Always Use wait_for_all()

One of the most common mistakes made in flow graph programming is to forget to call wait_for_all. The function graph::wait_for_all blocks until all tasks spawned by the graph are complete. This is not only useful when you want to wait until the computation is done, but it is necessary to call wait_for_all before destroying the graph, or any of its nodes. For example, the following function will lead to a program failure:

void no_wait_for_all() {
        graph g;
        function_node< int, int > f( g, 1, []( int i ) -> int {
                return spin_for(i);
        } );

        // program will fail when f and g are destroyed at the
        // end of the scope, since the body of f is not complete

In the function above, the graph g and its node f are destroyed at the end of the function's scope. However, the task spawned to execute f's body is still in flight. When the task completes, it will look for any successors connected to its node, but by then both the graph and the node have been deleted out from underneath it. Placing a g.wait_for_all() at the end of the function prevents the premature destruction of the graph and node.

If you use a flow graph and see mysterious behavior, check first to see that you have called wait_for_all.

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