ceph/src/boost/libs/graph/doc/EventVisitorList.html

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   9 <Head>
  10 <Title>Boost Graph Library: EventVisitorList</Title>
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  17
  18 <H1>EventVisitorList Concept</H1>
  19
  20 An EventVisitorList is either an <a
  21 href="./EventVisitor.html">EventVisitor</a>, or a list of
  22 EventVisitors combined using <tt>std::pair</tt>. Each graph algorithm
  23 defines visitor adaptors that convert an EventVisitorList into the
  24 particular kind of visitor needed by the algorithm.
  25
  26 In the following example we will show how to combine event visitors
  27 into a list using <tt>std::pair</tt> and how to use an algorithm's
  28 visitor adaptor class.
  29
  30 <p>
  31 Suppose we would like to print out the parenthesis
  32 structure of the discover/finish times of vertices in a <a
  33 href="./graph_theory_review.html#sec:dfs-algorithm">depth-first
  34 search</a>. We can use the BGL algorithm <a
  35 href="./depth_first_search.html"><tt>depth_first_search()</tt></a> and
  36 two event visitors to accomplish this. The complete source code for
  37 the following example is in <a href="../example/dfs_parenthesis.cpp">
  38 <tt>examples/dfs_parenthesis.cpp</tt></a>. First we define the two
  39 event visitors. We use <tt>on_discover_vertex</tt> and
  40 <tt>on_finish_vertex</tt> as the event points, selected from the list
  41 of event points specified in <a
  42 href="./DFSVisitor.html">DFSVisitor</a>.
  43
  44 <pre>
  45 struct open_paren : public base_visitor&lt;open_paren&gt; {
  46   typedef on_discover_vertex event_filter;
  47   template &lt;class Vertex, class Graph&gt;
  48   void operator()(Vertex v, Graph& G) {
  49     std::cout &lt;&lt; "(" &lt;&lt; v;
  50   }
  51 };
  52 struct close_paren : public base_visitor&lt;close_paren&gt; {
  53   typedef on_finish_vertex event_filter;
  54   template &lt;class Vertex, class Graph&gt;
  55   void operator()(Vertex v, Graph& G) {
  56     std::cout &lt;&lt; v &lt;&lt; ")";
  57   }
  58 };
  59 </pre>
  60
  61 Next we create two event visitor objects and make an EventVisitorList
  62 out of them using a <tt>std::pair</tt> which created by
  63 <tt>std::make_pair</tt>.
  64
  65 <pre>
  66 std::make_pair(open_paren(), close_paren())
  67 </pre>
  68
  69 Next we want to pass this list into <tt>depth_first_search()</tt>, but
  70 <tt>depth_first_search()</tt> is expecting a <a
  71 href="./DFSVisitor.html">DFSVisitor</a>, not a EventVisitorList.  We
  72 therefore use the <a
  73 href="./dfs_visitor.html"><tt>dfs_visitor</tt></a> adaptor which turns
  74 an EventVisitor list into a DFSVisitor. Like all of the visitor
  75 adaptors, <tt>dfs_visitor</tt> has a creation function called
  76 <tt>make_dfs_visitor()</tt>.
  77
  78 <pre>
  79 make_dfs_visitor(std::make_pair(open_paren(), close_paren()))
  80 </pre>
  81
  82 Now 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&lt;default_color_type&gt; 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
  94 For 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>
  98 std::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 &copy; 2000-2001</TD><TD>
 117 <A HREF="http://www.boost.org/people/jeremy_siek.htm">Jeremy Siek</A>,
 118 Indiana University (<A
 119 HREF="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="http://www.osl.iu.edu/~lums">Andrew Lumsdaine</A>,
 122 Indiana University (<A
 123 HREF="mailto:lums@osl.iu.edu">lums@osl.iu.edu</A>)
 124 </TD></TR></TABLE>
 125
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