How to convert variadic function arguments to array?

Question

How to convert variadic function arguments to array? I need something like this:

template <typename T>
struct Colordata
{
public:
    T dataFirst;
    T dataLast;

    template <typename... Ta>
    Colordata(Ta... args)
    {
        constexpr std::size_t n = sizeof...(Ta);
        std::cout << n << std::endl;
        dataFirst = (args..)[0];
        dataLast = (args...)[n - 1];
        return;
    }   
};

int main(int argc, char const *argv[])
{
    auto a = Colordata<float>(1.0f, 2.2f, 3.3f, 4.3f, 5.5f);
    return 0;
}

I tried to use variadic arguments as T args, ... and use functions from stdarg.h but then I can't get the count of arguments.

Answer 1

You can't convert a parameter pack into an array. You could create an array from one, but that would copy, and that would be wasteful. Instead, we can "convert" the parameter pack into a tuple of references, and then use get to index into that tuple. That would look like

template <typename... Ta>
Colordata(Ta... args)
{
    constexpr std::size_t n = sizeof...(Ta);
    static_assert(n >= 1, "must pass at least one argument");
    std::cout << n << std::endl;
    auto& tuple = std::tie(args...);
    dataFirst = std::get<0>(tuple)
    dataLast = std::get<n - 1>(tuple);
}