opposite

template <class Graph>
  typename graph_traits<Graph>::vertex_descriptor
  opposite(typename graph_traits<Graph>::edge_descriptor e,
           typename graph_traits<Graph>::vertex_descriptor v,
           const Graph& g)

Given an edge and a vertex which must be incident to the edge, this function returns the opposite vertex. So if v is the source vertex, this function returns the target vertex. If v is the target, then this function returns the source vertex.

Example

edge_descriptor e;
...
vertex_descriptor u, v;
boost::tie(u, v) = incident(e, g);
assert(v == opposite(e, u, g));
assert(u == opposite(e, v, g));