How can I make this variadic template code shorter using features from C++14 and C++1z?

Question

This is a code snippet that I am going to use in order to check whether the variadic template types are unique:

template 
struct is_one_of         


        

Answer 1

I'd (now) suggest using the std::conj/disj/nega family of STL functions:

#include 

template 
struct is_one_of : std::disjunction...> {};

template 
struct is_unique : std::conjunction>..., is_unique> {};

template 
struct is_unique : std::true_type {};

int main()
{
    static_assert(is_one_of::value);
    static_assert(is_unique::value);
    static_assert(!is_unique::value);
}

---

When fold-expressions, which were designed for these cases, are released into the language this will become trivial:

namespace stx = std::experimental;

template 
struct is_one_of {
    static constexpr bool value = (stx::is_same_v || ...);
};

template 
struct is_unique {
    static constexpr bool value = (!stx::is_same_v && ... && is_unique::value);
};

template 
struct is_unique : std::true_type {};