#

    predecessor_recorder<PredecessorMap, EventTag>

This is an EventVisitor that records the predecessor (or parent) of a vertex in a predecessor 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 predecessor recorder is typically used with the `on_tree_edge` or `on_relax_edge` events and cannot be used with vertex events.

`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 to the source vertex (which is the root of the search tree). It is often useful to initialize the source vertex's predecessor to itself, thereby identifying the root vertex as the only vertex which is its own parent. When using an algorithm like depth-first search that creates a forest (multiple search trees) it is useful to initialize the predecessor of every vertex to itself. This way all the root nodes can be distinguished.

### Example

See the example for `bfs_visitor`.

### Model of

EventVisitor

### Where Defined

`boost/graph/visitors.hpp`

### Template Parameters

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

## Associated Types

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

### Member Functions

Member | Description
--- | ---
`predecessor_recorder(PredecessorMap pa);` | Construct a 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 u as the predecessor (or parent) of v.

### Non-Member Functions

Function | Description
--- | ---
`template <class PredecessorMap, class Tag> predecessor_recorder<PredecessorMap, Tag> record_predecessors(PredecessorMap pa, Tag);` | A convenient way to create a `predecessor_recorder`.

### See Also

Visitor concepts

The following are other event visitors: <a `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)