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<header data-kunai-mdinfo="{"meta": {"header": ["latch"], "namespace": ["std"], "id-type": ["class"], "cpp": ["cpp20"]}, "sources": [{"id": "a300c35d4b4dda2735fa4b5bafdaf1d8f982ba12", "source": "#include <iostream>\n#include <latch>\n#include <mutex>\n#include <thread>\n\nconstexpr int NWORKER = 3; // \u30ef\u30fc\u30ab\u6570\n\nstd::mutex cout_mtx; // \u884c\u5358\u4f4dcout\u51fa\u529b\u7528\u30df\u30e5\u30fc\u30c6\u30c3\u30af\u30b9\n\nint main()\n{\n // 1\u56de\u3060\u3051\u521d\u671f\u5316\u3055\u308c\u308b\u5171\u6709\u30c7\u30fc\u30bf\n int shared_data = 0;\n\n // \u521d\u671f\u5316\u6e08\u307f\u3092\u8868\u3059\u30e9\u30c3\u30c1: \u521d\u671f\u30ab\u30a6\u30f3\u30c8\u5024=1\n std::latch initialized{1};\n // \u30bf\u30b9\u30af\u5b8c\u4e86\u3092\u8868\u3059\u30e9\u30c3\u30c1: \u521d\u671f\u30ab\u30a6\u30f3\u30c8\u5024=\u30ef\u30fc\u30ab\u6570+1(\u30e1\u30a4\u30f3\u30b9\u30ec\u30c3\u30c9)\n std::latch completed{NWORKER+1};\n\n // \u30ef\u30fc\u30ab\u30b9\u30ec\u30c3\u30c9\u7fa4\u3092Fire-and-Forget\u8d77\u52d5\n for (int id = 1; id <= NWORKER; id++) {\n std::thread([&,id]{\n // \u5171\u6709\u30c7\u30fc\u30bf\u306e\u521d\u671f\u5316\u5b8c\u4e86\u3092\u5f85\u6a5f\n initialized.wait();\n // \u30e1\u30a4\u30f3\u30b9\u30ec\u30c3\u30c9\u4e0a\u3067\u306eshared_data\u4ee3\u5165\u5b8c\u4e86\u306f\u4fdd\u8a3c\u3055\u308c\u3066\u304a\u308a\u3001\n // \u4ee5\u964d\u306fshared_data\u8aad\u51fa\u306e\u307f\u3067\u3042\u308c\u3070\u30c7\u30fc\u30bf\u7af6\u5408\u767a\u751f\u3057\u306a\u3044\u3002\n int local_data = shared_data;\n\n { // \u30ef\u30fc\u30ab\u30b9\u30ec\u30c3\u30c9\u306e\u30bf\u30b9\u30af\u3092\u5b9f\u884c\n std::lock_guard lk{cout_mtx};\n std::cout << \"Worker#\" << id << \":\" << local_data << std::endl;\n }\n\n // \u30bf\u30b9\u30af\u5b8c\u4e86\u3092\u901a\u77e5\n completed.count_down();\n // \u30ef\u30fc\u30ab\u30b9\u30ec\u30c3\u30c9\u306f\u30d6\u30ed\u30c3\u30af\u3055\u308c\u305a\u305d\u306e\u307e\u307e\u7d42\u4e86\u3059\u308b\n }).detach();\n }\n\n // \u5171\u6709\u30c7\u30fc\u30bf\u3092\u521d\u671f\u5316\n shared_data = 42;\n\n // \u5171\u6709\u30c7\u30fc\u30bf\u521d\u671f\u5316\u5b8c\u4e86\u3092\u30ef\u30fc\u30ab\u30b9\u30ec\u30c3\u30c9\u7fa4\u3078\u901a\u77e5\n initialized.count_down();\n // \u30e1\u30a4\u30f3\u30b9\u30ec\u30c3\u30c9\u306f\u30d6\u30ed\u30c3\u30af\u3055\u308c\u305a\u5f8c\u7d9a\u884c\u3092\u5b9f\u884c\u3059\u308b\n\n { // \u30e1\u30a4\u30f3\u30b9\u30ec\u30c3\u30c9\u306e\u30bf\u30b9\u30af\u3092\u5b9f\u884c\n std::lock_guard lk{cout_mtx};\n std::cout << \"Main:\" << shared_data << std::endl;\n }\n\n // \u30e1\u30a4\u30f3\u30b9\u30ec\u30c3\u30c9\u306e\u30bf\u30b9\u30af\u5b8c\u4e86\u901a\u77e5 \u517c \u5168\u30ef\u30fc\u30ab\u30b9\u30ec\u30c3\u30c9\u306e\u30bf\u30b9\u30af\u5b8c\u4e86\u5f85\u6a5f\n completed.arrive_and_wait();\n}\n"}], "page_id": ["reference", "latch", "latch"]}">
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<div class="identifier-type">class</div><div class="header"><latch></div><h1 itemprop="name"><span class="namespace" title="namespace std">std::</span><span class="token">latch</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">class</span><span class="w"> </span><span class="nc">latch</span><span class="p">;</span>
<span class="p">}</span>
</code></pre></div>
</p>
<h2>概要</h2>
<p><code>latch</code>クラスは、ラッチ通過/到達スレッドが期待する個数になるまで複数スレッドの進行をブロックする、スレッド調整機構（同期プリミティブ）である。</p>
<p>ラッチは1つのカウンタで状態管理される同期プリミティブとみなせる。カウンタは<code>0</code>以上かつ初期値以下の値をとり、このカウンタ値がラッチ未到達のスレッド個数と解釈される。
その動作仕様からカウントダウン・ラッチと呼ばれることもある。</p>
<ul>
<li><a href="latch/op_constructor.html">コンストラクタ</a>にてカウンタ値を設定する。</li>
<li><code><a href="latch/count_down.html">count_down()</a></code>によりカウンタ値の減算と通知を行う。（ラッチ通過）</li>
<li><code><a href="latch/wait.html">wait()</a></code>によりカウンタ値が<code>0</code>になるまで呼び出しスレッドをブロックする。（ラッチ待機）</li>
<li><code><a href="latch/arrive_and_wait.html">arrive_and_wait()</a></code>は<code>count_down()</code>＋<code>wait()</code>に相当する。（ラッチ到達 兼 待機）</li>
</ul>
<p>ラッチオブジェクトは1回だけ使用でき、カウンタ値が<code>0</code>に到達したオブジェクトの再利用はできない。
複数スレッド間同期を繰り返しとる場合はバリア<code><a href="../barrier/barrier.html">barrier</a></code>を利用する。</p>
<p>ラッチ<code>latch</code>はカウンタ値<code>0</code>を待機するが、類似機構のセマフォ<code><a href="../semaphore/counting_semaphore.html">counting_semaphore</a></code>はカウンタ値が<code>0</code>より大きいことを待機するという違いがある。</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><a href="latch/op_constructor.html">(constructor)</a></code></td>
<td>コンストラクタ</td>
<td>C++20</td>
</tr>
<tr>
<td><code>(destructor)</code></td>
<td>デストラクタ</td>
<td>C++20</td>
</tr>
<tr>
<td><code>operator=(const latch&) = delete;</code></td>
<td>代入演算子</td>
<td>C++20</td>
</tr>
<tr>
<td><code><a href="latch/count_down.html">count_down</a></code></td>
<td>通過通知</td>
<td>C++20</td>
</tr>
<tr>
<td><code><a href="latch/try_wait.html">try_wait</a></code></td>
<td>カウンタ値が<code>0</code>か否かを確認</td>
<td>C++20</td>
</tr>
<tr>
<td><code><a href="latch/wait.html">wait</a></code></td>
<td>待機処理</td>
<td>C++20</td>
</tr>
<tr>
<td><code><a href="latch/arrive_and_wait.html">arrive_and_wait</a></code></td>
<td>到達通知と待機処理</td>
<td>C++20</td>
</tr>
</tbody>
</table>
<h2>静的メンバ関数</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><a href="latch/max.html">max</a></code></td>
<td>処理系でサポートされるカウンタ最大値</td>
<td>C++20</td>
</tr>
</tbody>
</table>
<h2>例</h2>
<p><div class="yata" id="a300c35d4b4dda2735fa4b5bafdaf1d8f982ba12"><div class="codehilite"><pre><span></span><code><span class="cp">#include <a href="../iostream.html"><iostream></a></span>
<span class="cp">#include <a href="../latch.html"><latch></a></span>
<span class="cp">#include <a href="../mutex.html"><mutex></a></span>
<span class="cp">#include <a href="../thread.html"><thread></a></span>
<span class="k">constexpr</span><span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">NWORKER</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="c1">// ワーカ数</span>
<span class="n"><a href="../mutex/mutex.html">std::mutex</a></span><span class="w"> </span><span class="n">cout_mtx</span><span class="p">;</span><span class="w"> </span><span class="c1">// 行単位cout出力用ミューテックス</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回だけ初期化される共有データ</span>
<span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">shared_data</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="c1">// 初期化済みを表すラッチ: 初期カウント値=1</span>
<span class="w"> </span><span class="n"><span style="color:#ff0000">std::latch</span></span><span class="w"> </span><span class="n">initialized</span><span class="p">{</span><span class="mi">1</span><span class="p">};</span>
<span class="w"> </span><span class="c1">// タスク完了を表すラッチ: 初期カウント値=ワーカ数+1(メインスレッド)</span>
<span class="w"> </span><span class="n"><span style="color:#ff0000">std::latch</span></span><span class="w"> </span><span class="n">completed</span><span class="p">{</span><span class="n">NWORKER</span><span class="o">+</span><span class="mi">1</span><span class="p">};</span>
<span class="w"> </span><span class="c1">// ワーカスレッド群をFire-and-Forget起動</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">id</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">1</span><span class="p">;</span><span class="w"> </span><span class="n">id</span><span class="w"> </span><span class="o"><=</span><span class="w"> </span><span class="n">NWORKER</span><span class="p">;</span><span class="w"> </span><span class="n">id</span><span class="o">++</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n"><a href="../thread/thread.html">std::thread</a></span><span class="p">([</span><span class="o">&</span><span class="p">,</span><span class="n">id</span><span class="p">]{</span>
<span class="w"> </span><span class="c1">// 共有データの初期化完了を待機</span>
<span class="w"> </span><span class="n">initialized</span><span class="p">.</span><span class="n"><a href="latch/wait.html">wait()</a></span><span class="p">;</span>
<span class="w"> </span><span class="c1">// メインスレッド上でのshared_data代入完了は保証されており、</span>
<span class="w"> </span><span class="c1">// 以降はshared_data読出のみであればデータ競合発生しない。</span>
<span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">local_data</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">shared_data</span><span class="p">;</span>
<span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="c1">// ワーカスレッドのタスクを実行</span>
<span class="w"> </span><span class="n"><a href="../mutex/lock_guard.html">std::lock_guard</a></span><span class="w"> </span><span class="n">lk</span><span class="p">{</span><span class="n">cout_mtx</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">"Worker#"</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">id</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">local_data</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">// タスク完了を通知</span>
<span class="w"> </span><span class="n">completed</span><span class="p">.</span><span class="n"><a href="latch/count_down.html">count_down()</a></span><span class="p">;</span>
<span class="w"> </span><span class="c1">// ワーカスレッドはブロックされずそのまま終了する</span>
<span class="w"> </span><span class="p">}).</span><span class="n"><a href="../thread/thread/detach.html">detach()</a></span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span>
<span class="w"> </span><span class="c1">// 共有データを初期化</span>
<span class="w"> </span><span class="n">shared_data</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">42</span><span class="p">;</span>
<span class="w"> </span><span class="c1">// 共有データ初期化完了をワーカスレッド群へ通知</span>
<span class="w"> </span><span class="n">initialized</span><span class="p">.</span><span class="n"><a href="latch/count_down.html">count_down()</a></span><span class="p">;</span>
<span class="w"> </span><span class="c1">// メインスレッドはブロックされず後続行を実行する</span>
<span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="c1">// メインスレッドのタスクを実行</span>
<span class="w"> </span><span class="n"><a href="../mutex/lock_guard.html">std::lock_guard</a></span><span class="w"> </span><span class="n">lk</span><span class="p">{</span><span class="n">cout_mtx</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">"Main:"</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">shared_data</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">// メインスレッドのタスク完了通知 兼 全ワーカスレッドのタスク完了待機</span>
<span class="w"> </span><span class="n">completed</span><span class="p">.</span><span class="n"><a href="latch/arrive_and_wait.html">arrive_and_wait()</a></span><span class="p">;</span>
<span class="p">}</span>
</code></pre></div>
</div></p>
<h3>出力例</h3>
<p><pre><code>Worker#1:42
Worker#2:42
Main:42
Worker#3:42
</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>: 11.0 <span aria-label="検証済" role="img" title="検証済">✅</span></li>
<li><a href="../../implementation.html#gcc">GCC</a>: ??</li>
<li><a href="../../implementation.html#icc">ICC</a>: ??</li>
<li><a href="../../implementation.html#visual_cpp">Visual C++</a>: ??</li>
</ul>
<h2>関連項目</h2>
<ul>
<li><code><a href="../semaphore.html"><semaphore></a></code></li>
<li><code><a href="../barrier.html"><barrier></a></code></li>
</ul>
<h2>参照</h2>
<ul>
<li><a href="https://www.boost.org/doc/libs/1_73_0/doc/html/thread/synchronization.html#thread.synchronization.latches" target="_blank">Boost.Threadライブラリ, Latches</a></li>
<li><a href="https://docs.oracle.com/javase/jp/6/api/java/util/concurrent/CountDownLatch.html" target="_blank">Java標準ライブラリ, java.util.concurrent.CountDownLatch</a></li>
<li><a href="https://docs.microsoft.com/en-us/dotnet/api/system.threading.countdownevent" target="_blank">C#標準ライブラリ, System.Threading.CountdownEvent</a></li>
</ul></div>
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