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.)
