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<h1 itemprop="name"><span class="token">[[fallthrough]]属性 [P0188R1]</span><span class="cpp cpp17" title="C++17で追加">(C++17)</span></h1>
<div itemprop="articleBody"><p></p>
<p>このページはC++17に採用された言語機能の変更を解説しています。</p>
<p>のちのC++規格でさらに変更される場合があるため<a href="#relative-page">関連項目</a>を参照してください。</p>
<p></p>
<h2>概要</h2>
<p><code>[[fallthrough]]</code>属性は意図したフォールスルーであることをコンパイラに伝え、警告を抑制するための属性である。</p>
<p>switch-case文において意図しないフォールスルーによってバグが発生することを防止するため、コンパイラはコンパイル時にフォールスルーを検出して警告を出力する場合がある。</p>
<p>プログラマが意図してフォールスルーを行う場合、コンパイラの警告は無用である。しかし従来はコンパイラごとに警告を抑制する方法が異なり、標準的な方法は無かった。C++17では<code>[[fallthrough]]</code>属性により意図したフォールスルーであることをコンパイラに伝え、警告を抑制することができる。</p>
<h2>仕様</h2>
<p><code>[[fallthrough]]</code>属性はフォールスルーしたい各<code>case</code>の最後の式に記述する。ただし最後の<code>case</code>/<code>default</code>に記述するとコンパイルエラーとなる。</p>
<h2>例</h2>
<p><div class="yata" id="40c673ef9b10eb0de62802768ce1cbea5a287d3f"><div class="codehilite"><pre><span></span><code><span class="cp">#include <a href="../../reference/iostream.html"><iostream></a></span>
<span class="kt">int</span><span class="w"> </span><span class="nf">main</span><span class="p">()</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">n</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">3</span><span class="p">;</span>
<span class="w"> </span><span class="k">switch</span><span class="w"> </span><span class="p">(</span><span class="n">n</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">case</span><span class="w"> </span><span class="mi">1</span><span class="p">:</span>
<span class="w"> </span><span class="k">case</span><span class="w"> </span><span class="mi">2</span><span class="p">:</span><span class="w"> </span><span class="c1">//caseの間に1つも文がなければフォールスルーは警告されない</span>
<span class="w"> </span><span class="n"><a href="../../reference/iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="s">"case 2</span><span class="se">\n</span><span class="s">"</span><span class="p">;</span>
<span class="w"> </span><span class="p">[[</span><span class="n">fallthrough</span><span class="p">]];</span>
<span class="w"> </span><span class="k">case</span><span class="w"> </span><span class="mi">3</span><span class="p">:</span><span class="w"> </span><span class="c1">//[[fallthrough]]属性の記述によりフォールスルー警告は無効化される</span>
<span class="w"> </span><span class="n"><a href="../../reference/iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="s">"case 3</span><span class="se">\n</span><span class="s">"</span><span class="p">;</span>
<span class="w"> </span><span class="k">case</span><span class="w"> </span><span class="mi">4</span><span class="p">:</span><span class="w"> </span><span class="c1">//コンパイラがフォールスルーを警告する</span>
<span class="w"> </span><span class="n"><a href="../../reference/iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="s">"case 4</span><span class="se">\n</span><span class="s">"</span><span class="p">;</span>
<span class="w"> </span><span class="c1">//[[fallthrough]]; //最後のcaseには記述できない、コンパイルエラーになる</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">}</span>
</code></pre></div>
</div></p>
<h3>出力</h3>
<p><pre><code>case 3
case 4
</code></pre></p>
<h3>警告例</h3>
<p>g++ 7.1.0、<code>-Wextra</code> オプションでコンパイルした場合:</p>
<p><pre><code>fallthrough.cpp: In function ‘int main()’:
fallthrough.cpp:11:15: warning: this statement may fall through [-Wimplicit-fallthrough=]
<a href="../../reference/iostream/cout.html">std::cout</a> << "case 3\n";
~~~~~~~~~~^~~~~~~~~~~~~
fallthrough.cpp:12:3: note: here
case 4: //コンパイラがフォールスルーを警告する
^~~~
</code></pre></p>
<h2><a href="#relative-page" id="relative-page">関連項目</a></h2>
<ul>
<li><a href="../cpp11/attributes.html">C++11 属性構文</a></li>
</ul>
<h2>参照</h2>
<ul>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/p0068r0.pdf" target="_blank">P0068R0 Proposal of [[unused]], [[nodiscard]] and [[fallthrough]] attributes.</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0188r1.pdf" target="_blank">P0188R1 Wording for [[fallthrough]] attribute.</a></li>
</ul></div>
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