Intel Threading Building Blocks

Empty tasks

You might need a task that does not do anything but wait for its children to complete. The header file task.h defines class empty_task for this purpose. Example 9-10 shows its definition.

Example 9-10. empty_task

// Task that does nothing. Useful for synchronization.
class empty_task: public task {
    /*override*/ task* execute() {
        return NULL;
    }
};

A good example of empty_task in action shown in Example 9-11. It invokes parallel_for in the method start_for::execute(). The code there uses continuation-passing style. It creates two child tasks and uses an empty_task as the continuation when the child tasks complete. The top-level routine parallel_for waits on the root.

Example 9-11. empty_task usage from parallel_for

template<typename Range, typename Body>
task* start_for<Range,Body>::execute() {
  if( !my_range.is_divisible() ) {
   my_body( my_range );
   return NULL;
  } else {
   empty_task& c = *new(allocate_continuation()) empty_task;
   recycle_as_child_of(c);
   c.set_ref_count(2);
   start_for& b =
     *new(c.allocate_child()) start_for(Range(my_range,split()),my_body);
   c.spawn(b);
   return this; 
  }
}