On 26/05/2011 13:19, Larry Evans wrote:
On 05/26/11 04:47, tymofey wrote:
Robert Jones<robertgbjones@gmail.com> On Thu, May 26, 2011 at 8:34 AM, tymofey<tymofey@qip.ru> wrote:
While experimenting with varidic templates of the upcoming standard i discovered that quite often one would need to extract a type from parameter pack at specified position. [snip] Forgive me if I'm missing the point, but isn't this essentially Boost.MPL's 'at' metafunction?
- Rob.
It is, but without the need to drag in mpl::vector with the limitations it imposes.
Could you please describe those limitations, other than the one for maximum arity, which, IIUC, is modifiable by redefining the macro:
BOOST_MPL_LIMIT_VECTOR_SIZE
? Also, IIUC, one reason for using the macro, and the preprocessor, is to speed compile time by reducing the number of template instantiations. Thus, maybe one advantage of mpl::at vs your type_at is compile time. You might check that somehow.
You cannot expand a template parameter pack to instantiate a non-variadic template. Typically, this template<typename... T> struct make_vector { typedef boost::mpl::vector<T...> type; }: doesn't work. GCC gives the error: sorry, unimplemented: cannot expand 'T ...' into a fixed-length argument list And I believe the standard doesn't require it just because it would be difficult to implement in GCC's codebase.