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<h1 itemprop="name"><span class="token">コンパイル時のタプルやリストを展開処理する<code>template for</code>文 [P1306R5]</span><span class="cpp cpp26" title="C++26で追加">(C++26)</span></h1>
<div itemprop="articleBody"><p>
</p>
<p>このページはC++26に採用される見込みの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++26では、タプル、構造体、Range、パラメータパック展開リストなどの各要素に対して、コンパイル時に文を繰り返し展開する「展開文 (expansion statement)」として<code>template for</code>文を導入する。</p>
<p><div class="codehilite"><pre><span></span><code><span class="k">auto</span><span class="w"> </span><span class="n">tup</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n"><a href="../../reference/tuple/make_tuple.html">std::make_tuple</a></span><span class="p">(</span><span class="mi">0</span><span class="p">,</span><span class="w"> </span><span class="sc">'a'</span><span class="p">,</span><span class="w"> </span><span class="mf">3.14</span><span class="p">);</span>
<span class="k">template</span><span class="w"> </span><span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="k">auto</span><span class="w"> </span><span class="n">elem</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="n">tup</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n"><a href="../../reference/print/println.html">std::println</a></span><span class="p">(</span><span class="s">"{}"</span><span class="p">,</span><span class="w"> </span><span class="n">elem</span><span class="p">);</span>
<span class="p">}</span>
</code></pre></div>
</p>
<p>出力：</p>
<p><pre><code>0
a
3.14
</code></pre></p>
<p>この<code>template for</code>文は、本体の文をコンパイル時に要素数分だけ展開する。上記のコードは以下と等価である：</p>
<p><div class="codehilite"><pre><span></span><code><span class="p">{</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">elem</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">get</span><span class="o"><</span><span class="mi">0</span><span class="o">></span><span class="p">(</span><span class="n">tup</span><span class="p">);</span><span class="w"> </span><span class="n"><a href="../../reference/print/println.html">std::println</a></span><span class="p">(</span><span class="s">"{}"</span><span class="p">,</span><span class="w"> </span><span class="n">elem</span><span class="p">);</span><span class="w"> </span><span class="p">}</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">elem</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">get</span><span class="o"><</span><span class="mi">1</span><span class="o">></span><span class="p">(</span><span class="n">tup</span><span class="p">);</span><span class="w"> </span><span class="n"><a href="../../reference/print/println.html">std::println</a></span><span class="p">(</span><span class="s">"{}"</span><span class="p">,</span><span class="w"> </span><span class="n">elem</span><span class="p">);</span><span class="w"> </span><span class="p">}</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">elem</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">get</span><span class="o"><</span><span class="mi">2</span><span class="o">></span><span class="p">(</span><span class="n">tup</span><span class="p">);</span><span class="w"> </span><span class="n"><a href="../../reference/print/println.html">std::println</a></span><span class="p">(</span><span class="s">"{}"</span><span class="p">,</span><span class="w"> </span><span class="n">elem</span><span class="p">);</span><span class="w"> </span><span class="p">}</span>
<span class="p">}</span>
</code></pre></div>
</p>
<p>各展開において要素の型が異なりうるため、通常の<code>for</code>文では実現できない処理が可能となる。</p>
<h2>構文</h2>
<p><pre><code>template for ( init-statement(opt) for-<a href="../../reference/ranges/range.html">range</a>-declaration : expansion-initializer ) compound-statement
</code></pre></p>
<p><code>expansion-initializer</code>には以下の3種類の形式がある：</p>
<ul>
<li>列挙展開: <code>{ expression-list }</code> — 引数パック展開や、任意のタプル指定</li>
<li>反復展開 (Range) : <code>expression</code> — <code>begin()</code>/<code>end()</code>が定義され、要素数がコンパイル時に確定するRange</li>
<li>分解展開 (構造体・タプル) : <code>expression</code> — 構造化束縛可能なオブジェクト</li>
</ul>
<h2>展開の種類</h2>
<h3>列挙展開</h3>
<p>ブレース区切りの式リストやパック展開を展開する。</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">Ts</span><span class="o">></span>
<span class="kt">void</span><span class="w"> </span><span class="n">print_all</span><span class="p">(</span><span class="n">Ts</span><span class="p">...</span><span class="w"> </span><span class="n">elems</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="k">auto</span><span class="w"> </span><span class="n">elem</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="p">{</span><span class="n">elems</span><span class="p">...})</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n"><a href="../../reference/print/println.html">std::println</a></span><span class="p">(</span><span class="s">"{}"</span><span class="p">,</span><span class="w"> </span><span class="n">elem</span><span class="p">);</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">}</span>
<span class="n">print_all</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mf">2.0</span><span class="p">,</span><span class="w"> </span><span class="s">"three"</span><span class="p">);</span>
</code></pre></div>
</p>
<p>パック展開のほか、任意の式のリストも使用できる：</p>
<p><div class="codehilite"><pre><span></span><code><span class="k">template</span><span class="w"> </span><span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="k">auto</span><span class="w"> </span><span class="n">x</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="mf">2.0</span><span class="p">,</span><span class="w"> </span><span class="s">"three"</span><span class="p">})</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n"><a href="../../reference/print/println.html">std::println</a></span><span class="p">(</span><span class="s">"{}"</span><span class="p">,</span><span class="w"> </span><span class="n">x</span><span class="p">);</span>
<span class="p">}</span>
</code></pre></div>
</p>
<h3>反復展開</h3>
<p><code>begin()</code>/<code>end()</code>が定義され、<code>end - begin</code>がコンパイル時に確定するRangeを展開する。</p>
<p><div class="codehilite"><pre><span></span><code><span class="kt">void</span><span class="w"> </span><span class="nf">f</span><span class="p">()</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="k">for</span><span class="w"> </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">I</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="n"><a href="../../reference/array/array.html">std::array</a></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="p">{</span>
<span class="w"> </span><span class="k">static_assert</span><span class="p">(</span><span class="n">I</span><span class="w"> </span><span class="o"><</span><span class="w"> </span><span class="mi">4</span><span class="p">);</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">}</span>
</code></pre></div>
</p>
<h3>分解展開</h3>
<p>構造化束縛可能なオブジェクト（タプル、構造体など）を展開する。</p>
<p><div class="codehilite"><pre><span></span><code><span class="k">struct</span><span class="w"> </span><span class="nc">Point</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">x</span><span class="p">;</span><span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">y</span><span class="p">;</span><span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">z</span><span class="p">;</span><span class="w"> </span><span class="p">};</span>
<span class="k">consteval</span><span class="w"> </span><span class="kt">long</span><span class="w"> </span><span class="nf">total_size</span><span class="p">(</span><span class="n">Point</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="kt">long</span><span class="w"> </span><span class="n">result</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="k">auto</span><span class="w"> </span><span class="n">field</span><span class="w"> </span><span class="o">:</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="n">result</span><span class="w"> </span><span class="o">+=</span><span class="w"> </span><span class="k">sizeof</span><span class="p">(</span><span class="n">field</span><span class="p">);</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">result</span><span class="p">;</span>
<span class="p">}</span>
</code></pre></div>
</p>
<p>範囲としても分解としても解釈できる場合は、範囲が優先される。</p>
<h2><code>break</code>と<code>continue</code></h2>
<p><code>template for</code>文の本体内で<code>break</code>と<code>continue</code>を使用できる。これらは実行時の制御フローとして動作する。</p>
<ul>
<li><code>break</code> : 展開全体の末尾に飛ぶ</li>
<li><code>continue</code> : 次の展開の先頭に飛ぶ</li>
</ul>
<p>同じ型の要素のみのタプルであれば、通常の<code>if</code>文で条件分岐できる：</p>
<p><div class="codehilite"><pre><span></span><code><span class="k">auto</span><span class="w"> </span><span class="n">tup</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n"><a href="../../reference/tuple/make_tuple.html">std::make_tuple</a></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="k">template</span><span class="w"> </span><span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="k">auto</span><span class="w"> </span><span class="n">elem</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="n">tup</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">elem</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="mi">2</span><span class="p">)</span><span class="w"> </span><span class="k">break</span><span class="p">;</span>
<span class="w"> </span><span class="n"><a href="../../reference/print/println.html">std::println</a></span><span class="p">(</span><span class="s">"{}"</span><span class="p">,</span><span class="w"> </span><span class="n">elem</span><span class="p">);</span>
<span class="p">}</span>
<span class="c1">// 出力: 1</span>
</code></pre></div>
</p>
<p>異なる型の要素をもつタプルでは、型に依存する条件を<code>if constexpr</code>で分岐する必要がある：</p>
<p><div class="codehilite"><pre><span></span><code><span class="k">auto</span><span class="w"> </span><span class="n">tup</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n"><a href="../../reference/tuple/make_tuple.html">std::make_tuple</a></span><span class="p">(</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="s">"hello"</span><span class="p">,</span><span class="w"> </span><span class="mf">3.14</span><span class="p">);</span>
<span class="k">template</span><span class="w"> </span><span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="k">auto</span><span class="w"> </span><span class="n">elem</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="n">tup</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="c1">// 整数型の場合のみ値を判定してcontinueする</span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="p">(</span><span class="n"><a href="../../reference/type_traits/is_integral.html">std::is_integral_v</a></span><span class="o"><</span><span class="k">decltype</span><span class="p">(</span><span class="n">elem</span><span class="p">)</span><span class="o">></span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">elem</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="mi">1</span><span class="p">)</span><span class="w"> </span><span class="k">continue</span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="n"><a href="../../reference/print/println.html">std::println</a></span><span class="p">(</span><span class="s">"{}"</span><span class="p">,</span><span class="w"> </span><span class="n">elem</span><span class="p">);</span>
<span class="p">}</span>
<span class="c1">// 出力:</span>
<span class="c1">// hello</span>
<span class="c1">// 3.14</span>
</code></pre></div>
</p>
<h2><code>return</code></h2>
<p><code>template for</code>文の本体内で<code>return</code>を使用すると、所属する関数から直接返る。これは<code><a href="../../reference/tuple/apply.html">std::apply()</a></code>にラムダ式を渡す方法では実現できなかった利点である。</p>
<p><div class="codehilite"><pre><span></span><code><span class="c1">// タプルの要素から最初の整数型の値を探して返す</span>
<span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">Tuple</span><span class="o">></span>
<span class="n"><a href="../../reference/optional/optional.html">std::optional</a></span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="w"> </span><span class="n">find_first_int</span><span class="p">(</span><span class="n">Tuple</span><span class="o">&&</span><span class="w"> </span><span class="n">tup</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="k">auto</span><span class="o">&&</span><span class="w"> </span><span class="n">elem</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="n">tup</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="p">(</span><span class="n"><a href="../../reference/type_traits/is_integral.html">std::is_integral_v</a></span><span class="o"><</span><span class="n"><a href="../../reference/type_traits/remove_cvref.html">std::remove_cvref_t</a></span><span class="o"><</span><span class="k">decltype</span><span class="p">(</span><span class="n">elem</span><span class="p">)</span><span class="o">>></span><span class="p">)</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">elem</span><span class="p">;</span><span class="w"> </span><span class="c1">// 所属する関数から直接返る</span>
<span class="w"> </span><span class="p">}</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">std</span><span class="o">::</span><span class="n">nullopt</span><span class="p">;</span>
<span class="p">}</span>
<span class="k">auto</span><span class="w"> </span><span class="n">tup</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n"><a href="../../reference/tuple/make_tuple.html">std::make_tuple</a></span><span class="p">(</span><span class="s">"hello"</span><span class="p">,</span><span class="w"> </span><span class="mi">42</span><span class="p">,</span><span class="w"> </span><span class="mf">3.14</span><span class="p">);</span>
<span class="k">auto</span><span class="w"> </span><span class="n">result</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">find_first_int</span><span class="p">(</span><span class="n">tup</span><span class="p">);</span><span class="w"> </span><span class="c1">// 42</span>
</code></pre></div>
</p>
<h2><code>co_await</code></h2>
<p><code>template for</code>文の本体内で<code>co_await</code>を使用すると、所属するコルーチンを直接サスペンドできる。ラムダ式内の<code>co_await</code>は囲む関数ではなくラムダ自体をコルーチンにしてしまうため、従来の方法では実現が困難であった。</p>
<p><div class="codehilite"><pre><span></span><code><span class="c1">// タプルの各要素に対して非同期処理を順番に実行する</span>
<span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">Tuple</span><span class="o">></span>
<span class="n">Task</span><span class="w"> </span><span class="n">process_all</span><span class="p">(</span><span class="n">Tuple</span><span class="o">&&</span><span class="w"> </span><span class="n">tup</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="k">auto</span><span class="o">&&</span><span class="w"> </span><span class="n">elem</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="n">tup</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">co_await</span><span class="w"> </span><span class="n">async_process</span><span class="p">(</span><span class="n">elem</span><span class="p">);</span><span class="w"> </span><span class="c1">// 所属するコルーチンをサスペンドする</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">}</span>
</code></pre></div>
</p>
<h2>この機能が必要になった背景・経緯</h2>
<p>C++23以前では、<code><a href="../../reference/tuple/tuple.html">std::tuple</a></code>のような異種型コンテナの各要素を処理するために、テンプレートの再帰的インスタンス化や<code><a href="../../reference/tuple/apply.html">std::apply()</a></code>を使用する必要があった。これらの手法には以下の問題があった：</p>
<ul>
<li>コンパイル時間とメモリの消費が大きい</li>
<li>ラムダ式内での<code>return</code>や<code>co_await</code>が囲む関数に影響を与えない</li>
<li>コードが不自然で読みにくい</li>
</ul>
<p><code>template for</code>文はこれらの問題を解決し、異種型コンテナの反復処理を通常の<code>for</code>文と同様の構文で記述できるようにする。</p>
<h2><a href="#relative-page" id="relative-page">関連項目</a></h2>
<ul>
<li><a href="../cpp11/variadic_templates.html">C++11 可変引数テンプレート</a></li>
<li><a href="../cpp17/folding_expressions.html">C++17 畳み込み式</a></li>
<li><a href="../cpp17/structured_bindings.html">C++17 構造化束縛</a></li>
<li><a href="structured_bindings_can_introduce_a_pack.html">C++26 構造化束縛でパックを導入できるようにする</a></li>
<li><a href="reflection.html">C++26 静的リフレクション</a></li>
<li><code><a href="../../reference/tuple/tuple.html">std::tuple</a></code></li>
<li><code><a href="../../reference/tuple/apply.html">std::apply()</a></code></li>
</ul>
<h2>参照</h2>
<ul>
<li><a href="https://open-std.org/jtc1/sc22/wg21/docs/papers/2025/p1306r5.html" target="_blank">P1306R5 Expansion Statements</a></li>
</ul></div>
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