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<header data-kunai-mdinfo="{"meta": {"header": ["optional"], "namespace": ["std"], "id-type": ["class template"], "cpp": ["cpp17"]}, "sources": [{"id": "9a27469119d04e3872eb45ea3b0c032ed43a6503", "source": "#include <iostream>\n#include <optional>\n\n// \u9664\u7b97\u3092\u3059\u308b\u95a2\u6570\u3002\n// \u30bc\u30ed\u5272\u308a\u3092\u8a66\u307f\u305f\u5834\u5408\u3001\u7121\u52b9\u5024\u304c\u8fd4\u308b\nstd::optional<int> safe_divide(int a, int b)\n{\n if (b == 0)\n return std::nullopt;\n\n return a / b;\n}\n\nint main()\n{\n // 9/3\u3092\u8a08\u7b97\u3059\u308b\n std::optional<int> result1 = safe_divide(9, 3);\n if (result1) { // \u8a08\u7b97\u306b\u6210\u529f\u3057\u305f\u5834\u5408\u3001\u6709\u52b9\u5024\u304c\u8fd4\u308b\n int x = result1.value(); // \u6709\u52b9\u5024\u3092\u53d6\u308a\u51fa\u3059\n std::cout << x << std::endl;\n }\n\n // 3/0\u306e\u8a08\u7b97\u3092\u8a66\u307f\u308b\n std::optional<int> result2 = safe_divide(3, 0);\n if (!result2) { // \u8a08\u7b97\u306b\u5931\u6557\u3057\u305f\u5834\u5408\u3001\u7121\u52b9\u5024\u304c\u8fd4\u308b\n std::cout << \"error\" << std::endl;\n }\n}\n"}, {"id": "62a543c8e11ab322f321b4e612f57dde3292867f", "source": "#include <print>\n#include <optional>\n#include <map>\n#include <string>\n\n// \u30b3\u30f3\u30c6\u30ca\u304b\u3089\u306e\u8981\u7d20\u691c\u7d22\u3002\u898b\u3064\u304b\u3089\u306a\u3044\u5834\u5408\u306f\u7121\u52b9\u5024\u3092\u8fd4\u3059\nstd::optional<const std::string&>\n find_value(const std::map<int, std::string>& m, int key)\n{\n auto it = m.find(key);\n if (it != m.end()) {\n return it->second; // \u8981\u7d20\u3078\u306e\u53c2\u7167\u3092\u8fd4\u3059 (\u30b3\u30d4\u30fc\u306f\u767a\u751f\u3057\u306a\u3044)\n }\n return std::nullopt;\n}\n\nint main()\n{\n std::map<int, std::string> m = {\n {1, \"Alice\"},\n {2, \"Bob\"},\n {3, \"Charlie\"}\n };\n\n // \u5024\u304c\u898b\u3064\u304b\u3063\u305f\u5834\u5408\n if (auto name = find_value(m, 2)) {\n // \u30b3\u30d4\u30fc\u3067\u306f\u306a\u304f\u53c2\u7167\u3092\u4fdd\u6301\u3057\u3066\u3044\u308b\u305f\u3081\u3001\u52b9\u7387\u7684\u306b\u30a2\u30af\u30bb\u30b9\u3067\u304d\u308b\n std::println(\"{}\", name.value());\n }\n\n // \u5024\u304c\u898b\u3064\u304b\u3089\u306a\u304b\u3063\u305f\u5834\u5408\n if (auto name = find_value(m, 99); !name) {\n std::println(\"not found\");\n }\n\n // \u30e2\u30ca\u30c9\u64cd\u4f5c\u3068\u306e\u7d44\u307f\u5408\u308f\u305b\n auto result = find_value(m, 1)\n .transform([](const std::string& s) { return s.size(); })\n .value_or(0u);\n std::println(\"{}\", result);\n}\n"}], "page_id": ["reference", "optional", "optional"]}">
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最終更新日時(UTC):
<span itemprop="datePublished" content="2026-04-07T06:36:39">
2026年04月07日 06時36分39秒
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<span itemprop="author" itemscope itemtype="http://schema.org/Person">
<span itemprop="name">Akira Takahashi</span>
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<div class="identifier-type">class template</div><div class="header"><optional></div><h1 itemprop="name"><span class="namespace" title="namespace std">std::</span><span class="token">optional</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="k">namespace</span><span class="w"> </span><span class="nn">std</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </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">T</span><span class="o">></span>
<span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">optional</span><span class="p">;</span>
<span class="w"> </span><span class="c1">// 参照に対する部分特殊化 (C++26)</span>
<span class="w"> </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">T</span><span class="o">></span>
<span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">optional</span><span class="o"><</span><span class="n">T</span><span class="o">&></span><span class="p">;</span>
<span class="w"> </span><span class="c1">// <a href="../ranges/view.html">view</a>コンセプトを有効化する (C++26)</span>
<span class="w"> </span><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="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="kt">bool</span><span class="w"> </span><span class="n">ranges</span><span class="o">::</span><span class="n">enable_view</span><span class="o"><</span><span class="n">optional</span><span class="o"><</span><span class="n">T</span><span class="o">>></span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="nb">true</span><span class="p">;</span>
<span class="w"> </span><span class="c1">// std::formatによるフォーマットを無効化する (C++26)</span>
<span class="w"> </span><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="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="k">auto</span><span class="w"> </span><span class="n">format_kind</span><span class="o"><</span><span class="n">optional</span><span class="o"><</span><span class="n">T</span><span class="o">>></span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">range_format</span><span class="o">::</span><span class="n">disabled</span><span class="p">;</span>
<span class="w"> </span><span class="c1">// <a href="../ranges/borrowed_range.html">borrowed_range</a>コンセプトを有効化する (C++26)</span>
<span class="w"> </span><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="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="kt">bool</span><span class="w"> </span><span class="n">ranges</span><span class="o">::</span><span class="n">enable_borrowed_range</span><span class="o"><</span><span class="n">optional</span><span class="o"><</span><span class="n">T</span><span class="o">&>></span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="nb">true</span><span class="p">;</span>
<span class="p">}</span>
</code></pre></div>
</p>
<h2>概要</h2>
<p><code>optional</code>クラスは、任意の型<code>T</code>の値を有効値として、あらゆる型に共通の無効値状態を表現できる型である。</p>
<p>このクラスには、大きく2つの用途がある：</p>
<ol>
<li>オブジェクトの初期化タイミングを遅延させる</li>
<li>エラー報告のために、あらゆる型に共通の無効値表現を持たせる</li>
</ol>
<p>オブジェクトは定義した時点で初期化が行われるが、<code>optional</code>クラスはその初期化タイミングを遅延させるために使用できる。この用途には、<code><a href="../memory/shared_ptr.html">std::shared_ptr</a></code>のようなスマートポインタを使用することもできるが、このクラスは動的メモリ確保を行わないため、リソース管理ではなく初期化タイミングを遅延させるだけであれば、このクラスの方が適している。</p>
<p>エラー報告について、このクラスを使用しない場合、従来の方法として有効値と無効値は、以下のように表現されていた：</p>
<ul>
<li><code>int</code>型の場合、0以上の値を有効な値とし、エラーが起きたら負数を代入する</li>
<li>ポインタの場合、オブジェクトへのポインタを有効な値とし、どこも指さないヌルポインタを無効値として代入する</li>
</ul>
<p>このような有効値と無効値の表現は、変数単位もしくはAPI・ライブラリ単位での仕様である。<code>optional</code>クラスでは、<code><a href="nullopt_t.html">nullopt</a></code>という特殊な定数を無効値とし、あらゆる型に共通の無効状態を持たせられるようになっている。</p>
<p>このクラスは、ヒープからの動的メモリ確保を行わない。実装は配置newや<code><a href="../type_traits/aligned_storage.html">std::aligned_storage</a></code>のような機能によって、スタック領域のメモリのみを使用する。</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>
<p>プライマリテンプレート<code>optional<T></code>の型<code>T</code>が以下のいずれかに該当してはならない：</p>
<ul>
<li>参照型</li>
<li>(<a class="cpprefjp-defined-word" data-desc="型をconstおよび・もしくはvolatileで修飾すること">CV修飾</a>された)<code><a href="../utility/in_place_t.html">std::in_place_t</a></code></li>
<li>(<a class="cpprefjp-defined-word" data-desc="型をconstおよび・もしくはvolatileで修飾すること">CV修飾</a>された)<code><a href="nullopt_t.html">std::nullopt_t</a></code></li>
</ul>
<p>また、型<code>T</code>は<code><a href="../concepts/destructible.html">std::destructible</a></code>要件を満たすこと。</p>
<h2>参照に対する部分特殊化 (C++26)</h2>
<p>このクラスは、<code>optional<T&></code>のように参照を保持できる。内部的には<code>T</code>へのポインタとして保持され、代入時には参照先を再束縛 (rebind) する。</p>
<p><code>optional<T&></code>は以下の特徴をもつ：</p>
<ul>
<li><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>参照を防ぐため、一時オブジェクトから構築する<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>
<li>代入は参照先の再束縛 (rebind) を行う（参照先オブジェクトへの代入ではない）</li>
<li><code><a href="optional/emplace.html">emplace()</a></code>は参照先の再束縛を行う意味論であるため、可変長引数版や<code><a href="../initializer_list/initializer_list.html">std::initializer_list</a></code>版はなく、単一引数のみ受け取る</li>
<li><code>optional<T&></code>はトリビアルコピー可能 (trivially copyable) である</li>
<li>モナド操作 (<code><a href="optional/and_then.html">and_then()</a></code>, <code><a href="optional/transform.html">transform()</a></code>, <code><a href="optional/or_else.html">or_else()</a></code>) も使用可能</li>
</ul>
<h2>備考</h2>
<p>このクラスの前身となった<a href="https://boost.org/libs/optional" target="_blank">Boost Optional Library</a>では、<code>optional<int&></code>のように左辺値参照を要素型とした場合に、無効値の領域を最適化する機能が入っていた。</p>
<h2>メンバ関数</h2>
<h3>構築・破棄</h3>
<table border="1" bordercolor="#888" style="border-collapse:collapse">
<thead>
<tr>
<th>名前</th>
<th>説明</th>
<th>対応バージョン</th>
</tr>
</thead>
<tbody>
<tr>
<td><code><a href="optional/op_constructor.html">(constructor)</a></code></td>
<td>コンストラクタ</td>
<td>C++17</td>
</tr>
<tr>
<td><code><a href="optional/op_destructor.html">(destructor)</a></code></td>
<td>デストラクタ</td>
<td>C++17</td>
</tr>
</tbody>
</table>
<h3>代入</h3>
<table border="1" bordercolor="#888" style="border-collapse:collapse">
<thead>
<tr>
<th>名前</th>
<th>説明</th>
<th>対応バージョン</th>
</tr>
</thead>
<tbody>
<tr>
<td><code><a href="optional/op_assign.html">operator=</a></code></td>
<td>代入演算子</td>
<td>C++17</td>
</tr>
<tr>
<td><code><a href="optional/emplace.html">emplace</a></code></td>
<td>要素型のコンストラクタ引数から直接構築する</td>
<td>C++17</td>
</tr>
<tr>
<td><code><a href="optional/swap.html">swap</a></code></td>
<td>他の<code>optional</code>オブジェクトとデータを入れ替える</td>
<td>C++17</td>
</tr>
<tr>
<td><code><a href="optional/reset.html">reset</a></code></td>
<td>有効値を保持していない状態にする</td>
<td>C++17</td>
</tr>
</tbody>
</table>
<h3>イテレータ</h3>
<table border="1" bordercolor="#888" style="border-collapse:collapse">
<thead>
<tr>
<th>名前</th>
<th>説明</th>
<th>対応バージョン</th>
</tr>
</thead>
<tbody>
<tr>
<td><code><a href="optional/begin.html">begin</a></code></td>
<td><code>optional</code>をrangeとした時の先頭要素を指すイテレータを取得する</td>
<td>C++26</td>
</tr>
<tr>
<td><code><a href="optional/end.html">end</a></code></td>
<td><code>optional</code>をrangeとした時の末尾要素の次を指すイテレータを取得する</td>
<td>C++26</td>
</tr>
</tbody>
</table>
<h3>値の観測</h3>
<table border="1" bordercolor="#888" style="border-collapse:collapse">
<thead>
<tr>
<th>名前</th>
<th>説明</th>
<th>対応バージョン</th>
</tr>
</thead>
<tbody>
<tr>
<td><code><a href="optional/op_deref.html">operator*</a></code></td>
<td>間接参照</td>
<td>C++17</td>
</tr>
<tr>
<td><code><a href="optional/op_arrow.html">operator-></a></code></td>
<td>メンバアクセス</td>
<td>C++17</td>
</tr>
<tr>
<td><code><a href="optional/op_bool.html">operator bool</a></code></td>
<td>有効な値を保持しているかを判定する</td>
<td>C++17</td>
</tr>
<tr>
<td><code><a href="optional/has_value.html">has_value</a></code></td>
<td>有効な値を保持しているかを判定する</td>
<td>C++17</td>
</tr>
<tr>
<td><code><a href="optional/value.html">value</a></code></td>
<td>有効値を取得する</td>
<td>C++17</td>
</tr>
<tr>
<td><code><a href="optional/value_or.html">value_or</a></code></td>
<td>有効値もしくは指定された無効値を取得する</td>
<td>C++17</td>
</tr>
</tbody>
</table>
<h3>モナド操作</h3>
<table border="1" bordercolor="#888" style="border-collapse:collapse">
<thead>
<tr>
<th>名前</th>
<th>説明</th>
<th>対応バージョン</th>
</tr>
</thead>
<tbody>
<tr>
<td><code><a href="optional/and_then.html">and_then</a></code></td>
<td>有効値に対して関数を適用する</td>
<td>C++23</td>
</tr>
<tr>
<td><code><a href="optional/transform.html">transform</a></code></td>
<td>有効値を変換する</td>
<td>C++23</td>
</tr>
<tr>
<td><code><a href="optional/or_else.html">or_else</a></code></td>
<td>無効値に対して関数を適用する</td>
<td>C++23</td>
</tr>
</tbody>
</table>
<h2>メンバ型</h2>
<table border="1" bordercolor="#888" style="border-collapse:collapse">
<thead>
<tr>
<th>名前</th>
<th>説明</th>
<th>対応バージョン</th>
</tr>
</thead>
<tbody>
<tr>
<td><code>value_type</code></td>
<td>要素型<code>T</code></td>
<td>C++17</td>
</tr>
<tr>
<td><code>iterator</code></td>
<td><a class="cpprefjp-defined-word" data-desc="処理系定義の動作。処理系によって事前に定めた動作をする" href="../../implementation-compliance.html#dfn-implementation-defined-behavior">実装定義</a>のイテレータ型。<code><a href="../iterator/contiguous_iterator.html">contiguous_iterator</a></code>、<code><a href="../iterator/random_access_iterator.html">random_access_iterator</a></code>、constexprイテレータのモデルであり、コンテナのイテレータに対するすべての要件を満たす</td>
<td>C++26</td>
</tr>
<tr>
<td><code>const_iterator</code></td>
<td><a class="cpprefjp-defined-word" data-desc="処理系定義の動作。処理系によって事前に定めた動作をする" href="../../implementation-compliance.html#dfn-implementation-defined-behavior">実装定義</a>の読み取り専用イテレータ型。<code><a href="../iterator/contiguous_iterator.html">contiguous_iterator</a></code>、<code><a href="../iterator/random_access_iterator.html">random_access_iterator</a></code>、constexprイテレータのモデルであり、コンテナのイテレータに対するすべての要件を満たす</td>
<td>C++26</td>
</tr>
</tbody>
</table>
<h2>非メンバ関数</h2>
<h3>ヘルパ関数</h3>
<table border="1" bordercolor="#888" style="border-collapse:collapse">
<thead>
<tr>
<th>名前</th>
<th>説明</th>
<th>対応バージョン</th>
</tr>
</thead>
<tbody>
<tr>
<td><code><a href="make_optional.html">make_optional</a></code></td>
<td>有効な値を保持する<code>optional</code>オブジェクトを構築する</td>
<td>C++17</td>
</tr>
</tbody>
</table>
<h3>値の入れ替え</h3>
<table border="1" bordercolor="#888" style="border-collapse:collapse">
<thead>
<tr>
<th>名前</th>
<th>説明</th>
<th>対応バージョン</th>
</tr>
</thead>
<tbody>
<tr>
<td><code><a href="optional/swap_free.html">swap</a></code></td>
<td>2つの<code>optional</code>オブジェクトを入れ替える</td>
<td>C++17</td>
</tr>
</tbody>
</table>
<h3>比較演算子</h3>
<table border="1" bordercolor="#888" style="border-collapse:collapse">
<thead>
<tr>
<th>名前</th>
<th>説明</th>
<th>対応バージョン</th>
</tr>
</thead>
<tbody>
<tr>
<td><code><a href="optional/op_equal.html">operator==</a></code></td>
<td>等値比較</td>
<td>C++17</td>
</tr>
<tr>
<td><code><a href="optional/op_not_equal.html">operator!=</a></code></td>
<td>非等値比較</td>
<td>C++17</td>
</tr>
<tr>
<td><code><a href="optional/op_compare_3way.html">operator<=></a></code></td>
<td>三方比較</td>
<td>C++20</td>
</tr>
<tr>
<td><code><a href="optional/op_less.html">operator<</a></code></td>
<td>左辺が右辺より小さいかを判定する</td>
<td>C++17</td>
</tr>
<tr>
<td><code><a href="optional/op_less_equal.html">operator<=</a></code></td>
<td>左辺が右辺以下かを判定する</td>
<td>C++17</td>
</tr>
<tr>
<td><code><a href="optional/op_greater.html">operator></a></code></td>
<td>左辺が右辺より大きいかを判定する</td>
<td>C++17</td>
</tr>
<tr>
<td><code><a href="optional/op_greater_equal.html">operator>=</a></code></td>
<td>左辺が右辺以上かを判定する</td>
<td>C++17</td>
</tr>
</tbody>
</table>
<h2>推論補助</h2>
<table border="1" bordercolor="#888" style="border-collapse:collapse">
<thead>
<tr>
<th>名前</th>
<th>説明</th>
<th>対応バージョン</th>
</tr>
</thead>
<tbody>
<tr>
<td><code><a href="optional/op_deduction_guide.html">(deduction_guide)</a></code></td>
<td>クラステンプレートの推論補助</td>
<td>C++17</td>
</tr>
</tbody>
</table>
<h2>ハッシュサポート</h2>
<table border="1" bordercolor="#888" style="border-collapse:collapse">
<thead>
<tr>
<th>名前</th>
<th>説明</th>
<th>対応バージョン</th>
</tr>
</thead>
<tbody>
<tr>
<td><code>template <class T> struct hash;</code></td>
<td><code>hash</code>クラスの先行宣言</td>
<td>C++17</td>
</tr>
<tr>
<td><code>template <class T> struct hash<optional<T>>;</code></td>
<td><code>hash</code>クラスの<code>optional</code>に対する特殊化</td>
<td>C++17</td>
</tr>
</tbody>
</table>
<h2>例</h2>
<p><div class="yata" id="9a27469119d04e3872eb45ea3b0c032ed43a6503"><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="c1">// 除算をする関数。</span>
<span class="c1">// ゼロ割りを試みた場合、無効値が返る</span>
<span class="n"><a href="">std::optional</a></span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="w"> </span><span class="n">safe_divide</span><span class="p">(</span><span class="kt">int</span><span class="w"> </span><span class="n">a</span><span class="p">,</span><span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">b</span><span class="p">)</span>
<span class="p">{</span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">b</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="mi">0</span><span class="p">)</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n"><a href="nullopt_t.html">std::nullopt</a></span><span class="p">;</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">a</span><span class="w"> </span><span class="o">/</span><span class="w"> </span><span class="n">b</span><span class="p">;</span>
<span class="p">}</span>
<span class="kt">int</span><span class="w"> </span><span class="n">main</span><span class="p">()</span>
<span class="p">{</span>
<span class="w"> </span><span class="c1">// 9/3を計算する</span>
<span class="w"> </span><span class="n"><a href="">std::optional</a></span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="w"> </span><span class="n">result1</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">safe_divide</span><span class="p">(</span><span class="mi">9</span><span class="p">,</span><span class="w"> </span><span class="mi">3</span><span class="p">);</span>
<span class="w"> </span><span class="n"><a href="optional/op_bool.html">if (result1)</a></span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="c1">// 計算に成功した場合、有効値が返る</span>
<span class="w"> </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="optional/value.html">result1.value()</a></span><span class="p">;</span><span class="w"> </span><span class="c1">// 有効値を取り出す</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="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="c1">// 3/0の計算を試みる</span>
<span class="w"> </span><span class="n"><a href="">std::optional</a></span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="w"> </span><span class="n">result2</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">safe_divide</span><span class="p">(</span><span class="mi">3</span><span class="p">,</span><span class="w"> </span><span class="mi">0</span><span class="p">);</span>
<span class="w"> </span><span class="n"><a href="optional/op_bool.html">if (!result2)</a></span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="c1">// 計算に失敗した場合、無効値が返る</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">"error"</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="p">}</span>
</code></pre></div>
</div></p>
<h3>出力</h3>
<p><pre><code>3
error
</code></pre></p>
<h3>参照型を保持する例 (C++26)</h3>
<p><div class="yata" id="62a543c8e11ab322f321b4e612f57dde3292867f"><div class="codehilite"><pre><span></span><code><span class="cp">#include <a href="../print.html"><print></a></span>
<span class="cp">#include <a href="../optional.html"><optional></a></span>
<span class="cp">#include <a href="../map.html"><map></a></span>
<span class="cp">#include <a href="../string.html"><string></a></span>
<span class="c1">// コンテナからの要素検索。見つからない場合は無効値を返す</span>
<span class="n"><a href="">std::optional</a></span><span class="o"><</span><span class="k">const</span><span class="w"> </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">find_value</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="n"><a href="../map/map.html">std::map</a></span><span class="o"><</span><span class="kt">int</span><span class="p">,</span><span class="w"> </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">m</span><span class="p">,</span><span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">key</span><span class="p">)</span>
<span class="p">{</span>
<span class="w"> </span><span class="k">auto</span><span class="w"> </span><span class="n">it</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">m</span><span class="p">.</span><span class="n">find</span><span class="p">(</span><span class="n">key</span><span class="p">);</span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">it</span><span class="w"> </span><span class="o">!=</span><span class="w"> </span><span class="n">m</span><span class="p">.</span><span class="n">end</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">it</span><span class="o">-></span><span class="n">second</span><span class="p">;</span><span class="w"> </span><span class="c1">// 要素への参照を返す (コピーは発生しない)</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="nullopt_t.html">std::nullopt</a></span><span class="p">;</span>
<span class="p">}</span>
<span class="kt">int</span><span class="w"> </span><span class="n">main</span><span class="p">()</span>
<span class="p">{</span>
<span class="w"> </span><span class="n"><a href="../map/map.html">std::map</a></span><span class="o"><</span><span class="kt">int</span><span class="p">,</span><span class="w"> </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">m</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="p">{</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="s">"Alice"</span><span class="p">},</span>
<span class="w"> </span><span class="p">{</span><span class="mi">2</span><span class="p">,</span><span class="w"> </span><span class="s">"Bob"</span><span class="p">},</span>
<span class="w"> </span><span class="p">{</span><span class="mi">3</span><span class="p">,</span><span class="w"> </span><span class="s">"Charlie"</span><span class="p">}</span>
<span class="w"> </span><span class="p">};</span>
<span class="w"> </span><span class="c1">// 値が見つかった場合</span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="k">auto</span><span class="w"> </span><span class="n">name</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">find_value</span><span class="p">(</span><span class="n">m</span><span class="p">,</span><span class="w"> </span><span class="mi">2</span><span class="p">))</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="c1">// コピーではなく参照を保持しているため、効率的にアクセスできる</span>
<span class="w"> </span><span class="n"><a href="../print/println.html">std::println</a></span><span class="p">(</span><span class="s">"{}"</span><span class="p">,</span><span class="w"> </span><span class="n"><a href="optional/value.html">name.value()</a></span><span class="p">);</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="c1">// 値が見つからなかった場合</span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="k">auto</span><span class="w"> </span><span class="n">name</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">find_value</span><span class="p">(</span><span class="n">m</span><span class="p">,</span><span class="w"> </span><span class="mi">99</span><span class="p">);</span><span class="w"> </span><span class="o">!</span><span class="n">name</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n"><a href="../print/println.html">std::println</a></span><span class="p">(</span><span class="s">"not found"</span><span class="p">);</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="c1">// モナド操作との組み合わせ</span>
<span class="w"> </span><span class="k">auto</span><span class="w"> </span><span class="n">result</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">find_value</span><span class="p">(</span><span class="n">m</span><span class="p">,</span><span class="w"> </span><span class="mi">1</span><span class="p">)</span>
<span class="w"> </span><span class="n"><a href="optional/transform.html">.transform</a></span><span class="p">([](</span><span class="k">const</span><span class="w"> </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">s</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">s</span><span class="p">.</span><span class="n">size</span><span class="p">();</span><span class="w"> </span><span class="p">})</span>
<span class="w"> </span><span class="n"><a href="optional/value_or.html">.value_or</a></span><span class="p">(</span><span class="mi">0u</span><span class="p">);</span>
<span class="w"> </span><span class="n"><a href="../print/println.html">std::println</a></span><span class="p">(</span><span class="s">"{}"</span><span class="p">,</span><span class="w"> </span><span class="n">result</span><span class="p">);</span>
<span class="p">}</span>
</code></pre></div>
</div></p>
<h4>出力</h4>
<p><pre><code>Bob
not found
5
</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/2005/n1878.htm" target="_blank">N1878 A proposal to add an utility class to represent optional objects (Revision 1)</a></li>
<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="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3527.html" target="_blank">N3527 A proposal to add a utility class to represent optional objects (Revision 3)</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3672.html" target="_blank">N3672 A proposal to add a utility class to represent optional objects (Revision 4)</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3793.html" target="_blank">N3793 A proposal to add a utility class to represent optional objects (Revision 5)</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n3966.html" target="_blank">N3966 Fixes for optional objects</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n3982.html" target="_blank">N3982 Rvalue reference overloads for optional</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4480.html" target="_blank">N4480 Programming Languages — C++ Extensions for Library Fundamentals</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0220r0.html" target="_blank">P0220R0 Adopt Library Fundamentals TS for C++17</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0220r1.html" target="_blank">P0220R1 Adopt Library Fundamentals V1 TS Components for C++17 (R1)</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/p0032r0.pdf" target="_blank">P0032R0 Homogeneous interface for <code>variant</code>, <code>any</code> and <code>optional</code></a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/p0032r1.pdf" target="_blank">P0032R1 Homogeneous interface for <code>variant</code>, <code>any</code> and <code>optional</code> (Revision 1)</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0032r2.pdf" target="_blank">P0032R2 Homogeneous interface for <code>variant</code>, <code>any</code> and <code>optional</code> (Revision 2)</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0032r3.pdf" target="_blank">P0032R3 Homogeneous interface for <code>variant</code>, <code>any</code> and <code>optional</code> (Revision 3)</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0307r0.pdf" target="_blank">P0307R0 Making Optional Greater Equal Again</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0307r2.pdf" target="_blank">P0307R2 Making Optional Greater Equal Again</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0504r0.html" target="_blank">P0504R0 Revisiting in-place tag types for <code>any</code>/<code>optional</code>/<code>variant</code></a></li>
<li><a href="https://wg21.cmeerw.net/lwg/issue3196" target="_blank">LWG Issue 3196. <code>std::optional<T></code> is ill-formed is <code>T</code> is an array</a></li>
<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<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://open-std.org/jtc1/sc22/wg21/docs/papers/2025/p3836r2.html" target="_blank">P3836R2 <code>optional<T&></code> Should Be Trivially Copyable</a><ul>
<li>C++26で<code>optional<T&></code>がトリビアルコピー可能であることを保証</li>
</ul>
</li>
</ul></div>
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