On Thu, Dec 16, 2010 at 9:18 AM, Dave Abrahams <dave@boostpro.com> wrote:
That sort of sounds like concept map templates with SFINAE "tacked on" (just the same way we tacked on SFINAE to partial template specialization—though most people don't know that usage of enable_if).
That's exactly how it works. Everything is done with specializations and there is also an implicit "Enabler" for convenience, which is why the macro can optionally use those "if" and "try" conditions (try just checks expression validity).
Would you support the analogue of concept map templates that can be partially-ordered (sounds from what you write below like you would), or would we be writing SFINAE conditions like "if (InputIterator<X> && !BidirectionalIterator<X>)" to avoid ambiguities?
You can do everything that you'd expect from C++03 template specializations that also happen to implicitly include a "SFINAE enabler". So, for instance, you can have a concept map for std::vector< bool, Alloc > that will be picked as a better match over one for std::vector< T, Alloc >, but you cannot do the equivalent of concept overloading, which I think you may be trying to get at with your example. If it came down to it, that could be simulated in a manner similar to how "switch" works with Auto_Function, but that'd be really pushing things to their limits and I'm not sure it's worth trying to support, especially not at this stage. -- -Matt Calabrese