Catching Exceptions Inside the Node that Throws the Exception

If you catch an exception within the node’s body, execution continues normally, as you might expect. If an exception is thrown but is not caught before it propagates beyond the node’s body, the execution of all of the graph’s nodes are canceled and the exception is rethrown at the call site of graph::wait_for_all(). Take the graph below as an example:

graph g;


function_node< int, int > f1( g, 1, []( int i ) {  return i; } );


function_node< int, int > f2( g, 1,
    []( const int i ) -> int {
    throw i;
    return i;
} );


function_node< int, int > f3( g, 1, []( int i ) {  return i; } );


make_edge( f1, f2 );
make_edge( f2, f3 );
f1.try_put(1);
f1.try_put(2);
g.wait_for_all();

In the code above, the second function_node, f2, throws an exception that is not caught within the body. This will cause the execution of the graph to be canceled and the exception to be rethrown at the call to g.wait_for_all(). Since it is not handled there either, the program will terminate. If desirable, the exception could be caught and handled within the body:

function_node< int, int > f2( g, 1,
    []( const int i ) -> int {
        try {
            throw i;
        } catch (int j) {
            cout << "Caught " << j << "\n";
        }
        return i;
} );

If the exception is caught and handled in the body, then there is no effect on the overall execution of the graph. However, you could choose instead to catch the exception at the call to wait_for_all:

try {
    g.wait_for_all();
} catch ( int j ) {
    cout << "Caught " << j << "\n";
}

In this case, the execution of the graph is canceled. For our example, this means that the input 1 never reaches f3 and that input 2 never reaches either f2 or f3.