The has_xxx way may return a superset. For example, 'boost::iterator_range' might be wrongly identified as container, and a program might be ill-formed. So it may have little choice but to return a subset, IMO.
IIRC, a forward declaration of standard container is, strictly speaking, not allowed. Also, vector is allowed to have extra template parameters.
Understood. So, would a more correct implementation of is_container forward declare STL containers in a portable way and specialize the traits class for each one? I'm working with STLport and VC 7.1/8.0 so I can have the forward declarations working for these, but I don't know about other stl implementations. Thanks a lot for your input. On 14/05/07, shunsuke <pstade.mb@gmail.com> wrote:
Christian Holmquist wrote:
It must be useful. It could be more useful if it supports 'is_std_pair' and 'is_std_vector' etc. Also, <std_pair_fwd.hpp> and <std_vector_fwd.hpp> etc could be useful.
My first attempt was implemented this way, by simply forward declaring std containers:
namespace std { template<class T, class Alloc> class vector; }
template<class T, class Alloc> struct is_container< std::vector<T, Alloc> > : public boost::true_type { typedef sequence_tag container_category; };
This of course didn't scale very well, since it required is_container<> to be manually specialized for every container class. I needed something that worked automatically, so I decided for the following implementation.. Do you think it would be better to forward declare classes instead? (i.e ., multi_index, ptr_container, array etc..)
The has_xxx way may return a superset. For example, 'boost::iterator_range' might be wrongly identified as container, and a program might be ill-formed. So it may have little choice but to return a subset, IMO.
IIRC, a forward declaration of standard container is, strictly speaking, not allowed. Also, vector is allowed to have extra template parameters. A portable layer to bell a cat can be valuable.
Regards,
-- Shunsuke Sogame
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