Re: [Boost-users] BGL: filtered_graph and breadth_first_search.

On 4/20/07, Dima F. wrote:

Aaron Windsor wrote:

...

The predicates used in a filtered_graph can have non-static members. In particular, I don't see anything wrong with the example you included in your previous email. The predicate needs to be default constructable because in the implementation of filtered_graph, it's stored by value in an iterator and the iterator must be default constructable. But you can still create a non-default instance of your predicate and pass it in to filtered_graph and you should get the results you expect - just make sure it has the correct semantics when default constructed.

Here is an example which (it seems to me) shows that non-static members in predicate can cause a problem (see code below). The program causes segmentation fault, and it happens when pointer to one of non-static members is dereferenced. When I tried to find the reason, it seems that a pointer, when used in operator() during an execution of BFS, contains garbage, that is - it doesn't point to an object it was pointing to after a predicate object was constructed.

When you run the program, it is possible to see according to a log messages that sometimes predicate is constructed using a default constructor, which means that it's members are not copied to a new object.

P.S.: Also, I see that copy-constructor is used, which is somewhat strange - it is written (http://www.sgi.com/tech/stl/DefaultConstructible.html):

"[1] The form X x = X() is not guaranteed to be a valid expression, because it uses a copy constructor. A type that is DefaultConstructible is not necessarily Assignable"

<snip>

cout << "\nAnd in filtered graph:\n\n";

typedef induced_graph_filter filter_t; filter_t filter(&g, &cont);

filtered_graph g2(g, filter);

Hi Dima, Well, you do need a copy constructor - look a the line above, where you pass filter by value in the filtered_graph constructor. The copy constructor is called there (and anywhere else filter is passed by value). But even though your filtered graph is declared as a filtered_graph You're only passing the constructor one filter (for the edges). So the vertex filter is default constructed, then used, and you get a segmentation fault. If you either (1) remove the extra filter_t template parameter or (2) add an extra filter argument to the constructor, the segfault goes away. Regards, Aaron