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| <Head> |
| <Title>Boost Graph Library: BFSVisitor</Title> |
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| <IMG SRC="../../../boost.png" |
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| <BR Clear> |
| |
| <H1><img src="figs/python.gif" alt="(Python)"/>BFS Visitor Concept</H1> |
| |
| This concept defines the visitor interface for <a |
| href="./breadth_first_search.html"><tt>breadth_first_search()</tt></a>. |
| Users can define a class with the BFS Visitor interface and pass and |
| object of the class to <tt>breadth_first_search()</tt>, thereby |
| augmenting the actions taken during the graph search. |
| |
| <h3>Refinement of</h3> |
| |
| <a href="../../utility/CopyConstructible.html">Copy Constructible</a> |
| (copying a visitor should be a lightweight operation). |
| |
| |
| <h3>Notation</h3> |
| |
| <Table> |
| <TR> |
| <TD><tt>V</tt></TD> |
| <TD>A type that is a model of BFS Visitor.</TD> |
| </TR> |
| |
| <TR> |
| <TD><tt>vis</tt></TD> |
| <TD>An object of type <tt>V</tt>.</TD> |
| </TR> |
| |
| <TR> |
| <TD><tt>G</tt></TD> |
| <TD>A type that is a model of Graph.</TD> |
| </TR> |
| |
| <TR> |
| <TD><tt>g</tt></TD> |
| <TD>An object of type <tt>G</tt>.</TD> |
| </TR> |
| |
| <TR> |
| <TD><tt>e</tt></TD> |
| <TD>An object of type <tt>boost::graph_traits<G>::edge_descriptor</tt>.</TD> |
| </TR> |
| |
| <TR> |
| <TD><tt>s,u</tt></TD> |
| <TD>An object of type <tt>boost::graph_traits<G>::vertex_descriptor</tt>.</TD> |
| </TR> |
| |
| </table> |
| |
| <h3>Associated Types</h3> |
| |
| none |
| <p> |
| |
| <h3>Valid Expressions</h3> |
| |
| <table border> |
| <tr> |
| <th>Name</th><th>Expression</th><th>Return Type</th><th>Description</th> |
| </tr> |
| |
| <tr> |
| <td>Initialize Vertex</td> |
| <td><tt>vis.initialize_vertex(s, g)</tt></td> |
| <td><tt>void</tt></td> |
| <td> |
| This is invoked on every vertex of the graph before the start of the |
| graph search. |
| </td> |
| </tr> |
| |
| <tr> |
| <td>Discover Vertex</td> |
| <td><tt>vis.discover_vertex(u, g)</tt></td> |
| <td><tt>void</tt></td> |
| <td> |
| This is invoked when a vertex is encountered for the first time. |
| </td> |
| </tr> |
| |
| <tr> |
| <td>Examine Vertex</td> |
| <td><tt>vis.examine_vertex(u, g)</tt></td> |
| <td><tt>void</tt></td> |
| <td> |
| This is invoked on a vertex as it is popped from the queue. This |
| happens immediately before <tt>examine_edge()</tt> is invoked |
| on each of the out-edges of vertex <tt>u</tt>. |
| </td> |
| </tr> |
| |
| <tr> |
| <td>Examine Edge</td> |
| <td><tt>vis.examine_edge(e, g)</tt></td> |
| <td><tt>void</tt></td> |
| <td> |
| This is invoked on every out-edge of each vertex after it is discovered. |
| </td> |
| </tr> |
| |
| |
| <tr> |
| <td>Tree Edge</td> |
| <td><tt>vis.tree_edge(e, g)</tt></td> |
| <td><tt>void</tt></td> |
| <td> |
| This is invoked on each edge as it becomes a member of the edges that |
| form the search tree.</td> |
| </tr> |
| |
| <tr> |
| <td>Non-Tree Edge</td> |
| <td><tt>vis.non_tree_edge(e, g)</tt></td> |
| <td><tt>void</tt></td> |
| <td> |
| This is invoked on back or cross edges for directed graphs and cross |
| edges for undirected graphs. |
| </td> |
| </tr> |
| |
| <tr> |
| <td>Gray Target</td> |
| <td><tt>vis.gray_target(e, g)</tt></td> |
| <td><tt>void</tt></td> |
| <td> |
| This is invoked on the subset of non-tree edges whose target vertex is |
| colored gray at the time of examination. The color gray indicates |
| that the vertex is currently in the queue. |
| </td> |
| </tr> |
| |
| <tr> |
| <td>Black Target</td> |
| <td><tt>vis.black_target(e, g)</tt></td> |
| <td><tt>void</tt></td> |
| <td> |
| This is invoked on the subset of non-tree edges whose target vertex is |
| colored black at the time of examination. The color black indicates |
| that the vertex has been removed from the queue. |
| </td> |
| </tr> |
| |
| <tr> |
| <td>Finish Vertex</td> |
| <td><tt>vis.finish_vertex(u, g)</tt></td> |
| <td><tt>void</tt></td> |
| <td> |
| This invoked on a vertex after all of its out edges have been added to the |
| search tree and all of the adjacent vertices have been discovered |
| (but before the out-edges of the adjacent vertices have been examined). |
| </td> |
| </tr> |
| |
| </table> |
| |
| <h3>Models</h3> |
| |
| <ul> |
| <li><a href="./bfs_visitor.html"><tt>bfs_visitor</tt></a> |
| </ul> |
| |
| <a name="python"></a><h3>Python</h3> |
| To implement a model of the <tt>BFSVisitor</tt> concept in Python, |
| create a new class that derives from the <tt>BFSVisitor</tt> type of |
| the graph, which will be |
| named <tt><i>GraphType</i>.BFSVisitor</tt>. The events and syntax are |
| the same as with visitors in C++. Here is an example for the |
| Python <tt>bgl.Graph</tt> graph type: |
| |
| <pre> |
| class count_tree_edges_bfs_visitor(bgl.Graph.BFSVisitor): |
| def __init__(self, name_map): |
| bgl.Graph.BFSVisitor.__init__(self) |
| self.name_map = name_map |
| |
| def tree_edge(self, e, g): |
| (u, v) = (g.source(e), g.target(e)) |
| print "Tree edge ", |
| print self.name_map[u], |
| print " -> ", |
| print self.name_map[v] |
| </pre> |
| |
| <h3>See also</h3> |
| |
| <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> |
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