Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. — Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

BFS Visitor Concept

This concept defines the visitor interface for breadth_first_search(). Users can define a class with the BFS Visitor interface and pass and object of the class to breadth_first_search(), thereby augmenting the actions taken during the graph search.

Refinement of

Copy Constructible (copying a visitor should be a lightweight operation).

Notation

`V`	A type that is a model of BFS Visitor.
`vis`	An object of type `V`.
`G`	A type that is a model of Graph.
`g`	An object of type `G`.
`e`	An object of type `boost::graph_traits<G>::edge_descriptor`.
`s,u`	An object of type `boost::graph_traits<G>::vertex_descriptor`.

Associated Types

none

Valid Expressions

Name	Expression	Return Type	Description
Initialize Vertex	`vis.initialize_vertex(s, g)`	`void`	This is invoked on every vertex of the graph before the start of the graph search.
Discover Vertex	`vis.discover_vertex(u, g)`	`void`	This is invoked when a vertex is encountered for the first time.
Examine Vertex	`vis.examine_vertex(u, g)`	`void`	This is invoked on a vertex as it is popped from the queue. This happens immediately before `examine_edge()` is invoked on each of the out-edges of vertex `u`.
Examine Edge	`vis.examine_edge(e, g)`	`void`	This is invoked on every out-edge of each vertex after it is discovered.
Tree Edge	`vis.tree_edge(e, g)`	`void`	This is invoked on each edge as it becomes a member of the edges that form the search tree.
Non-Tree Edge	`vis.non_tree_edge(e, g)`	`void`	This is invoked on back or cross edges for directed graphs and cross edges for undirected graphs.
Gray Target	`vis.gray_target(e, g)`	`void`	This is invoked on the subset of non-tree edges whose target vertex is colored gray at the time of examination. The color gray indicates that the vertex is currently in the queue.
Black Target	`vis.black_target(e, g)`	`void`	This is invoked on the subset of non-tree edges whose target vertex is colored black at the time of examination. The color black indicates that the vertex has been removed from the queue.
Finish Vertex	`vis.finish_vertex(u, g)`	`void`	This invoked on a vertex after all of its out edges have been added to the search tree and all of the adjacent vertices have been discovered (but before the out-edges of the adjacent vertices have been examined).

Models

bfs_visitor

Python

To implement a model of the BFSVisitor concept in Python, create a new class that derives from the BFSVisitor type of the graph, which will be named GraphType.BFSVisitor. The events and syntax are the same as with visitors in C++. Here is an example for the Python bgl.Graph graph type:

class count_tree_edges_bfs_visitor(bgl.Graph.BFSVisitor):
  def __init__(self, name_map):
    bgl.Graph.BFSVisitor.__init__(self)
    self.name_map = name_map

  def tree_edge(self, e, g):
    (u, v) = (g.source(e), g.target(e))
    print "Tree edge ",
    print self.name_map[u],
    print " -> ",
    print self.name_map[v]