Cookbook » Exception Handling

Catch an Exception from a Running Taskflow
Catch an Exception from a Subflow
Catch an Exception from an Async Task
Catch an Exception from a Corun Loop

This chapters discusses how to handle exceptions from a submitted taskflow so you can properly catch or propagate exceptions in your workload.

Catch an Exception from a Running Taskflow

When a task throws an exception, the executor will store that exception in the shared state referenced by the tf::Future handle. You can catch that exception via calling the get method:

tf::Executor executor;
tf::Taskflow taskflow;

taskflow.emplace([](){ throw std::runtime_error("exception"); });

try {
  executor.run(taskflow).get();
}
catch(const std::runtime_error& e) {
  std::cerr << e.what() << std::endl;
}

Attention

As tf::Future is derived from std::future, it inherits all the exception handling behaviors defined by the C++ standard.

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, A and B, where B runs after A. When A throws an exception, the executor will cancel the execution of the taskflow, stopping every tasks that run after A. In this case, B will not run.

tf::Executor executor;
tf::Taskflow taskflow;

tf::Task A = taskflow.emplace([](){ throw std::runtime_error("exception on A"); });
tf::Task B = taskflow.emplace([](){ std::cout << "Task B\n"; });
A.precede(B);

try {
  executor.run(taskflow).get();
}
catch(const std::runtime_error& e) {
  std::cerr << e.what() << std::endl;
}

~$ exception on A
# execution of taskflow is cancelled after an execution is thrown

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 B and task C. Only one exception, either B or C, will be propagated.

tf::Executor executor;
tf::Taskflow taskflow;

auto [A, B, C, D] = taskflow.emplace(
  []() { std::cout << "TaskA\n"; },
  []() { 
    std::cout << "TaskB\n";
    throw std::runtime_error("Exception on Task B");
  },
  []() { 
    std::cout << "TaskC\n"; 
    throw std::runtime_error("Exception on Task C");
  },
  []() { std::cout << "TaskD will not be printed due to exception\n"; }
);

A.precede(B, C);  // A runs before B and C
D.succeed(B, C);  // D runs after  B and C

try {
  executor.run(taskflow).get();
}
catch(const std::runtime_error& e) {
  // caught either B's or C's exception
  std::cout << e.what() << std::endl;
}

Catch an Exception from a Subflow

When you join a subflow using tf::Subflow::join, you can catch an exception thrown by its children tasks. For example, the following code catches an exception from the child task A of the subflow sf:

tf::Executor executor;
tf::Taskflow taskflow;

taskflow.emplace([](tf::Subflow& sf) {
  tf::Task A = sf.emplace([]() { 
    std::cout << "Task A\n";
    throw std::runtime_error("exception on A"); 
  });
  tf::Task B = sf.emplace([]() { 
    std::cout << "Task B\n"; 
  });
  A.precede(B);
  
  // catch the exception
  try {
    sf.join();
  }
  catch(const std::runtime_error& re) {
    std::cout << "exception thrown during subflow joining: " << re.what() << '\n';
  }
});

executor.run(taskflow).get();

When an exception is thrown, it will cancel the execution of the parent subflow. All the subsequent tasks that depend on that exception task will not run. The above code example has the following output:

Task A
exception thrown during subflow joining: exception on A

Uncaught exception will be propagated to the parent level until being explicitly caught. For example, the code below will propagate the exception to the parent of the subflow, which in this case in its taskflow.

tf::Executor executor;
tf::Taskflow taskflow;

taskflow.emplace([](tf::Subflow& sf) {
  tf::Task A = sf.emplace([]() {
    std::cout << "Task A\n";
    throw std::runtime_error("exception on A");
  });
  tf::Task B = sf.emplace([]() {
    std::cout << "Task B\n"; 
  }); 
  A.precede(B);

  // uncaught exception will propagate to the parent
  sf.join();
});

try
{
  executor.run(taskflow).get();
}
catch (const std::runtime_error& re)
{
  std::cout << "exception thrown from running the taskflow: " << re.what() << '\n';
}

Task A
exception thrown from running the taskflow: exception on A

Catch an Exception from an Async Task

Similar to std::future, tf::Executor::async will store the exception in the shared state referenced by the returned std::future handle.

tf::Executor executor;
auto fu = executor.async([](){ throw std::runtime_error("exception"); });
try {
  fu.get();
}
catch(const std::runtime_error& e) {
  std::cerr << e.what() << std::endl;
}

Running the program will show the exception message on the async task:

~$ exception

On the other hand, since tf::Executor::silent_async 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.

tf::Taskflow taskflow;
tf::Executor executor;

// exception will be silently ignored
executor.silent_async([](){ throw std::runtime_error("exception"); });

// exception will be propagated to the parent tf::Runtime task and then its Taskflow
taskflow.emplace([&](tf::Runtime& rt){
  rt.silent_async([](){ throw std::runtime_error("exception"); });
});
try {
  taskflow.get();
}
catch(const std::runtime_error& re) {
  std::cout << re.what() << std::endl;
}

Catch an Exception from a Corun Loop

When you corun a graph via tf::Executor::corun or tf::Runtime::corun, any exception will be thrown during the execution. For example, the code below will throw an exception during the execution of taskflow1:

tf::Executor executor;
tf::Taskflow taskflow1;
tf::Taskflow taskflow2;

taskflow1.emplace([](){
  throw std::runtime_error("exception");
}); 
taskflow2.emplace([&](){
  try {
    executor.corun(taskflow1);
  } catch(const std::runtime_error& re) {
    std::cout << re.what() << std::endl;
  }
}); 
executor.run(taskflow2).get();

We can observe the same behavior when using tf::Runtime::corun:

tf::Executor executor;
tf::Taskflow taskflow1;
tf::Taskflow taskflow2;

taskflow1.emplace([](){
  throw std::runtime_error("exception");
}); 
taskflow2.emplace([&](tf::Runtime& rt){
  try {
    rt.corun(taskflow1);
  } catch(const std::runtime_error& re) {
    std::cout << re.what() << std::endl;
  }
}); 
executor.run(taskflow2).get();

For the above example, if the exception is not caught with tf::Runtime::corun, it will be propagated to its parent task, which is the tf::Runtime object rt in this case. Then, the exception will be propagated to taskflow2.