DFS Visitor Concept
This concept defines the visitor interface for depth_first_search().
Users can define a class with the DFS Visitor interface and pass an
object of the class to depth_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 DFS 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.
|
Start Vertex |
vis.start_vertex(s, g) |
void |
This is invoked on the source vertex once before the start of the
search.
|
Discover Vertex |
vis.discover_vertex(u, g) |
void |
This is invoked when a vertex is encountered for the first time.
|
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. |
Back Edge |
vis.back_edge(e, g) |
void |
This is invoked on the back edges in the graph. For an undirected
graph there is some ambiguity between tree edges and back edges since
the edge (u,v) and (v,u) are the same edge, but both the
tree_edge() and back_edge() functions will be
invoked. One way to resolve this ambiguity is to record the tree
edges, and then disregard the back-edges that are already marked as
tree edges. An easy way to record tree edges is to record
predecessors at the tree_edge event point.
|
Forward or Cross Edge |
vis.forward_or_cross_edge(e, g) |
void |
This is invoked on forward or cross edges in the graph. In an
undirected graph this method is never called.
|
Finish Vertex |
vis.finish_vertex(u, g) |
void |
This is invoked on vertex u after finish_vertex has
been called for all the vertices in the DFS-tree rooted at vertex
u. If vertex u is a leaf in the DFS-tree, then
the finish_vertex function is call on u after
all the out-edges of u have been examined.
|
Models
Python
To implement a model of the DFSVisitor concept in Python,
create a new class that derives from the DFSVisitor type of
the graph, which will be
named GraphType.DFSVisitor. 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_dfs_visitor(bgl.Graph.DFSVisitor):
def __init__(self, name_map):
bgl.Graph.DFSVisitor.__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]
|