在boost graph lib中，如何在不遍历该顶点的所有边缘的情况下获得该顶点的特定边缘？

Question

Let's say I have a graph, with edges each containing a char. From a vertex, I want to get a specific out-edge with a specific char. Since the edge container can be set to a set or a hash-set, I assume there is a way to do this without iterating through the vertex's out-edges. I'm also assuming/hoping the edge container is keyed on the type the edge contains.

#include <boost/graph/adjacency_list.hpp>

using namespace boost;
typedef boost::adjacency_list<setS, vecS, directedS, std::string, char> MyGraph;
typedef boost::graph_traits<MyGraph>::vertex_descriptor Vertex;
typedef boost::graph_traits<MyGraph>::edge_descriptor Edge;

MyGraph g;

//setup
add_vertex(std::string("xxx"), g);
Vertex currentVertex = g.vertex_set()[0];
Vertex endVertex = add_vertex(std::string("yyy"), g);
add_edge(currentVertex, endVertex, 'i', g);

//later...
//Now I want that edge containing the letter 'i'.

//out_edges returns a pair of edge iterators.
std::pair<iterator, iterator> iterators = out_edges(currentVertex, g);  // do not want!

Edge iEdge = how_do_I_get_This?(currentVertex, g); // want!

Is there a way to do this, or is iterating through the out-edges the only option?

update:

I think this will get me the container.

std::set<?> edges = g.out_edge_list(currentVertex);

Now I cannot figure out what the ? template type is.

update2:

This seems to compile, but I need an edge_descriptor, not an edge_property to pass to target.

 std::set<boost::detail::stored_edge_property<long unsigned int, char> > edges = fGraph.out_edge_list(currentVertex);

update3:

Guess I don't need an edge descriptor. Got what I needed like this:

 std::set<boost::detail::stored_edge_property<long unsigned int, char> > edges = fGraph.out_edge_list(currentVertex);
 std::_Rb_tree_const_iterator<boost::detail::stored_edge_property<long unsigned int, char> >  edge = edges.find(*i);

 Vertex target = edge.get_target();

This all compiles and seems to work, but it is massively ugly.

Answer 1

Are you looking for how to use edge descriptors?

Edge i_edge = add_edge(currentVertex, endVertex, 'i', g).first;

i_edge is the vertex-descriptor for the 'i' edge.

// later...
// Now I want that edge containing the letter 'i'.
char yougotit = g[i_edge];

Check it:

assert('i' == yougotit);

See it Live On Coliru

---

If you really want to search, and can use c++1y you might find this elegant: Also Live

#include <boost/graph/adjacency_list.hpp>
#include <boost/range/algorithm.hpp>
#include <boost/range/adaptors.hpp>
#include <iostream>

using namespace boost::adaptors;

using namespace boost;
typedef boost::adjacency_list<setS, vecS, directedS, std::string, char> MyGraph;
typedef boost::graph_traits<MyGraph>::vertex_descriptor Vertex;
typedef boost::graph_traits<MyGraph>::edge_descriptor Edge;

int main() {
    MyGraph g;

    // setup
    add_vertex(std::string("xxx"), g);
    Vertex currentVertex = g.vertex_set()[0];
    Vertex endVertex = add_vertex(std::string("yyy"), g);
    add_edge(currentVertex, endVertex, 'i', g);

    for (auto matching : boost::edges(g) | filtered([&g](auto const& e) { return g[e] == 'i'; }))
        std::cout << matching << " --> " << g[matching] << "\n";
}

Output:

(0,1) --> i