How does boost::copy_graph's vertex_copy work?

Question

I am using boost::copy_graph to copy an adjacency_list into another adjacency_list with a different VertexProperties template. To do this, I am trying to use the vertex_copy parameter (doc here). I am running into a compiler error that tells the I have a wrong type for the vertex properties of the second (to-be-copied) graph.

Minimal Example

#include <boost/graph/adjacency_list.hpp>
#include <boost/graph/copy.hpp>

typedef boost::adjacency_list<boost::vecS,
                              boost::vecS,
                              boost::undirectedS,
                              uint32_t,
                              float> AdjacencyList;

typedef AdjacencyList::vertex_descriptor VertexID;

struct custom_property
{
    uint32_t label;
    float f;
};

typedef boost::adjacency_list<boost::vecS,
                              boost::vecS,
                              boost::undirectedS,
                              custom_property,
                              float> AdjacencyListCustom;

struct vertex_copier
{
    void operator() (uint32_t &input, custom_property &output)
    {
        output.label = input;
        output.f = 0.;
    }
};


int main(int argc, char** argv)
{
    AdjacencyList adj;
    VertexID id_0 = boost::add_vertex(0, adj);
    VertexID id_1 = boost::add_vertex(1, adj);
    VertexID id_2 = boost::add_vertex(2, adj);
    VertexID id_3 = boost::add_vertex(4, adj);
    boost::add_edge(id_0, id_1, 1.0f, adj);
    boost::add_edge(id_2, id_3, 2.0f, adj);

    AdjacencyListCustom adj_custom;
    boost::copy_graph(adj, adj_custom,  boost::vertex_copy(vertex_copier()));

}

g++ compilation error

...
/usr/include/boost/graph/copy.hpp:164:22: error: no match for call to '(vertex_copier) (boost::iterators::detail::iterator_facade_base<boost::range_detail::integer_iterator<long unsigned int>, long unsigned int, boost::iterators::random_access_traversal_tag, long unsigned int, long int, false, false>::reference, boost::graph_traits<boost::adjacency_list<boost::vecS, boost::vecS, boost::undirectedS, custom_property, float> >::vertex_descriptor&)'
           copy_vertex(*vi, new_v);
           ~~~~~~~~~~~^~~~~~~~~~~~
/path/to/file.cpp: note: candidate: void vertex_copier::operator()(uint32_t&, custom_property&)
     void operator() (uint32_t &input, custom_property &output)
          ^~~~~~~~
/path/to/file.cpp: note:   no known conversion for argument 2 from 'boost::graph_traits<boost::adjacency_list<boost::vecS, boost::vecS, boost::undirectedS, custom_property, float> >::vertex_descriptor {aka long unsigned int}' to 'custom_property&'

This tells me that that vertex_copy is actually trying to copy the vertex_descriptor, which is a long unsigned int, not a custom_property. This seems to go against what is stated in the docs:

This is a Binary Function that copies the properties of a vertex in the original graph into the corresponding vertex in the copy.

How does vertex_copy work? Can it be used to set/define vertex properties in the copied graph that are not in the original? If not, do these properties have to be set after the copying by iterating through the graph? Can I apply a mapping en masse or does each vertex have to be visited and updated?

EDIT: If I attempt to use copy_graph without specifying vertex_copy, then there is an error because the = operator does not exist between custom_property and uint32_t.

Answer 1

Sidestepping the question for a second, the simplest way to achieve things would be to add the appropriate conversion constructor to the custom property:

struct custom_property {
    custom_property(uint32_t label = 0, float f = 0) : label(label), f(f) {}
    uint32_t label;
    float f;
};

In which case, a simple copy will work:

boost::copy_graph(adj, adj_custom);

See it Live On Coliru

---

Regarding the vertex copier, it receives the vertex decriptors. To access the vertex properties, you need to have the graph reference:

struct vertex_copier {
    AdjacencyList& from;
    AdjacencyListCustom& to;

    void operator()(AdjacencyList::vertex_descriptor input, AdjacencyListCustom::vertex_descriptor output) const {
        to[output] = { from[input], 0.f };
    }
};

In which case you'd invoke things:

boost::copy_graph(adj, adj_custom, boost::vertex_copy(vertex_copier{adj, adj_custom}));

Again, Live On Coliru

#include <boost/graph/adjacency_list.hpp>
#include <boost/graph/copy.hpp>
#include <iostream>

typedef boost::adjacency_list<boost::vecS, boost::vecS, boost::undirectedS, uint32_t, float> AdjacencyList;

typedef AdjacencyList::vertex_descriptor VertexID;

    struct custom_property {
        //custom_property(uint32_t label = 0, float f = 0) : label(label), f(f) {}
        uint32_t label;
        float f;
    };

typedef boost::adjacency_list<boost::vecS, boost::vecS, boost::undirectedS, custom_property, float> AdjacencyListCustom;

struct vertex_copier {
    AdjacencyList& from;
    AdjacencyListCustom& to;

    void operator()(AdjacencyList::vertex_descriptor input, AdjacencyListCustom::vertex_descriptor output) const {
        to[output] = { from[input], 0.f };
    }
};

int main(int argc, char **argv) {
    AdjacencyList adj;
    VertexID id_0 = boost::add_vertex(0, adj);
    VertexID id_1 = boost::add_vertex(1, adj);
    VertexID id_2 = boost::add_vertex(2, adj);
    VertexID id_3 = boost::add_vertex(4, adj);
    boost::add_edge(id_0, id_1, 1.0f, adj);
    boost::add_edge(id_2, id_3, 2.0f, adj);

    AdjacencyListCustom adj_custom;
    boost::copy_graph(adj, adj_custom, boost::vertex_copy(vertex_copier{adj, adj_custom}));
}