parameter_pack.h

#pragma once
#include <cstddef>
#include <utility>
 
namespace asyncpp::detail {
    class parameter_pack {
        template<size_t Index, typename Arg0, typename... Args>
        static constexpr decltype(auto) get_at_index_impl(Arg0&& first_arg, Args&&... args) noexcept {
            if constexpr (Index == 0) {
                return std::forward<Arg0>(first_arg);
            } else {
                return get_at_index_impl<Index - 1>(std::forward<Args>(args)...);
            }
        }
 
    public:
        parameter_pack() = delete;
 
        template<size_t Index, typename... Args>
        static constexpr decltype(auto) get_at_index(Args&&... args) noexcept {
            static_assert(Index < sizeof...(Args), "index out of range");
            return get_at_index_impl<Index>(std::forward<Args>(args)...);
        }
 
        template<typename... Args>
        static constexpr decltype(auto) get_last(Args&&... args) noexcept {
            static_assert(0 < sizeof...(Args), "parameter pack is empty");
            return get_at_index_impl<sizeof...(Args) - 1>(std::forward<Args>(args)...);
        }
    };
 
} // namespace asyncpp::detail