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 <Head>
 <Title>Boost Graph Library: BFSVisitor</Title>
 <BODY BGCOLOR="#ffffff" LINK="#0000ee" TEXT="#000000" VLINK="#551a8b"
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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&lt;G&gt;::edge_descriptor</tt>.</TD>
 </TR>

 <TR>
 <TD><tt>s,u</tt></TD>
 <TD>An object of type <tt>boost::graph_traits&lt;G&gt;::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 &copy; 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>
	<HTML>
	<!--
	Copyright (c) Jeremy Siek, Lie-Quan Lee, and Andrew Lumsdaine 2000

	Distributed under the Boost Software License, Version 1.0.
	(See accompanying file LICENSE_1_0.txt or copy at
	http://www.boost.org/LICENSE_1_0.txt)
	-->
	<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="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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	</HTML>