[boost] Re: [graph] Adjacency bug after swapping undirected adjacency_list

From: Eelis van der Weegen (gmane_at_[hidden])
Date: 2005-02-06 23:36:03

Analysis:

vec_adj_list_impl's operator= calls this->clear() followed by copy_impl(x), 
where x is the vec_adj_list_impl that is being assigned from. copy_impl(x) 
_adds_ x's vertices and edges to this->m_vertices and this->m_edges. Therefore, 
it would seem that clear() should clear both of those. While clear() does 
immediately do m_vertices.clear(), it doesn't clear m_edges directly, but 
instead calls clear_dispatch(edge_property_type()). There are two overloads for 
clear_dispatch(): one function template parameterized by the property type, and 
a non-template overload that accepts a reference to const no_property. While the 
former immediately calls m_edges.clear(), the latter does nothing (empty 
function body). Consequently, in graphs whose edge_property_type is no_property, 
the edges from 'x' mentioned above are added to the currently existing edges, 
while they should replace them.

Since I don't know the purpose of the clear_dispatch() overload for no_property, 
I have no idea what the solution should be.

I've also attached a simpler testcase.

Regards,

Eelis


#include <algorithm>

#include <boost/graph/adjacency_list.hpp>

int main ()
{
  typedef boost::adjacency_list<boost::vecS, boost::vecS, boost::undirectedS> G;

  G g (2);

  add_edge(0, 1, g);

  G h = g;
  g = h;

  assert(num_edges(g) == num_edges(h)); // fails
}