# Visitor Concepts

The visitor concepts plays the same role in BGL as functors play in the STL. Functors provide a mechanism for extending an algorithm; for customizing what is done at each step of the algorithm. Visitors allow the user to insert their own operations at various steps within a graph algorithm. Unlike the STL algorithms, graph algorithms typically have multiple event points where one may want to insert a call-back via a functor. Therefore visitors do not have a single `operator()` method like a functor, but instead have several methods that correspond to the various event points. Each algorithm has a different set of event points, which are described by the following visitor concepts:

- BFS Visitor
- DFS Visitor
- Uniform Cost Visitor
- Bellman Ford Visitor
- Event Visitor

Copyright © 2000 | Jeremy Siek, Univ.of Notre Dame (jsiek@lsc.nd.edu) Lie-Quan Lee, Univ.of Notre Dame (llee1@lsc.nd.edu) Andrew Lumsdaine, Univ.of Notre Dame (lums@lsc.nd.edu)