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<h1 itemprop="name"><span class="token">型の文脈でtypenameの省略を許可 [P0634R3]</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>型しか現れない文脈では、依存名を解決するための<code>typename</code>キーワードを省略できるようにする。</p>
<h2>仕様</h2>
<p>修飾名は、以下の条件を満たすときに型名とみなされる。</p>
<ul>
<li>型しか現れない文脈にある<ul>
<li>new演算子に指定する型名</li>
<li><a href="../cpp11/alias_templates.html">エイリアス宣言またはエイリアステンプレート宣言</a>に現れる型名</li>
<li><a href="../cpp11/trailing_return_types.html">後置の戻り値型</a></li>
<li>テンプレート型引数のデフォルト引数</li>
<li><code>static_cast</code>、<code>const_cast</code>、<code>reinterpret_cast</code>、<code>dynamic_cast</code>の型名</li>
</ul>
</li>
<li>以下の宣言における型指定子である<ul>
<li>名前空間スコープの単純宣言(≒変数宣言または構造化束縛宣言)または関数定義</li>
<li>メンバ宣言</li>
<li>メンバ宣言の中の引数宣言(デフォルト引数に現れる場合を除く)</li>
<li>識別子が修飾名である関数または関数テンプレートの引数宣言(デフォルト引数に現れる場合を除く)</li>
<li><a href="../cpp11/lambda_expressions.html">ラムダ式</a>または<a href="concepts.html">requires式</a>の引数宣言(デフォルト引数に現れる場合を除く)</li>
<li><a class="cpprefjp-defined-word" data-desc="定数テンプレートパラメータ。constant template parameter。テンプレートパラメータのうち、型ではなく値をとるもの。C++23以前は「非型テンプレートパラメータ (non-type template parameter)」と呼ばれていた">非型テンプレートパラメータ</a>ーの宣言</li>
</ul>
</li>
</ul>
<p><div class="codehilite"><pre><span></span><code><span class="c1">// <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0634r3.html" target="_blank">P0634R3</a>より引用</span>
<span class="k">template</span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">T</span><span class="o">></span><span class="w"> </span><span class="n">T</span><span class="o">::</span><span class="n">R</span><span class="w"> </span><span class="n">f</span><span class="p">();</span><span class="w"> </span><span class="c1">// OK, グローバルスコープにおける関数の戻り値</span>
<span class="k">template</span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">T</span><span class="o">></span><span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="n">f</span><span class="p">(</span><span class="n">T</span><span class="o">::</span><span class="n">R</span><span class="p">);</span><span class="w"> </span><span class="c1">// Ill-formed, void型の変数テンプレートの宣言</span>
<span class="k">template</span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">T</span><span class="o">></span><span class="w"> </span><span class="k">struct</span><span class="w"> </span><span class="nc">S</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="c1">// クラススコープ</span>
<span class="w"> </span><span class="k">using</span><span class="w"> </span><span class="n">Ptr</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">PtrTraits</span><span class="o"><</span><span class="n">T</span><span class="o">>::</span><span class="n">Ptr</span><span class="p">;</span><span class="w"> </span><span class="c1">// OK, エイリアス宣言</span>
<span class="w"> </span><span class="n">T</span><span class="o">::</span><span class="n">R</span><span class="w"> </span><span class="nf">f</span><span class="p">(</span><span class="n">T</span><span class="o">::</span><span class="n">P</span><span class="w"> </span><span class="n">p</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="c1">// OK, メンバ宣言、およびメンバ宣言における仮引数宣言</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="k">static_cast</span><span class="o"><</span><span class="n">T</span><span class="o">::</span><span class="n">R</span><span class="o">></span><span class="p">(</span><span class="n">p</span><span class="p">);</span><span class="w"> </span><span class="c1">// OK, static_castの型名</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="k">auto</span><span class="w"> </span><span class="n">g</span><span class="p">()</span><span class="w"> </span><span class="o">-></span><span class="w"> </span><span class="n">S</span><span class="o"><</span><span class="n">T</span><span class="o">*>::</span><span class="n">Ptr</span><span class="p">;</span><span class="w"> </span><span class="c1">// OK, 後置戻り値型</span>
<span class="p">};</span>
<span class="k">template</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="w"> </span><span class="kt">void</span><span class="w"> </span><span class="n">f</span><span class="p">()</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="c1">// ブロックスコープ</span>
<span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="p">(</span><span class="o">*</span><span class="n">pf</span><span class="p">)(</span><span class="n">T</span><span class="o">::</span><span class="n">X</span><span class="p">);</span><span class="w"> </span><span class="c1">// T::Xで初期化されるvoid*型の変数pf</span>
<span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="nf">g</span><span class="p">(</span><span class="n">T</span><span class="o">::</span><span class="n">X</span><span class="p">);</span><span class="w"> </span><span class="c1">// エラー: void型の変数宣言</span>
<span class="p">}</span>
</code></pre></div>
</p>
<h2>例</h2>
<p><div class="yata" id="886eb523609aaff9a495049c9899119d408d7074"><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/vector.html"><vector></a></span>
<span class="k">template</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">auto</span><span class="w"> </span><span class="n">Begin</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="n">T</span><span class="o">&</span><span class="w"> </span><span class="n">t</span><span class="p">)</span><span class="w"> </span><span class="o">-></span><span class="w"> </span><span class="n">T</span><span class="o">::</span><span class="n">const_iterator</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">t</span><span class="p">.</span><span class="n">begin</span><span class="p">();</span>
<span class="p">}</span>
<span class="kt">int</span><span class="w"> </span><span class="n">main</span><span class="p">()</span>
<span class="p">{</span>
<span class="w"> </span><span class="n"><a href="../../reference/vector/vector.html">std::vector</a></span><span class="w"> </span><span class="n">v</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="mi">3</span><span class="p">,</span><span class="w"> </span><span class="mi">4</span><span class="p">,</span><span class="w"> </span><span class="mi">5</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">Begin</span><span class="p">(</span><span class="n">v</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>1
</code></pre></p>
<h2>この機能が必要になった背景・経緯</h2>
<p>以前から基底クラスの指定では<code>typename</code>を省略することができた。</p>
<p><div class="codehilite"><pre><span></span><code><span class="k">template</span><span class="o"><</span><span class="k">class</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">Derived</span><span class="o">:</span><span class="w"> </span><span class="n">T</span><span class="o">::</span><span class="n">Base</span><span class="w"> </span><span class="c1">// typename不要</span>
<span class="p">{};</span>
</code></pre></div>
</p>
<p>しかし、他にも型しか現れない文脈は存在するので、それらでも<code>typename</code>を省略できるようになった。</p>
<h2>参照</h2>
<ul>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0634r3.html" target="_blank">P0634R3 Down with <code>typename</code>!</a></li>
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