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<div class="identifier-type">class</div><div class="header"><functional></div><h1 itemprop="name"><span class="namespace" title="namespace std">std::</span><span class="token">identity</span><span class="cpp cpp20" title="C++20で追加">(C++20)</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">struct</span><span class="w"> </span><span class="nc"><a href="">identity</a></span><span class="w"> </span><span class="p">{</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="n">T</span><span class="o">&&</span><span class="w"> </span><span class="k">operator</span><span class="p">()(</span><span class="n">T</span><span class="o">&&</span><span class="w"> </span><span class="n">t</span><span class="p">)</span><span class="w"> </span><span class="k">const</span><span class="w"> </span><span class="k">noexcept</span><span class="p">;</span>
<span class="w"> </span><span class="k">using</span><span class="w"> </span><span class="n">is_transparent</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n"><i>unspecified</i></span><span class="p">;</span>
<span class="w"> </span><span class="p">};</span>
<span class="p">}</span>
</code></pre></div>
</p>
<h2>概要</h2>
<p><code>identity</code>クラスは、受け取った引数をそのまま返す（恒等変換を行う）関数オブジェクトである。この恒等変換はC++の意味論においてのものであり、値そのもの及び<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>や値カテゴリも含めて引数をそのまま返す。</p>
<p>この関数オブジェクトは一切のメンバ変数を持たず、状態を保持しない。</p>
<h2>メンバ関数</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>operator()</code></td>
<td><code>return std::forward<T>(t);</code></td>
<td>C++20</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>is_transparent</code></td>
<td><code>operator()</code> が関数テンプレートである事を示すタグ型。<br />実装依存の型であるがあくまでタグ型であり、型そのものには意味はない。</td>
<td>C++20</td>
</tr>
</tbody>
</table>
<h2>備考</h2>
<p>これは主にRangeをとるアルゴリズムにおいて、カスタマイズ可能な射影操作のデフォルトとして利用される。</p>
<h2>例</h2>
<p><div class="yata" id="1c55128fa1aa6b39b9a9bbc3887d941cffbc62c7"><div class="codehilite"><pre><span></span><code><span class="cp">#include <a href="../iostream.html"><iostream></a></span>
<span class="cp">#include <a href="../functional.html"><functional></a></span>
<span class="c1">//intを受け取ってその値と射影した値を足して返す</span>
<span class="c1">//デフォルトはnを2倍にする</span>
<span class="k">template</span><span class="o"><</span><span class="k">typename</span><span class="w"> </span><span class="nc">Projection</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n"><span style="color:#ff0000">std::identity</span></span><span class="o">></span>
<span class="k">auto</span><span class="w"> </span><span class="n">f</span><span class="p">(</span><span class="kt">int</span><span class="w"> </span><span class="n">n</span><span class="p">,</span><span class="w"> </span><span class="n">Projection</span><span class="w"> </span><span class="n">proj</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{})</span><span class="w"> </span><span class="o">-></span><span class="w"> </span><span class="kt">int</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">n</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="n">proj</span><span class="p">(</span><span class="n">n</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">//デフォルトの射影で呼び出し</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">f</span><span class="p">(</span><span class="mi">10</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="c1">//何も足さないようにカスタマイズ</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">f</span><span class="p">(</span><span class="mi">10</span><span class="p">,</span><span class="w"> </span><span class="p">[](</span><span class="kt">int</span><span class="p">){</span><span class="k">return</span><span class="w"> </span><span class="mi">0</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="c1">//3倍にする</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="n">f</span><span class="p">(</span><span class="mi">10</span><span class="p">,</span><span class="w"> </span><span class="p">[](</span><span class="kt">int</span><span class="w"> </span><span class="n">m</span><span class="p">){</span><span class="k">return</span><span class="w"> </span><span class="n">m</span><span class="w"> </span><span class="o">+</span><span class="w"> </span><span class="n">m</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="p">}</span>
</code></pre></div>
</div></p>
<h3>出力</h3>
<p><pre><code>20
10
30
</code></pre></p>
<h2>バージョン</h2>
<h3>言語</h3>
<ul>
<li>C++20</li>
</ul>
<h3>処理系</h3>
<ul>
<li><a href="../../implementation.html#clang">Clang</a>: ??</li>
<li><a href="../../implementation.html#gcc">GCC</a>: 10.1 <span aria-label="検証済" role="img" title="検証済">✅</span></li>
<li><a href="../../implementation.html#visual_cpp">Visual C++</a>: 2019 Update 1 <span aria-label="検証済" role="img" title="検証済">✅</span></li>
</ul>
<h2>参照</h2>
<ul>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0898r3.pdf" target="_blank">P0898R3 Standard Library Concepts</a></li>
</ul></div>
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