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- <Head>
- <Title>Boost Graph Library: BFSVisitor</Title>
- <BODY BGCOLOR="#ffffff" LINK="#0000ee" TEXT="#000000" VLINK="#551a8b"
- ALINK="#ff0000">
- <IMG SRC="../../../boost.png"
- ALT="C++ Boost" width="277" height="86">
- <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="https://homes.cs.washington.edu/~al75">Andrew Lumsdaine</A>,
- Indiana University (<A
- HREF="mailto:lums@osl.iu.edu">lums@osl.iu.edu</A>)
- </TD></TR></TABLE>
- </BODY>
- </HTML>
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