128 lines
4.1 KiB
HTML
128 lines
4.1 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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Distributed under the Boost Software License, Version 1.0.
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(See accompanying file LICENSE_1_0.txt or copy at
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http://www.boost.org/LICENSE_1_0.txt)
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-->
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<Head>
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<Title>Boost Graph Library: EventVisitorList</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="../../../boost.png"
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ALT="C++ Boost" width="277" height="86">
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<BR Clear>
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<H1>EventVisitorList Concept</H1>
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An EventVisitorList is either an <a
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href="./EventVisitor.html">EventVisitor</a>, or a list of
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EventVisitors combined using <tt>std::pair</tt>. Each graph algorithm
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defines visitor adaptors that convert an EventVisitorList into the
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particular kind of visitor needed by the algorithm.
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In the following example we will show how to combine event visitors
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into a list using <tt>std::pair</tt> and how to use an algorithm's
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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="../example/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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For creating a list of more than two event visitors, you can nest calls to
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<tt>std::make_pair</tt> in the following way:
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<pre>
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std::make_pair(<i>visitor1</i>,
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std::make_pair(<i>visitor2</i>,
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...
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std::make_pair(<i>visitorN-1</i>, <i>visitorN</i>)...));
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</pre>
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<h3>See Also</h3>
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<a href="./EventVisitor.html">EventVisitor</a>,
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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 © 2000-2001</TD><TD>
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<A HREF="http://www.boost.org/people/jeremy_siek.htm">Jeremy Siek</A>,
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Indiana University (<A
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HREF="mailto:jsiek@osl.iu.edu">jsiek@osl.iu.edu</A>)<br>
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<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>
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<A HREF="https://homes.cs.washington.edu/~al75">Andrew Lumsdaine</A>,
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Indiana University (<A
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HREF="mailto:lums@osl.iu.edu">lums@osl.iu.edu</A>)
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</TD></TR></TABLE>
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</BODY>
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</HTML>
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