Re: [boost] Is the bellman_ford_shortest_paths function missing an overload with the root vertex?

Subject: Re: [boost] Is the bellman_ford_shortest_paths function missing an overload with the root vertex?
From: Geoff Hilton (geoff.hilton_at_[hidden])
Date: 2008-11-28 13:17:53

Geoff Hilton wrote:
> Quick question, was the fact that there is no 
> bellman_ford_shortest_paths overload in the BGL that allows for the 
> specification of the root vertex intentional? I'm using MSVC9 and tried 
> using the named parameters overload but it won't compile. Ditto for 
> dijkstra_shortest_paths and dag_shortest_paths actually.
> 
> Here's what I have that won't compile with (twice) the error message:
> error C2248: 'Weight<long long>::Weight': cannot access private member 
> declared in class 'Weight<long long>'    c:\program 
> files\boost\boost_1_37_0\boost\graph\bellman_ford_shortest_paths.hpp    155
> 
> 
> class ShortestPathBellmanFordForward: public ShortestPathFunction
> {
>     public:
>         virtual ~ShortestPathBellmanFordForward(){}
>         virtual AlgoResult operator()(
>                 const vertex_descriptor startingVertex,
>                 const WeightComparer<long long>& comparer,
>                 Graph& graph) const {
>     const size_t numVertices = boost::num_vertices(graph);
>     AlgoResult retval;
>     retval.first = PredecessorMap(numVertices);
>     retval.second = DistanceMap(numVertices);
>     vertex_index_prop_t vindex = get(boost::vertex_index, graph);
>     boost::bellman_ford_shortest_paths(graph, numVertices,
>         weight_map(boost::get(&EdgeProp::weights, graph)).
>         predecessor_map(boost::make_iterator_property_map(
>             retval.first.begin(), vindex)).
>         distance_map(boost::make_iterator_property_map(
>             retval.second.begin(), vindex)).
>         root_vertex(startingVertex).
>         distance_combine(WeightCombiner<long long>()).
>         distance_compare(comparer));
>     return retval;
>     }
> };
> 
> Thanks,
> Geoff

Hmm, so I realized that the above error was because my Weight datatype 
happens to have a private constructor which takes a 
template-type-specified value in the constructor for default 
initialization (it's really only used (in my code) by my named 
constructors CreateInfinity() and CreateZero() though).
When I comment out the word "private:" which hides the constructor in 
question, everything compiles fine, but when I comment out the 
constructor entirely (and adjust the named constructors accordingly) the 
compiler goes nuts with 30 errors (all of which are attributed by the 
compiler to bellman_ford_shortest_paths.hpp), the first of which is:

error C2440: '<function-style-cast>': cannot convert from 'int' to 
'weight_type'	c:\program 
files\boost\boost_1_37_0\boost\graph\bellman_ford_shortest_paths.hpp	155

"cannot convert from 'int' to 'weight_type'"... well duh. But notice 
line 155 as mentioned above... is this really desirable? That line does 
indeed force the requirement that the weight type have a constructor 
which takes a POD as a parameter.

I'd like to propose replacing the above mentioned behaviour with that 
which is similar to dijkstra or dag; namely that a weight of value 
infinity and zero be supplied to the algorithm by named/unnamed parameter.

Thanks,
Geoff