# Bundled Properties

Class templates `adjacency_list` and `adjacency_matrix` support the introduction of named properties via internal properties. However, this method is cumbersome in many uses, where it would be more intuitive to just specify a structure or class that contains internal properties for edges or vertices. Bundled properties allow one to use `adjacency_list` and `adjacency_matrix` in this manner, providing a simple way to introduce and access any number of internal properties for vertices and edges.

One can introduce bundled properties into an either graph type by providing a user-defined class type for the `VertexProperties` or `EdgeProperties` template arguments. The user-defined class may alternatively be placed at the end of a `property` list, replacing the (implicit) `boost::no_property` argument.

## Example: Route planning

Consider the implementation of a simple route planner that should find the shortest directions from one city to another via a set of highways. The vertices of the graph are cities, and we may wish to store several bits of information about the city within each vertex:

    struct City
    {
      string name;
      int population;
      vector<int> zipcodes;
    };

The edges in the graph represent highways, which also have several interesting attributes:

    struct Highway
    {
      string name;
      double miles;
      int speed_limit;
      int lanes;
      bool divided;
    };

Without bundled properties, translating this example directly into an instantiation of `adjacency_list` would involve several custom properties and would result in a type like this:

    typedef boost::adjacency_list<
        boost::listS, boost::vecS, boost::bidirectionalS,
        // Vertex properties
        boost::property<boost::vertex_name_t, std::string,
        boost::property<population_t, int,
        boost::property<zipcodes_t, std::vector<int> > > >,
        // Edge properties
        boost::property<boost::edge_name_t, std::string,
        boost::property<boost::edge_length_t, double,
        boost::property<edge_speed_limit_t, int,
        boost::property<edge_lanes_t, int,
        boost::property<edge_divided, bool> > > > > >
      Map;

With bundled properties, we can directly use the `City` and `Highway` structures:

    typedef boost::adjacency_list<
        boost::listS, boost::vecS, boost::bidirectionalS,
        City, Highway> Map;

## Accessing bundled properties

To access a bundled property for a particular edge or vertex, subscript your graph with the descriptor of the edge or vertex whose bundled property you wish to access. For instance:

    Map map; // load the map
    Map::vertex_descriptor v = *vertices(map).first;
    map[v].name = "Troy";
    map[v].population = 49170;
    map[v].zipcodes.push_back(12180);
    Map::edge_descriptor e = *out_edges(v, map).first;
    map[e].name = "I-87";
    map[e].miles = 10;
    map[e].speed_limit = 65;
    map[e].lanes = 4;
    map[e].divided = true;

## Properties maps from bundled properties

Often one needs to create a property map from an internal property for use in a generic algorithm. For instance, using the graph without bundled properties we might invoke Dijkstra's shortest paths algorithm like this:

    vector<double> distances(num_vertices(map));
    dijkstra_shortest_paths(map, from,
          weight_map(get(edge_length, map))
          .distance_map(make_iterator_property_map(distances.begin(),
                                                   get(vertex_index, map))));

With bundled properties, we can just pass a member pointer as the property for `get`. The equivalent example using bundled properties is:

    vector<double> distances(num_vertices(map));
    dijkstra_shortest_paths(map, from,
          weight_map(get(&Highway::miles, map))
          .distance_map(make_iterator_property_map(distances.begin(),
                                                   get(vertex_index, map))));

The type of the returned property map is `property_map<Map, int Highway::*>::type` or `property_map<Map, int Highway::*>::const_type`, depending on whether the graph `map` is non-constant or constant.

You may also access the entire vertex or edge bundle as a property map using the `vertex_bundle` or `edge_bundle` properties, respectively. For instance, the property map returned by `get(vertex_bundle, map)` is an Lvalue Property Map providing access to the `City` values stored in each vertex.

## Compatibility

Bundled properties will only work properly on compilers that support class template partial specialization.

Copyright © 2004 Doug Gregor.
Last modified: Fri May 7 10:56:01 EDT 2004