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2026年08月14日 16時05分16秒 (JST)
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<div class="identifier-type">function template</div><div class="header"><optional></div><h1 itemprop="name"><span class="namespace" title="namespace std">std::</span><span class="class" title="class optional">optional::</span><span class="token">emplace</span><span class="cpp cpp17" title="C++17で追加">(C++17)</span></h1>
<div itemprop="articleBody"><p><div class="codehilite"><pre><span></span><code><span class="c1">// optional<T>版のオーバーロード</span>
<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">Args</span><span class="o">></span>
<span class="n">T</span><span class="o">&</span><span class="w"> </span><span class="n">emplace</span><span class="p">(</span><span class="n">Args</span><span class="o">&&</span><span class="p">...</span><span class="w"> </span><span class="n">args</span><span class="p">);</span><span class="w"> </span><span class="c1">// (1) C++17</span>
<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">Args</span><span class="o">></span>
<span class="k">constexpr</span><span class="w"> </span><span class="n">T</span><span class="o">&</span><span class="w"> </span><span class="n">emplace</span><span class="p">(</span><span class="n">Args</span><span class="o">&&</span><span class="p">...</span><span class="w"> </span><span class="n">args</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"><</span><span class="k">class</span><span class="w"> </span><span class="nc">U</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="p">...</span><span class="w"> </span><span class="n">Args</span><span class="o">></span>
<span class="n">T</span><span class="o">&</span><span class="w"> </span><span class="n">emplace</span><span class="p">(</span><span class="n"><a href="../../initializer_list/initializer_list.html">std::initializer_list</a></span><span class="o"><</span><span class="n">U</span><span class="o">></span><span class="w"> </span><span class="n">il</span><span class="p">,</span><span class="w"> </span><span class="n">Args</span><span class="o">&&</span><span class="p">...</span><span class="w"> </span><span class="n">args</span><span class="p">);</span><span class="w"> </span><span class="c1">// (2) C++17</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">U</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="p">...</span><span class="w"> </span><span class="n">Args</span><span class="o">></span>
<span class="k">constexpr</span><span class="w"> </span><span class="n">T</span><span class="o">&</span><span class="w"> </span><span class="n">emplace</span><span class="p">(</span><span class="n"><a href="../../initializer_list/initializer_list.html">std::initializer_list</a></span><span class="o"><</span><span class="n">U</span><span class="o">></span><span class="w"> </span><span class="n">il</span><span class="p">,</span><span class="w"> </span><span class="n">Args</span><span class="o">&&</span><span class="p">...</span><span class="w"> </span><span class="n">args</span><span class="p">);</span><span class="w"> </span><span class="c1">// (2) C++23</span>
<span class="c1">// optional<T&>版のオーバーロード (C++26)</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">U</span><span class="o">></span>
<span class="k">constexpr</span><span class="w"> </span><span class="n">T</span><span class="o">&</span><span class="w"> </span><span class="n">emplace</span><span class="p">(</span><span class="n">U</span><span class="o">&&</span><span class="w"> </span><span class="n">u</span><span class="p">)</span><span class="w"> </span><span class="k">noexcept</span><span class="p">(</span><span class="n"><i>see below</i></span><span class="p">);</span><span class="w"> </span><span class="c1">// (3) C++26</span>
</code></pre></div>
</p>
<h2>概要</h2>
<p>要素型のコンストラクタ引数から直接構築する。</p>
<ul>
<li>(1) : 可変個の引数をとり、それを型<code>T</code>のコンストラクタ引数として渡して、この関数内で型<code>T</code>の有効値を構築して保持する</li>
<li>(2) : 初期化子リストと可変個の引数をとり、それらを型<code>T</code>のコンストラクタ引数として渡して、この関数内で型<code>T</code>の有効値を構築して保持する</li>
<li>(3) : <code>optional<T&></code>の参照先を再束縛する。変換可能な単一引数を受け取る</li>
</ul>
<p><code>optional<T></code>では (1), (2) が定義され、<code>optional<T&></code>では (3) のみが定義される。</p>
<h2><a class="cpprefjp-defined-word" data-desc="関数等の意味論を構成する要素の1つ。Constraints。関数であればオーバーロード解決に参加するための条件、それ以外であれば受け付ける型の条件">テンプレートパラメータ制約</a></h2>
<ul>
<li>(1) : <code><a href="../../type_traits/is_constructible.html">is_constructible_v</a><T, Args&&...> == true</code>であること</li>
<li>(3) : <code><a href="../../type_traits/is_constructible.html">is_constructible_v</a><T&, U> == true</code>であり、<code><a href="../../type_traits/reference_constructs_from_temporary.html">reference_constructs_from_temporary_v</a><T&, U> == false</code>であること</li>
</ul>
<h2>効果</h2>
<ul>
<li>(1), (2) : まず<code><a href="reset.html">reset()</a></code>メンバ関数を呼び出す<ul>
<li>(1) : 型<code>T</code>の有効値を、<code><a href="../../utility/forward.html">std::forward<Args></a>(args)...</code>を引数として構築する</li>
<li>(2) : 型<code>T</code>の有効値を、<code>il</code>と<code><a href="../../utility/forward.html">std::forward<Args></a>(args)...</code>を引数として構築する</li>
</ul>
</li>
<li>(3) : 参照先を<code>u</code>に再束縛する</li>
</ul>
<h2><a class="cpprefjp-defined-word" data-desc="関数呼び出し式の評価結果となるオブジェクト・値">戻り値</a></h2>
<p>代入された有効値への参照を返す。</p>
<h2><a class="cpprefjp-defined-word" data-desc="問題が発生したときに、現在実行位置を過去に通過・記録した位置に戻し、文脈情報を添えて紐づけられた処理(例外ハンドラー)を呼び出す仕組み。またはその事態">例外</a></h2>
<p>型<code>T</code>の選択されたコンストラクタが、任意の<a class="cpprefjp-defined-word" data-desc="問題が発生したときに、現在実行位置を過去に通過・記録した位置に戻し、文脈情報を添えて紐づけられた処理(例外ハンドラー)を呼び出す仕組み。またはその事態">例外</a>を送出する可能性がある。</p>
<h2><a class="cpprefjp-defined-word" data-desc="問題が発生したときに、現在実行位置を過去に通過・記録した位置に戻し、文脈情報を添えて紐づけられた処理(例外ハンドラー)を呼び出す仕組み。またはその事態">例外</a>安全性</h2>
<p>型<code>T</code>のコンストラクタが<a class="cpprefjp-defined-word" data-desc="問題が発生したときに、現在実行位置を過去に通過・記録した位置に戻し、文脈情報を添えて紐づけられた処理(例外ハンドラー)を呼び出す仕組み。またはその事態">例外</a>を送出した場合、<code>*this</code>は有効値を含まない状態となり、元々保持していた有効値は破棄される。</p>
<h2>備考</h2>
<ul>
<li>(2) : この<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>は主に、コンテナをアロケータ付きで、初期化子リスト代入するためにある</li>
<li>(3) : 一時オブジェクトから参照を構築すると<a class="cpprefjp-defined-word" data-desc="dangling。寿命が尽きた無効なオブジェクトやメモリ領域を、ポインタ・参照・イテレータなどが指し続けている状態のこと。これらにアクセスすると未定義の動作を引き起こす">ダングリング</a>になるケースでは、この<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>は削除定義される</li>
</ul>
<h2>例</h2>
<p><div class="yata" id="bcff7a8df9df6c695e13160026226671b9cf5350"><div class="codehilite"><pre><span></span><code><span class="cp">#include <a href="../../iostream.html"><iostream></a></span>
<span class="cp">#include <a href="../../optional.html"><optional></a></span>
<span class="cp">#include <a href="../../string.html"><string></a></span>
<span class="cp">#include <a href="../../vector.html"><vector></a></span>
<span class="kt">int</span><span class="w"> </span><span class="nf">main</span><span class="p">()</span>
<span class="p">{</span>
<span class="w"> </span><span class="c1">// (1)</span>
<span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n"><a href="../optional.html">std::optional</a></span><span class="o"><</span><span class="n"><a href="../../string/basic_string.html">std::string</a></span><span class="o">></span><span class="w"> </span><span class="n">p</span><span class="p">;</span>
<span class="w"> </span><span class="c1">// <a href="../../string/basic_string.html">std::string</a>のコンストラクタ引数を渡して、</span>
<span class="w"> </span><span class="c1">// <span style="color:#ff0000">emplace</span>関数内でstringオブジェクトを構築する。</span>
<span class="w"> </span><span class="n">p</span><span class="p">.</span><span class="n"><span style="color:#ff0000">emplace</span></span><span class="p">(</span><span class="mi">3</span><span class="p">,</span><span class="w"> </span><span class="sc">'A'</span><span class="p">);</span>
<span class="w"> </span><span class="n"><a href="../../iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="s">"(1) : "</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="value.html">p.value()</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../../ostream/endl.html">std::endl</a></span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="c1">// (2)</span>
<span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n"><a href="../optional.html">std::optional</a></span><span class="o"><</span><span class="n"><a href="../../vector/vector.html">std::vector</a></span><span class="o"><</span><span class="kt">int</span><span class="o">>></span><span class="w"> </span><span class="n">p</span><span class="p">;</span>
<span class="w"> </span><span class="c1">// 初期化子リストとアロケータから、</span>
<span class="w"> </span><span class="c1">// <span style="color:#ff0000">emplace</span>関数内でvectorオブジェクトを構築する。</span>
<span class="w"> </span><span class="n"><a href="../../memory/allocator.html">std::allocator</a></span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="w"> </span><span class="n">alloc</span><span class="p">;</span>
<span class="w"> </span><span class="n">p</span><span class="p">.</span><span class="n"><span style="color:#ff0000">emplace</span></span><span class="p">({</span><span class="mi">3</span><span class="p">,</span><span class="w"> </span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">4</span><span class="p">},</span><span class="w"> </span><span class="n">alloc</span><span class="p">);</span>
<span class="w"> </span><span class="n"><a href="../../iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="s">"(2) :"</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../../ostream/endl.html">std::endl</a></span><span class="p">;</span>
<span class="w"> </span><span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="kt">int</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="o">:</span><span class="w"> </span><span class="n"><a href="value.html">p.value()</a></span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n"><a href="../../iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="s">" "</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">x</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../../ostream/endl.html">std::endl</a></span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">}</span>
</code></pre></div>
</div></p>
<h3>出力</h3>
<p><pre><code>(1) : AAA
(2) :
3
1
4
</code></pre></p>
<h2>バージョン</h2>
<h3>言語</h3>
<ul>
<li>C++17</li>
</ul>
<h3>処理系</h3>
<ul>
<li><a href="../../../implementation.html#clang">Clang</a>: 4.0.1 <span aria-label="検証済" role="img" title="検証済">✅</span></li>
<li><a href="../../../implementation.html#gcc">GCC</a>: 7.2 <span aria-label="検証済" role="img" title="検証済">✅</span></li>
<li><a href="../../../implementation.html#icc">ICC</a>: ??</li>
<li><a href="../../../implementation.html#visual_cpp">Visual C++</a>: ??</li>
</ul>
<h2>参照</h2>
<ul>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3406.html" target="_blank">N3406 A proposal to add a utility class to represent optional objects (Revision 2)</a></li>
<li><a href="https://wg21.cmeerw.net/lwg/issue2857" target="_blank">LWG Issue 2857. {<code>variant</code>,<code>optional</code>,<code>any</code>}<code>::emplace</code> should return the constructed value</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0084r2.pdf" target="_blank">P0084R2 Emplace Return Type (Revision 2)</a></li>
<li><a href="https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p2231r1.html" target="_blank">P2231R1 Missing <code>constexpr</code> in <code>std::optional</code> and <code>std::variant</code></a></li>
<li><a href="https://open-std.org/jtc1/sc22/wg21/docs/papers/2025/p2988r12.pdf" target="_blank">P2988R12 <code>std::optional<T&></code></a><ul>
<li>C++26で参照型<code>T&</code>に対する部分特殊化を追加</li>
</ul>
</li>
<li><a href="https://cplusplus.github.io/LWG/issue2746" target="_blank">LWG Issue 2746. Inconsistency between requirements for <code>emplace</code> between <code>optional</code> and <code>variant</code></a><ul>
<li>C++26で、<code>emplace</code>の制約が<code>Requires</code>（ハードエラー）から<code>Constraints</code>（SFINAE）に統一された（cpprefjpでは「<a class="cpprefjp-defined-word" data-desc="関数等の意味論を構成する要素の1つ。Constraints。関数であればオーバーロード解決に参加するための条件、それ以外であれば受け付ける型の条件">テンプレートパラメータ制約</a>」として記載済み）</li>
</ul>
</li>
<li><a href="https://cplusplus.github.io/LWG/issue4300" target="_blank">LWG Issue 4300. Missing <code>Returns:</code> element in <code>optional<T&>::emplace</code></a><ul>
<li>C++26で、<code>optional<T&></code>版 (3) の<a class="cpprefjp-defined-word" data-desc="関数呼び出し式の評価結果となるオブジェクト・値">戻り値</a>が<code>*val</code>（参照先）であることが明記された（cpprefjpの<a class="cpprefjp-defined-word" data-desc="関数呼び出し式の評価結果となるオブジェクト・値">戻り値</a>記述は全<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>共通で既にこれを含む）</li>
</ul>
</li>
</ul></div>
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