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‎lang/cpp26.html‎

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<p class="text-right"><small>
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最終更新日時(UTC):
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<span itemprop="datePublished" content="2025-04-03T07:03:23">
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2025年04月03日 07時03分23秒
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<span itemprop="datePublished" content="2025-04-03T08:20:34">
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2025年04月03日 08時20分34秒
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<h3>型特性</h3>
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<ul>
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<li><code><a href="../reference/type_traits.html">&lt;type_traits&gt;</a></code>に、共用体のどのメンバがアクティブかを判定するための関数として<code><span href="https://cpprefjp.github.io/reference/type_traits/is_within_lifetime.md.nolink">std::is_within_lifetime()</span></code>を追加</li>
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<li><code><a href="../reference/type_traits.html">&lt;type_traits&gt;</a></code>に、仮想継承の関係を判定する<code><span href="https://cpprefjp.github.io/reference/type_traits/is_virtual_base_of.md.nolink">std::is_virtual_base_of</span></code>を追加</li>
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<li><code><a href="../reference/type_traits.html">&lt;type_traits&gt;</a></code>に、仮想継承の関係を判定する<code><a href="../reference/type_traits/is_virtual_base_of.html">std::is_virtual_base_of</a></code>を追加</li>
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<h3>制約</h3>
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<ul>

‎reference/type_traits.html‎

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<span itemprop="datePublished" content="2025-01-10T08:54:05">
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2025年01月10日 08時54分05秒
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<span itemprop="datePublished" content="2025-04-03T08:20:34">
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<td>C++11</td>
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</tr>
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<tr>
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<td><code><a href="type_traits/is_virtual_base_of.html">is_virtual_base_of</a></code></td>
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<td>ある型が別の型の仮想基底クラスか調べる (class template)</td>
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<td>C++26</td>
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</tr>
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<tr>
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<td><code><a href="type_traits/is_convertible.html">is_convertible</a></code></td>
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<td>ある型から別の型へ変換可能か調べる (class template)</td>
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<td>C++11</td>

‎reference/type_traits/is_virtual_base_of.html‎

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2025年04月03日 08時20分34秒
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<span itemprop="name">Akira Takahashi</span>
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が更新
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<div class="identifier-type">class template</div><div class="header">&lt;type_traits&gt;</div><h1 itemprop="name"><span class="namespace" title="namespace std">std::</span><span class="token">is_virtualbase_of</span><span class="cpp cpp26" title="C++26で追加">(C++26)</span></h1>
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<div itemprop="articleBody"><p><div class="codehilite"><pre><span></span><code><span class="k">namespace</span> <span class="n">std</span> <span class="p">{</span>
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<span class="k">template</span> <span class="o">&lt;</span><span class="k">class</span> <span class="nc">Base</span><span class="p">,</span> <span class="k">class</span> <span class="nc">Derived</span><span class="o">&gt;</span>
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<span class="k">struct</span> <span class="n">is_virtual_base_of</span><span class="p">;</span>
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<span class="k">template</span> <span class="o">&lt;</span><span class="k">class</span> <span class="nc">Base</span><span class="p">,</span> <span class="k">class</span> <span class="nc">Derived</span><span class="o">&gt;</span>
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<span class="kr">inline</span> <span class="k">constexpr</span> <span class="kt">bool</span> <span class="n">is_virtual_base_of_v</span> <span class="o">=</span> <span class="n">is_virtual_base_of</span><span class="o">&lt;</span><span class="n">Base</span><span class="p">,</span> <span class="n">Derived</span><span class="o">&gt;::</span><span class="n">value</span><span class="p">;</span>
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<span class="p">}</span>
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</code></pre></div>
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</p>
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<h2>概要</h2>
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<p>型<code>Base</code>が型<code>Derived</code>の仮想基底クラスか調べる。</p>
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<h2>要件</h2>
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<p><code>Base</code>と<code>Derived</code>が非共用体のクラスであり、異なる型である場合(cv修飾は無視される)、<code>Derived</code>は<a class="cpprefjp-defined-word" data-desc="型のサイズを決定できる型。不完全型ではない型">完全型</a>でなければならない。</p>
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<h2>効果</h2>
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<p><code>is_virtual_base_of</code>は、型<code>Base</code>が型<code>Derived</code>の仮想基底クラス (cv修飾は無視される) であるならば<code><a href="true_type.html">true_type</a></code>から派生し、そうでなければ<code><a href="false_type.html">false_type</a></code>から派生する。</p>
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<h2>備考</h2>
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<p>派生時の<code>private</code>、<code>protected</code>指定は、派生関係の判定に影響しない。</p>
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<h2>例</h2>
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<p><div class="yata" id="53bc45345087e71cf0846acac2f5f96bd4fa1521"><div class="codehilite"><pre><span></span><code><span class="cp">#include</span> <span class="cpf"><a href="../type_traits.html">&lt;type_traits&gt;</a></span><span class="cp"></span>
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<span class="k">struct</span> <span class="n">B</span> <span class="p">{};</span>
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<span class="k">struct</span> <span class="nl">B1</span> <span class="p">:</span> <span class="k">virtual</span> <span class="k">public</span> <span class="n">B</span> <span class="p">{};</span>
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<span class="k">struct</span> <span class="nl">B2</span> <span class="p">:</span> <span class="k">virtual</span> <span class="k">public</span> <span class="n">B</span> <span class="p">{};</span>
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<span class="k">struct</span> <span class="nl">D</span> <span class="p">:</span> <span class="k">public</span> <span class="n">B1</span><span class="p">,</span> <span class="k">public</span> <span class="n">B2</span> <span class="p">{};</span>
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<span class="k">static_assert</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">is_virtual_base_of_v</span><span class="o">&lt;</span><span class="n">B</span><span class="p">,</span> <span class="n">D</span><span class="o">&gt;</span> <span class="o">==</span> <span class="nb">true</span><span class="p">);</span>
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<span class="k">static_assert</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">is_virtual_base_of_v</span><span class="o">&lt;</span><span class="n">B</span><span class="p">,</span> <span class="n">B1</span><span class="o">&gt;</span> <span class="o">==</span> <span class="nb">true</span><span class="p">);</span>
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<span class="k">static_assert</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">is_virtual_base_of_v</span><span class="o">&lt;</span><span class="n">B</span><span class="p">,</span> <span class="n">B2</span><span class="o">&gt;</span> <span class="o">==</span> <span class="nb">true</span><span class="p">);</span>
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<span class="c1">// cv修飾は無視される</span>
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<span class="k">static_assert</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">is_virtual_base_of_v</span><span class="o">&lt;</span><span class="k">const</span> <span class="n">B</span><span class="p">,</span> <span class="k">const</span> <span class="n">D</span><span class="o">&gt;</span> <span class="o">==</span> <span class="nb">true</span><span class="p">);</span>
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<span class="c1">// クラス型ではない</span>
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<span class="k">static_assert</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">is_virtual_base_of_v</span><span class="o">&lt;</span><span class="kt">int</span><span class="p">,</span> <span class="kt">char</span><span class="o">&gt;</span> <span class="o">==</span> <span class="nb">false</span><span class="p">);</span>
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<span class="kt">int</span> <span class="nf">main</span><span class="p">()</span> <span class="p">{}</span>
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</code></pre></div>
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</div></p>
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<h3>出力</h3>
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<p><pre><code></code></pre></p>
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<h2>バージョン</h2>
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<h3>言語</h3>
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<ul>
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<li>C++26</li>
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<h3>処理系</h3>
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<li><a href="../../implementation.html#clang">Clang</a>: 21.0 <span aria-label="検証済" role="img" title="検証済">✅</span></li>
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<li><a href="../../implementation.html#visual_cpp">Visual C++</a>: 2022 Update 10 <span aria-label="未実装" role="img" title="未実装">❌</span></li>
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</ul>
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<h2>参照</h2>
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<ul>
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<li><a href="https://open-std.org/jtc1/sc22/wg21/docs/papers/2023/p2985r0.html" target="_blank">P2985R0 A type trait for detecting virtual base classes</a></li>
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</body>
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</html>

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