# AStar Heuristic Concept

This concept defines the interface for the heuristic function of an A* search, which is responsible for estimating the remaining cost from some vertex to a goal. The user can create a class that matches this interface, and then pass objects of the class into `astar_search()` to guide the order of vertex examination of the search. The heuristic instance must incorporate any necessary information about goal vertices in the graph. For further discussion of the use of heuristics in an A* search, see the documentation of astar_search().

### Refinement of

Unary Function (must take a single argument -- a graph vertex -- and return a cost value) and Copy Constructible (copying a heuristic should be a lightweight operation).

### Notation

`H` | A type that is a model of AStar Heuristic.
`h` | An object of type `H`.
`G` | A type that is a model of Graph.
`g` | An object of type `G`.
`u` | An object of type `boost::graph_traits<G>::vertex_descriptor`.
`CostType` | A type that can be used with the `compare` and `combine` functions passed to A*.
`c` | An object of type `CostType`.

### Associated Types

none

### Valid Expressions

Name | Expression | Return Type | Description
--- | --- | --- | ---
Call Heuristic | `CostType c = h(u)` | `CostType` | Called for the target of every out edge of a vertex being examined.

### Models

- `astar_heuristic`

### Concept Checking Class

      template <class Heuristic, class Graph>
      struct AStarHeuristicConcept {
        void constraints()
        {
          function_requires< CopyConstructibleConcept<Heuristic> >();
          h(u);
        }
        Heuristic h;
        typename graph_traits<Graph>::vertex_descriptor u;
      };

Copyright © 2004 | Kristopher Beevers, Rensselaer Polytechnic Institute (beevek@cs.rpi.edu)