Re: [Boost-users] [BGL] using biconnected_components with an exteriorproperty map

From: Doug Gregor (dgregor_at_[hidden])
Date: 2005-09-06 15:24:55

On Aug 24, 2005, at 10:56 AM, Greg Landrum wrote:

> Janusz Piwowarski wrote:
>> Greg Landrum wrote:
>>> [..]
>>> So my question is: how can I make a call to biconnected_components in
>>> a manner that doesn't modify my graph (or any of its associated
>>> property maps)?
>>
>> If you want to use external property map for edges, you must use 
>> edge_index
>> instead of edge_descriptor. But, you must provide edge_index property 
>> by
>> yourself. See exterior_properties.cpp for example.
>
> The example in exterior_properties.cpp seems to require that the graph
> have a boost::edge_index_t property map associated with its edges. If
> this is true, it means that in order to call biconnected_components on 
> a
> graph that truly must be const, I need to modify the definition of my
> graph. Is this truly intended?

Yes and no.

It is intended because there is no way to efficiently access a property 
map on edges without having an edge_index property or storing it the 
property on the edges themselves. The unintended consequence is that 
you have to modify your graph to use edge properties or create a 
special kind of property map that isn't entirely efficient.

I'm hoping we can make exterior property maps for edges easier to use 
in the future, but for now you have only a few choices. Add an 
edge_index property (be sure to renumber the edges when you change the 
graph!), add properties directly to the edges in the graph, or use an 
associative property map.

The problem (and, thus, the answer to your original question) is that 
edges can't be compared using "<", so the std::map fails. We can define 
a not-quite-correct-but-still-works function object for std::map like 
this:

struct order_edges
{
   template<typename T>
   bool operator()(const T& x, const T& y) const
   {
     return x.get_property() < y.get_property();
   }
};

Pass this as the third argument to std::map and the 
associative_property_map would work fine:

   typedef std::map< graph_traits<graph_t>::edge_descriptor, int,
                     order_edges> map_t;
   map_t base_map;
   associative_property_map< map_t > component(base_map);

Using this code, I was able to get the biconnected_components example 
working with exterior property maps.

        Doug