../../property_map/doc/ReadWritePropertyMap.html">Read/Write
  Property Map</a>. The vertex descriptor type of the graph needs to
  be usable as the key type of the distance map. 

  The value type of the distance map is the element type of a <a
  href="./Monoid.html">Monoid</a> formed with the <tt>distance_weight_combine</tt>
  function object and the <tt>distance_zero</tt> object for the identity
  element. Also the distance value type must have a <a
  href="http://www.sgi.com/tech/stl/StrictWeakOrdering.