#

    edge_predecessor_recorder<PredEdgeMap, EventTag>

This is an EventVisitor that records the predecessor (or parent) edge of a vertex in a property map. This is particularly useful in graph search algorithms where recording the predecessors is an efficient way to encode the search tree that was traversed during the search. The edge predecessor recorder is typically used with the `on_tree_edge` or `on_relax_edge` events and cannot be used with vertex events. This visitor is meant to be used instead of `predecessor_recorder` when a graph has parallel edges and it is necessary to know which incoming edge a search algorithm used to get to a particular vertex.

`edge_predecessor_recorder` can be used with graph algorithms by wrapping it with an algorithm-specific adaptor, such as `bfs_visitor` and `dfs_visitor`. Also, this event visitor can be combined with other event visitors using `std::pair` to form an EventVisitorList.

Algorithms such as Dijkstra's and breadth-first search will not assign a predecessor edge to the source vertex (which is the root of the search tree). It is often useful to initialize the source vertex's predecessor to a special value, thereby identifying the root vertex. When using an algorithm like depth-first search that creates a forest (multiple search trees) it is useful to do the same for every vertex. This way all the root nodes can be distinguished.

### Model of

EventVisitor

### Where Defined

`boost/graph/visitors.hpp`

### Template Parameters

Parameter | Description | Default
--- | --- | ---
`PredEdgeMap` | A WritablePropertyMap where the key and value types are the vertex and edge descriptor types, respectively, of the graph. |
`EventTag` | The tag to specify when the `edge_predecessor_recorder` should be applied during the graph algorithm. `EventTag` must be an edge event. |

## Associated Types

Type | Description
--- | ---
`edge_predecessor_recorder::event_filter` | This will be the same type as the template parameter `EventTag`.

### Member Functions

Member | Description
--- | ---
`edge_predecessor_recorder(PredEdgeMap pa);` | Construct an edge predecessor recorder object with predecessor property map `pa`.
`template <class Edge, class Graph> void operator()(Edge e, const Graph& g);` | Given edge e = (u,v), this records e as the predecessor (or parent) edge of v.

### Non-Member Functions

Function | Description
--- | ---
`template <class PredEdgeMap, class Tag> edge_predecessor_recorder<PredEdgeMap, Tag> record_edge_predecessors(PredEdgeMap pa, Tag);` | A convenient way to create a `edge_predecessor_recorder`.

### See Also

Visitor concepts

The following are other event visitors: `distance_recorder`, `time_stamper`, and `property_writer`.

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)