1<HTML> 2<!-- 3 Copyright (c) Jeremy Siek, Lie-Quan Lee, and Andrew Lumsdaine 2000 4 5 Distributed under the Boost Software License, Version 1.0. 6 (See accompanying file LICENSE_1_0.txt or copy at 7 http://www.boost.org/LICENSE_1_0.txt) 8 --> 9<Head> 10<Title>Boost Graph Library: EventVisitorList</Title> 11<BODY BGCOLOR="#ffffff" LINK="#0000ee" TEXT="#000000" VLINK="#551a8b" 12 ALINK="#ff0000"> 13<IMG SRC="../../../boost.png" 14 ALT="C++ Boost" width="277" height="86"> 15 16<BR Clear> 17 18<H1>EventVisitorList Concept</H1> 19 20An EventVisitorList is either an <a 21href="./EventVisitor.html">EventVisitor</a>, or a list of 22EventVisitors combined using <tt>std::pair</tt>. Each graph algorithm 23defines visitor adaptors that convert an EventVisitorList into the 24particular kind of visitor needed by the algorithm. 25 26In the following example we will show how to combine event visitors 27into a list using <tt>std::pair</tt> and how to use an algorithm's 28visitor adaptor class. 29 30<p> 31Suppose we would like to print out the parenthesis 32structure of the discover/finish times of vertices in a <a 33href="./graph_theory_review.html#sec:dfs-algorithm">depth-first 34search</a>. We can use the BGL algorithm <a 35href="./depth_first_search.html"><tt>depth_first_search()</tt></a> and 36two event visitors to accomplish this. The complete source code for 37the following example is in <a href="../example/dfs_parenthesis.cpp"> 38<tt>examples/dfs_parenthesis.cpp</tt></a>. First we define the two 39event visitors. We use <tt>on_discover_vertex</tt> and 40<tt>on_finish_vertex</tt> as the event points, selected from the list 41of event points specified in <a 42href="./DFSVisitor.html">DFSVisitor</a>. 43 44<pre> 45struct open_paren : public base_visitor<open_paren> { 46 typedef on_discover_vertex event_filter; 47 template <class Vertex, class Graph> 48 void operator()(Vertex v, Graph& G) { 49 std::cout << "(" << v; 50 } 51}; 52struct close_paren : public base_visitor<close_paren> { 53 typedef on_finish_vertex event_filter; 54 template <class Vertex, class Graph> 55 void operator()(Vertex v, Graph& G) { 56 std::cout << v << ")"; 57 } 58}; 59</pre> 60 61Next we create two event visitor objects and make an EventVisitorList 62out of them using a <tt>std::pair</tt> which created by 63<tt>std::make_pair</tt>. 64 65<pre> 66std::make_pair(open_paren(), close_paren()) 67</pre> 68 69Next we want to pass this list into <tt>depth_first_search()</tt>, but 70<tt>depth_first_search()</tt> is expecting a <a 71href="./DFSVisitor.html">DFSVisitor</a>, not a EventVisitorList. We 72therefore use the <a 73href="./dfs_visitor.html"><tt>dfs_visitor</tt></a> adaptor which turns 74an EventVisitor list into a DFSVisitor. Like all of the visitor 75adaptors, <tt>dfs_visitor</tt> has a creation function called 76<tt>make_dfs_visitor()</tt>. 77 78<pre> 79make_dfs_visitor(std::make_pair(open_paren(), close_paren())) 80</pre> 81 82Now we can pass the resulting visitor object into 83<tt>depth_first_search()</tt> as follows. 84 85<pre> 86 // graph object G is created ... 87 88 std::vector<default_color_type> color(num_vertices(G)); 89 90 depth_first_search(G, make_dfs_visitor(std::make_pair(open_paren(), close_paren())), 91 color.begin()); 92</pre> 93 94For creating a list of more than two event visitors, you can nest calls to 95<tt>std::make_pair</tt> in the following way: 96 97<pre> 98std::make_pair(<i>visitor1</i>, 99 std::make_pair(<i>visitor2</i>, 100 ... 101 std::make_pair(<i>visitorN-1</i>, <i>visitorN</i>)...)); 102</pre> 103 104 105 106<h3>See Also</h3> 107 108<a href="./EventVisitor.html">EventVisitor</a>, 109<a href="./visitor_concepts.html">Visitor concepts</a> 110 111 112<br> 113<HR> 114<TABLE> 115<TR valign=top> 116<TD nowrap>Copyright © 2000-2001</TD><TD> 117<A HREF="http://www.boost.org/people/jeremy_siek.htm">Jeremy Siek</A>, 118Indiana University (<A 119HREF="mailto:jsiek@osl.iu.edu">jsiek@osl.iu.edu</A>)<br> 120<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> 121<A HREF="https://homes.cs.washington.edu/~al75">Andrew Lumsdaine</A>, 122Indiana University (<A 123HREF="mailto:lums@osl.iu.edu">lums@osl.iu.edu</A>) 124</TD></TR></TABLE> 125 126</BODY> 127</HTML> 128
