update automatically

Author: cpprefjp-autoupdate

reference/optional/optional.html

@@ -200,8 +200,8 @@
 
     <p class="text-right"><small>
       最終更新日時(UTC):
-      <span itemprop="datePublished" content="2025-07-08T15:33:39">
-        2025年07月08日 15時33分39秒
+      <span itemprop="datePublished" content="2026-04-07T06:36:39">
+        2026年04月07日 06時36分39秒
       </span>
       <br/>
       <span itemprop="author" itemscope itemtype="http://schema.org/Person">
@@ -225,15 +225,27 @@
       <div class="col-sm-12 content-body">
 
   <div class="identifier-type">function template</div><div class="header">&lt;optional&gt;</div><h1 itemprop="name"><span class="namespace" title="namespace std">std::</span><span class="class" title="class optional">optional::</span><span class="token">and_then</span><span class="cpp cpp23" title="C++23で追加">(C++23)</span></h1>
-<div itemprop="articleBody"><p><div class="codehilite"><pre><span></span><code><span class="k">template</span><span class="w"> </span><span class="o">&lt;</span><span class="k">class</span><span class="w"> </span><span class="nc">F</span><span class="o">&gt;</span><span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="k">auto</span><span class="w"> </span><span class="n">and_then</span><span class="p">(</span><span class="n">F</span><span class="o">&amp;&amp;</span><span class="w"> </span><span class="n">f</span><span class="p">)</span><span class="w"> </span><span class="o">&amp;</span><span class="p">;</span><span class="w">       </span><span class="c1">// (1)</span>
-<span class="k">template</span><span class="w"> </span><span class="o">&lt;</span><span class="k">class</span><span class="w"> </span><span class="nc">F</span><span class="o">&gt;</span><span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="k">auto</span><span class="w"> </span><span class="n">and_then</span><span class="p">(</span><span class="n">F</span><span class="o">&amp;&amp;</span><span class="w"> </span><span class="n">f</span><span class="p">)</span><span class="w"> </span><span class="o">&amp;&amp;</span><span class="p">;</span><span class="w">      </span><span class="c1">// (2)</span>
-<span class="k">template</span><span class="w"> </span><span class="o">&lt;</span><span class="k">class</span><span class="w"> </span><span class="nc">F</span><span class="o">&gt;</span><span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="k">auto</span><span class="w"> </span><span class="n">and_then</span><span class="p">(</span><span class="n">F</span><span class="o">&amp;&amp;</span><span class="w"> </span><span class="n">f</span><span class="p">)</span><span class="w"> </span><span class="k">const</span><span class="o">&amp;</span><span class="p">;</span><span class="w">  </span><span class="c1">// (3)</span>
-<span class="k">template</span><span class="w"> </span><span class="o">&lt;</span><span class="k">class</span><span class="w"> </span><span class="nc">F</span><span class="o">&gt;</span><span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="k">auto</span><span class="w"> </span><span class="n">and_then</span><span class="p">(</span><span class="n">F</span><span class="o">&amp;&amp;</span><span class="w"> </span><span class="n">f</span><span class="p">)</span><span class="w"> </span><span class="k">const</span><span class="o">&amp;&amp;</span><span class="p">;</span><span class="w"> </span><span class="c1">// (4)</span>
+<div itemprop="articleBody"><p><div class="codehilite"><pre><span></span><code><span class="c1">// optional&lt;T&gt;版のオーバーロード</span>
+<span class="k">template</span><span class="w"> </span><span class="o">&lt;</span><span class="k">class</span><span class="w"> </span><span class="nc">F</span><span class="o">&gt;</span><span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="k">auto</span><span class="w"> </span><span class="n">and_then</span><span class="p">(</span><span class="n">F</span><span class="o">&amp;&amp;</span><span class="w"> </span><span class="n">f</span><span class="p">)</span><span class="w"> </span><span class="o">&amp;</span><span class="p">;</span><span class="w">       </span><span class="c1">// (1) C++23</span>
+<span class="k">template</span><span class="w"> </span><span class="o">&lt;</span><span class="k">class</span><span class="w"> </span><span class="nc">F</span><span class="o">&gt;</span><span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="k">auto</span><span class="w"> </span><span class="n">and_then</span><span class="p">(</span><span class="n">F</span><span class="o">&amp;&amp;</span><span class="w"> </span><span class="n">f</span><span class="p">)</span><span class="w"> </span><span class="o">&amp;&amp;</span><span class="p">;</span><span class="w">      </span><span class="c1">// (2) C++23</span>
+<span class="k">template</span><span class="w"> </span><span class="o">&lt;</span><span class="k">class</span><span class="w"> </span><span class="nc">F</span><span class="o">&gt;</span><span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="k">auto</span><span class="w"> </span><span class="n">and_then</span><span class="p">(</span><span class="n">F</span><span class="o">&amp;&amp;</span><span class="w"> </span><span class="n">f</span><span class="p">)</span><span class="w"> </span><span class="k">const</span><span class="o">&amp;</span><span class="p">;</span><span class="w">  </span><span class="c1">// (3) C++23</span>
+<span class="k">template</span><span class="w"> </span><span class="o">&lt;</span><span class="k">class</span><span class="w"> </span><span class="nc">F</span><span class="o">&gt;</span><span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="k">auto</span><span class="w"> </span><span class="n">and_then</span><span class="p">(</span><span class="n">F</span><span class="o">&amp;&amp;</span><span class="w"> </span><span class="n">f</span><span class="p">)</span><span class="w"> </span><span class="k">const</span><span class="o">&amp;&amp;</span><span class="p">;</span><span class="w"> </span><span class="c1">// (4) C++23</span>
+
+<span class="c1">// optional&lt;T&amp;&gt;版のオーバーロード (C++26)</span>
+<span class="k">template</span><span class="w"> </span><span class="o">&lt;</span><span class="k">class</span><span class="w"> </span><span class="nc">F</span><span class="o">&gt;</span><span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="k">auto</span><span class="w"> </span><span class="n">and_then</span><span class="p">(</span><span class="n">F</span><span class="o">&amp;&amp;</span><span class="w"> </span><span class="n">f</span><span class="p">)</span><span class="w"> </span><span class="k">const</span><span class="p">;</span><span class="w">   </span><span class="c1">// (5) C++26</span>
 </code></pre></div>
 </p>
 <h2>概要</h2>
 <p>有効値を保持していれば、値に対して<code>f</code>を適用した結果を<code>optional</code>として返す。
 有効値を保持していなければ、<code><a href="../nullopt_t.html">std::nullopt</a></code>を返す。</p>
+<ul>
+<li>(1) : <code>*this</code>が非<code>const</code>左辺値の場合</li>
+<li>(2) : <code>*this</code>が非<code>const</code>右辺値の場合</li>
+<li>(3) : <code>*this</code>が<code>const</code>左辺値の場合</li>
+<li>(4) : <code>*this</code>が<code>const</code>右辺値の場合</li>
+<li>(5) : <code>optional&lt;T&amp;&gt;</code>の場合</li>
+</ul>
+<p><code>optional&lt;T&gt;</code>では (1)～(4) が定義され、<code>optional&lt;T&amp;&gt;</code>では (5) のみが定義される。</p>
 <p>実際には複数<a class="cpprefjp-defined-word" data-desc="同名の関数を異なる引数・テンプレート・制約などで複数定義すること。または同名の関数の集合">オーバーロード</a>が提供されるが、大まかには下記シグニチャのようにみなせる。
 <code>and_then</code>へは、引数リストに1個の<code>T</code>型をとり<code>std::optional&lt;Return&gt;</code>型を返す関数や関数オブジェクトを与える。</p>
 <p><div class="codehilite"><pre><span></span><code><span class="k">template</span><span class="w"> </span><span class="o">&lt;</span><span class="k">class</span><span class="w"> </span><span class="nc">T</span><span class="o">&gt;</span>
@@ -248,6 +260,7 @@ <h2><a class="cpprefjp-defined-word" data-desc="関数等の意味論を構成
 <ul>
 <li>(1), (3) : <code><a href="../../type_traits/invoke_result.html">invoke_result_t</a>&lt;F, decltype(<a href="value.html">value()</a>)&gt;</code></li>
 <li>(2), (4) : <code><a href="../../type_traits/invoke_result.html">invoke_result_t</a>&lt;F, decltype(<a href="../../utility/move.html">std::move</a>(<a href="value.html">value()</a>))&gt;</code></li>
+<li>(5) : <code><a href="../../type_traits/invoke_result.html">invoke_result_t</a>&lt;F, decltype(*val)&gt;</code> (説明専用メンバ<code>val</code>は<code>T*</code>型)</li>
 </ul>
 <p><code><a href="../../type_traits/remove_cvref.html">remove_cvref_t</a>&lt;U&gt;</code>は<code>optional</code>の特殊化であること</p>
 <h2>効果</h2>
@@ -272,6 +285,16 @@ <h2>効果</h2>
 </code></pre></div>
 </p>
 </li>
+<li>
+<p>(5) : 次と等価</p>
+<p><div class="codehilite"><pre><span></span><code><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="o">*</span><span class="k">this</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"><a href="../../functional/invoke.html">invoke</a></span><span class="p">(</span><span class="n"><a href="../../utility/forward.html">std::forward</a></span><span class="o">&lt;</span><span class="n">F</span><span class="o">&gt;</span><span class="p">(</span><span class="n">f</span><span class="p">),</span><span class="w"> </span><span class="o">*</span><span class="n">val</span><span class="p">);</span>
+<span class="p">}</span><span class="w"> </span><span class="k">else</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"><a href="../../type_traits/remove_cvref.html">remove_cvref_t</a></span><span class="o">&lt;</span><span class="n">U</span><span class="o">&gt;</span><span class="p">();</span>
+<span class="p">}</span>
+</code></pre></div>
+</p>
+</li>
 </ul>
 <h2>備考</h2>
 <p><code>and_then</code>は、メソッドチェーンをサポートするモナド風(monadic)操作として導入された。
@@ -325,6 +348,10 @@ <h2>関連項目</h2>
 <h2>参照</h2>
 <ul>
 <li><a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p0798r8.html" target="_blank">P0798R8 Monadic operations for std::optional</a></li>
+<li><a href="https://open-std.org/jtc1/sc22/wg21/docs/papers/2025/p2988r12.pdf" target="_blank">P2988R12 <code>std::optional&lt;T&amp;&gt;</code></a><ul>
+<li>C++26で参照型<code>T&amp;</code>に対する部分特殊化を追加</li>
+</ul>
+</li>
 </ul></div>
 
       </div>