#

    graph_traits<Graph>

Just like the iterators of STL, graphs have associated types. As stated in the various graph concepts, a graph has quite a few associated types: `vertex_descriptor`, `edge_descriptor`, `out_edge_iterator`, etc.. Any particular graph concepts will not require that all of the following associated types be defined. When implementing a graph class that fullfils one or more graph concepts, for associated types that are not required by the concepts, it is ok to use `void` as the type (when using nested typedefs inside the graph class), or to leave the typedef out of the `graph_traits` specialization for the graph class. Note that because of the use of traits classes rather than member types, it is not safe (and often will not work) to define subclasses of BGL graph types; those types may be missing important traits and properties that were defined externally to the class definition.

      template <typename Graph>
      struct graph_traits {
        typedef typename Graph::vertex_descriptor      vertex_descriptor;
        typedef typename Graph::edge_descriptor        edge_descriptor;
        typedef typename Graph::adjacency_iterator     adjacency_iterator;
        typedef typename Graph::out_edge_iterator      out_edge_iterator;
        typedef typename Graph::in_edge_iterator       in_edge_iterator;
        typedef typename Graph::vertex_iterator        vertex_iterator;
        typedef typename Graph::edge_iterator          edge_iterator;

        typedef typename Graph::directed_category      directed_category;
        typedef typename Graph::edge_parallel_category edge_parallel_category;
        typedef typename Graph::traversal_category     traversal_category;

        typedef typename Graph::vertices_size_type     vertices_size_type;
        typedef typename Graph::edges_size_type        edges_size_type;
        typedef typename Graph::degree_size_type       degree_size_type;
      };

### Where Defined

`boost/graph/graph_traits.hpp`

### Template Parameters

Parameter | Description
--- | ---
`Graph` | The graph type whose associated types are being accessed.

### Model of

DefaultConstructible and Assignable

### Type Requirements

- `Graph` is a model of one of the graph concepts.

## Members

Member | Description
--- | ---
`vertex_descriptor` | The type for the objects used to identity vertices in the graph.
`edge_descriptor` | The type for the objects used to identity edges in the graph.
`adjacency_iterator` | The type for the iterators that traverse the vertices adjacent to a vertex.
`out_edge_iterator` | The type for the iterators that traverse through the out-edges of a vertex.
`in_edge_iterator` | The type for the iterators that traverse through the in-edges of a vertex.
`vertex_iterator` | The type for the iterators that traverse through the complete vertex set of the graph.
`edge_iterator` | The type for the iterators that traverse through the complete edge set of the graph.
`directed_category` | This says whether the graph is undirected (`undirected_tag`) or directed (`directed_tag`).
`edge_parallel_category` | This says whether the graph allows parallel edges to be inserted (`allow_parallel_edge_tag`) or if it automatically removes parallel edges (`disallow_parallel_edge_tag`).
`traversal_category` | The ways in which the vertices in the graph can be traversed. The traversal category tags are: `incidence_graph_tag, adjacency_graph_tag, bidirectional_graph_tag, vertex_list_graph_tag, edge_list_graph_tag, vertex_and_edge_list_graph_tag, adjacency_matrix_tag`. You can also create your own tag which should inherit from one of the above.
`vertices_size_type` | The unsigned integer type used for representing the number of vertices in the graph.
`edge_size_type` | The unsigned integer type used for representing the number of edge in the graph.
`degree_size_type` | The unsigned integer type used for representing the degree of vertices in the graph.

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)