Re: [boost] "Simple C++11 metaprogramming"

On 6/4/2015 4:30 PM, Peter Dimov wrote:

Eric Niebler wrote:

...

I made Meta SFINAE-friendly on a lark and found it useful in practice. (See this thread[*] where Louis and I compared common_type implementations with Meta and Hana.) ... [*] http://lists.boost.org/Archives/boost/2015/03/220446.php

Having a SFINAE-friendly fold helps, but here's my implementation of this challenge.

// common_type

template struct common_type { using type_not_present = void***[]; };

template<class T> struct common_type<T>: std::decay<T> { };

template using common_type_impl = common_type;

template struct common_type: eval_or_default, common_type_impl, T1, T2, T...> { };

#include <iostream> #include <typeinfo>

int main() { std::cout << typeid( common_type::type ).name() << std::endl; std::cout << typeid( common_type::type_not_present ).name() << std::endl; }

where eval_or_default checks is_evaluable (from Bruno Dutra's earlier message) and returns F if evaluable, Def if not.

Getting it right was a bit tricky, but I find the final result quite readable. (This implements the specification in the latest C++17 draft.)

I'm not so sure. If a user defines a specialization for common_type, it won't get used when computing common_type. I think that would complicate your implementation somewhat. I like the mutual recursion between common_type and common_type_impl, but really you've just implemented fold, and it would be better to have fold as a separate reusable algorithm. I see that you've isolated the SFINAE-friendliness in your library to the eval_or_default utility. It's an interesting choice, but I think it's why you can't use a fold here, am I right? -- Eric Niebler Boost.org http://www.boost.org