c++ - How to make a variadic is_same?

Question

How can I make a class template that returns whether any of its variadic types are equal to the first type. I want to be able to do this:

is_same<T, A, B, C>::value; // true if T is one of A, B or C

And if T is equal to any one of those types, its static value member will be true, otherwise false. How can I do this?

Answer 1

Nice and concise with C++17:

template <class T, class... Ts>
struct is_any : std::disjunction<std::is_same<T, Ts>...> {};

And the dual:

template <class T, class... Ts>
struct are_same : std::conjunction<std::is_same<T, Ts>...> {};

A variation that uses fold expressions:

template <class T, class... Ts>
struct is_any : std::bool_constant<(std::is_same_v<T, Ts> || ...)> {};

template <class T, class... Ts>
struct are_same : std::bool_constant<(std::is_same_v<T, Ts> && ...)> {};