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511 lines
9.6 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: Visitor Classes</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><A NAME="sec:foundation-visitor-classes"></A>
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Foundation Visitor Classes
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</H1>
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<P>
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Foundation visitor is just the name we use for visitor classes that are
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used inside the implementation of BGL algorithms. Typically you will
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never need to see or use these classes, however perhaps they will be
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of interest as examples or can reused in some unforeseen
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circumstances.
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<P>
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<UL>
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<LI><TT>coloring_visitor</TT>
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<BR>
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This visitor is used inside the <TT>breadth_first_search()</TT>
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family of algorithms. The main responsibility of this visitor is to
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keep track of the progress of the algorithm through the graph by
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coloring the graph.
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<P>
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</LI>
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<LI><TT>dfs_visitor</TT>
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<BR>
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This visitor is used inside the <TT>depth_first-_search()</TT> family of
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algorithms. Like the <TT>coloring_visitor</TT>, the responsibility
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of this visitor is to color the graph to control the progress
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of the algorithm.
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<P>
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</LI>
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<LI><TT>weighted_edge_visitor</TT>
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<BR>
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This visitor is used for Prim's algorithm for minimum spanning tree and
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Dijkstra's algorithm for single-source shortest paths. This visitor
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is responsible for calculating and <I>relaxing</I> the distances
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to the vertices.
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<P>
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</LI>
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<LI><TT>topo_sort_visitor</TT>
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<BR>
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This visitor is used inside the <TT>topological_sort()</TT> algorithm,
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and is responsible for feeding vertices to the output-iterator
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during the execution of a <TT>depth_first_search()</TT>.
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<P>
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</LI>
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<LI><TT>components_visitor</TT>
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<BR>
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This visitor is used in the strongly connected components algorithm
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for recording to which component a vertex belongs.
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<P>
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</LI>
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</UL>
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<H1><A NAME="sec:user-visitor-classes"></A>
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User-Level Visitor Classes
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</H1>
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<P>
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There are a handful of visitors defined by BGL, but the visitor
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mechanism is mainly intended as a way for users to extend BGL with
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their own custom visitors. Use the BGL visitors as examples for how to
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implement you own. The visitors in this section are models of <a
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href="./UserVisitor.html">UserVisitor</a>.
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<P>
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<hr>
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<H2><TT>null_visitor</TT></H2>
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<P>
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This is a visitor that does nothing. It is useful as a base class for
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user-defined visitors that only implement some of the visitor
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methods. The unimplemented methods will default to the empty ones in
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the <TT>base_visitor</TT> class. Due to deficiencies in VC++ this class
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can not be used with that compiler. Use <TT>base_visitor</TT> instead
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if you need VC++ portability.
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<P>
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<H3>Members</H3>
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<P>
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<P> <P>
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<PRE>
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null_visitor::category
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</PRE>
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The category for this visitor is <TT>every_edge_tag</TT>.
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<P>
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<P> <P>
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<PRE>
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template <class Vertex>
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void null_visitor::initialize(Vertex u) { }
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</PRE>
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<P>
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<P> <P>
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<PRE>
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template <class Vertex>
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void null_visitor::start(Vertex s) { }
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</PRE>
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<P>
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<P> <P>
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<PRE>
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template <class Vertex>
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void null_visitor::discover(Vertex s) { }
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</PRE>
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<P>
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<P> <P>
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<PRE>
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template <class Vertex>
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void null_visitor::finish(Vertex s) { }
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</PRE>
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<P>
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<P> <P>
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<PRE>
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template <class Edge, class Graph>
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void null_visitor::process(Edge e, Graph& g) { }
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</PRE>
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<P>
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<P>
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<hr>
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<H2><A NAME="SECTION001542000000000000000">
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<TT>base_visitor</TT></A>
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</H2>
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<P>
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<PRE>
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base_visitor<Visitor>
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</PRE>
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<P>
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This is a visitor that does nothing. It is useful as a base class for
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user-defined visitors that only implement some of the visitor
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methods. The unimplemented methods will default to the empty ones in
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the <TT>base_visitor</TT> class. The template argument <TT>Visitor</TT>
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must be the deriving visitor class. This class is a workaround for
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VC++. If VC++ portability is not an issue use <TT>null_visitor</TT>
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instead.
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<P>
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<H3><A NAME="SECTION001542100000000000000">
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Members</A>
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</H3>
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<P>
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<P> <P>
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<PRE>
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base_visitor::category
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</PRE>
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The category for this visitor is <TT>every_edge_tag</TT>.
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<P>
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<P> <P>
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<PRE>
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template <class Vertex>
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void base_visitor::initialize(Vertex u) { }
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</PRE>
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<P>
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<P> <P>
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<PRE>
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template <class Vertex>
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void base_visitor::start(Vertex s) { }
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</PRE>
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<P>
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<P> <P>
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<PRE>
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template <class Vertex>
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void base_visitor::discover(Vertex s) { }
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</PRE>
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<P>
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<P> <P>
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<PRE>
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template <class Vertex>
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void base_visitor::finish(Vertex s) { }
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</PRE>
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<P>
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<P> <P>
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<PRE>
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template <class Edge, class Graph>
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void base_visitor::process(Edge e, Graph& g) { }
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</PRE>
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<P>
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<P>
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<hr>
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<H2><A NAME="SECTION001543000000000000000">
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<TT>distance_visitor</TT></A>
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</H2>
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<P>
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<PRE>
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distance_visitor<Distance>
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</PRE>
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<P>
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The distance visitor records the distance from the starting vertex to
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each vertex in a search tree. The type of search tree depends on the
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algorithm being applied.
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<P>
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<H3>Example</H3>
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<P>
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<H3>Template Parameters</H3>
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<P>
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<TABLE border>
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<TR><TD><TT>Distance</TT></TD>
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<TD>Must be a model of <a
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href="./ReadWritePropertyAccessor.html">ReadWritePropertyAccessor</a>.</TD>
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</TR>
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</TABLE>
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<P>
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<H3>Model of</H3>
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<P>
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<a href="./Visitor.html">Visitor</a>
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<P>
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<H3>Members</H3>
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<P>
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<P> <P>
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<PRE>
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distance_visitor::category
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</PRE>
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The visitor category for this class is <TT>tree_edge_tag</TT>.
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<P>
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<P> <P>
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<PRE>
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distance_visitor::distance_visitor(Distance p)
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</PRE>
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Constructor.
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<P>
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<P> <P>
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<PRE>
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template <class Vertex>
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distance_visitor::initialize(Vertex u)
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</PRE>
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This initializes the distance property for all the vertices
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to <TT>std::numeric_limits<T>::max()</TT> where <TT>T</TT> is
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the value type of the distance property accessor.
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<P>
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<P> <P>
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<PRE>
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template <class Vertex>
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distance_visitor::start(Vertex u)
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</PRE>
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This initializes the distance of the starting vertex to zero.
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<P>
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<P> <P>
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<PRE>
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template <class Edge, class Graph>
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distance_visitor::process(Edge e, Graph& g)
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</PRE>
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This records the distance of for the <TT>target(e,g)</TT> as
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<TT>d[source(e,g)] + 1</TT>.
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<P>
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<P>
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<hr>
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<H2><TT>predecessor_visitor</TT>
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</H2>
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<P>
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<PRE>
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predecessor_visitor<Predecessor>
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</PRE>
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<P>
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The predecessor visitor records the predecessors (or parents) of the
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nodes in a search tree, where the search tree depends on the kind of
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graph search algorithm being applied.
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<P>
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<H3><A NAME="SECTION001544100000000000000">
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Example</A>
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</H3>
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<P>
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<H3><A NAME="SECTION001544200000000000000">
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Template Parameters</A>
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</H3>
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<P>
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<TABLE border>
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<TR><TD><TT>Predecessor</TT></TD>
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<TD>Must be a model of <a
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href="./WritablePropertyAccessor.html">WritablePropertyAccessor</a>.</TD>
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</TR>
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</TABLE>
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<P>
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<H3>Model of</H3>
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<P>
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<a href="./Visitor.html">Visitor</a>
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<P>
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<H3>Members</H3>
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<P>
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<P> <P>
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<PRE>
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predecessor_visitor::category
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</PRE>
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The visitor category for this class is <TT>tree_edge_tag</TT>.
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<P>
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<P> <P>
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<PRE>
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predecessor_visitor::predecessor_visitor(Predecessor p)
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</PRE>
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Constructor.
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<P>
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<P> <P>
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<PRE>
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template <class Edge, class Graph>
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predecessor_visitor::process(Edge e, Graph& g)
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</PRE>
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This method records the predecessor (or parent), where
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<TT>target(e)</TT> is the parent and <TT>source(e)</TT> is the child.
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The implementation is simply <TT>p[target(e,g)] = source(e,g)</TT>.
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<P>
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<hr>
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<H2><TT>timestamp_visitor</TT></H2>
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<P>
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<PRE>
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timestamp_visitor<DiscoverTime,FinishTime>
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</PRE>
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<P>
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The time-stamp visitor is for recording the ``discover'' and
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``finish'' order for vertices during a graph search. The time-stamp
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starts at zero and increments one each time a vertex is discovered or
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finished.
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<P>
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<H3>Example</H3>
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<P>
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<H3>Template Parameters</H3>
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<P>
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<TABLE border>
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<TR><TD><TT>DiscoverTime</TT></TD>
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<TD>Must be a model of <a
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href="./WritablePropertyAccessor.html">WritablePropertyAccessor</a>.</TD>
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</TR>
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<TR><TD><TT>FinishTime</TT></TD>
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<TD>Must be a model of <a
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href="./WritablePropertyAccessor.html">WritablePropertyAccessor</a>.</TD>
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</TR>
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</TABLE>
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<P>
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<H3>Model of</H3>
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<P>
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<a href="./Visitor.html">Visitor</a>
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<P>
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<H3>Members</H3>
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<P>
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<P> <P>
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<PRE>
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timestamp_visitor::category
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</PRE>
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The visitor category for this class is <TT>tree_edge_tag</TT>.
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<P>
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<P> <P>
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<PRE>
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timestamp_visitor::timestamp_visitor(DiscoverTime d, FinishTime f)
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</PRE>
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Constructor.
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<P>
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<P> <P>
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<PRE>
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template <class Vertex>
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timestamp_visitor::discover(Vertex u)
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</PRE>
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This method records the discover time for vertex <TT>u</TT>. The
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implementation is simply <TT>d[u] = ++time</TT>.
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<P>
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<P> <P>
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<PRE>
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template <class Vertex>
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timestamp_visitor::finish(Vertex u)
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</PRE>
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This method records the finish time for vertex <TT>u</TT>. The
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implementation is simply <TT>f[u] = ++time</TT>.
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<P>
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<P>
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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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