# EventVisitor Concept

This concept defines the interface for single-event visitors. An EventVisitor has an apply member function (`operator()`) which is invoked within the graph algorithm at the event-point specified by the `event_filter` typedef within the EventVisitor. EventVisitor's can be combined into an EventVistorList.

The following is the list of event tags that can be invoked in BGL algorithms. Each tag corresponds to a member function of the visitor for an algorithm. For example, the BFSVisitor of `breadth_first_search()` has a `cycle_edge()` member function. The corresponding tag is `on_cycle_edge`. The first argument is the event visitor's `operator()` must be either an edge or vertex descriptor depending on the event tag.

    namespace boost {
      struct on_initialize_vertex { };
      struct on_start_vertex { };
      struct on_discover_vertex { };
      struct on_examine_edge { };
      struct on_tree_edge { };
      struct on_cycle_edge { };
      struct on_finish_vertex { };
      struct on_forward_or_cross_edge { };
      struct on_back_edge { };
      struct on_edge_relaxed { };
      struct on_edge_not_relaxed { };
      struct on_edge_minimized { };
      struct on_edge_not_minimized { };
    } // namespace boost

### Refinement of

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

### Notation

`G` | A type that is a model of Graph.
`g` | An object of type `G`.
`V` | A type that is a model of EventVisitor.
`vis` | An object of type `V`.
`x` | An object of type `boost::graph_traits<G>::vertex_descriptor` or `boost::graph_traits<G>::edge_descriptor`.

### Associated Types

Event Filter | `V::event_filter` | A tag struct to specify on which event the visitor should be invoked.

### Valid Expressions

Name | Expression | Return Type | Description
--- | --- | --- | ---
Apply Visitor | `vis(x, g)` | `void` | Invokes the visitor operation on object `x`, which is either a vertex or edge descriptor of the graph.

### Models

- `predecessor_recorder`
- `distance_recorder`
- `time_stamper`
- `property_writer`
- `null_visitor`

### See Also

EventVisitorList, Visitor concepts

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)