Attach an external property map to a graph

Question

It is straight forward to add edge weights to a graph as internal properties:

void InternalProperties()
{
    std::cout << "InternalProperties()" << std::endl;

    // Graph with internal edge weights
    using EdgeWeightProperty = boost::property<boost::edge_weight_t, double>; // <tag, type>

    using GraphWithInternalEdgeWeightsType =  boost::adjacency_list<boost::setS, // out edge container
                                                                    boost::vecS, // vertex container
                                                                    boost::undirectedS, // directed or undirected
                                                                    boost::no_property, // vertex properites
                                                                    EdgeWeightProperty> // edge properties
                                                                    ;

    // Create a graph object
    GraphWithInternalEdgeWeightsType g(3);

    // add two edges with edge weights
    EdgeWeightProperty e1 = 5;
    add_edge(0, 1, e1, g);

    EdgeWeightProperty e2 = 3;
    add_edge(1, 2, e2, g);

    boost::property_map<GraphWithInternalEdgeWeightsType, boost::edge_weight_t>::type edgeWeightMap = get(boost::edge_weight_t(), g);

    using edge_iter = boost::graph_traits<GraphWithInternalEdgeWeightsType>::edge_iterator;
    std::pair<edge_iter, edge_iter> edgePair;
    for(edgePair = edges(g); edgePair.first != edgePair.second; ++edgePair.first) {
      std::cout << edgeWeightMap[*edgePair.first] << " ";
    }
}

Now if I want to do the same thing and demonstrate using "external properties", I came up with this, but there is really no link at all back to the original graph:

void ExternalProperties()
{
    std::cout << std::endl << "ExternalProperties()" << std::endl;

    // Graph with external edge weights
    using GraphWithExternalEdgeWeightsType =  boost::adjacency_list<boost::setS, // out edge container
                                                                    boost::vecS, // vertex container
                                                                    boost::undirectedS> // directed or undirected
                                                                    ;

    // Create a graph object
    GraphWithExternalEdgeWeightsType g(3);

    // add edges (without edge weights)
    add_edge(0, 1, g);
    add_edge(1, 2, g);

    // create a map from edge_descriptors to weights and populate it
    std::map<GraphWithExternalEdgeWeightsType::edge_descriptor, double> edgeWeightMap;
    edgeWeightMap[boost::edge(0,1,g).first] = 5;
    edgeWeightMap[boost::edge(1,2,g).first] = 3;

    using edge_iter = boost::graph_traits<GraphWithExternalEdgeWeightsType>::edge_iterator;
    std::pair<edge_iter, edge_iter> edgePair;
    for(edgePair = edges(g); edgePair.first != edgePair.second; ++edgePair.first) {
      std::cout << edgeWeightMap[*edgePair.first] << " ";
    }
}

Is there any way to make something like get(boost::edge_weight_t(), g); (from the internal example) return this map? Like to say g.setPropertyMap(boost::edge_weight_t, edgeWeightMap) in this external example?

Answer 1

I'm not sure what the gain is, but perhaps this helps for inspiration:

#include <boost/graph/adjacency_list.hpp>
#include <boost/property_map/property_map.hpp>
#include <map>
#include <iostream>

namespace MyLib {
    struct MyGraph : boost::adjacency_list<boost::setS, boost::vecS, boost::undirectedS> {
        using base_type = boost::adjacency_list<boost::setS, boost::vecS, boost::undirectedS>;
        using base_type::adjacency_list;

        std::map<edge_descriptor, double> m_weights;
    };

    auto get(boost::edge_weight_t, MyGraph& g)       { return boost::make_assoc_property_map(g.m_weights); }
    auto get(boost::edge_weight_t, MyGraph const& g) { return boost::make_assoc_property_map(g.m_weights); }
}

namespace boost {
    template <> struct graph_traits<MyLib::MyGraph> : graph_traits<adjacency_list<setS, vecS, undirectedS> > {};

    template <> struct property_map<MyLib::MyGraph, edge_weight_t, void> {
        using Traits = graph_traits<MyLib::MyGraph>;

        using Edge       = Traits::edge_descriptor;
        using type       = boost::associative_property_map<std::map<Edge, double> >;
        using const_type = boost::associative_property_map<std::map<Edge, double> > const;
    };
}

void ExternalProperties() {
    std::cout << "ExternalProperties()" << std::endl;

    // Graph with external edge weights
    // Create a graph object
    using Graph = MyLib::MyGraph;
    Graph g(3);

    // add edges (without edge weights)
    add_edge(0, 1, g);
    add_edge(1, 2, g);

    // create a map from edge_descriptors to weights and populate it
    auto edgeWeightMap = MyLib::get(boost::edge_weight, g);
    edgeWeightMap[boost::edge(0, 1, g).first] = 5;
    edgeWeightMap[boost::edge(1, 2, g).first] = 3;

    using edge_iter = boost::graph_traits<Graph>::edge_iterator;
    std::pair<edge_iter, edge_iter> edgePair;

    for (edgePair = edges(g); edgePair.first != edgePair.second; ++edgePair.first) {
        std::cout << edgeWeightMap[*edgePair.first] << " ";
    }
}

int main() {
    ExternalProperties();
}

I've not been able to avoid ambiguity with boost::get in such a way that you can trust ADL to pick the "best" overload without namespace qualification.

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