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.
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().
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:
