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<h1 itemprop="name"><span class="token">可変引数テンプレートでfriend宣言をできるようにする [P2893R3]</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、可変引数テンプレートのパラメータパックを、friend宣言に指定できるようになる。</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">class</span><span class="p">...</span><span class="w"> </span><span class="n">Ts</span><span class="o">></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">friend</span><span class="w"> </span><span class="n">Ts</span><span class="p">...;</span><span class="w"> </span><span class="c1">// C++26: OK</span>
<span class="p">};</span>
</code></pre></div>
</p>
<p>この指定により、パラメータパック<code>Ts</code>内のすべての型をfriend指定したことになる。</p>
<h3>Passkeyイディオム</h3>
<p>Passkeyイディオムを使用すると、メンバ関数ごとにアクセス許可をだすことができる。以下のプログラムでは、クラス<code>C</code>はクラス<code>A</code>にfriend関係を付与している。そのため、<code>A::m()</code>は<code>C</code>の内部構造すべてにアクセスできる。</p>
<p>しかし、<code>C</code>はPasskeyイディオムを使用して、<code>intentionalA()</code>と<code>intentionalB()</code>へのアクセスも許可している。<code>Passkey<B></code>クラスには、フレンド関係の<code>B</code>のみがアクセスできるプライベートコンストラクタがあるため、<code>B</code>のみが<code>Passkey<B></code>のインスタンスを作成できる。最初の引数として<code>Passkey<B></code>のインスタンスを指定せずに<code>intentionalB()</code>を呼び出すことはできない。そのため、<code>intentionalB()</code>は公開メンバ関数ではあるが、<code>B</code>からのみ呼び出すことができる。</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">class</span><span class="w"> </span><span class="nc">Passkey</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">friend</span><span class="w"> </span><span class="n">T</span><span class="p">;</span>
<span class="w"> </span><span class="n">Passkey</span><span class="p">()</span><span class="w"> </span><span class="p">{}</span>
<span class="p">};</span>
<span class="k">class</span><span class="w"> </span><span class="nc">A</span><span class="p">;</span>
<span class="k">class</span><span class="w"> </span><span class="nc">B</span><span class="p">;</span>
<span class="k">class</span><span class="w"> </span><span class="nc">C</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">friend</span><span class="w"> </span><span class="n">A</span><span class="p">;</span>
<span class="k">private</span><span class="o">:</span>
<span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="n">internal</span><span class="p">();</span>
<span class="k">public</span><span class="o">:</span>
<span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="n">intentionalA</span><span class="p">(</span><span class="n">Passkey</span><span class="o"><</span><span class="n">A</span><span class="o">></span><span class="p">);</span>
<span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="nf">intentionalB</span><span class="p">(</span><span class="n">Passkey</span><span class="o"><</span><span class="n">B</span><span class="o">></span><span class="p">);</span>
<span class="p">};</span>
<span class="k">class</span><span class="w"> </span><span class="nc">A</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">m</span><span class="p">(</span><span class="n">C</span><span class="o">&</span><span class="w"> </span><span class="n">c</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">c</span><span class="p">.</span><span class="n">internal</span><span class="p">();</span><span class="w"> </span><span class="c1">// OK</span>
<span class="w"> </span><span class="n">c</span><span class="p">.</span><span class="n">intentionalA</span><span class="p">({});</span><span class="w"> </span><span class="c1">// OK</span>
<span class="w"> </span><span class="n">c</span><span class="p">.</span><span class="n">intentionalB</span><span class="p">({});</span><span class="w"> </span><span class="c1">// エラー！Passkey<B>のコンストラクタにアクセスできない</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">};</span>
<span class="k">class</span><span class="w"> </span><span class="nc">B</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">m</span><span class="p">(</span><span class="n">C</span><span class="o">&</span><span class="w"> </span><span class="n">c</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">c</span><span class="p">.</span><span class="n">intentionalB</span><span class="p">({});</span><span class="w"> </span><span class="c1">// OK</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">};</span>
</code></pre></div>
</p>
<p>C++26ではこのイディオムを拡張し、複数のクラスに同時にアクセス許可を付与できるようになる。</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="p">...</span><span class="w"> </span><span class="n">Ts</span><span class="o">></span>
<span class="k">class</span><span class="w"> </span><span class="nc">Passkey</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">friend</span><span class="w"> </span><span class="n">Ts</span><span class="p">...;</span>
<span class="w"> </span><span class="n">Passkey</span><span class="p">()</span><span class="w"> </span><span class="p">{}</span>
<span class="p">};</span>
<span class="k">class</span><span class="w"> </span><span class="nc">C</span><span class="w"> </span><span class="p">{</span>
<span class="k">public</span><span class="o">:</span>
<span class="w"> </span><span class="c1">// Blarg, Blip, Bazクラスからのみ呼び出せる</span>
<span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="n">intentional</span><span class="p">(</span><span class="n">Passkey</span><span class="o"><</span><span class="n">Blarg</span><span class="p">,</span><span class="w"> </span><span class="n">Blip</span><span class="p">,</span><span class="w"> </span><span class="n">Baz</span><span class="o">></span><span class="p">);</span>
<span class="p">};</span>
</code></pre></div>
</p>
<h3>派生クラスへのCRTPアクセス</h3>
<p>クラステンプレートから継承する際、派生クラスの型を基底クラスのテンプレート引数として指定するイディオムが「CRTP (Curiously Recurring Template Pattern、奇妙に再帰したテンプレートパターン)」と呼ばれている。このイディオムでも、可変引数テンプレートでfriend宣言できるようになることが有用である。</p>
<p>派生クラスの一部APIが基底クラスで必要となる場合があり、基底クラスでのみ必要であるためそのAPIはprivateにして、基底クラスにのみアクセスを許可したいことがある。そういった状況で、C++26での可変引数テンプレートでのfriend宣言が必要になる。</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">Crtp</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">MsgT</span><span class="o">></span>
<span class="k">class</span><span class="w"> </span><span class="nc">Receiver</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">receive</span><span class="p">(</span><span class="n">MsgT</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">static_cast</span><span class="o"><</span><span class="n">Crtp</span><span class="o">*></span><span class="p">(</span><span class="k">this</span><span class="p">)</span><span class="o">-></span><span class="n">private_</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="p">}</span>
<span class="p">};</span>
<span class="k">template</span><span class="o"><</span><span class="k">class</span><span class="p">...</span><span class="w"> </span><span class="n">MsgTs</span><span class="o">></span>
<span class="k">struct</span><span class="w"> </span><span class="nc">Dispatcher</span><span class="w"> </span><span class="o">:</span>
<span class="w"> </span><span class="k">public</span><span class="w"> </span><span class="n">Receiver</span><span class="o"><</span><span class="n">Dispatcher</span><span class="o"><</span><span class="n">MsgTs</span><span class="p">...</span><span class="o">></span><span class="p">,</span><span class="w"> </span><span class="n">MsgTs</span><span class="o">></span><span class="p">...</span><span class="w"> </span><span class="c1">// OK。可変引数テンプレートでの継承</span>
<span class="p">{</span>
<span class="w"> </span><span class="k">using</span><span class="w"> </span><span class="n">Receiver</span><span class="o"><</span><span class="n">Dispatcher</span><span class="p">,</span><span class="w"> </span><span class="n">MsgTs</span><span class="o">>::</span><span class="n">Receiver</span><span class="p">...;</span><span class="w"> </span><span class="c1">// OK。可変引数テンプレートでのusing宣言</span>
<span class="w"> </span><span class="k">friend</span><span class="w"> </span><span class="n">Receiver</span><span class="o"><</span><span class="n">Dispatcher</span><span class="p">,</span><span class="w"> </span><span class="n">MsgTs</span><span class="o">></span><span class="p">...;</span><span class="w"> </span><span class="c1">// C++26: OK</span>
<span class="k">private</span><span class="o">:</span>
<span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">private_</span><span class="p">;</span>
<span class="p">};</span>
</code></pre></div>
</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>
</ul>
<h2>参照</h2>
<ul>
<li><a href="https://open-std.org/jtc1/sc22/wg21/docs/papers/2024/p2893r3.html" target="_blank">P2893R3 Variadic friends</a></li>
</ul></div>
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