## VertexListGraph

The VertexListGraph concept refines the Graph concept, and adds the requirement for efficient traversal of all the vertices in the graph.

### Refinement of

Graph

### Associated Types

`boost::graph_traits<G>::traversal_category` This tag type must be convertible to `vertex_list_graph_tag`.
`boost::graph_traits<G>::vertex_iterator` A vertex iterator (obtained via `vertices(g)`) provides access to all of the vertices in a graph. A vertex iterator type must meet the requirements of MultiPassInputIterator. The value type of the vertex iterator must be the vertex descriptor of the graph.
`boost::graph_traits<G>::vertices_size_type` The unsigned integer type used to represent the number of vertices in the graph.

### Valid Expressions

Name | Expression | Return Type | Description
--- | --- | --- | ---
Vertex Set of the Graph | `vertices(g)` | `std::pair<vertex_iterator, vertex_iterator>` | Returns an iterator-range providing access to all the vertices in the graph`g`.
Number of Vertices in the Graph | `num_vertices(g)` | `vertices_size_type` | Returns the number of vertices in the graph `g`.

### Complexity guarantees

The `vertices()` function must return in constant time.

### See Also

Graph concepts

### Design Rationale

One issue in the design of this concept is whether to include the refinement from the IncidenceGraph and AdjacencyGraph concepts. The ability to traverse the vertices of a graph is orthogonal to traversing out-edges, so it would make sense to have a VertexListGraph concept that only includes vertex traversal. However, such a concept would no longer really be a graph, but would just be a set, and the STL already has concepts for dealing with such things. However, there are many BGL algorithms that need to traverse the vertices and out-edges of a graph, so for convenience a concept is needed that groups these requirements together, hence the VertexListGraph concept.

### Concept Checking Class

      template <class G>
      struct VertexListGraphConcept
      {
        typedef typename boost::graph_traits<G>::vertex_iterator
          vertex_iterator;
        void constraints() {
          BOOST_CONCEPT_ASSERT(( IncidenceGraphConcept<G> ));
          BOOST_CONCEPT_ASSERT(( AdjacencyGraphConcept<G> ));
          BOOST_CONCEPT_ASSERT(( MultiPassInputIteratorConcept<vertex_iterator> ));

          p = vertices(g);
          V = num_vertices(g);
          v = *p.first;
          const_constraints(g);
        }
        void const_constraints(const G& g) {
          p = vertices(g);
          V = num_vertices(g);
          v = *p.first;
        }
        std::pair<vertex_iterator, vertex_iterator> p;
        typename boost::graph_traits<G>::vertex_descriptor v;
        typename boost::graph_traits<G>::vertices_size_type V;
        G g;
      };

Copyright © 2000-2001 | Jeremy Siek, Indiana University (jsiek@osl.iu.edu)