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From: Anand Shankar (anand.shankar.k_at_[hidden])
Date: 2008-04-12 15:13:40


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.


On Sat, Apr 12, 2008 at 2:38 PM, Eric Niebler <eric_at_[hidden]>

> 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
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