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<div class="identifier-type">function</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">value</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">constexpr</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">T</span><span class="o">&</span><span class="w"> </span><span class="nf">value</span><span class="p">()</span><span class="w"> </span><span class="k">const</span><span class="o">&</span><span class="p">;</span><span class="w"> </span><span class="c1">// (1) C++17</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="nf">value</span><span class="p">()</span><span class="w"> </span><span class="o">&</span><span class="p">;</span><span class="w"> </span><span class="c1">// (2) C++17</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="nf">value</span><span class="p">()</span><span class="w"> </span><span class="o">&&</span><span class="p">;</span><span class="w"> </span><span class="c1">// (3) C++17</span>
<span class="k">constexpr</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="n">T</span><span class="o">&&</span><span class="w"> </span><span class="nf">value</span><span class="p">()</span><span class="w"> </span><span class="k">const</span><span class="o">&&</span><span class="p">;</span><span class="w"> </span><span class="c1">// (4) C++17</span>
<span class="c1">// optional<T&>版のオーバーロード (C++26)</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="nf">value</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>有効値を取得する。</p>
<ul>
<li>(1) : <code>*this</code>が<code>const</code>左辺値である場合に、有効値への<code>const</code>左辺値参照を返す</li>
<li>(2) : <code>*this</code>が非<code>const</code>左辺値である場合に、有効値への非<code>const</code>左辺値参照を返す</li>
<li>(3) : <code>*this</code>が右辺値である場合に、有効値への右辺値参照を返す</li>
<li>(4) : <code>*this</code>が<code>const</code>右辺値である場合に、有効値への<code>const</code>右辺値参照を返す</li>
<li>(5) : <code>optional<T&></code>の場合に、参照先への左辺値参照を返す</li>
</ul>
<p><code>optional<T></code>では (1)～(4) が定義され、<code>optional<T&></code>では (5) のみが定義される。</p>
<h2>効果</h2>
<ul>
<li>(1), (2), (5) : 有効値を保持している場合、有効値への参照を返す</li>
<li>(3), (4) : 有効値を保持している場合、有効値をムーブして返す</li>
</ul>
<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>有効値を保持していない状態でこの関数が呼び出された場合、<code><a href="../bad_optional_access.html">bad_optional_access</a></code><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。</p>
<h2>例</h2>
<p><div class="yata" id="38bb3c82bde49188669a008fea09281ce8944a7c"><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="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="n"><a href="../optional.html">std::optional</a></span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="w"> </span><span class="n">p1</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">3</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">p1</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="kt">int</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">p1</span><span class="p">.</span><span class="n"><span style="color:#ff0000">value()</span></span><span class="p">;</span><span class="w"> </span><span class="c1">// 保持している値を取得する</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">"p1 : "</span><span class="w"> </span><span class="o"><<</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"><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="n"><a href="../optional.html">std::optional</a></span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="w"> </span><span class="n">p2</span><span class="p">;</span>
<span class="w"> </span><span class="k">try</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">p2</span><span class="p">.</span><span class="n"><span style="color:#ff0000">value()</span></span><span class="p">;</span><span class="w"> </span><span class="c1">// 有効値を保持していないのに、有効値を取り出そうとした</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="k">catch</span><span class="w"> </span><span class="p">(</span><span class="n"><a href="../bad_optional_access.html">std::bad_optional_access</a></span><span class="o">&</span><span class="w"> </span><span class="n">e</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">"p2 exception : "</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">e</span><span class="p">.</span><span class="n">what</span><span class="p">()</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>p1 : 3
p2 exception : bad optional access
</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://faithandbrave.hateblo.jp/entry/2014/08/05/151146" target="_blank">getかvalueか - Faith and Brave - C++で遊ぼう</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>
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