In keeping with your comments, I have tried with both out edge iterators and
adjacency iterators in the below simplified examples. Both indeed exhibit
the same behavior, but they seem to invalidate the iterators, when
(according to the documentation) the iterators should remain valid. There
must be a good technical reason why these iterators are invalidated. If
anyone can explain that to me, I would greatly appreciate it. In any event,
if someone else can confirm this, I think the documentation should be
changed.
I think the problem comes from the graph being undirected, which can cause some interesting problems.
The graph actually doubles up on out edges so that a single undirected edge is actually two distinct edges that are stored in the out edge list of each vertex-- one (u, v) the other (v, u). Each of these out edges refers to the same edge stored on the graph (via a ptr or something like that). Basically, its giving the impression that its undirected, but it's not really.
When you clear a vertex, you're removing all of the corresponding edges from each adjacent vertex and the "globally" stored edge. What happens, when you see the failures, is that you're actually removing a reciprical edge out from underneath the avinext or oeinext iterator, invalidating it.
Basically, you can probably assume that clear_vertex will invalidate any iterators referring to the vertex being cleared for an undirected graph.