Empty Tasks

You might need a task that does not do anything but wait for its children to complete. The header task.h defines class empty_task for this purpose. Its definition is as follows:

// 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 is provided in tbb/parallel_for.h, in 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 (in tbb/parallel_for.h) waits on the root.

For more complete information about compiler optimizations, see our Optimization Notice.