Make static_assert for every other type in the template

Question

How can I do static_assert (or other checks) for every other type in the template?

 template<typename... Ts> //T1,T2,T3,... struct foo { //How can I //for T1,T3,T5,T7,... //do some checks, for example: //static_assert(std::is_default_constructible<Tn>::value,"invalid type"); //static_assert(std::is_copy_constructible<Tn>::value,"invalid type"); }; 

Answer 1

Please try the following:

 #include <type_traits> template <typename... Ts> struct default_constructible; template <typename T> struct default_constructible<T> { static constexpr bool value = std::is_default_constructible<T>::value; }; template <> struct default_constructible<> { static constexpr bool value = true; }; template <typename T, typename U, typename... Ts> struct default_constructible<T, U, Ts...> { static constexpr bool value = std::is_default_constructible<T>::value && default_constructible<Ts...>::value; }; template <typename... Ts> struct foo { static_assert(default_constructible<Ts...>::value, ""); }; class A { A() = delete; }; template class foo<int, bool>; // Compiles template class foo<int, bool, A>; // Does not compile 

Demo

Answer 2

You can try the following:

 template <template <typename...> class Pred, typename ...Args> struct check_odd; template <template <typename...> class Pred> struct check_odd<Pred> : std::true_type { }; template <template <typename...> class Pred, typename T> struct check_odd<Pred, T> : Pred<T> { }; template <template <typename...> class Pred, typename T1, typename T2, typename ...Args> struct check_odd<Pred, T1, T2, Args...> { static constexpr bool value = Pred<T1>::value && check_odd<Pred, Args...>::value; }; 

Using:

 static_assert(check_odds<std::is_arithmetic, T1, T2, T3, T4, T5, T6>::value, "Not all odd types are arithmetic."); 

Obvious generalization routes parameterize the combiner (currently hardcoded as "AND" or && ) and provide additional predicate parameters.