Re: [Boost-users] Could the BGL graph algorithms be improved to automatically work with VertexList = listS graphs?

From: Douglas Gregor (doug.gregor_at_[hidden])
Date: 2005-09-14 21:01:55

On Sep 14, 2005, at 7:26 PM, Jeremy Siek wrote:
> Part of the problem is the language (C++), not the library. However,
> we may be able to fix this by going to extraordinary
> lengths. Though I can't say when one of the developers will have the
> time to do this. It's quite tedious trying to imagine
> all the ways an algorithm can be misused and try to trick the
> compiler into giving a good error message.
> This is one of the reasons I wrote a Ph.D. thesis about a new  
> language.

And Jeremy neglected to mention that we're trying *very* hard to get  
some extensions into C++0x so that wretched error messages like these  
will be a thing of the past.

>> What do to BGL experts/implementers say? Would it not be **saner**
>> to get these algorithms to work automatically no matter what graph
>> type was chosen?
>>
> The problem with std::map is that it does not provide constant time
> access to the vertex index
> (it is logarithmic). Thus, the graph algorithm will run slower; it
> won't have the advertised
> time complexity. Then we'll get bug reports from people that are
> surprised by how slow the
> algorithm is.

It's definitely slower, but I think we need to give the usability  
issue more weight. I only recall ever having seen two cases where  
someone complained about the BGL being to slow: one was with  
betweenness_centrality, until we learned that the person had compiled  
without any optimizations; the other was with bundled properties  
slowing things down (due to the extra level of dispatch).  On the  
other hand, we get questions every week about how to create property  
maps, especially edge property maps (because there's no way to avoid  
managing your own edge_index_t). We should get users hooked on the  
BGL first, then we can worry about getting maximal performance for  
them later.

I've been thinking of a few ways to try to help users with creating  
and using property maps. It's possible that "all of them" is the  
right answer:

     1) Make edge descriptors ordered (via <), so that users can make  
associative property maps more easily

     2) Create templates vertex_property_map<T, Graph> and  
edge_property_map<T, Graph> that create external property maps  
without much effort from the user; in particular, that'll use an  
associative property map or an iterator property map depending on  
whether a vertex_index property is available.

     3) As Tony mentions, make the algorithms a bit more lenient  
about the input graphs. They could use something like the class  
templates in #2, so long as we can still get maximal efficiency out  
of them when index maps are available.

     4) Create new graph types (say, directed_graph<Vertex, Edge> and  
undirected_graph<Vertex, Edge>) that maintain vertex and edge index  
maps internally (as does the Python graph type).

     Doug