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<h1 itemprop="name"><span class="token">using宣言のパック展開 [P0195R2]</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>C++17にて<code>using</code>宣言の仕様が拡張され、パラメータパックが指定できるようになった。
具体的には<code>using</code>宣言の識別子をカンマで区切ること、
パラメータパックに省略記号<code>...</code>を指定してパック展開が可能になる。</p>
<p>C++14では下記のように1つずつ宣言していたが、</p>
<p><div class="codehilite"><pre><span></span><code><span class="k">using</span><span class="w"> </span><span class="n"><a href="../../reference/iostream/cout.html">std::cout</a></span><span class="p">;</span>
<span class="k">using</span><span class="w"> </span><span class="n"><a href="../../reference/ostream/endl.html">std::endl</a></span><span class="p">;</span>
</code></pre></div>
</p>
<p>下記のように一度に宣言できるようになった。
1つ目の識別子と名前空間が同じであっても省略はできないため、
2番目のような書き方はできない。</p>
<p><div class="codehilite"><pre><span></span><code><span class="k">using</span><span class="w"> </span><span class="n"><a href="../../reference/iostream/cout.html">std::cout</a></span><span class="p">,</span><span class="w"> </span><span class="n"><a href="../../reference/ostream/endl.html">std::endl</a></span><span class="p">;</span>
<span class="c1">//using <a href="../../reference/iostream/cout.html">std::cout</a>, endl; // NG</span>
</code></pre></div>
</p>
<p>省略記号<code>...</code>を指定しパック展開を行う例は下記の通り。</p>
<p><div class="codehilite"><pre><span></span><code><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">typename</span><span class="p">...</span><span class="w"> </span><span class="n">T</span><span class="o">></span>
<span class="k">struct</span><span class="w"> </span><span class="nc">A</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="n">T</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="n">T</span><span class="o">::</span><span class="k">operator</span><span class="p">()...;</span><span class="w"> </span><span class="c1">// 受け取ったクラスのoperator()を全て使えるようにする</span>
<span class="p">};</span>
</code></pre></div>
</p>
<h2>仕様</h2>
<p>文法の仕様は下記の通り。</p>
<p><pre><code> using-declaration:
using using-declarator-list ;
using-declarator-list:
using-declarator ...opt
using-declarator-list , using-declarator ...opt
</code></pre></p>
<h2>例</h2>
<p><code>ForAll</code>は<code>operator()(int)</code>メンバ関数を持つクラステンプレートである。
複数のクラスをテンプレートパラメータに受け取り、受け取った全てのクラスを継承する。
テンプレートパラメータのパック展開を行う<code>using</code>宣言により、
継承した全てのクラスの<code>operator()</code>を使えるようにしている。</p>
<p>この例では<code>long</code>や<code>std::string</code>を引数として渡すと<code>ForAll::operator()(int)</code>ではなく、
<code>using</code>宣言した<code>ForLong::operator()(long)</code>や<code>ForString::operator()(const std::string&)</code>が呼び出される。</p>
<p><div class="yata" id="f835a37e872080ad499565d8a6969d43ff540708"><div class="codehilite"><pre><span></span><code><span class="cp">#include <a href="../../reference/iostream.html"><iostream></a></span>
<span class="cp">#include <a href="../../reference/string.html"><string></a></span>
<span class="k">struct</span><span class="w"> </span><span class="nc">ForLong</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="nf">operator</span><span class="p">()(</span><span class="kt">long</span><span class="w"> </span><span class="n">v</span><span class="p">)</span><span class="w"> </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="s">"ForLong:"</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">v</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="p">}</span>
<span class="p">};</span>
<span class="k">struct</span><span class="w"> </span><span class="nc">ForString</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="nf">operator</span><span class="p">()(</span><span class="k">const</span><span class="w"> </span><span class="n"><a href="../../reference/string/basic_string.html">std::string</a></span><span class="o">&</span><span class="w"> </span><span class="n">v</span><span class="p">)</span><span class="w"> </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="s">"ForString:"</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">v</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="p">}</span>
<span class="p">};</span>
<span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">typename</span><span class="p">...</span><span class="w"> </span><span class="n">T</span><span class="o">></span>
<span class="k">struct</span><span class="w"> </span><span class="nc">ForAll</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="n">T</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="n">T</span><span class="o">::</span><span class="k">operator</span><span class="p">()...;</span>
<span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="nf">operator</span><span class="p">()(</span><span class="kt">int</span><span class="w"> </span><span class="n">v</span><span class="p">)</span><span class="w"> </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="s">"ForAll:"</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">v</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="p">}</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="p">{</span>
<span class="w"> </span><span class="n">ForAll</span><span class="o"><</span><span class="n">ForLong</span><span class="p">,</span><span class="w"> </span><span class="n">ForString</span><span class="o">></span><span class="w"> </span><span class="n">p</span><span class="p">;</span>
<span class="w"> </span><span class="n">p</span><span class="p">(</span><span class="mi">10</span><span class="p">);</span>
<span class="w"> </span><span class="n">p</span><span class="p">(</span><span class="mf">100L</span><span class="p">);</span>
<span class="w"> </span><span class="n">p</span><span class="p">(</span><span class="s">"hello"</span><span class="p">);</span>
<span class="p">}</span>
</code></pre></div>
</div></p>
<h3>出力</h3>
<p><pre><code>ForAll:10
ForLong:100
ForString:hello
</code></pre></p>
<h2>この機能が必要になった背景・経緯</h2>
<p>C++11にて可変引数テンプレートが導入され、
テンプレートパラメータに渡されたクラスをパック展開し、一度に全て継承することができるようになった。</p>
<p><div class="codehilite"><pre><span></span><code><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">typename</span><span class="p">...</span><span class="w"> </span><span class="n">T</span><span class="o">></span>
<span class="k">struct</span><span class="w"> </span><span class="nc">ForAll2</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="n">T</span><span class="p">...</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="c1">// Tに指定されたクラスを全て継承</span>
<span class="p">};</span>
</code></pre></div>
</p>
<p>しかしC++11やC++14ではパラメータパックに指定されたクラスが持つメンバ関数を、
全て<code>using</code>宣言する簡単な方法がなかった。
このためクラステンプレートが基底クラスと派生クラスでメンバ関数を<a class="cpprefjp-defined-word" data-desc="同名の関数を異なる引数・テンプレート・制約などで複数定義すること。または同名の関数の集合">オーバーロード</a>する場合、
実装が煩雑になってしまう問題があった。</p>
<p><div class="yata" id="d1e9b99ec705ad34bec4bf5fefa60e2e74e07d13"><div class="codehilite"><pre><span></span><code><span class="cp">#include <a href="../../reference/iostream.html"><iostream></a></span>
<span class="k">struct</span><span class="w"> </span><span class="nc">ForLong</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="nf">operator</span><span class="p">()(</span><span class="kt">long</span><span class="w"> </span><span class="n">v</span><span class="p">)</span><span class="w"> </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="s">"ForLong:"</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">v</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="p">}</span>
<span class="p">};</span>
<span class="k">struct</span><span class="w"> </span><span class="nc">ForString</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="nf">operator</span><span class="p">()(</span><span class="k">const</span><span class="w"> </span><span class="n"><a href="../../reference/string/basic_string.html">std::string</a></span><span class="o">&</span><span class="w"> </span><span class="n">v</span><span class="p">)</span><span class="w"> </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="s">"ForString:"</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">v</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="p">}</span>
<span class="p">};</span>
<span class="c1">// C++14までの方法、C++17でも使えるが冗長</span>
<span class="c1">// クラスを2つ以上受け取る場合、先頭のクラス T とそれ以外 Ts に分割する</span>
<span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">typename</span><span class="w"> </span><span class="nc">T</span><span class="p">,</span><span class="w"> </span><span class="k">typename</span><span class="p">...</span><span class="w"> </span><span class="n">Ts</span><span class="o">></span>
<span class="k">struct</span><span class="w"> </span><span class="nc">ForAll2</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="n">T</span><span class="p">,</span><span class="w"> </span><span class="n">ForAll2</span><span class="o"><</span><span class="n">Ts</span><span class="p">...</span><span class="o">></span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">using</span><span class="w"> </span><span class="n">T</span><span class="o">::</span><span class="k">operator</span><span class="p">();</span><span class="w"> </span><span class="c1">// 先頭のクラス T の operator() を使えるようにする</span>
<span class="w"> </span><span class="k">using</span><span class="w"> </span><span class="n">ForAll2</span><span class="o"><</span><span class="n">Ts</span><span class="p">...</span><span class="o">>::</span><span class="k">operator</span><span class="p">();</span><span class="w"> </span><span class="c1">// 残りのクラスを再帰的に使えるようにする</span>
<span class="p">};</span>
<span class="c1">// クラスを1つだけ受け取る場合、特殊な処理を行う（部分特殊化）</span>
<span class="c1">//</span>
<span class="c1">// クラステンプレートのみだと、クラスを1つしか受け取らない場合に</span>
<span class="c1">// Ts... が空になって ForAll2<Ts...> が文法エラーになる</span>
<span class="c1">// このためクラスが1つの場合は特別扱いする必要がある</span>
<span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">typename</span><span class="w"> </span><span class="nc">T</span><span class="o">></span>
<span class="k">struct</span><span class="w"> </span><span class="nc">ForAll2</span><span class="o"><</span><span class="n">T</span><span class="o">></span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="n">T</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">using</span><span class="w"> </span><span class="n">T</span><span class="o">::</span><span class="k">operator</span><span class="p">();</span>
<span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="nf">operator</span><span class="p">()(</span><span class="kt">int</span><span class="w"> </span><span class="n">v</span><span class="p">)</span><span class="w"> </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="s">"ForAll2:"</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">v</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="p">}</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="p">{</span>
<span class="w"> </span><span class="n">ForAll2</span><span class="o"><</span><span class="n">ForLong</span><span class="p">,</span><span class="w"> </span><span class="n">ForString</span><span class="o">></span><span class="w"> </span><span class="n">p</span><span class="p">;</span>
<span class="w"> </span><span class="n">p</span><span class="p">(</span><span class="mi">20</span><span class="p">);</span>
<span class="w"> </span><span class="n">p</span><span class="p">(</span><span class="mf">200L</span><span class="p">);</span>
<span class="w"> </span><span class="n">p</span><span class="p">(</span><span class="s">"hello2"</span><span class="p">);</span>
<span class="p">}</span>
</code></pre></div>
</div></p>
<h3>出力</h3>
<p><pre><code>ForAll2:20
ForLong:200
ForString:hello2
</code></pre></p>
<p>この問題を解決するためC++17では<code>using</code>でパック展開ができるようになった。</p>
<h2><a href="#relative-page" id="relative-page">関連項目</a></h2>
<ul>
<li><a href="../cpp11/variadic_templates.html">可変引数テンプレート</a></li>
</ul>
<h2>参照</h2>
<ul>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0195r2.html" target="_blank">P0195R2 Pack expansions in using-declarations</a></li>
</ul></div>
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