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<span itemprop="name">_at_thread_exit系の関数が存在している理由</span>
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最終更新日時(UTC):
<span itemprop="datePublished" content="2025-07-11T17:16:44">
2025年07月11日 17時16分44秒
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<span itemprop="author" itemscope itemtype="http://schema.org/Person">
<span itemprop="name">Koichi Murase</span>
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が更新
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<h1 itemprop="name"><span class="token">_at_thread_exit系の関数が存在している理由</span></h1>
<div itemprop="articleBody"><p><code><a href="../../reference/future.html"><future></a></code> や <code><a href="../../reference/condition_variable.html"><condition_variable></a></code> には、<code>*_at_thread_exit</code> という名前の関数が定義されている。</p>
<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="kt">void</span><span class="w"> </span><span class="nf"><a href="../../reference/condition_variable/condition_variable/notify_all_at_thread_exit.html">notify_all_at_thread_exit</a></span><span class="p">(</span><span class="n"><a href="../../reference/condition_variable/condition_variable.html">condition_variable</a></span><span class="o">&</span><span class="w"> </span><span class="n">cond</span><span class="p">,</span><span class="w"> </span><span class="n"><a href="../../reference/mutex/unique_lock.html">unique_lock</a></span><span class="o"><</span><span class="n"><a href="../../reference/mutex/mutex.html">mutex</a></span><span class="o">></span><span class="w"> </span><span class="n">lk</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">R</span><span class="o">></span>
<span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc"><a href="../../reference/future/promise.html">promise</a></span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">public</span><span class="o">:</span>
<span class="w"> </span><span class="p">...</span>
<span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="n"><a href="../../reference/future/promise/set_value_at_thread_exit.html">set_value_at_thread_exit</a></span><span class="p">(</span><span class="k">const</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="kt">void</span><span class="w"> </span><span class="nf"><a href="../../reference/future/promise/set_exception_at_thread_exit.html">set_exception_at_thread_exit</a></span><span class="p">(</span><span class="n"><a href="../../reference/exception/exception_ptr.html">exception_ptr</a></span><span class="w"> </span><span class="n">p</span><span class="p">);</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">R</span><span class="p">,</span><span class="w"> </span><span class="k">class</span><span class="p">...</span><span class="w"> </span><span class="n">ArgTypes</span><span class="o">></span>
<span class="w"> </span><span class="k">class</span><span class="w"> </span><span class="nc"><a href="../../reference/future/packaged_task.html">packaged_task</a></span><span class="o"><</span><span class="n">R</span><span class="p">(</span><span class="n">ArgTypes</span><span class="p">...)</span><span class="o">></span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">public</span><span class="o">:</span>
<span class="w"> </span><span class="p">...</span>
<span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="n"><a href="../../reference/future/packaged_task/make_ready_at_thread_exit.html">make_ready_at_thread_exit</a></span><span class="p">(</span><span class="n">ArgTypes</span><span class="p">...</span><span class="w"> </span><span class="n">args</span><span class="p">);</span>
<span class="w"> </span><span class="p">};</span>
<span class="p">}</span>
</code></pre></div>
</p>
<p>これらの関数は、スレッドローカル記憶域が破棄された後に通知を行なったり、状態を変更する。 </p>
<p>また、<code><a href="../../reference/thread/thread.html">thread</a>::<a href="../../reference/thread/thread/detach.html">detach</a>()</code> されたスレッド上で、
スレッドローカル記憶域との同期を取る唯一の方法でもある。</p>
<p>デタッチされたスレッドにおいて、スレッドローカル記憶域にあるオブジェクトがいつ破棄されるかという規定は無い。
そのため、<a class="cpprefjp-defined-word" data-desc="未定義の動作。処理系は予期せぬ動作をする可能性がある。要するに動作保証対象外。undefined behavior (UB)。" href="../../implementation-compliance.html#dfn-undefined-behavior">未定義動作</a>を含まずにこれらのオブジェクトを破棄するのは難しい。 </p>
<p>例えば、以下のようなケースで問題になる。</p>
<p><div class="yata" id="171e623d248d83490f9f10888099cc4ccbfdcc88"><div class="codehilite"><pre><span></span><code><span class="cp">#include <a href="../../reference/type_traits.html"><type_traits></a></span>
<span class="cp">#include <a href="../../reference/future.html"><future></a></span>
<span class="cp">#include <a href="../../reference/thread.html"><thread></a></span>
<span class="cp">#include <a href="../../reference/iostream.html"><iostream></a></span>
<span class="k">template</span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">F</span><span class="o">></span>
<span class="n">std</span><span class="o">::</span><span class="n">future</span><span class="o"><</span><span class="k">typename</span><span class="w"> </span><span class="nc">std</span><span class="o">::</span><span class="n">result_of</span><span class="o"><</span><span class="n">F</span><span class="p">()</span><span class="o">>::</span><span class="n">type</span><span class="o">></span><span class="w"> </span><span class="n">spawn_task</span><span class="p">(</span><span class="n">F</span><span class="w"> </span><span class="n">f</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">using</span><span class="w"> </span><span class="n">result_type</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">typename</span><span class="w"> </span><span class="nc">std</span><span class="o">::</span><span class="n">result_of</span><span class="o"><</span><span class="n">F</span><span class="p">()</span><span class="o">>::</span><span class="n">type</span><span class="p">;</span>
<span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">packaged_task</span><span class="o"><</span><span class="n">result_type</span><span class="w"> </span><span class="p">()</span><span class="o">></span><span class="w"> </span><span class="n">task</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="n">f</span><span class="p">));</span>
<span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">future</span><span class="o"><</span><span class="n">result_type</span><span class="o">></span><span class="w"> </span><span class="n">future</span><span class="p">(</span><span class="n">task</span><span class="p">.</span><span class="n">get_future</span><span class="p">());</span>
<span class="w"> </span><span class="n"><a href="../../reference/thread/thread.html">std::thread</a></span><span class="w"> </span><span class="nf">th</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="n">task</span><span class="p">));</span>
<span class="w"> </span><span class="n">th</span><span class="p">.</span><span class="n">detach</span><span class="p">();</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">future</span><span class="p">;</span>
<span class="p">}</span>
<span class="k">struct</span><span class="w"> </span><span class="nc">Hoge</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="o">~</span><span class="n">Hoge</span><span class="p">()</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="n"><a href="../../reference/iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="s">"Hoge destructor"</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../../reference/ostream/endl.html">std::endl</a></span><span class="p">;</span><span class="w"> </span><span class="p">}</span>
<span class="p">};</span>
<span class="kt">int</span><span class="w"> </span><span class="nf">f</span><span class="p">()</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n"><a href="../../lang/cpp11/thread_local_storage.html">thread_local</a></span><span class="w"> </span><span class="n">Hoge</span><span class="w"> </span><span class="n">h</span><span class="p">;</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="mi">42</span><span class="p">;</span>
<span class="p">}</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">std</span><span class="o">::</span><span class="n">future</span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="w"> </span><span class="n">res</span><span class="p">(</span><span class="n">spawn_task</span><span class="p">(</span><span class="n">f</span><span class="p">));</span>
<span class="w"> </span><span class="n"><a href="../../reference/iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">res</span><span class="p">.</span><span class="n">get</span><span class="p">()</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../../reference/ostream/endl.html">std::endl</a></span><span class="p">;</span>
<span class="p">}</span>
</code></pre></div>
</div></p>
<p>出力:</p>
<p><pre><code>42Hoge destructor
</code></pre></p>
<p><code>spawn_task</code> は、渡された任意の処理を別スレッドで行なう一般的な関数である。関数内部でスレッドを作り、デタッチを行なっている。 </p>
<p>出力は、<code>main()</code> 関数での出力と、<code>Hoge</code> デストラクタでの出力が混在している。これはスレッドローカル記憶域と <code>future</code> オブジェクトが正しく同期されていないからである。そのため、これ以外の出力も起こり得る。</p>
<p>これは <code>*_at_thread_exit</code> 系の関数を利用することで修正できる。</p>
<p><div class="yata" id="af87758ee2bf0e113465ddcd28c0cff86e7107c1"><div class="codehilite"><pre><span></span><code><span class="cp">#include <a href="../../reference/type_traits.html"><type_traits></a></span>
<span class="cp">#include <a href="../../reference/future.html"><future></a></span>
<span class="cp">#include <a href="../../reference/thread.html"><thread></a></span>
<span class="cp">#include <a href="../../reference/iostream.html"><iostream></a></span>
<span class="k">struct</span><span class="w"> </span><span class="nc">task_executor</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">R</span><span class="o">></span>
<span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="k">operator</span><span class="p">()(</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">packaged_task</span><span class="o"><</span><span class="n">R</span><span class="o">></span><span class="w"> </span><span class="n">task</span><span class="w"> </span><span class="p">)</span>
<span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">task</span><span class="p">.</span><span class="n"><span style="color:#ff0000">make_ready_at_thread_exit</span></span><span class="p">();</span><span class="w"> </span><span class="c1">// operator() を呼び出す代わりに <span style="color:#ff0000">make_ready_at_thread_exit</span>() を呼び出す。</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">};</span>
<span class="k">template</span><span class="o"><</span><span class="k">class</span><span class="w"> </span><span class="nc">F</span><span class="o">></span>
<span class="n">std</span><span class="o">::</span><span class="n">future</span><span class="o"><</span><span class="k">typename</span><span class="w"> </span><span class="nc">std</span><span class="o">::</span><span class="n">result_of</span><span class="o"><</span><span class="n">F</span><span class="p">()</span><span class="o">>::</span><span class="n">type</span><span class="o">></span><span class="w"> </span><span class="n">spawn_task</span><span class="p">(</span><span class="n">F</span><span class="w"> </span><span class="n">f</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">using</span><span class="w"> </span><span class="n">result_type</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="k">typename</span><span class="w"> </span><span class="nc">std</span><span class="o">::</span><span class="n">result_of</span><span class="o"><</span><span class="n">F</span><span class="p">()</span><span class="o">>::</span><span class="n">type</span><span class="p">;</span>
<span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">packaged_task</span><span class="o"><</span><span class="n">result_type</span><span class="w"> </span><span class="p">()</span><span class="o">></span><span class="w"> </span><span class="n">task</span><span class="p">(</span><span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="n">f</span><span class="p">));</span>
<span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">future</span><span class="o"><</span><span class="n">result_type</span><span class="o">></span><span class="w"> </span><span class="n">future</span><span class="p">(</span><span class="n">task</span><span class="p">.</span><span class="n">get_future</span><span class="p">());</span>
<span class="w"> </span><span class="n"><a href="../../reference/thread/thread.html">std::thread</a></span><span class="w"> </span><span class="nf">th</span><span class="p">(</span><span class="n">task_executor</span><span class="p">{},</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">move</span><span class="p">(</span><span class="n">task</span><span class="p">));</span>
<span class="w"> </span><span class="n">th</span><span class="p">.</span><span class="n">detach</span><span class="p">();</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">future</span><span class="p">;</span>
<span class="p">}</span>
<span class="k">struct</span><span class="w"> </span><span class="nc">Hoge</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="o">~</span><span class="n">Hoge</span><span class="p">()</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="n"><a href="../../reference/iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="s">"Hoge destructor"</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../../reference/ostream/endl.html">std::endl</a></span><span class="p">;</span><span class="w"> </span><span class="p">}</span>
<span class="p">};</span>
<span class="kt">int</span><span class="w"> </span><span class="nf">f</span><span class="p">()</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n"><a href="../../lang/cpp11/thread_local_storage.html">thread_local</a></span><span class="w"> </span><span class="n">Hoge</span><span class="w"> </span><span class="n">h</span><span class="p">;</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="mi">42</span><span class="p">;</span>
<span class="p">}</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">std</span><span class="o">::</span><span class="n">future</span><span class="o"><</span><span class="kt">int</span><span class="o">></span><span class="w"> </span><span class="n">res</span><span class="p">(</span><span class="n">spawn_task</span><span class="p">(</span><span class="n">f</span><span class="p">));</span>
<span class="w"> </span><span class="n"><a href="../../reference/iostream/cout.html">std::cout</a></span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n">res</span><span class="p">.</span><span class="n">get</span><span class="p">()</span><span class="w"> </span><span class="o"><<</span><span class="w"> </span><span class="n"><a href="../../reference/ostream/endl.html">std::endl</a></span><span class="p">;</span>
<span class="p">}</span>
</code></pre></div>
</div></p>
<p>出力:</p>
<p><pre><code>Hoge destructor
42
</code></pre></p>
<p>このプログラムの出力は、必ずこの通りになる。つまり、確実にスレッドローカル記憶域のオブジェクトが破棄された後に <code>res.get()</code> の結果が出力される。</p>
<h2>参考</h2>
<ul>
<li><a href="http://d.hatena.ne.jp/yohhoy/20120131/p1" target="_blank">futureとpromiseのあれこれ（理論編） - yohhoyの日記</a></li>
<li><a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2010/n3070.html" target="_blank">N3070 - Handling Detached Threads and thread_local Variables</a></li>
</ul></div>
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