How to detect the first and the last argument in the variadic templates?

Question

How to detect the first and the last argument in the variadic templates?

For the 1st argument it is easy (just compare sizeof...(T) with 0), but is ther

Answer 1

Here's another set of code with a convenience function return_type that you could use to access any type at a specific index in a varadic template list ... you could then adapt the call to return_type so that you get the first and the last arguments (i.e., the first argument will be at 0, and the last argument will be at sizeof...(TypeList)):

template
struct type_id_struct
{
    typedef T type;
    T object_instance;
};

template
struct reduce {};

template
struct reduce
{
    typedef typename reduce::type type;
};

template
struct reduce<0, T1, TypeList...>
{
    typedef T1 type;
};

//convenience function
template
type_id_struct::type> return_type()
{
        return type_id_struct::type>();
}

Here's an example of using the convenience function return_type in actual code to determine the Nth template argument in a variadic template:

int main()
{
    auto type_returned = return_type<2, int, double, char>();
    std::cout << typeid(type_returned.object_instance).name() << std::endl;

    return 0;
}

In this case, since the int template argument to return_type is 2, you'll get the char type as the output. Any number over 2 will cause an overflow that will create a compile rather than runtime error. As noted, you could adapt it so that it's wrapped inside a function in a structure that will allow you to access the types in the variadic template for that specific structure instance using the sizeof...(TypeList) - 1 applied to an enum. For instance:

template
struct an_object
{
    enum { first = 0, last = (sizeof...(TypeList) - 1) };

    template
    auto wrapper() -> decltype(return_type())
    {
            return return_type();
    }
};

//...more code

int main()
{
    an_object a;

    auto r_type1 = a.wrapper::first>();
    std::cout << typeid(r_type1.object_instance).name() << std::endl;

    auto r_type2 = a.wrapper::last>();
    std::cout << typeid(r_type2.object_instance).name() << std::endl;

    return 0;
}