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 am following this page: http://fireflyblue.blogspot.com/2008/01/boost-graph-library.html. )

I understand that vertS vertex lists in Boost does not need explicit vertex id creations, and I have also seen this very related question in Stackoverflow (how provide a vertex_index property for my graph) which discusses how to use an associative_property_map to assign vertex indices. The following though - getting the vertex_index map, and assigning the key value pairs - seems a fairly straightforward thing to do, and I would like to understand why it fails. Any help is greatly appreciated!

The compile error is as below:

error: expression is not assignable vertIndx[v] = i;

//Define graph
typedef boost::property<boost::vertex_name_t, std::string> sv_namePty;
typedef boost::property<boost::vertex_index_t, int, sv_namePty > sv_indx_n_name_pty;
typedef boost::property<boost::edge_weight_t, int> se_weightPty;
typedef boost::adjacency_list<boost::vecS, boost::vecS, boost::undirectedS, 
        sv_indx_n_name_pty, se_weightPty> ScafGraph;

//descriptors
typedef boost::graph_traits<ScafGraph>::vertex_descriptor SV;
typedef boost::graph_traits<ScafGraph>::edge_descriptor SE;

//Graph Object
ScafGraph SG;

//property accessors
boost::property_map<ScafGraph, 
     boost::vertex_name_t>::type vertName = boost::get(boost::vertex_name, SG);
boost::property_map<ScafGraph, 
     boost::vertex_index_t>::type vertIndx = boost::get(boost::vertex_index, SG);
boost::property_map<ScafGraph, 
     boost::edge_weight_t>::type edgeWeight = boost::get(boost::edge_weight, SG);

//Populate Graph
std::vector<SV> svlist;
for(int i=0; i<4; i++) {
    SV v = boost::add_vertex(SG);
    svlist.push_back(v);
    vertName[v] = std::to_string(i);
    vertIndx[v] = 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 <boost\graph\adjacency_list.hpp>

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

    //descriptors
    typedef boost::graph_traits<ScafGraph>::vertex_descriptor SV;
    typedef boost::graph_traits<ScafGraph>::edge_descriptor SE;

    //Graph Object
    ScafGraph SG;

    //property accessors
    boost::property_map<ScafGraph,
        boost::vertex_name_t>::type vertName = boost::get(boost::vertex_name, SG);
    boost::property_map<ScafGraph,
        boost::vertex_index_t>::type vertIndx = boost::get(boost::vertex_index, SG);
    boost::property_map<ScafGraph,
        boost::edge_weight_t>::type edgeWeight = boost::get(boost::edge_weight, SG);

    //Populate Graph
    std::vector<SV> 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;
}