EventVisitorList Concept

The EventVisitorList is either an EventVisitor, or a list of EventVisitor's combined using std::pair. Each graph algorithm defines visitor adaptors that convert an EventVisitorList into the particular kind of visitor needed by the algorithm. The following example shows how to combine event visitors into a list using std::pair and how to use an algorithm's visitor adaptor class.

For example, to print out the parenthesis structure of the discover/finish times of vertices in a depth-first search. To accomplish this, use the BGL algorithm depth_first_search() and two event visitors. The complete source code for the following example is in examples/dfs_parenthesis.cpp. First, define the two event visitors. Use on_discover_vertex and on_finish_vertex as the event points, selected from the list of event points specified in DFSVisitor.

struct open_paren : public base_visitor<open_paren> {
  typedef on_discover_vertex event_filter;
  template <class Vertex, class Graph>
  void operator()(Vertex v, Graph& G) {
    std::cout << "(" << v;
  }
};
struct close_paren : public base_visitor<close_paren> {
  typedef on_finish_vertex event_filter;
  template <class Vertex, class Graph>
  void operator()(Vertex v, Graph& G) {
    std::cout << v << ")";
  }
};

Next, create two event visitor objects and make an EventVisitorList out of them using a std::pair; created by std::make_pair.

std::make_pair(open_paren(), close_paren()))

Then, pass this list into depth_first_search(), but depth_first_search() is expecting a DFSVisitor, not a EventVisitorList. Hence use  the dfs_visitor adaptor which turns an EventVisitor list into a DFSVisitor. Like all of the visitor adaptors, dfs_visitor has a creation function called make_dfs_visitor().

make_dfs_visitor(std::make_pair(open_paren(), close_paren()))

Now pass the resulting visitor object into depth_first_search() as follows.

  // graph object G is created ...

  std::vector<default_color_type> color(num_vertices(G));

  depth_first_search(G, make_dfs_visitor(std::make_pair(open_paren(), close_paren())),
                     color.begin());

For creating a list of more than two event visitors, nest calls to std::make_pair in the following way:

std::make_pair(visitor1,
  std::make_pair(visitor2,
    ...
    std::make_pair(visitorN-1, visitorN)...));

See Also

EventVisitor, Visitor concepts