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<h1 itemprop="name"><span class="token">関数宣言を削除する理由を指定できるようにする [P2573R2]</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、関数をdelete宣言する際に、その理由をコンパイル時文字列で指定できるようになる。</p>
<p><div class="codehilite"><pre><span></span><code><span class="c1">// 別名の新たなAPIを用意する場合に、そちらに誘導する</span>
<span class="kt">void</span><span class="w"> </span><span class="nf">newapi</span><span class="p">();</span>
<span class="kt">void</span><span class="w"> </span><span class="n">oldapi</span><span class="p">()</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">delete</span><span class="p">(</span><span class="s">"This old API is outdated and already been removed. Please use newapi() instead."</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="w"> </span><span class="nc">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="p">{</span><span class="cm">/* … */</span><span class="p">};</span>
<span class="c1">// 一部のオーバーロードが危険であるために禁止する場合</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="n">A</span><span class="o"><</span><span class="n">T</span><span class="o">></span><span class="w"> </span><span class="n">factory</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="p">)</span><span class="w"> </span><span class="p">{</span><span class="cm">/* process lvalue */</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="n">A</span><span class="o"><</span><span class="n">T</span><span class="o">></span><span class="w"> </span><span class="n">factory</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="p">)</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">delete</span><span class="p">(</span><span class="s">"Using rvalue to construct A may result in dangling reference"</span><span class="p">);</span>
<span class="c1">// 一部の自動定義される特殊メンバ関数が</span>
<span class="c1">// 低パフォーマンスを引き起こすために禁止する場合</span>
<span class="k">struct</span><span class="w"> </span><span class="nc">MoveOnly</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="c1">// … (with move members defaulted or defined)</span>
<span class="w"> </span><span class="n">MoveOnly</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="n">MoveOnly</span><span class="o">&</span><span class="p">)</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">delete</span><span class="p">(</span><span class="s">"Copy-construction is expensive; please use move construction instead."</span><span class="p">);</span>
<span class="w"> </span><span class="n">MoveOnly</span><span class="o">&</span><span class="w"> </span><span class="k">operator</span><span class="o">=</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="n">MoveOnly</span><span class="o">&</span><span class="p">)</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">delete</span><span class="p">(</span><span class="s">"Copy-assignment is expensive; please use move assignment instead."</span><span class="p">);</span>
<span class="p">};</span>
</code></pre></div>
</p>
<p>これまでは削除理由を指定できなかったために、ユーザーが誤って削除済み関数にアクセスした際に「エラー！：削除された関数を呼び出そうとした」という情報しか得られなかった。削除理由が指定できるようになることで、その設計意図をユーザーに伝えられるようになる。</p>
<h2><a href="#relative-page" id="relative-page">関連項目</a></h2>
<ul>
<li><a href="../cpp11/defaulted_and_deleted_functions.html">C++11 関数のdefault／delete宣言</a></li>
</ul>
<h2>参照</h2>
<ul>
<li><a href="https://open-std.org/jtc1/sc22/wg21/docs/papers/2024/p2573r2.html" target="_blank">P2573R2 <code>= delete("should have a reason");</code></a></li>
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