Test if std::common_type exists

问题

I want to write a helper template that checks if a template parameter pack has a common type, i.e., if applying std::common_type to the pack defines a type.

Using std::void_t with SFINAE I came up with the following definition:

template<typename... Types, typename Enable = void>
struct has_common_type : std::false_type
{ };

template<typename... Types>
struct has_common_type<Types..., std::void_t<std::common_type<Types...>::type>> : std::true_type
{ };

This however does not work, because the template parameter pack must be the last parameter. The compiler raises the following error:

error: template parameter pack must be the last template parameter
                        template<typename... Types, typename Enable = void>

How can one go about defining such a template?

回答1:

Option #1

template <typename... Ts>
using has_common_type = std::experimental::is_detected<std::common_type_t, Ts...>;

DEMO

---

Option #2

template <typename AlwaysVoid, typename... Ts>
struct has_common_type_impl : std::false_type {};

template <typename... Ts>
struct has_common_type_impl<std::void_t<std::common_type_t<Ts...>>, Ts...> : std::true_type {};

template <typename... Ts>
using has_common_type = typename has_common_type_impl<void, Ts...>::type;

DEMO 2

来源：https://stackoverflow.com/questions/42748199/test-if-stdcommon-type-exists