Comments inline, commentary at bottom. David Abrahams wrote:
on Sun Aug 17 2008, Raindog <raindog-AT-macrohmasheen.com> wrote:
Hello,
This will be the 2nd time I've tried to use boost::graph in a project, the first time I ended up scrapping the effort and using a different graph implementation because of performance reasons,
Note that it is one of the BGL's main features that if for any reason you need your own graph data structure, you can still use BGL's algorithms by retroactively and non-intrusively adapting your structure to the BGL's graph concepts. So from what you're saying above I wouldn't necessarily conclude that you couldn't use BGL.
but I was also considering scrapping it for the reasons I have listed here. My newest project will be using much smaller graphs. First, I *want* to use bgl,
Just curious: why? What do you value about this library?
I value boost because of its general high quality and I did not necessarily want to bring more dependencies to my project. Additionally I wanted to use a well known library that others I may work with might use or already be familiar with.
the problem I have is that it is by far the most complex and difficult to use library I have ever seen. I equate it to something harder than COM using only C.
The majority of my difficulties with BGL are related to the extensive use of template magic that does two things for me: makes code impossible to follow and secondly, it makes debugging even more difficult. For example, someone tell me from looking at this, what *values* will actually be passed to astar_search:
astar_search (g, s, h, choose_param(get_param(params, graph_visitor), make_astar_visitor(null_visitor())), choose_param(get_param(params, vertex_predecessor), p_map), cost, distance, weight, index_map, color, choose_param(get_param(params, distance_compare_t()), std::less<C>()), choose_param(get_param(params, distance_combine_t()), closed_plus<C>()), choose_param(get_param(params, distance_inf_t()), std::numeric_limits<C>::max BOOST_PREVENT_MACRO_SUBSTITUTION ()), choose_param(get_param(params, distance_zero_t()), C())); }
I would be hard pressed to believe that anyone can answer that in a short amount of time even assuming you have memorized all of the template parameters of g and s and even h.
The answer to that depends on how you called astar_search in the first place. Show me your call, and I can easily tell you the answer. But I wouldn't need to know the answer in order to use the library.
Anyway, if tracing that code (which is an implementation detail of the graph library) bothers you, then you have several choices other than dropping the BGL again:
1. don't used the named-parameter interface. Just use the overload of astar_search at http://svn.boost.org/trac/boost/browser/trunk/boost/graph/astar_search.hpp#L...
It just occurred to me what all that hubbub does now, which is far more understandable, but still increases debugging difficulty simply because of all the parameters that must be passed to the function.
2. Keep using the named parameter interface and set a breakpoint at the line I indicated. It is eventually called by the code you're quoting anyway.
In any case, if you want to be effective with Boost libraries I strongly suggest you don't use reading their implementation code as the primary way of understanding how to use them. Most Boost libraries are very carefully constructed so that they are usable just by reading the documentation.
I'm able to use every other boost library that I have tried quite effectively, and they've assisted me greatly in solving various problems in an efficient manner.
On with more issues.
One of the biggest issues I have is that the the property_map concept is exceptionally vague
I would like to know in what way it is vague. You don't say below.
and exceedingly difficult to work with. For example, I have created a graph, it's concise typedef:
typedef boost::adjacency_list < boost::vecS, boost::vecS, boost::directedS, boost::property<boost::vertex_name_t, Ogre::Vector3>, boost::property<boost::edge_weight3_t, edge_cost> > GraphT;
You might consider whether bundled properties would save you some effort here (http://www.boost.org/doc/libs/1_35_0/libs/graph/doc/bundles.html). I'm not sure they would, on reflection, but it might be worth looking into.
I had looked at the bundled properties and had at the time decided to write my own property which took me a very long time to do.
Because of the use of free functions, it's hard to quickly distinguish "get" calls on a property_map from "get" calls on a graph.
The use of the same name for both functions is an annoying design decision (partially my fault for encouraging it way back when), but I don't think using member functions would make a huge difference. One of the reasons it's hard to understand is that the latter kind of "get" is just a special "convenience" feature of boost::graph::adjacency_list, so you can't really use it in generic algorithms, while the former is actually part of the property map concept.
In any case, the use of free functions in generic libraries is essential for allowing the non-intrusive concept adaptation I mentioned above.
I must be missing something here, as concepts to me seem just like policies. In Loki, where I first learned of policies, policies are required to implement certain member functions, such as Clone and Release for the Ownership policy. Perhaps there is some lookup magic that I don't understand that requires free functions.
Additionally, the property_map installation and retrieval to and from a BGL graph is also hard to follow. One might see the following code in the examples directory of bgl:
typename property_map<Graph, edge_mycapacity_t>::const_type capacity = get(edge_mycapacity, G); typename property_map<Graph, edge_myflow_t>::const_type flow = get(edge_myflow, G);
Where "edge_mycapacity" is an enum; and how exactly enums of the same value can be used to look up different property_maps from the graph is a gaping hole left in the documentation and I still have no idea how it works.
It uses the fact that the two enums are values of different types (imagine overloads on int and long -- 0 and 0L could select different property maps). But what you're looking at is, I think, an outdated approach. It still needs to be supported for backward compatibility, but IIUC you should use bundled properties now.
Another example is the following:
get(vertex_color_t(), g, vertex(2,g))
Here we seem to be left to our imaginations as to how this works.
How it works is not important if you're just trying to use the library. You know what it's supposed to do, don't you?
The reason I brought that specific example up was that it seemed default constructing a property that is then used to retrieve a property_map from the graph which is then used to look up a property on a vertex, the construction of the vertex_color_t makes one believe that it is somehow efficient to do this for each property retrieval. Secondly, it appears to have different semantics than using get(property,graph); get(key,property); A key thing I find hard to track is how expensive are these operations. It seems that some properties can be stored and retrieved in O(1) space/time, yet others are obviously more expensive to work with. After going through the bgl documentation again
So, for some real questions and to end the rant.
1. What language mechanism is allowing property_maps to be stored and retrieved from an instance of a graph?
The question is too broad to answer usefully. The BGL uses many of the language's mechanisms in any given bit of code.
How does "BOOST_DEF_PROPERTY(edge, index)" work exactly?
My advice: don't ask. I don't know the answer, but it doesn't inhibit my ability to use the library.
It inhibited my understanding of how to best go about creating my own property.
2. What *is* the required interface for a property_map? get,put, operator[] of course, but the BGL properties appear to function nothing like the example given in the documentation.
The exact requirements are documented at http://www.boost.org/doc/libs/1_35_0/libs/property_map/property_map.html
3. Why is using get(p,g) better than using myPropMap.get_from(myGraph) and myGraph.get_property_map(prop_map_type) and myGraph.get_edge(edge_descriptor). What benefit does the chosen method provide?
An arbitrary type that has the functional _capabilities_ of a property map can be non-intrusively adapted so that it actually *is* a property map just by defining the appropriate free functions and traits specializations. That's how, for example, std::vector can be a property map with an integer key type.
4. The edge property example contains the following: typedef property<edge_mycapacity_t, int> Cap; typedef property<edge_myflow_t, int, Cap> Flow; typedef adjacency_list<vecS, vecS, bidirectionalS, no_property, Flow> Graph;
But accesses the Cap property_map as if it is directly associated with the graph, not a nested property of Flow:
typename property_map<Graph, edge_mycapacity_t>::const_type capacity = get(edge_mycapacity, G);
Can someone explain how that works in better detail?
It's only "nested" in a type sense. Logically, these properties are both at the top level of the graph. It's just the old way of "chaining" properties so you can attach multiple ones to edges or vertices. See http://www.boost.org/doc/libs/1_35_0/libs/graph/doc/property.html
HTH,
Thanks for responding. To put some more context behind my usage of bgl, I was writing an application that would read in a terrain height map and produce a navigation mesh that could be used for pathfinding. The resolution of the graph was about 4 units distance between each node and a maximum degree of 8. The terrain file was about 35k x 35k units in size so before any of the weeding of non-navigable nodes took place, the graph was quite sizable. The main grid was broken down into a grid of a grid of a grid IE: Outer grid (A) of 16x16 and each grid there was another grid of (B)16x16 and that grid was again broken into another grid (C) of 16x16. Each cell in grid A was stored in a separate file for optimization reasons so that only portions of the terrain had to be loaded at a time. This also made for a complex but efficient way of operating on the graphs nodes when it was loaded; it enabled integer arithmetic to be used for astar searches and then that path could be converted to through additional calculations based on a property associated with each node, into its actual floating point location. Similarly, given a 3d vector in world coordinates, an operation could be performed that would quickly locate the nearest node in the graph and additionally it could be used to determine which files needed to be loaded. It all worked somewhat like how a paging table is used to translate a 32 bit virtual memory address into a physical memory address. The reason I was unable to just "use" the bgl as you mentioned was because of the performance issues I was having. I needed to know why for example, so many copies of 400,000 element vectors were being made, which required better understandings of the implementation details of BGL. Thanks.