I cut out most of the stuff not directly related to accessing the graph. It segfaults in the line marked by !!!, so it would seem that out_edges returns an invalid range (I did some debugging and the this line executes quite a few times before the iterator becomes invalid. I can't seem to force gdb to recognise nn1e as a symbol so I can't tell which edge/node is causing the problem.  All this is ran after the graph is initialised (it's quite large, ab. 22k nodes and 900k edges).