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<div class="identifier-type">function template</div><div class="header"><algorithm></div><h1 itemprop="name"><span class="namespace" title="namespace std::ranges">std::ranges::</span><span class="token">find_if</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="o">::</span><span class="nn">ranges</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="n"><a href="../iterator/input_iterator.html">input_iterator</a></span><span class="w"> </span><span class="n">I</span><span class="p">,</span>
<span class="w"> </span><span class="n"><a href="../iterator/sentinel_for.html">sentinel_for</a></span><span class="o"><</span><span class="n">I</span><span class="o">></span><span class="w"> </span><span class="n">S</span><span class="p">,</span>
<span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Proj</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n"><a href="../functional/identity.html">identity</a></span><span class="p">,</span>
<span class="w"> </span><span class="n"><a href="../iterator/indirect_unary_predicate.html">indirect_unary_predicate</a></span><span class="o"><</span><span class="n"><a href="../iterator/projected.html">projected</a></span><span class="o"><</span><span class="n">I</span><span class="p">,</span><span class="w"> </span><span class="n">Proj</span><span class="o">>></span><span class="w"> </span><span class="n">Pred</span><span class="o">></span>
<span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="n">I</span>
<span class="w"> </span><span class="n">find_if</span><span class="p">(</span><span class="n">I</span><span class="w"> </span><span class="n">first</span><span class="p">,</span>
<span class="w"> </span><span class="n">S</span><span class="w"> </span><span class="n">last</span><span class="p">,</span>
<span class="w"> </span><span class="n">Pred</span><span class="w"> </span><span class="n">pred</span><span class="p">,</span>
<span class="w"> </span><span class="n">Proj</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="c1">// (1) C++20</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="n"><a href="../ranges/input_range.html">input_range</a></span><span class="w"> </span><span class="n">R</span><span class="p">,</span>
<span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Proj</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n"><a href="../functional/identity.html">identity</a></span><span class="p">,</span>
<span class="w"> </span><span class="n"><a href="../iterator/indirect_unary_predicate.html">indirect_unary_predicate</a></span><span class="o"><</span><span class="n"><a href="../iterator/projected.html">projected</a></span><span class="o"><</span><span class="n"><a href="../ranges/iterator_t.html">iterator_t</a></span><span class="o"><</span><span class="n">R</span><span class="o">></span><span class="p">,</span><span class="w"> </span><span class="n">Proj</span><span class="o">>></span><span class="w"> </span><span class="n">Pred</span><span class="o">></span>
<span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="n"><a href="../ranges/borrowed_iterator_t.html">borrowed_iterator_t</a></span><span class="o"><</span><span class="n">R</span><span class="o">></span>
<span class="w"> </span><span class="n">find_if</span><span class="p">(</span><span class="n">R</span><span class="o">&&</span><span class="w"> </span><span class="n">r</span><span class="p">,</span>
<span class="w"> </span><span class="n">Pred</span><span class="w"> </span><span class="n">pred</span><span class="p">,</span>
<span class="w"> </span><span class="n">Proj</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="c1">// (2) C++20</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="n"><a href="../execution/execution-policy.html">execution-policy</a></span><span class="w"> </span><span class="n">Ep</span><span class="p">,</span>
<span class="w"> </span><span class="n"><a href="../iterator/random_access_iterator.html">random_access_iterator</a></span><span class="w"> </span><span class="n">I</span><span class="p">,</span>
<span class="w"> </span><span class="n"><a href="../iterator/sized_sentinel_for.html">sized_sentinel_for</a></span><span class="o"><</span><span class="n">I</span><span class="o">></span><span class="w"> </span><span class="n">S</span><span class="p">,</span>
<span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Proj</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n"><a href="../functional/identity.html">identity</a></span><span class="p">,</span>
<span class="w"> </span><span class="n"><a href="../iterator/indirect_unary_predicate.html">indirect_unary_predicate</a></span><span class="o"><</span><span class="n"><a href="../iterator/projected.html">projected</a></span><span class="o"><</span><span class="n">I</span><span class="p">,</span><span class="w"> </span><span class="n">Proj</span><span class="o">>></span><span class="w"> </span><span class="n">Pred</span><span class="o">></span>
<span class="w"> </span><span class="n">I</span><span class="w"> </span><span class="n">find_if</span><span class="p">(</span><span class="n">Ep</span><span class="o">&&</span><span class="w"> </span><span class="n">exec</span><span class="p">,</span>
<span class="w"> </span><span class="n">I</span><span class="w"> </span><span class="n">first</span><span class="p">,</span>
<span class="w"> </span><span class="n">S</span><span class="w"> </span><span class="n">last</span><span class="p">,</span>
<span class="w"> </span><span class="n">Pred</span><span class="w"> </span><span class="n">pred</span><span class="p">,</span>
<span class="w"> </span><span class="n">Proj</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="c1">// (3) C++26</span>
<span class="w"> </span><span class="k">template</span><span class="w"> </span><span class="o"><</span><span class="n"><a href="../execution/execution-policy.html">execution-policy</a></span><span class="w"> </span><span class="n">Ep</span><span class="p">,</span>
<span class="w"> </span><span class="n"><a href="../ranges/sized-random-access-range.html">sized-random-access-range</a></span><span class="w"> </span><span class="n">R</span><span class="p">,</span>
<span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Proj</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n"><a href="../functional/identity.html">identity</a></span><span class="p">,</span>
<span class="w"> </span><span class="n"><a href="../iterator/indirect_unary_predicate.html">indirect_unary_predicate</a></span><span class="o"><</span><span class="n"><a href="../iterator/projected.html">projected</a></span><span class="o"><</span><span class="n"><a href="../ranges/iterator_t.html">iterator_t</a></span><span class="o"><</span><span class="n">R</span><span class="o">></span><span class="p">,</span><span class="w"> </span><span class="n">Proj</span><span class="o">>></span><span class="w"> </span><span class="n">Pred</span><span class="o">></span>
<span class="w"> </span><span class="n"><a href="../ranges/borrowed_iterator_t.html">borrowed_iterator_t</a></span><span class="o"><</span><span class="n">R</span><span class="o">></span>
<span class="w"> </span><span class="n">find_if</span><span class="p">(</span><span class="n">Ep</span><span class="o">&&</span><span class="w"> </span><span class="n">exec</span><span class="p">,</span>
<span class="w"> </span><span class="n">R</span><span class="o">&&</span><span class="w"> </span><span class="n">r</span><span class="p">,</span>
<span class="w"> </span><span class="n">Pred</span><span class="w"> </span><span class="n">pred</span><span class="p">,</span>
<span class="w"> </span><span class="n">Proj</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="c1">// (4) C++26</span>
<span class="p">}</span>
</code></pre></div>
</p>
<h2>概要</h2>
<p>範囲の中から、指定された条件を満たす最初の要素を検索する。</p>
<ul>
<li>(1): <a class="cpprefjp-defined-word" data-desc="2つのイテレータで表現される半開区間を表す。C++20からイテレータと番兵で異なる型であっても指定可能になる">イテレータ範囲</a>を指定する</li>
<li>(2): Rangeを直接指定する</li>
<li>(3): (1)の並列アルゴリズム版。実行ポリシーを指定する</li>
<li>(4): (2)の並列アルゴリズム版。実行ポリシーを指定する</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><code>[first,last)</code> あるいは <code>r</code> 内のイテレータ <code>i</code> について、<code><a href="../functional/invoke.html">invoke</a>(pred, <a href="../functional/invoke.html">invoke</a>(proj, *i)) != false</code> である最初のイテレータを返す。そのようなイテレータが見つからなかった場合は <code>last</code> を返す。</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。Complexity。関数が使うアルゴリズムの空間計算量・時間計算量">計算量</a></h2>
<p>最大で <code>last - first</code> 回<a class="cpprefjp-defined-word" data-desc="boolを返す関数・関数オブジェクト。predicate">述語</a>による比較を行う</p>
<h2>例</h2>
<h3>基本的な使い方</h3>
<p><div class="yata" id="79c599fb3f443cbe69160d07e59905b683e983ac"><div class="codehilite"><pre><span></span><code><span class="cp">#include <a href="../algorithm.html"><algorithm></a></span>
<span class="cp">#include <a href="../iostream.html"><iostream></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="w"> </span><span class="p">{</span>
<span class="w"> </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">v</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">{</span><span class="w"> </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="w"> </span><span class="p">};</span>
<span class="w"> </span><span class="c1">// 3ではない最初の要素を検索する</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"><span style="color:#ff0000">std::ranges::find_if</span></span><span class="p">(</span><span class="n">v</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">x</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">x</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="p">});</span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">result</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="n"><a href="../vector/vector/end.html">v.end()</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">"not found"</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="k">else</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">"found: "</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="o">*</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="p">}</span>
</code></pre></div>
</div></p>
<h4>出力</h4>
<p><pre><code>found: 1
</code></pre></p>
<h3>並列アルゴリズムの例 (C++26)</h3>
<p><div class="yata" id="89946b3392293991a989233b87a9ce2cf97bf675"><div class="codehilite"><pre><span></span><code><span class="cp">#include <a href="../algorithm.html"><algorithm></a></span>
<span class="cp">#include <a href="../execution.html"><execution></a></span>
<span class="cp">#include <a href="../iostream.html"><iostream></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="w"> </span><span class="p">{</span>
<span class="w"> </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">v</span><span class="w"> </span><span class="o">=</span><span class="w"> </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="mi">1</span><span class="p">,</span><span class="w"> </span><span class="mi">5</span><span class="p">,</span><span class="w"> </span><span class="mi">9</span><span class="p">};</span>
<span class="w"> </span><span class="c1">// 並列に5以上の最初の要素を検索する</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"><span style="color:#ff0000">std::ranges::find_if</span></span><span class="p">(</span><span class="n"><a href="../execution/execution/execution_policy.html">std::execution::par</a></span><span class="p">,</span><span class="w"> </span><span class="n">v</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">x</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">x</span><span class="w"> </span><span class="o">>=</span><span class="w"> </span><span class="mi">5</span><span class="p">;</span><span class="w"> </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">result</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="n"><a href="../vector/vector/end.html">v.end()</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">"not found"</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="k">else</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">"found: "</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="o">*</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="p">}</span>
</code></pre></div>
</div></p>
<h4>出力</h4>
<p><pre><code>found: 5
</code></pre></p>
<h2>実装例</h2>
<p><div class="codehilite"><pre><span></span><code><span class="k">struct</span><span class="w"> </span><span class="nc">find_if_impl</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">template</span><span class="o"><</span><span class="n"><a href="../iterator/input_iterator.html">input_iterator</a></span><span class="w"> </span><span class="n">I</span><span class="p">,</span><span class="w"> </span><span class="n"><a href="../iterator/sentinel_for.html">sentinel_for</a></span><span class="o"><</span><span class="n">I</span><span class="o">></span><span class="w"> </span><span class="n">S</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Proj</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n"><a href="../functional/identity.html">identity</a></span><span class="p">,</span><span class="w"> </span><span class="n"><a href="../iterator/indirect_unary_predicate.html">indirect_unary_predicate</a></span><span class="o"><</span><span class="n"><a href="../iterator/projected.html">projected</a></span><span class="o"><</span><span class="n">I</span><span class="p">,</span><span class="w"> </span><span class="n">Proj</span><span class="o">>></span><span class="w"> </span><span class="n">Pred</span><span class="o">></span>
<span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="n">I</span><span class="w"> </span><span class="k">operator</span><span class="p">()(</span><span class="n">I</span><span class="w"> </span><span class="n">first</span><span class="p">,</span><span class="w"> </span><span class="n">S</span><span class="w"> </span><span class="n">last</span><span class="p">,</span><span class="w"> </span><span class="n">Pred</span><span class="w"> </span><span class="n">pred</span><span class="p">,</span><span class="w"> </span><span class="n">Proj</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="k">const</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="w"> </span><span class="p">;</span><span class="w"> </span><span class="n">first</span><span class="w"> </span><span class="o">!=</span><span class="w"> </span><span class="n">last</span><span class="p">;</span><span class="w"> </span><span class="o">++</span><span class="n">first</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"><a href="../functional/invoke.html">invoke</a></span><span class="p">(</span><span class="n">pred</span><span class="p">,</span><span class="w"> </span><span class="n"><a href="../functional/invoke.html">invoke</a></span><span class="p">(</span><span class="n">proj</span><span class="p">,</span><span class="w"> </span><span class="o">*</span><span class="n">first</span><span class="p">)))</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">first</span><span class="p">;</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">first</span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="k">template</span><span class="o"><</span><span class="n"><a href="../ranges/input_range.html">input_range</a></span><span class="w"> </span><span class="n">R</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc">Proj</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n"><a href="../functional/identity.html">identity</a></span><span class="p">,</span><span class="w"> </span><span class="n"><a href="../iterator/indirect_unary_predicate.html">indirect_unary_predicate</a></span><span class="w"> </span><span class="o"><</span><span class="n"><a href="../iterator/projected.html">projected</a></span><span class="o"><</span><span class="n"><a href="../ranges/iterator_t.html">iterator_t</a></span><span class="o"><</span><span class="n">R</span><span class="o">></span><span class="p">,</span><span class="w"> </span><span class="n">Proj</span><span class="o">>></span><span class="w"> </span><span class="n">Pred</span><span class="o">></span>
<span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="n"><a href="../ranges/borrowed_iterator_t.html">borrowed_iterator_t</a></span><span class="o"><</span><span class="n">R</span><span class="o">></span><span class="w"> </span><span class="k">operator</span><span class="p">()(</span><span class="n">R</span><span class="o">&&</span><span class="w"> </span><span class="n">r</span><span class="p">,</span><span class="w"> </span><span class="n">Pred</span><span class="w"> </span><span class="n">pred</span><span class="p">,</span><span class="w"> </span><span class="n">Proj</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="k">const</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="p">(</span><span class="o">*</span><span class="k">this</span><span class="p">)(</span><span class="n"><a href="../ranges/begin.html">begin</a></span><span class="p">(</span><span class="n">r</span><span class="p">),</span><span class="w"> </span><span class="n"><a href="../ranges/end.html">end</a></span><span class="p">(</span><span class="n">r</span><span class="p">),</span><span class="w"> </span><span class="n"><a href="../functional/ref.html">ref</a></span><span class="p">(</span><span class="n">pred</span><span class="p">),</span><span class="w"> </span><span class="n"><a href="../functional/ref.html">ref</a></span><span class="p">(</span><span class="n">proj</span><span class="p">));</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">};</span>
<span class="kr">inline</span><span class="w"> </span><span class="k">constexpr</span><span class="w"> </span><span class="n">find_if_impl</span><span class="w"> </span><span class="n">find_if</span><span class="p">;</span>
</code></pre></div>
</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>: 10.1.0 <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>: 2019 Update 10 <span aria-label="検証済" role="img" title="検証済">✅</span></li>
</ul>
<h2>参照</h2>
<ul>
<li><a href="https://timsong-cpp.github.io/cppwp/n4861/algorithms" target="_blank">N4861 25 Algorithms library</a></li>
<li><a href="https://open-std.org/jtc1/sc22/wg21/docs/papers/2025/p3179r9.html" target="_blank">P3179R9 C++ parallel range algorithms</a></li>
</ul></div>
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