On 09/23/09 14:47, Christopher Schmidt wrote:
Larry Evans schrieb:
by using the "tagged multiple inheritance"(TMI) method shown around here:
https://svn.boost.org/trac/boost/browser/sandbox/variadic_templates/libs/mpl...
...
The only other objection might be to limit the number of instantiations because the more template instantiations, the slower the compile time; however, Doug Gregor suggested he wasn't too worried about that at the bottom of:
http://groups.google.com/group/comp.std.c++/msg/6449d909fd3d5cdc
-regards, Larry
Thank you very much for your feedback.
I did consider that specific method. I decided against using it due to the enormous amount of template instantiations and the expensive type_at. The current implementation is 4-unrolled recursive. I am pretty certain that it is faster - there are about 4 to 8 times less template instantiations.
There's no type_at in: https://svn.boost.org/trac/boost/browser/sandbox/variadic_templates/libs/mpl... There is, however, a tuple<,>::at_type<I> nested struct. Is that what you're referring to? If so, I don't understand why that would be expensive since neither at_type or what it calls, at_elem, use recursion. Could you explain why at_type would be expensive? Or maybe you've run some timing tests on it. If so, could you provide the test driver so I could test to see why it's so expensive? Now, AFAICT, the number of template instantiations is, neglecting those needed for the package_range_c at: https://svn.boost.org/trac/boost/browser/sandbox/variadic_templates/libs/mpl... which (as Doug mentioned near the bottom of his 12 Jan 2009 post) are amortized, the number of "overhead" template instantiations is N+C where N is the number of elements and C is a small constant. I think C would be around 1, i.e. 1 for _Tuple_impl. The N is for the N _Tuple_element<Tag,Value> values where Tag is some instance of mpl::integral_c<IndexType,IndexValue>. By "overhead" I means the number needed besides the N for each element in the tuple and the 1 for the overall tuple (in the case of vector, that would be mpl::vector). Does that make sense? TIA. -Larry