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<h1 itemprop="name"><span class="token">スコープ付き列挙型のusing宣言 [P1099R5]</span><span class="cpp cpp20" title="C++20で追加">(C++20)</span></h1>
<div itemprop="articleBody"><p></p>
<p>このページはC++20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>名前空間のように、<code>using enum EnumType;</code>もしくは<code>using EnumType::enumerator</code>とすることで、列挙値のスコープ指定を省略できる。</p>
<h2>仕様</h2>
<p>以下のusing enum宣言を導入する。</p>
<p><pre><code>using enum EnumType;
</code></pre></p>
<p>using enum宣言はトップレベル、ブロック内、クラスメンバの宣言として書くことができる。</p>
<p>using enum宣言した列挙型の列挙値は、そのスコープにおいて、スコープ指定を省略して列挙値の名前のみで参照できる。</p>
<p><div class="codehilite"><pre><span></span><code><span class="c1">// <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1099r5.html" target="_blank">P1099R5</a>より引用</span>
<span class="k">enum</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">rgba_color_channel</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="n">red</span><span class="p">,</span><span class="w"> </span><span class="n">green</span><span class="p">,</span><span class="w"> </span><span class="n">blue</span><span class="p">,</span><span class="w"> </span><span class="n">alpha</span><span class="p">};</span>
<span class="n"><a href="../../reference/string_view/basic_string_view.html">std::string_view</a></span><span class="w"> </span><span class="nf">to_string</span><span class="p">(</span><span class="n">rgba_color_channel</span><span class="w"> </span><span class="n">channel</span><span class="p">)</span><span class="w"> </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">my_channel</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">using</span><span class="w"> </span><span class="k">enum</span><span class="w"> </span><span class="nc">rgba_color_channel</span><span class="p">;</span>
<span class="w"> </span><span class="k">case</span><span class="w"> </span><span class="no">red</span><span class="p">:</span><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="s">"red"</span><span class="p">;</span>
<span class="w"> </span><span class="k">case</span><span class="w"> </span><span class="no">green</span><span class="p">:</span><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="s">"green"</span><span class="p">;</span>
<span class="w"> </span><span class="k">case</span><span class="w"> </span><span class="no">blue</span><span class="p">:</span><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="s">"blue"</span><span class="p">;</span>
<span class="w"> </span><span class="k">case</span><span class="w"> </span><span class="no">alpha</span><span class="p">:</span><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="s">"alpha"</span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">}</span>
</code></pre></div>
</p>
<p>また、using宣言で列挙値を指定できるようになる。このときは、指定した列挙値だけがスコープ指定を省略できるようになる。</p>
<p>対象とする列挙型は<a href="../cpp11/scoped_enum.html">スコープを持つ列挙型</a>でなくても構わない。</p>
<p><div class="yata" id="19ea57c4dad31905b836128128f820898a41213a"><div class="codehilite"><pre><span></span><code><span class="k">namespace</span><span class="w"> </span><span class="nn">ns</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">foo</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">enum</span><span class="w"> </span><span class="nc">bar</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">A</span><span class="p">,</span>
<span class="w"> </span><span class="n">B</span><span class="p">,</span>
<span class="w"> </span><span class="n">C</span>
<span class="w"> </span><span class="p">};</span>
<span class="w"> </span><span class="p">};</span>
<span class="p">}</span>
<span class="k">using</span><span class="w"> </span><span class="k">enum</span><span class="w"> </span><span class="nc">ns</span><span class="o">::</span><span class="n">foo</span><span class="o">::</span><span class="n">bar</span><span class="p">;</span>
</code></pre></div>
</div></p>
<h2>例</h2>
<p><div class="yata" id="4b5f62f64b069281d880659b5876a2573686a674"><div class="codehilite"><pre><span></span><code><span class="cp">#include <a href="../../reference/iostream.html"><iostream></a></span>
<span class="k">enum</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Enum1</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">Value1</span>
<span class="p">};</span>
<span class="k">struct</span><span class="w"> </span><span class="nc">Enum2</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">enum</span><span class="w"> </span><span class="nc">Enum</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">Value2</span>
<span class="w"> </span><span class="p">};</span>
<span class="p">};</span>
<span class="k">enum</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Enum3</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">Value3</span><span class="p">,</span>
<span class="w"> </span><span class="n">Value4</span>
<span class="p">};</span>
<span class="k">struct</span><span class="w"> </span><span class="nc">Type</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">using</span><span class="w"> </span><span class="k">enum</span><span class="w"> </span><span class="nc">Enum3</span><span class="p">;</span>
<span class="p">};</span>
<span class="k">constexpr</span><span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">Value1</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">1</span><span class="p">;</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="k">using</span><span class="w"> </span><span class="k">enum</span><span class="w"> </span><span class="nc">Enum1</span><span class="p">;</span>
<span class="w"> </span><span class="k">using</span><span class="w"> </span><span class="k">enum</span><span class="w"> </span><span class="nc">Enum2</span><span class="o">::</span><span class="n">Enum</span><span class="p">;</span>
<span class="w"> </span><span class="k">using</span><span class="w"> </span><span class="n">Enum3</span><span class="o">::</span><span class="n">Value3</span><span class="p">;</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="k">static_cast</span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="p">(</span><span class="n">Value1</span><span class="p">)</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../../reference/ostream/endl.html">std::endl</a></span><span class="p">;</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="o">::</span><span class="n">Value1</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../../reference/ostream/endl.html">std::endl</a></span><span class="p">;</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="n">Value2</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../../reference/ostream/endl.html">std::endl</a></span><span class="p">;</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="k">static_cast</span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="p">(</span><span class="n">Value3</span><span class="p">)</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../../reference/ostream/endl.html">std::endl</a></span><span class="p">;</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="k">static_cast</span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="p">(</span><span class="n">Enum3</span><span class="o">::</span><span class="n">Value4</span><span class="p">)</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../../reference/ostream/endl.html">std::endl</a></span><span class="p">;</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="k">static_cast</span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="p">(</span><span class="n">Type</span><span class="o">::</span><span class="n">Value4</span><span class="p">)</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../../reference/ostream/endl.html">std::endl</a></span><span class="p">;</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="k">static_cast</span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="p">(</span><span class="n">Type</span><span class="p">().</span><span class="n">Value4</span><span class="p">)</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../../reference/ostream/endl.html">std::endl</a></span><span class="p">;</span>
<span class="p">}</span>
</code></pre></div>
</div></p>
<h3>出力</h3>
<p><pre><code>0
1
0
0
1
1
1
</code></pre></p>
<h2>この機能が必要になった背景・経緯</h2>
<p>C言語形式の列挙型を改善するために<a href="../cpp11/scoped_enum.html">スコープを持つ列挙型</a>が導入されたが、スコープを持っているために列挙値を参照するときに必ずスコープ解決演算子を使う必要があり、冗長に感じられる場合もあった。</p>
<p>そこで、列挙型のスコープを名前空間と同じようなものと捉えて、using宣言によって省略できるようにした。</p>
<p>これにより、C言語形式の列挙型を<a href="../cpp11/scoped_enum.html">スコープを持つ列挙型</a>へ置き換えるというリファクタリングをする際、(整数との暗黙変換を別とすれば)ソースコードレベルの互換性を保って置き換えることができるようにもなった。</p>
<h2><a href="#relative-page" id="relative-page">関連項目</a></h2>
<ul>
<li><a href="../cpp11/scoped_enum.html">スコープを持つ列挙型</a></li>
</ul>
<h2>参照</h2>
<ul>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1099r5.html" target="_blank">P1099R5 Using Enum</a></li>
</ul></div>
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