Re: [boost] property map, boost::identity_property_map

From: Doug Gregor (dgregor_at_[hidden])
Date: 2004-07-29 09:11:21

On Wednesday 28 July 2004 5:11 pm, Val Samko wrote:
> is there any reason, why boost::identity_property_map
> implementation in <boost\property_map.hpp>
> is completely different from how it is documented
> in http://www.boost.org/libs/property_map/identity_property_map.html
> ?
>
> As mentioned in the implementation, boost::identity_property_map
> is a property map that applies the identity function to integers.
>
> On the other hand, documentation clearly states that one can use
> identity_property_map with any types.

I don't know why the two are out-of-synch, but identity_property_map is 
essentially only used for adjacency lists when the vertices are stored in a 
vecS. In any case, I think the implementation is wrong (not generic enough) 
and the interface in the documentation is wrong (doesn't create property maps 
that we can adapt). I'd much rather see something like:

template<typename T = void>
  struct identity_property_map
  { 
    typedef T key_type;
    typedef T value_type;
    typedef T reference;
    typedef boost::readable_property_map_tag category;
  };

template<typename T, typename Key>
  T get(identity_property_map<T>, const Key& x) { return x; }

The "identity_property_map<>" follows the behavior stated in the 
documentation, but "identity_property_map<T>" gives you a property map with 
useful key/value/reference types. But, of course, that's not 
backward-compatible.

        Doug