"Eric Niebler" <eric@boostpro.com> wrote in message news:< 4BF5CA3B.8090807@boostpro.com>...

On 5/20/2010 10:54 AM, John Bytheway wrote:

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On 19/05/10 07:02, Eric Niebler wrote: <snip>

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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!

I attach my solution for completeness, though I think Daniel's is neater. I used a similar idea to him, but without the use of sizeof/decltype, and with the distance-to-not_a_domain implemented as a metafunction, rather than injected into the definition of domain.

I think we use about the same number of instantiations, but Daniel's lends itself to unrolling much more than mine. I might argue that my implementation is a little easier to follow than his, but it would be a pretty feeble claim.

Thanks, John. I'll need to investigate more closely. Everybody's code is easy to follow when they wrote it! :-) Though I admit, Daniel's solution is subtle and had me scratching my head for a while.

And for completeness, here is my (decltype-based) solution. I tried to make one giant expression that calculates the result in one shot. Like Daniel, I use inheritance and overloading, but I build a parallel hierarchy instead of reusing the domains. (For reasons that aren't relevant, I don't want domains to actually inherit from other domains in Proto.)

-- Eric Niebler BoostPro Computing http://www.boostpro.com

Here's another try. No inheritance or function overloading. I have no idea how many template instantiations are there in this code and if it is acceptable. There is some influence from Daniel's solution in getting the default_domain resolutions right. - Krishna Achuthan