on Wed Jul 04 2012, Eric Niebler <eric-AT-boostpro.com> wrote:
He's using a variant of the variadic tuple trick. He wants to find the Nth element in a parameter pack. So he puts all the elements in a tuple-like thingy (select_impl) which holds individual elements, each in its own base class (select_base). But only the Nth base actually holds onto its argument. (See the partial specialization of select_base.) His solution instantiates O(N) instances of select_base, but gets you the Nth element in O(1). Fiendishly clever.
That explanation doesn't match what I'm seeing in his latest attachment (http://llvm.org/bugs/attachment.cgi?id=8838&action=edit), which doesn't require storing anything. Which is pretty cool! -- Dave Abrahams BoostPro Computing Software Development Training http://www.boostpro.com Clang/LLVM/EDG Compilers C++ Boost