Is Parameter pack contained in other Parameter pack

Question

I'm writing a class that manages a function object(for fun:)).

I want a templated member function of a templated class to check if all of the types given to it were already given to the class.

template<typename... Args>
class funclass
{
    public:
    template<typename... types>
    void funfun ()
    {
        static_assert(/*is every type in types contained in Args*/);
    }
};

Answer 1

You can use a helper to check if a given T is part of the Args... pack:

template <typename T, typename ...Args>
struct contains {
    static constexpr bool value = (std::is_same<T,Args>::value || ...);
};

Then check it for all types in types...:

template<typename... Args>
struct funclass
{
    template<typename... types>
    void funfun ()
    {
        constexpr bool contains_all = (contains<types,Args...>::value && ...);
        static_assert( contains_all  );
    }
};

int main() {
    funclass<int,double>{}.funfun<int>(); // OK
    funclass<int,double>{}.funfun<float>(); // error
}