[Boost-users] [BGL] DistanceMatrix type for johnson_all_pairs_shortest_paths

Subject: [Boost-users] [BGL] DistanceMatrix type for johnson_all_pairs_shortest_paths
From: Ralf Goertz (R_Goertz_at_[hidden])
Date: 2009-09-09 03:45:37

Hi,

I spent a lot of time to figure out what the problem was. But the reason
why there is a problem is still not clear.

I modified the johnson_all_pairs_shortest_paths example program to allow
for non-constant graphs. The example prog contained

std::vector < int >d(V, (std::numeric_limits < int >::max)());
int D[V][V];
johnson_all_pairs_shortest_paths(g, D, distance_map(&d[0]));

with V being const int whose value is known at compile time. In my
program this is obviously not the case, therefore I defined

const int V(num_vertices(g));
std::vector < int >d(V, (std::numeric_limits < int >::max)());
int D[V][V];
johnson_all_pairs_shortest_paths(g, D, distance_map(&d[0]));

But this didn't compile, I got the error

error: no matching function for call
to ‘johnson_all_pairs_shortest_paths(Graph&, int [(long int)V][(long
int)V], boost::bgl_named_params<int*, boost::vertex_distance_t,
boost::no_property>)’

However, it is possible to access elements of D as D[i][j], even if V
isn't known at compile time. This is (according to what I read at
http://www.boost.org/doc/libs/1_39_0/libs/graph/doc/BasicMatrix.html )
the only constrained for D. Substituting 

int D[V][V]

with

vector<vector<int> > D(V,vector<int>(V))

solves the problem, but still why is there a problem in the first place?

Ralf