Hi Johnny, On Apr 2, 2004, at 8:01 PM, Johnny Yune wrote:
That seems like an intuitive way for the back-end to work. Why should I, as a client, need to concern myself with those details, though?
Because you need to choose which data structure has the right time/space efficiency tradeoffs for the problem you are trying to solve. (I could imagine a graph library that makes that decision for the user, however, I don't think the field of artificial intelligence has progress far enough to make that possible) Of course, after you've chosen which graph type you want, the BGL hides the implementation details from you. That's part of the job of the vertex_descriptor.
Actually, what I really wanted clarification on was why the task of managing this mapping was left to the client instead of the underlying graph structure.
I'm not really sure what you mean by "the task of managing". Could you clarify what that consists of? In the BGL, there are two kinds of property maps, internal and external. Internal properties are managed by the graph object, in the sense that the lifetime of the property objects is tied to the lifetime of the graph object. The client obtains the internal property map from the graph, using one of the get() functions. External properties are not managed by the graph object, but created by the client. Have you tried using internal properties, or only external? Perhaps the internal property maps would fulfill what you are looking for? BTW, it is possible to bypass using the property map when using adjacency_list. Here's the syntax: get(vertex_color_t(), g, v)
The differences in syntax, above, are only indications of my confusion about the semantics. To wit, given your examples, I wouldnt necessarily expect v->color to have anything to do with the color_map. v->color looks like a property of a vertex, while get(color_map, v) looks like a property of a graph.
Both access the property of a vertex.
(not to mention the trouble of setting up the
color_map > to begin with)
Well, this does make things a lot more complex. The need for such mappings is quite clear to me the need for the client to set them up as opposed to the graph is unclear to me, though. If I were to ask an algorithm to generate a chromatic number over a graph, I could envision having a property map of some possible coloring returned to me. Why I would have to set one up or pass one in, though, I am unclear on.
Yes, that would seem annoying. That is why the BGL provides defaults for the property map parameters in many of the algorithms. Usually they default to asking the graph object for an internal property map. So the client only needs to pass in the property map separately when the graph object does not provide it.
In reviewing my post, I am realizing just how astute you were to identify the property map as a major source of confusion for me I am only beginning to be able to articulate why it feels unnatural for me. I guess the big question, then, is why use property maps instead of functors?
I fail to see how functors could be used to solve this problem. Perhaps you could explain in more detail.
Thanks, again, for entertaining my questions. I am highly pleased to be soliciting rationale and intent instead of the rabid defensiveness or dismissal that I probably deserve.
You're welcome. Cheers, Jeremy _______________________________________________ Jeremy Siek <jsiek@osl.iu.edu> http://www.osl.iu.edu/~jsiek Ph.D. Student, Indiana University Bloomington Graduating in August 2004 and looking for work C++ Booster (http://www.boost.org) _______________________________________________