Destroying Graphs That Run Outside the Main Thread

Make sure to enqueue a task to wait for and destroy graphs that run outside the main thread.

You may not always want to block the main application thread by calling wait_for_all(). However, it is safest to call wait_for_all on a graph before destroying it. A common solution is to enqueue a task to build and wait for the graph to complete. For example, assume you really do not want to call a wait_for_all in the example from Always Use wait_for_all(), Instead you can enqueue a task that creates the graph and waits for it:

class background_task : public tbb::task {
  /* override */ task *execute() {
    graph g;
    function_node< int, int > f( g, 1, []( int i ) -> int {
      return spin_for(i);
    } );
    return NULL;

void no_wait_for_all_enqueue() {
  task::enqueue( * (new( task::allocate_root() ) background_task()) );
  // do other things without waiting…

In the code snippet above, the enqueued task executes at some point, but it's not clear when. If you need to use the results of the enqueued task, or even ensure that it completes before the program ends, you will need to use some mechanism to signal from the enqueued task that the graph is complete. See the Developer Reference for more information on enqueued tasks.

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