(Please don't top-post.) On 5/19/2010 2:08 PM, Marco wrote:
On Wed, 19 May 2010 08:02:56 +0200, Eric Niebler <eric@boostpro.com> wrote: <snip>
Your job: implement the deduce_domain3 template that finds the common domain of 3 domains. You're allowed to use decltype, but you get bonus points for a solution that doesn't. Bonus also for instantiating fewest templates. The challenge is for the ternary case, but your solution should scale to N domains. If your solution is better than mine, I'll use it in Proto and credit you!
Good luck.
P.S. I haven't checked my solution in yet, so you can't look in trunk and cheat. ;-)
I am a bit surprised by:
// These should be ambiguous. BOOST_MPL_ASSERT((is_same<deduce_domain3<DD1, DD0, DD0>::type, not_a_domain>)); BOOST_MPL_ASSERT((is_same<deduce_domain3<DD0, DD1, DD0>::type, not_a_domain>)); BOOST_MPL_ASSERT((is_same<deduce_domain3<DD0, DD0, DD1>::type, not_a_domain>));
I was expecting something like: BOOST_MPL_ASSERT((is_same<deduce_domain3<DD1, DD0, DD0>::type, default_domain));
Am I missing something ?
This is a tricky case, and to understand why I want that result, you need to know what proto uses domains for. In proto, you put an expression in a domain to give it certain domain-specific behaviors. (E.g. lambda expressions should have an operator() that evaluates the lambda.) However, there are no domain-specific behaviors to the default_domain. (E.g. "42" is in the default_domain. Obviously, it has no domain-specific behavior itself, but "lambda::_1 + 42" should be in the lambda domain.) If you combine two expressions in different domains, and the only super-domain they have in common is default_domain, then that would effectively strip all domain-specific behavior from the new expression. IMO, this would be surprising. Instead, I'd rather make this case ambiguous -- proto doesn't know what domain-specific behaviors to give the new expression so it gives up. I acknowledge that it is a special case and that this result doesn't follow naturally from the other rules. -- Eric Niebler BoostPro Computing http://www.boostpro.com