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<span class="m-breadcrumb"><a href="Cookbook.html">Cookbook</a> &raquo;</span>
Exception Handling
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<h3>Contents</h3>
<ul>
<li><a href="#CatchAnExceptionFromARunningTaskflow">Catch an Exception from a Running Taskflow</a></li>
<li><a href="#CatchAnExceptionFromAnAsyncTask">Catch an Exception from an Async Task</a></li>
<li><a href="#CatchAnExceptionFromACorunLoop">Catch an Exception from a Corun Loop</a></li>
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<p>This chapters discusses how to handle exceptions from a submitted taskflow so you can properly catch or propagate exceptions in your workload.</p><section id="CatchAnExceptionFromARunningTaskflow"><h2><a href="#CatchAnExceptionFromARunningTaskflow">Catch an Exception from a Running Taskflow</a></h2><p>When a task throws an exception, the executor will store that exception in the shared state referenced by the <a href="classtf_1_1Future.html" class="m-doc">tf::<wbr />Future</a> handle. You can catch that exception via calling the <code>get</code> method:</p><pre class="m-code"><span class="n">tf</span><span class="o">::</span><span class="n">Executor</span><span class="w"> </span><span class="n">executor</span><span class="p">;</span>
<span class="n">tf</span><span class="o">::</span><span class="n">Taskflow</span><span class="w"> </span><span class="n">taskflow</span><span class="p">;</span>
<span class="n">taskflow</span><span class="p">.</span><span class="n">emplace</span><span class="p">([](){</span><span class="w"> </span><span class="k">throw</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">runtime_error</span><span class="p">(</span><span class="s">&quot;exception&quot;</span><span class="p">);</span><span class="w"> </span><span class="p">});</span>
<span class="k">try</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">executor</span><span class="p">.</span><span class="n">run</span><span class="p">(</span><span class="n">taskflow</span><span class="p">).</span><span class="n">get</span><span class="p">();</span>
<span class="p">}</span>
<span class="k">catch</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">runtime_error</span><span class="o">&amp;</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">std</span><span class="o">::</span><span class="n">cerr</span><span class="w"> </span><span class="o">&lt;&lt;</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">&lt;&lt;</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span>
<span class="p">}</span></pre><aside class="m-note m-info"><h4>Note</h4><p>As <a href="classtf_1_1Future.html" class="m-doc">tf::<wbr />Future</a> is derived from <a href="https://en.cppreference.com/w/cpp/thread/future">std::<wbr />future</a>, it inherits all the exception handling behaviors defined by the C++ standard.</p></aside><p>An exception will automatically cancel the execution of its parent taskflow. All the subsequent tasks that have dependencies on that exception task will not run. For instance, the following code defines two tasks, <code>A</code> and <code>B</code>, where <code>B</code> runs after <code>A</code>. When <code>A</code> throws an exception, the executor will cancel the execution of the taskflow, stopping every tasks that run after <code>A</code>. In this case, <code>B</code> will not run.</p><pre class="m-code"><span class="n">tf</span><span class="o">::</span><span class="n">Executor</span><span class="w"> </span><span class="n">executor</span><span class="p">;</span>
<span class="n">tf</span><span class="o">::</span><span class="n">Taskflow</span><span class="w"> </span><span class="n">taskflow</span><span class="p">;</span>
<span class="n">tf</span><span class="o">::</span><span class="n">Task</span><span class="w"> </span><span class="n">A</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">taskflow</span><span class="p">.</span><span class="n">emplace</span><span class="p">([](){</span><span class="w"> </span><span class="k">throw</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">runtime_error</span><span class="p">(</span><span class="s">&quot;exception on A&quot;</span><span class="p">);</span><span class="w"> </span><span class="p">});</span>
<span class="n">tf</span><span class="o">::</span><span class="n">Task</span><span class="w"> </span><span class="n">B</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">taskflow</span><span class="p">.</span><span class="n">emplace</span><span class="p">([](){</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">cout</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="s">&quot;Task B</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">;</span><span class="w"> </span><span class="p">});</span>
<span class="n">A</span><span class="p">.</span><span class="n">precede</span><span class="p">(</span><span class="n">B</span><span class="p">);</span>
<span class="k">try</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">executor</span><span class="p">.</span><span class="n">run</span><span class="p">(</span><span class="n">taskflow</span><span class="p">).</span><span class="n">get</span><span class="p">();</span>
<span class="p">}</span>
<span class="k">catch</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">runtime_error</span><span class="o">&amp;</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">std</span><span class="o">::</span><span class="n">cerr</span><span class="w"> </span><span class="o">&lt;&lt;</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">&lt;&lt;</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span>
<span class="p">}</span></pre><pre class="m-console"><span class="go">~$ exception on A</span>
<span class="gp"># </span>execution of taskflow is cancelled after an execution is thrown</pre><p>When multiple tasks throw exceptions simultaneously, the executor will only catch one exception and store it in the shared state. Other exceptions will be silently ignored. For example, the following taskflow may concurrently throw two exceptions from task <code>B</code> and task <code>C</code>. Only one exception, either <code>B</code> or <code>C</code>, will be propagated.</p><pre class="m-code"><span class="n">tf</span><span class="o">::</span><span class="n">Executor</span><span class="w"> </span><span class="n">executor</span><span class="p">;</span>
<span class="n">tf</span><span class="o">::</span><span class="n">Taskflow</span><span class="w"> </span><span class="n">taskflow</span><span class="p">;</span>
<span class="k">auto</span><span class="w"> </span><span class="p">[</span><span class="n">A</span><span class="p">,</span><span class="w"> </span><span class="n">B</span><span class="p">,</span><span class="w"> </span><span class="n">C</span><span class="p">,</span><span class="w"> </span><span class="n">D</span><span class="p">]</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">taskflow</span><span class="p">.</span><span class="n">emplace</span><span class="p">(</span>
<span class="w"> </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">cout</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="s">&quot;TaskA</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">;</span><span class="w"> </span><span class="p">},</span>
<span class="w"> </span><span class="p">[]()</span><span class="w"> </span><span class="p">{</span><span class="w"> </span>
<span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">cout</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="s">&quot;TaskB</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">;</span>
<span class="w"> </span><span class="k">throw</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">runtime_error</span><span class="p">(</span><span class="s">&quot;Exception on Task B&quot;</span><span class="p">);</span>
<span class="w"> </span><span class="p">},</span>
<span class="w"> </span><span class="p">[]()</span><span class="w"> </span><span class="p">{</span><span class="w"> </span>
<span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">cout</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="s">&quot;TaskC</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">;</span><span class="w"> </span>
<span class="w"> </span><span class="k">throw</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">runtime_error</span><span class="p">(</span><span class="s">&quot;Exception on Task C&quot;</span><span class="p">);</span>
<span class="w"> </span><span class="p">},</span>
<span class="w"> </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">cout</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="s">&quot;TaskD will not be printed due to exception</span><span class="se">\n</span><span class="s">&quot;</span><span class="p">;</span><span class="w"> </span><span class="p">}</span>
<span class="p">);</span>
<span class="n">A</span><span class="p">.</span><span class="n">precede</span><span class="p">(</span><span class="n">B</span><span class="p">,</span><span class="w"> </span><span class="n">C</span><span class="p">);</span><span class="w"> </span><span class="c1">// A runs before B and C</span>
<span class="n">D</span><span class="p">.</span><span class="n">succeed</span><span class="p">(</span><span class="n">B</span><span class="p">,</span><span class="w"> </span><span class="n">C</span><span class="p">);</span><span class="w"> </span><span class="c1">// D runs after B and C</span>
<span class="k">try</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">executor</span><span class="p">.</span><span class="n">run</span><span class="p">(</span><span class="n">taskflow</span><span class="p">).</span><span class="n">get</span><span class="p">();</span>
<span class="p">}</span>
<span class="k">catch</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">runtime_error</span><span class="o">&amp;</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="c1">// caught either B&#39;s or C&#39;s exception</span>
<span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">cout</span><span class="w"> </span><span class="o">&lt;&lt;</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">&lt;&lt;</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span>
<span class="p">}</span></pre></section><section id="CatchAnExceptionFromAnAsyncTask"><h2><a href="#CatchAnExceptionFromAnAsyncTask">Catch an Exception from an Async Task</a></h2><p>Similar to <a href="https://en.cppreference.com/w/cpp/thread/future">std::<wbr />future</a>, <a href="classtf_1_1Executor.html#af960048056f7c6b5bc71f4f526f05df7" class="m-doc">tf::<wbr />Executor::<wbr />async</a> will store the exception in the shared state referenced by the returned <a href="https://en.cppreference.com/w/cpp/thread/future">std::<wbr />future</a> handle.</p><pre class="m-code"><span class="n">tf</span><span class="o">::</span><span class="n">Executor</span><span class="w"> </span><span class="n">executor</span><span class="p">;</span>
<span class="k">auto</span><span class="w"> </span><span class="n">fu</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">executor</span><span class="p">.</span><span class="n">async</span><span class="p">([](){</span><span class="w"> </span><span class="k">throw</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">runtime_error</span><span class="p">(</span><span class="s">&quot;exception&quot;</span><span class="p">);</span><span class="w"> </span><span class="p">});</span>
<span class="k">try</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">fu</span><span class="p">.</span><span class="n">get</span><span class="p">();</span>
<span class="p">}</span>
<span class="k">catch</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">runtime_error</span><span class="o">&amp;</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">std</span><span class="o">::</span><span class="n">cerr</span><span class="w"> </span><span class="o">&lt;&lt;</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">&lt;&lt;</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span>
<span class="p">}</span></pre><p>Running the program will show the exception message on the async task:</p><pre class="m-console"><span class="go">~$ exception</span></pre><p>On the other hand, since <a href="classtf_1_1Executor.html#a0461cb2c459c9f9473c72af06af9c701" class="m-doc">tf::<wbr />Executor::<wbr />silent_async</a> does not return any future handle, any exception thrown from a silent-async task will be silently caught by the executor and (1) propagated to the its parent task if the parent task exists or (2) ignored if the parent task does not exist.</p><pre class="m-code"><span class="n">tf</span><span class="o">::</span><span class="n">Taskflow</span><span class="w"> </span><span class="n">taskflow</span><span class="p">;</span>
<span class="n">tf</span><span class="o">::</span><span class="n">Executor</span><span class="w"> </span><span class="n">executor</span><span class="p">;</span>
<span class="c1">// exception will be silently ignored</span>
<span class="n">executor</span><span class="p">.</span><span class="n">silent_async</span><span class="p">([](){</span><span class="w"> </span><span class="k">throw</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">runtime_error</span><span class="p">(</span><span class="s">&quot;exception&quot;</span><span class="p">);</span><span class="w"> </span><span class="p">});</span>
<span class="c1">// exception will be propagated to the parent tf::Runtime task and then its Taskflow</span>
<span class="n">taskflow</span><span class="p">.</span><span class="n">emplace</span><span class="p">([</span><span class="o">&amp;</span><span class="p">](</span><span class="n">tf</span><span class="o">::</span><span class="n">Runtime</span><span class="o">&amp;</span><span class="w"> </span><span class="n">rt</span><span class="p">){</span>
<span class="w"> </span><span class="n">rt</span><span class="p">.</span><span class="n">silent_async</span><span class="p">([](){</span><span class="w"> </span><span class="k">throw</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">runtime_error</span><span class="p">(</span><span class="s">&quot;exception&quot;</span><span class="p">);</span><span class="w"> </span><span class="p">});</span>
<span class="p">});</span>
<span class="k">try</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">taskflow</span><span class="p">.</span><span class="n">get</span><span class="p">();</span>
<span class="p">}</span>
<span class="k">catch</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">runtime_error</span><span class="o">&amp;</span><span class="w"> </span><span class="n">re</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">cout</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="n">re</span><span class="p">.</span><span class="n">what</span><span class="p">()</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span>
<span class="p">}</span></pre></section><section id="CatchAnExceptionFromACorunLoop"><h2><a href="#CatchAnExceptionFromACorunLoop">Catch an Exception from a Corun Loop</a></h2><p>When you corun a graph via <a href="classtf_1_1Executor.html#a8fcd9e0557922bb8194999f0cd433ea8" class="m-doc">tf::<wbr />Executor::<wbr />corun</a> or <a href="classtf_1_1Runtime.html#a1c772e90614302024cfa52fa86d75cac" class="m-doc">tf::<wbr />Runtime::<wbr />corun</a>, any exception will be thrown during the execution. For example, the code below will throw an exception during the execution of <code>taskflow1</code>:</p><pre class="m-code"><span class="n">tf</span><span class="o">::</span><span class="n">Executor</span><span class="w"> </span><span class="n">executor</span><span class="p">;</span>
<span class="n">tf</span><span class="o">::</span><span class="n">Taskflow</span><span class="w"> </span><span class="n">taskflow1</span><span class="p">;</span>
<span class="n">tf</span><span class="o">::</span><span class="n">Taskflow</span><span class="w"> </span><span class="n">taskflow2</span><span class="p">;</span>
<span class="n">taskflow1</span><span class="p">.</span><span class="n">emplace</span><span class="p">([](){</span>
<span class="w"> </span><span class="k">throw</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">runtime_error</span><span class="p">(</span><span class="s">&quot;exception&quot;</span><span class="p">);</span>
<span class="p">});</span><span class="w"> </span>
<span class="n">taskflow2</span><span class="p">.</span><span class="n">emplace</span><span class="p">([</span><span class="o">&amp;</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">executor</span><span class="p">.</span><span class="n">corun</span><span class="p">(</span><span class="n">taskflow1</span><span class="p">);</span>
<span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="k">catch</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">runtime_error</span><span class="o">&amp;</span><span class="w"> </span><span class="n">re</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">cout</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="n">re</span><span class="p">.</span><span class="n">what</span><span class="p">()</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">});</span><span class="w"> </span>
<span class="n">executor</span><span class="p">.</span><span class="n">run</span><span class="p">(</span><span class="n">taskflow2</span><span class="p">).</span><span class="n">get</span><span class="p">();</span></pre><p>We can observe the same behavior when using <a href="classtf_1_1Runtime.html#a1c772e90614302024cfa52fa86d75cac" class="m-doc">tf::<wbr />Runtime::<wbr />corun</a>:</p><pre class="m-code"><span class="n">tf</span><span class="o">::</span><span class="n">Executor</span><span class="w"> </span><span class="n">executor</span><span class="p">;</span>
<span class="n">tf</span><span class="o">::</span><span class="n">Taskflow</span><span class="w"> </span><span class="n">taskflow1</span><span class="p">;</span>
<span class="n">tf</span><span class="o">::</span><span class="n">Taskflow</span><span class="w"> </span><span class="n">taskflow2</span><span class="p">;</span>
<span class="n">taskflow1</span><span class="p">.</span><span class="n">emplace</span><span class="p">([](){</span>
<span class="w"> </span><span class="k">throw</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">runtime_error</span><span class="p">(</span><span class="s">&quot;exception&quot;</span><span class="p">);</span>
<span class="p">});</span><span class="w"> </span>
<span class="n">taskflow2</span><span class="p">.</span><span class="n">emplace</span><span class="p">([</span><span class="o">&amp;</span><span class="p">](</span><span class="n">tf</span><span class="o">::</span><span class="n">Runtime</span><span class="o">&amp;</span><span class="w"> </span><span class="n">rt</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">rt</span><span class="p">.</span><span class="n">corun</span><span class="p">(</span><span class="n">taskflow1</span><span class="p">);</span>
<span class="w"> </span><span class="p">}</span><span class="w"> </span><span class="k">catch</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">runtime_error</span><span class="o">&amp;</span><span class="w"> </span><span class="n">re</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">cout</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="n">re</span><span class="p">.</span><span class="n">what</span><span class="p">()</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">});</span><span class="w"> </span>
<span class="n">executor</span><span class="p">.</span><span class="n">run</span><span class="p">(</span><span class="n">taskflow2</span><span class="p">).</span><span class="n">get</span><span class="p">();</span></pre><p>For the above example, if the exception is not caught with <a href="classtf_1_1Runtime.html#a1c772e90614302024cfa52fa86d75cac" class="m-doc">tf::<wbr />Runtime::<wbr />corun</a>, it will be propagated to its parent task, which is the <a href="classtf_1_1Runtime.html" class="m-doc">tf::<wbr />Runtime</a> object <code>rt</code> in this case. Then, the exception will be propagated to <code>taskflow2</code>.</p><pre class="m-code"><span class="n">tf</span><span class="o">::</span><span class="n">Executor</span><span class="w"> </span><span class="n">executor</span><span class="p">;</span>
<span class="n">tf</span><span class="o">::</span><span class="n">Taskflow</span><span class="w"> </span><span class="n">taskflow1</span><span class="p">;</span>
<span class="n">tf</span><span class="o">::</span><span class="n">Taskflow</span><span class="w"> </span><span class="n">taskflow2</span><span class="p">;</span>
<span class="n">taskflow1</span><span class="p">.</span><span class="n">emplace</span><span class="p">([](){</span>
<span class="w"> </span><span class="k">throw</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">runtime_error</span><span class="p">(</span><span class="s">&quot;exception&quot;</span><span class="p">);</span>
<span class="p">});</span><span class="w"> </span>
<span class="n">taskflow2</span><span class="p">.</span><span class="n">emplace</span><span class="p">([</span><span class="o">&amp;</span><span class="p">](</span><span class="n">tf</span><span class="o">::</span><span class="n">Runtime</span><span class="o">&amp;</span><span class="w"> </span><span class="n">rt</span><span class="p">){</span>
<span class="w"> </span><span class="n">rt</span><span class="p">.</span><span class="n">corun</span><span class="p">(</span><span class="n">taskflow1</span><span class="p">);</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">executor</span><span class="p">.</span><span class="n">run</span><span class="p">(</span><span class="n">taskflow2</span><span class="p">).</span><span class="n">get</span><span class="p">();</span>
<span class="p">}</span>
<span class="k">catch</span><span class="p">(</span><span class="k">const</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">runtime_error</span><span class="o">&amp;</span><span class="w"> </span><span class="n">re</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">cout</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="n">re</span><span class="p">.</span><span class="n">what</span><span class="p">()</span><span class="w"> </span><span class="o">&lt;&lt;</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">endl</span><span class="p">;</span>
<span class="p">}</span></pre><p>For the above example, if the exception is not caught with <a href="classtf_1_1Runtime.html#a1c772e90614302024cfa52fa86d75cac" class="m-doc">tf::<wbr />Runtime::<wbr />corun</a>, it will be propagated to its parent task, which is the <a href="classtf_1_1Runtime.html" class="m-doc">tf::<wbr />Runtime</a> object <code>rt</code> in this case. Then, the exception will be propagated to <code>taskflow2</code>.</p></section>
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