c++ - Weight map as function in Boost Graph Dijkstra algorithm

Question

I'm using Boost Graph Libraries and need to use a weightmap which is not constant, but which is a function of a parameter K (i.e. the edge costs depend on K). In practice, given the following code:

#include <boost/config.hpp>
#include <iostream>
#include <fstream>
#include <boost/graph/graph_traits.hpp>
#include <boost/graph/dijkstra_shortest_paths.hpp>
#include <boost/graph/adjacency_list.hpp>

struct Edge {
        Edge(float weight_) : weight(weight_) {}
        float weight;
        float getWeight(int K)
        {
            return K*weight;
        }
};



int main(int, char**){
        typedef boost::adjacency_list < boost::vecS, boost::vecS, boost::directedS, boost::no_property, Edge > graph_t;
        typedef boost::graph_traits < graph_t >::vertex_descriptor vertex_t;
        graph_t g;
        vertex_t a = boost::add_vertex(g);
        vertex_t b = boost::add_vertex(g);
        vertex_t c = boost::add_vertex(g);
        vertex_t d = boost::add_vertex(g);
        boost::add_edge(a, b, Edge(3), g);
        boost::add_edge(b, c, Edge(3), g);
        boost::add_edge(a, d, Edge(1), g);
        boost::add_edge(d, c, Edge(4), g);

        std::vector<vertex_t> preds(4);

        // Traditional dijsktra (sum)
        boost::dijkstra_shortest_paths(g, a, boost::predecessor_map(&preds[0]).weight_map(boost::get(&Edge::weight,g)));

        return 0;
}

I'd like to call Dijkstra algorithm as follows:

boost::dijkstra_shortest_paths(g, a, boost::predecessor_map(&preds[0]).weight_map(boost::get(&Edge::getWeight(2),g)));

But the error is the following

cannot call member function ‘float Edge::getWeight(int)’ without object

Does anyone know how to solve this?

Answer 1

There are a number of property map flavours. In particular one is the transform_value_property_map can be used here.

Simple Approach C++03

Assuming c++03 you'd write:

Live On Coliru

#include <boost/property_map/transform_value_property_map.hpp>
#include <boost/bind.hpp>

// ...

boost::dijkstra_shortest_paths(g, a, boost::predecessor_map(&preds[0]).weight_map(
            boost::make_transform_value_property_map(
                boost::bind(&Edge::getWeight ,_1, 2), 
                boost::get(boost::edge_bundle, g))
        ));

Cleaner C++11

Live On Coliru

auto wmap = make_transform_value_property_map([](Edge& e) { return e.getWeight(2); }, get(boost::edge_bundle, g));
boost::dijkstra_shortest_paths(g, a, boost::predecessor_map(&preds[0]).weight_map(wmap));

You can drop the boost/bind.hpp include.

Bonus: drop the getWeight() member function

Live On Coliru

You don't actually need it. You could write a Phoenix actor in-place:

#include <boost/phoenix.hpp>
using boost::phoenix::arg_names::arg1;

auto wmap = make_transform_value_property_map(2 * (&arg1->*&Edge::weight), get(boost::edge_bundle, g));

Or use c++11 again:

Live On Coliru

auto wmap = make_transform_value_property_map([](Edge& e) { return e.weight * 2; }, get(boost::edge_bundle, g));