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| <Head> |
| <Title>Boost Graph Library: EventVisitorList</Title> |
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| <BR Clear> |
| |
| <H1>EventVisitorList Concept</H1> |
| |
| An EventVisitorList is either an <a |
| href="./EventVisitor.html">EventVisitor</a>, or a list of |
| EventVisitors combined using <tt>std::pair</tt>. 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 <tt>std::pair</tt> and how to use an algorithm's |
| visitor adaptor class. |
| |
| <p> |
| Suppose we would like to print out the parenthesis |
| structure of the discover/finish times of vertices in a <a |
| href="./graph_theory_review.html#sec:dfs-algorithm">depth-first |
| search</a>. We can use the BGL algorithm <a |
| href="./depth_first_search.html"><tt>depth_first_search()</tt></a> and |
| two event visitors to accomplish this. The complete source code for |
| the following example is in <a href="../example/dfs_parenthesis.cpp"> |
| <tt>examples/dfs_parenthesis.cpp</tt></a>. First we define the two |
| event visitors. We use <tt>on_discover_vertex</tt> and |
| <tt>on_finish_vertex</tt> as the event points, selected from the list |
| of event points specified in <a |
| href="./DFSVisitor.html">DFSVisitor</a>. |
| |
| <pre> |
| 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 << ")"; |
| } |
| }; |
| </pre> |
| |
| Next we create two event visitor objects and make an EventVisitorList |
| out of them using a <tt>std::pair</tt> which created by |
| <tt>std::make_pair</tt>. |
| |
| <pre> |
| std::make_pair(open_paren(), close_paren()) |
| </pre> |
| |
| Next we want to pass this list into <tt>depth_first_search()</tt>, but |
| <tt>depth_first_search()</tt> is expecting a <a |
| href="./DFSVisitor.html">DFSVisitor</a>, not a EventVisitorList. We |
| therefore use the <a |
| href="./dfs_visitor.html"><tt>dfs_visitor</tt></a> adaptor which turns |
| an EventVisitor list into a DFSVisitor. Like all of the visitor |
| adaptors, <tt>dfs_visitor</tt> has a creation function called |
| <tt>make_dfs_visitor()</tt>. |
| |
| <pre> |
| make_dfs_visitor(std::make_pair(open_paren(), close_paren())) |
| </pre> |
| |
| Now we can pass the resulting visitor object into |
| <tt>depth_first_search()</tt> as follows. |
| |
| <pre> |
| // 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()); |
| </pre> |
| |
| For creating a list of more than two event visitors, you can nest calls to |
| <tt>std::make_pair</tt> in the following way: |
| |
| <pre> |
| std::make_pair(<i>visitor1</i>, |
| std::make_pair(<i>visitor2</i>, |
| ... |
| std::make_pair(<i>visitorN-1</i>, <i>visitorN</i>)...)); |
| </pre> |
| |
| |
| |
| <h3>See Also</h3> |
| |
| <a href="./EventVisitor.html">EventVisitor</a>, |
| <a href="./visitor_concepts.html">Visitor concepts</a> |
| |
| |
| <br> |
| <HR> |
| <TABLE> |
| <TR valign=top> |
| <TD nowrap>Copyright © 2000-2001</TD><TD> |
| <A HREF="http://www.boost.org/people/jeremy_siek.htm">Jeremy Siek</A>, |
| Indiana University (<A |
| HREF="mailto:jsiek@osl.iu.edu">jsiek@osl.iu.edu</A>)<br> |
| <A HREF="http://www.boost.org/people/liequan_lee.htm">Lie-Quan Lee</A>, Indiana University (<A HREF="mailto:llee@cs.indiana.edu">llee@cs.indiana.edu</A>)<br> |
| <A HREF="http://www.osl.iu.edu/~lums">Andrew Lumsdaine</A>, |
| Indiana University (<A |
| HREF="mailto:lums@osl.iu.edu">lums@osl.iu.edu</A>) |
| </TD></TR></TABLE> |
| |
| </BODY> |
| </HTML> |
