Re: [Boost-users] boost::graph issues.

From: David Abrahams (dave_at_[hidden])
Date: 2008-08-18 00:42:24

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?

> 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#L276

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.

> 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.

> 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.

> 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?

> "get" in this case is obviously operating on a graph in this call, but
> why are we no longer using an enum and instead constructing a
> temporary "vertex_color_t".  Looking at the definition of "get", we
> see the following:
>
>      template<typename OutEdgeListS, typename VertexListS, typename
> DirectedS, typename VertexProperty,
>               typename EdgeProperty, typename GraphProperty, typename
> EdgeListS, typename T, typename Bundle,
>               typename Key>
>      inline T
>      get(T Bundle::* p, adjacency_list<OutEdgeListS, VertexListS,
> DirectedS, VertexProperty, EdgeProperty,
>                                        GraphProperty, EdgeListS> const & g,
> const Key& key)
>      {
>        return get(get(p, g), key);
>      }

Don't look at the implementation.  That's not even the overload of get
that you're calling in the line you quoted.

> 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.

> 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,

-- 
Dave Abrahams
BoostPro Computing
http://www.boostpro.com