On 04/07/12 15:12, Lorenzo Caminiti wrote: <snip>
Also, if I forward declare the templates I need from STL, I don't need to #include the STL. This way there is no draw back in declaring the specializations directly in boost/type_traits/has_equal_to.hpp (or, is there?)
As discussed elsewhere in this thread, unfortunately it's not conformant to forward-declare things in the std namespace. There's a boost header boost/detail/container_fwd.hpp that will do it for those libraries which allow it, but as you can see if you look in it there are lots of cases where it's not possible. So some users will end up paying the cost of including those headers.
instead that in a separate header boost/type_traits/std/vector.hpp. The advantage of this is that boost::has_equal_to<vector<T> > will do what the user expects "out of the box" and without requiring the user to know and remember to include the extra header type_traits/std/vector.hpp.
I think the situation is a bit worse than you suggest. If the user sometimes forgets to include the extra header then his program might well end up violating the one definition rule. For that reason I would strongly recommend including the specializations along with the definition of has_equal_to, which unfortunately has the aforementioned cost of extra #includes in some cases. John Bytheway