76 lines
6.6 KiB
HTML
76 lines
6.6 KiB
HTML
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<title>C++ 属性： fallthrough (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> fallthrough <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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<div id="mw-content-text" lang="zh" dir="ltr" class="mw-content-ltr" xml:lang="zh">
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<p>指示从前一标号直落是有意的，而在发生直落时给出警告的编译器不应诊断它。</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>[[fallthrough]]</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>仅可应用到<a href="cpp-language-statements.html#.E8.A1.A8.E8.BE.BE.E5.BC.8F.E8.AF.AD.E5.8F.A5">空语句</a>以创建<i>直落语句（ fallthrough statement ）</i>： <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">[</span><span class="br0">[</span>fallthrough<span class="br0">]</span><span class="br0">]</span><span class="sy4">;</span></span></span> 。</p>
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<p>直落语句仅可用于 <a href="cpp-language-switch.html"><tt>switch</tt></a> 语句中，其中待执行的下个语句是该 switch 语句的带 case 或 default 标号的语句。若直落语句在循环中，则下个（带标号）语句必须是该循环的同一迭代的一部分。</p>
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<p>指示从前一标号直落是有意的，而在发生直落时给出警告的编译器不应诊断它。</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="cpp source-cpp">
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<pre class="de1"><span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span> n<span class="br0">)</span> <span class="br0">{</span>
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<span class="kw4">void</span> g<span class="br0">(</span><span class="br0">)</span>, h<span class="br0">(</span><span class="br0">)</span>, i<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">switch</span> <span class="br0">(</span>n<span class="br0">)</span> <span class="br0">{</span>
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<span class="kw1">case</span> <span class="nu0">1</span><span class="sy4">:</span>
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<span class="kw1">case</span> <span class="nu0">2</span><span class="sy4">:</span>
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g<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">[</span><span class="br0">[</span>fallthrough<span class="br0">]</span><span class="br0">]</span><span class="sy4">;</span>
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<span class="kw1">case</span> <span class="nu0">3</span><span class="sy4">:</span> <span class="co1">// 直落时不警告</span>
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h<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="kw1">case</span> <span class="nu0">4</span><span class="sy4">:</span> <span class="co1">// 编译器可在发生直落时警告</span>
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<span class="kw1">if</span><span class="br0">(</span>n <span class="sy1"><</span> <span class="nu0">3</span><span class="br0">)</span> <span class="br0">{</span>
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i<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">[</span><span class="br0">[</span>fallthrough<span class="br0">]</span><span class="br0">]</span><span class="sy4">;</span> <span class="co1">// OK</span>
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<span class="br0">}</span>
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<span class="kw1">else</span> <span class="br0">{</span>
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<span class="kw1">return</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw1">case</span> <span class="nu0">5</span><span class="sy4">:</span>
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<span class="kw1">while</span> <span class="br0">(</span><span class="kw2">false</span><span class="br0">)</span> <span class="br0">{</span>
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<span class="br0">[</span><span class="br0">[</span>fallthrough<span class="br0">]</span><span class="br0">]</span><span class="sy4">;</span> <span class="co1">// 非良构：下一语句不是同一迭代的一部分</span>
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<span class="br0">}</span>
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<span class="kw1">case</span> <span class="nu0">6</span><span class="sy4">:</span>
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<span class="br0">[</span><span class="br0">[</span>fallthrough<span class="br0">]</span><span class="br0">]</span><span class="sy4">;</span> <span class="co1">// 非良构：无后继的 case 或 default 标号</span>
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<span class="br0">}</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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