Since there is a restriction on allowed non-type variadic templates, I am trying to write a function taking an arbitrary number of doubles using enable_if. In essence, I want to do something like:
template<typename... T,
typename = typename std::enable_if<std::is_convertible<T, double>::value, T>::type>
foo(T... t){ /* code here */ }
I'm opting to put the enable_if as a default value for an unnamed parameter since my function is actually a constructor and will not have a return value. This would work for a single parameter, but as it's a variadic template T is a parameter pack, and the above code is not valid. So, how can I check every parameter is convertible to a double?
The bool_pack trick again.
template<bool...> struct bool_pack;
template<bool... bs>
using all_true = std::is_same<bool_pack<bs..., true>, bool_pack<true, bs...>>;
Then
template<class R, class... Ts>
using are_all_convertible = all_true<std::is_convertible<Ts, R>::value...>;
and finally
template<typename... T,
typename = typename enable_if<are_all_convertible<double, T...>::value>::type>
foo(T... t){ /* code here */}