Dean Michael Berris wrote:
On Fri, Mar 21, 2008 at 9:17 AM, Eric Niebler <eric@boost-consulting.com> wrote:
Larry Evans wrote:
However, I'd imagine some people would want this restricted to just two elements, which could be done with something like: <snip> And now, if size<Args>::value is restricted to the Arity in tag_arity<shift_right,Arity> then it would be restricted to 2. Users may very well want to restrict tag::plus to arity 2 *in their domain*, but they shouldn't be foisting that decision on other consumers of Proto. The best way to ensure that expressions have the arity you expect is to encode that in the grammar for your domain, and check it with proto::matches.
I'm nowhere near being an expert, but how would a BOOST_STATIC_ASSERT fit in ensuring that Args has the correct arity provided in the
What do you mean by "correct" here?
construction of expr<>? Would there be a way to statically assert that 'Args' has arity 'Arity' from within Proto? Or would that be too much
Oh, Arity is the size of Args by default. expr<> is defined like: struct expr<Tag, Args, long Arity = Args::size> Nowhere does Proto enforce that Arity really *is* Args::size, though.
compile time trouble on top of all the compile time burdens Proto+Fusion+MPL already bring?
I don't think a static assert would be very expensive, but I also don't think it would add much safety. In fact, it would force me to remove an optimization in matches.hpp, where I fudge the Arity parameter to save a few template instantiations. -- Eric Niebler Boost Consulting www.boost-consulting.com