# 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 called on `u` after all the out-edges of `u` have been examined.

### Models

- `dfs_visitor`

### 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]

Copyright © 2000-2001 | Jeremy Siek, Indiana University (jsiek@osl.iu.edu) Lie-Quan Lee, Indiana University (llee@cs.indiana.edu) Andrew Lumsdaine, Indiana University (lums@osl.iu.edu)