On Nov 19, 2005, at 12:12 PM, Aaron Windsor wrote:
Auto index property maps are external property maps that were created with ease of use in mind. In short, whenever a vertex index map is expected for the graph g, you can substitute the expression "make_auto_vertex_index(g)" and everything will work as expected. Similarly, if an edge index property map is expected, you can use the expression "make_auto_edge_index(g)" instead.
Cool!
It's my hope that the auto index property maps will provide a quick solution to those looking to get started with the BGL who aren't yet worried about delving into the details of working with internal properties, which seems to be a difficult issue for a lot of beginners. Auto index property maps can be used while prototyping and replaced later with interior properties if efficiency is an issue.
This will definitely be very helpful. I've only had a few minutes to look over this, so I only have two questions on the code itself: 1) edge_less_than seems more complicated than it needs to be. Instead of creating an integer inside edge_to_index_dispatch, then comparing the integers for two edges to order them, why not just have edge_less_than produce an ordering itself? That would avoid having to store the number of vertices in the edge_less_than predicate. 2) For the auto_index_property_map that allows duplicate keys, was there any particular reason to use a map of vectors instead of a multimap? I also have some higher-level, non-code comments. The primary concern I have is that we're adding more functionality to the BGL to make it easier. It's the same thing we did with bundled properties: add a new, easier-to-use mechanism on top of what we already have. Unfortunately, features interact and I'm not entirely sure that we've managed to make life easier overall. Auto-indexing maps are a great feature, but overall will they make it easier to learn and use the BGL? They will make some things easier: when users ask "why can't I call this algorithm with my adjacency_list?" we'll have the simple answer of "add edge_index(auto_edge_index_map(g))", followed by the obligatory "if you find that it's too slow, do this other thing" comment. It seems that the way to make the BGL easier to use would be to make auto-indexing property maps automatic. When a BGL algorithm tries to pull out a vertex_index map, it checks the parameter list, then the graph itself, then falls back to generating an auto-indexing map. This would be convenient, but it also means that there are hidden performance penalties, which we've tried to avoid in the BGL. What to do? It's becoming more and more important to make the BGL easier to use out-of-the-box. I even think that most users will understand if at a later point in time they need to do a little work to get their code to give maximum performance, but we need to give them the tools to do so. For instance, I can imagine a macro BOOST_GRAPH_PERFORMANCE_WARNINGS that produces run-time warnings when the library is secretly building an auto-indexing map behind-the-scenes and a macro BOOST_GRAPH_PERFORMANCE_WARNINGS_ARE_ERRORS that causes compile-time errors instead. But for now, I think once I understand (1) and (2), the auto-indexing property maps should go into CVS HEAD and we can discuss just how automatic we want to make them. Doug