</span>
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<span class="br0">}</span> <span class="co1">// 未使用参数 thing1 与 thing2，无警告</span>
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<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span><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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