Re: [Boost-users] boost.graph question concerning edge descriptor details

23 Aug 2004

      I may have answered my own question.
The Compare functor that does the trick is:
  struct ltExternalEdge : public 
std::binary_function {
    bool operator()(const ExternalEdge& __x, const ExternalEdge& __y) const
    {
    return (__x.first < __y.first || (!(__y.first < __x.first) && 
__x.second.get_property() < __y.second.get_property())) ? true : false;
    }
  };

Is such a Compare operator safe?  Note that I'm note using internal 
properties.

It would be really nice if the object that edge_iterator references
implemenented a operator< method.
Then it would work seamlessly with the std::pair operator< method, which
I pretty much copied in the above code.

Doug Gregor wrote:
...
On Aug 20, 2004, at 3:53 PM, Jeff Holle wrote:
...
I'm using gcc v3.4.1 and boost_CVS captured on Tue Jun 29 05:25:17 
EDT 2004.
I'm developing a boost.graph application that must deal with multiple 
graphs.
The following typedefs describe how its using boost.graph:
 typedef 
boost::adjacency_listboost::vecS,boost::vecS,boost::bidirectionalS 
GraphT;
[snip]
...
I've used ExternalVertex successfully in two different maps.
Since the vertices are stored in an std::vector, the vertex descriptor 
is actually an unsigned integer in [0:num_vertices(g)).
...
Now I'm having trouble populating the first map that uses ExternalEdge.
The compiler warnings that I'm getting sugguest that Edge does have 
an "operator<" method.  Specifically its:
 error: no match for 'operator<' in '__x->std::pair::second < __y->std::pair::second'
Right. Edge descriptors do not have a < operator.
...
I'm wondering why Edge is different from Vertex.
It's not always clear how to order edges, nor is it clear how to order 
vertices except when the vertices are stored in an std::vector.
...
Any guidance that assists me to, at worst case, write a Compare 
operator for my "map" containers would be appreciated.
You have a few options here:
    1) You could just add the property as an internal property
    2) You could add (and maintain!) an edge_index internal property 
and order based on that
    3) If you don't have parallel edges, you could order based on 
std::make_pair(source(e, g), target(e, g)).
Doug
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