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232 lines
6.5 KiB
HTML
<HTML>
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<!--
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-- Copyright (c) Jeremy Siek, Lie-Quan Lee, and Andrew Lumsdaine 2000
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--
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-- Permission to use, copy, modify, distribute and sell this software
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-- and its documentation for any purpose is hereby granted without fee,
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-- provided that the above copyright notice appears in all copies and
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-- that both that copyright notice and this permission notice appear
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-- in supporting documentation. We make no
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-- representations about the suitability of this software for any
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-- purpose. It is provided "as is" without express or implied warranty.
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-->
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<Head>
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<Title>Boost Graph Library: EventVisitor</Title>
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<BODY BGCOLOR="#ffffff" LINK="#0000ee" TEXT="#000000" VLINK="#551a8b"
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ALINK="#ff0000">
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<IMG SRC="../../../c++boost.gif"
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ALT="C++ Boost">
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<BR Clear>
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<H1>EventVisitor Concept</H1>
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This concept defines the interface for single-event visitors. The
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visitor has a single apply member function (<tt>operator()</tt>).
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This function is invoked within the graph algorithm at the event-point
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specified by the <tt>event_filter</tt> typedef within the
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EventVisitor. The following is the list of event tags that can be
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invoked in BGL algorithms. Each tag corresponds to a member function
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of the visitor for an algorithm. For example, the <a
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href="./BFSVisitor.html">BFSVisitor</a> of <a
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href="./breadth_first_search.html"><tt>breadth_first_search()</tt></a>
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has a <tt>cycle_edge()</tt> member function. The corresponding tag is
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<tt>on_cycle_edge</tt>. The first argument is the event visitor's
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<tt>operator()</tt> must be either an edge or vertex descriptor
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depending on the event tag.
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<pre>
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namespace boost {
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struct on_initialize_vertex { };
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struct on_start_vertex { };
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struct on_discover_vertex { };
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struct on_examine_edge { };
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struct on_tree_edge { };
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struct on_cycle_edge { };
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struct on_finish_vertex { };
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struct on_forward_or_cross_edge { };
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struct on_back_edge { };
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struct on_edge_relaxed { };
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struct on_edge_not_relaxed { };
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struct on_edge_minimized { };
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struct on_edge_not_minimized { };
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} // namespace boost
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</pre>
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<h3>Creating an EventVisitorList</h3>
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Often times one would like to use multiple single-event vistors in a
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call to a particular algorithm. In the following example we will show
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how to combine event visitors into a list using <tt>std::pair</tt> and
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the algorithm's visitor adaptor class.
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<p>
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Suppose we would like to print out the parenthesis
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structure of the discover/finish times of vertices in a <a
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href="./graph_theory_review.html#sec:dfs-algorithm">depth-first
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search</a>. We can use the BGL algorithm <a
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href="./depth_first_search.html"><tt>depth_first_search()</tt></a> and
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two event visitors to accomplish this. The complete source code for
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the following example is in <a href="../examples/dfs_parenthesis.cpp">
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<tt>examples/dfs_parenthesis.cpp</tt></a>. First we define the two
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event visitors. We use <tt>on_discover_vertex</tt> and
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<tt>on_finish_vertex</tt> as the event points, selected from the list
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of event points specified in <a
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href="./DFSVisitor.html">DFSVisitor</a>.
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<pre>
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struct open_paren : public base_visitor<open_paren> {
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typedef on_discover_vertex event_filter;
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template <class Vertex, class Graph>
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void operator()(Vertex v, Graph& G) {
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std::cout << "(" << v;
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}
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};
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struct close_paren : public base_visitor<close_paren> {
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typedef on_finish_vertex event_filter;
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template <class Vertex, class Graph>
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void operator()(Vertex v, Graph& G) {
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std::cout << v << ")";
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}
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};
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</pre>
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Next we create two event visitor objects and make an EventVisitorList
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out of them using a <tt>std::pair</tt> which created by
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<tt>std::make_pair</tt>.
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<pre>
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std::make_pair(open_paren(), close_paren())
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</pre>
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Next we want to pass this list into <tt>depth_first_search()</tt>, but
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<tt>depth_first_search()</tt> is expecting a <a
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href="./DFSVisitor.html">DFSVisitor</a>, not a EventVisitorList. We
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therefore use the <a
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href="./dfs_visitor.html"><tt>dfs_visitor</tt></a> adaptor which turns
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an EventVisitor list into a DFSVisitor. Like all of the visitor
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adaptors, <tt>dfs_visitor</tt> has a creation function called
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<tt>make_dfs_visitor()</tt>.
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<pre>
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make_dfs_visitor(std::make_pair(open_paren(), close_paren()))
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</pre>
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Now we can pass the resulting visitor object into
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<tt>depth_first_search()</tt> as follows.
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<pre>
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// graph object G is created ...
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std::vector<default_color_type> color(num_vertices(G));
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depth_first_search(G, make_dfs_visitor(std::make_pair(open_paren(), close_paren())),
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color.begin());
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</pre>
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<h3>Refinement of</h3>
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none
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<p>
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<h3>Notation</h3>
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<Table>
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<TR>
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<TD><tt>G</tt></TD>
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<TD>A type that is a model of <a href="./Graph.html">Graph</a>.</TD>
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</TR>
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<TR>
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<TD><tt>g</tt></TD>
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<TD>An object of type <tt>G</tt>.</TD>
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</TR>
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<TR>
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<TD><tt>V</tt></TD>
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<TD>A type that is a model of EventVisitor.</TD>
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</TR>
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<TR>
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<TD><tt>vis</tt></TD>
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<TD>An object of type <tt>V</tt>.</TD>
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</TR>
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<TR>
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<TD><tt>x</tt></TD>
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<TD>An object of type
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<tt>boost::graph_traits<G>::vertex_descriptor</tt>
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or <tt>boost::graph_traits<G>::edge_descriptor</tt>.</TD>
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</TR>
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</table>
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<h3>Associated Types</h3>
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<Table border>
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<TR>
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<TD>Event Filter </TD>
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<TD><TT>V::event_filter</TT></TD>
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<TD>
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A tag struct to specify on which event the visitor should be invoked.
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</TD>
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</TR>
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</table>
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<h3>Valid Expressions</h3>
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<Table border>
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<tr>
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<th>Name</th><th>Expression</th><th>Return Type</th><th>Description</th>
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</tr>
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<tr>
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<td>Apply Visitor</td>
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<td><TT>vis(x, g)</TT></TD>
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<TD><TT>void</TT></TD>
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<TD>
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Invokes the visitor operation on object <tt>x</tt>, which is
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either a vertex or edge descriptor of the graph.
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</TD>
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</TR>
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</table>
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<h3>Models</h3>
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<ul>
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<li><a
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href="./predecessor_recorder.html"><tt>predecessor_recorder</tt></a>
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<li><a href="./distance_recorder.html"><tt>distance_recorder</tt></a>
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<li><a href="./time_stamper.html"><tt>time_stamper</tt></a>
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<li><a href="./property_writer.html"><tt>property_writer</tt></a>
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<li><a href="./null_visitor.html"><tt>null_visitor</tt></a>
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</ul>
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<h3>See Also</h3>
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<a href="./visitor_concepts.html">Visitor concepts</a>
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<br>
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<HR>
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<TABLE>
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<TR valign=top>
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<TD nowrap>Copyright © 2000</TD><TD>
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<A HREF=http://www.boost.org/people/jeremy_siek.htm>Jeremy Siek</A>,
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Univ.of Notre Dame (<A
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HREF="mailto:jsiek@lsc.nd.edu">jsiek@lsc.nd.edu</A>)<br>
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<A HREF=http://www.lsc.nd.edu/~llee1>Lie-Quan Lee</A>, Univ.of Notre Dame (<A HREF="mailto:llee1@lsc.nd.edu">llee1@lsc.nd.edu</A>)<br>
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<A HREF=http://www.lsc.nd.edu/~lums>Andrew Lumsdaine</A>,
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Univ.of Notre Dame (<A
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HREF="mailto:lums@lsc.nd.edu">lums@lsc.nd.edu</A>)
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</TD></TR></TABLE>
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</BODY>
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</HTML>
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