On C++ Boost Graph Creation and the vertex_index Property.

Question

I am boost noob. I am wondering why compilation fails in the following code. I am creating a set of vertices, and trying to assign my own vertex indices and vertex names. (I

Answer 1

The expression vertIndx[v] returns a Vertex by value. Thus you get the error because it's not an lvalue when you try to assign to it.

Furthermore, it actually returns v. Here's the code run by vertIndx[v]:

inline value_type operator[](key_type v) const { return v; }

Here's a version that is hopefully clear about how it works:

#include 

int main()
{
    //Define graph
    typedef boost::adjacency_list
        <
            boost::vecS                                        //! edge list 
          , boost::vecS                                        //! vertex list
          , boost::undirectedS                                 //! undirected graph  
          , boost::property //! vertex properties : name                
          , boost::property         //! edge properties : weight 
        >   ScafGraph;

    //descriptors
    typedef boost::graph_traits::vertex_descriptor SV;
    typedef boost::graph_traits::edge_descriptor SE;

    //Graph Object
    ScafGraph SG;

    //property accessors
    boost::property_map::type vertName = boost::get(boost::vertex_name, SG);
    boost::property_map::type vertIndx = boost::get(boost::vertex_index, SG);
    boost::property_map::type edgeWeight = boost::get(boost::edge_weight, SG);

    //Populate Graph
    std::vector svlist;
    for (int i = 0; i < 4; i++) {
        SV v = boost::add_vertex(ScafGraph::vertex_property_type(std::to_string(i)), SG);
        svlist.push_back(v);
        assert(vertName[v] == std::to_string(i));
        assert(vertIndx[v] == i);
    }
    return 0;
}