Re: [Boost-users] [BGL] iterator/descriptor invalidation

10 Dec 2008

      Dmitry Bufistov  writes:
...
Julius Ziegler escribió:
...
Dmitry Bufistov  writes:
...
...
It strikes to me now that the matter is not so subtle at all, since 
add_vertex()
with vecS is very likely to invalidate vertex_iterators (clearly contrary 
to 
the
documentation). In my case the issue was a bit obscured since it is not
perfectly clear that source(edge_descriptor) and target(edge_descriptor)
use
vertex_iterators. I had expected that edge_descriptor stores two
vertex_descriptors.
The problem is not with edge_descriptor it is in the operator++() of the 
edge_iterator class.
I think that edge_iterators are not directly affected, and its still save
to 
use them to access, e.g., edge properties. Its also save to use 
operator++(). 
The problem is that calling source(...) on an edge descriptor dereferences a
vertex_iterator, and THAT one may have been invalidated. Is that right?
Well, you can try to remove source() and target() calls from your 
example and to see if the problem is still reproducible.
The problem is still there! To reproduce (added "double" props to vertices and
edges):

#include <iostream>
#include 

using namespace boost;

typedef adjacency_list Graph;

int main() {
  Graph g(3);

  add_edge( 0, 1, 7.0,  g );
  add_edge( 1, 2, 2.0,  g );
  add_edge( 2, 0, 13.0, g );

  Graph::edge_iterator ei, ei_end;  

  for( tie( ei, ei_end ) = edges( g ); ei != ei_end; ++ei )
    {
      std::cout << g[*ei] << "\n";
      add_vertex( 1.0, g );             
    }  
}

Looks like things are not at all as I imagined! However, my problem is solved
well by reserving enough space for the vertices, and avoiding invalidation in
the first place, as you proposed.

Best regards, 
Julius