>
<blockquote>
  Maps each vertex of the graph to a unique integer in the range
  <tt>[0, num_vertices(g))</tt>. This property map is only needed
  if the default for the distance or predecessor map is used.
  The vertex index map must be a model of <a
  href="../../property_map/doc/ReadablePropertyMap.html">Readable Property
  Map</a>. The key type of the map must be the graph's vertex
  descriptor type.<br>
  <b>Default:</b> <tt>get(vertex_index, g)</tt>
    Note: if you use this default, make sure your g