# EventVisitorList Concept

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

Suppose we would like to print out the parenthesis structure of the discover/finish times of vertices in a depth-first search. We can use the BGL algorithm `depth_first_search()` and two event visitors to accomplish this. The complete source code for the following example is in `examples/dfs_parenthesis.cpp`. First we define the two event visitors. We 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 we create two event visitor objects and make an EventVisitorList out of them using a `std::pair` which created by `std::make_pair`.

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

Next we want to pass this list into `depth_first_search()`, but `depth_first_search()` is expecting a DFSVisitor, not a EventVisitorList. We therefore 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 we can 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, you can 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

Copyright © 2000-2001 | Jeremy Siek, Indiana University (jsiek@osl.iu.edu) Lie-Quan Lee, Indiana University (llee@cs.indiana.edu) Andrew Lumsdaine, Indiana University (lums@osl.iu.edu)