Hi, Here's a solution similar to Marco's - finds the first non default type using template specialization. It uses sizeof to check whether the types following first non-default type are conforming. Anand. On Sat, Apr 12, 2008 at 2:38 PM, Eric Niebler <eric@boost-consulting.com> wrote:
Eric Niebler wrote:
I recently had a C++ problem and found an answer that tickled my brain. In the spirit of Car Talk on NPR, I thought I'd share it in the form of a puzzler. <snip> Answer next week, unless someone beats me to it.
Wow, everybody loves a puzzle! This is good fun.
Joaquin and Marco Co.: virtual inheritance. Clever! I think it instantiates O(N) templates though.
Matus: This works but has scalability problems. To handle a set with N types, you need 2^N specializations of common_type_helper_picker.
Giovalli: conditional operator. Wish I thought of that. But it requires types to be associated with integers via a global registry, so I can't use it. Is there a way to avoid select() and result<>? It's easy with typeof, but can you do it without?
Marco Ce: This is pretty good! I think this instantiates O(1) templates in the case that a common type exists. Awesome. It's simpler that what I came up with, too.
Steven W: Wow, makes my head hurt. I think I see what's going on -- 2^N specializations? Might not scale so well.
Daniel Fey: Similar to what I came up with, but there's a trick that can (a) knock the 2^N overloads down to N, and (b) eliminate the need for typeof.
-- Eric Niebler Boost Consulting www.boost-consulting.com _______________________________________________ Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost