[Boost-users] [graph] Extending bgl_named_params

Subject: [Boost-users] [graph] Extending bgl_named_params
From: Philipp Moeller (philipp.moeller_at_[hidden])
Date: 2012-10-22 12:30:48

Hi,

I maintain a library that introduces its own graph properties for domain
specific algorithms and provides it's own implementation of
bgl_named_parameters by essentially mimicking bgl_named_parameters
one-to-one. With the refactoring in 1.51 that implementation broke and I
had to add another implementation of named parameters which essentially
looks like this:

enum my_property_t { my_property };

template <typename T, typename Tag, typename Base = boost::no_property>
struct foo_bgl_named_params : boost::bgl_named_params<T, Tag, Base>
{
  typedef boost::bgl_named_params<T, Tag, Base> base;
  typedef foo_bgl_named_params self;

  foo_bgl_named_params(T v = T()) : base(v) {}
  foo_bgl_named_params(T v, const Base& b) : base(v, b) {}

  template <typename PMap>
  foo_bgl_named_params<PMap, my_property_t, self>
  my_property_map(const PMap& p) const 
  {
    typedef foo_bgl_named_params<PMap, my_property_t, self> Params;
    return Params(p, *this);
  }
};

// the respective free functions...
template <typename PMap>
foo_bgl_named_params<PMap, my_property_t, self>
my_property_map(const Pmap& p) const 
{
  typedef foo_bgl_named_params<PMap, my_property_t, self> Params;
  return Params(p);
}

// partial specialization for lookup_named_param_def
namespace boost {
template <typename T, typename Tag, typename Base, typename Def>
struct lookup_named_param_def<Tag, foo_bgl_named_params<T, Tag, Base>, Def> {
  typedef T type;
  static const type& get(const bgl_named_params<T, Tag, Base>& p, const Def&) {
    return p.m_value;
  }
};

template <typename Tag1, typename T, typename Tag, typename Base, typename Def>
struct lookup_named_param_def<Tag1, foo_bgl_named_params<T, Tag, Base>, Def> {
  typedef typename lookup_named_param_def<Tag1, Base, Def>::type type;
  static const type& get(const bgl_named_params<T, Tag, Base>& p, const Def& def) {
    return lookup_named_param_def<Tag1, Base, Def>::get(p.m_base, def);
  }
};
} // boost

This is brittle because it the type returned by the inherited functions
is not the same and chaining will not work if one of those functions is
called first, e.g. all custom properties have to be chained together first.

>From the documentation it is not really obvious to me if other libraries
are even supposed to provide their own properties and how/if they
actually should be able to extend the named parameters API.

Is that simply not an intended feature of the BGL or can that be done
differently?

Cheers,
Philipp