edge_index zero for all edges?

Defining my boost::graph like the following, I get edge indices zero for all edges. Why? What am I doing wrong?

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

int main() {
    typedef boost::adjacency_list<boost::vecS, boost::vecS, boost::directedS, boost::no_property, boost::property<boost::edge_index_t, std::size_t> > Graph;
    typedef boost::graph_traits<Gra开发者_如何学编程ph>::edge_descriptor Edge;

    Graph g(3);
    Edge e1 = boost::add_edge(0, 1, g).first;
    Edge e2 = boost::add_edge(1, 2, g).first;
    Edge e3 = boost::add_edge(2, 0, g).first;

    boost::property_map<Graph, boost::edge_index_t>::type eim = boost::get(boost::edge_index, g);
    size_t e1n = eim[e1],
           e2n = eim[e2],
           e3n = eim[e3];

    return 0;
}

As far as I can tell from documentation and examples, this should work.

An adjacency_list doesn't have an edge index associated with it, only a vertex index. Which is quite logical once you think about how the graph is stored.

To have an edge index, you need to manually add it to the graph description, and then manually handle it.