function_traits.cpp

#include <type_traits>
#include <string>
#include <tuple>

template <class F>
struct function_traits : function_traits<decltype(&F::operator())> {};

template <class R, class... Args>
struct function_traits<R(Args...)> {
    using args = std::tuple<Args...>;
    using result_type = R;
};

template <class R, class T, class... Args>
struct function_traits<R(T::*)(Args...)> {
    using args = std::tuple<Args...>;
    using result_type = R;
};

template <class R, class T, class... Args>
struct function_traits<R(T::*)(Args...) const> {
    using args = std::tuple<Args...>;
    using result_type = R;
};

int f(int, double, std::string) { return 0; }

struct functor {
    int operator()(const int&, const double&, std::string) { return 1; }
};

int main() {
    auto g = [](std::string, double, int) { return 0; };

    static_assert(std::is_same_v<
        function_traits<decltype(f)>::args,
        std::tuple<int, double, std::string>
    >);

    static_assert(std::is_same_v<
        function_traits<decltype(g)>::args,
        std::tuple<std::string, double, int>
    >);
    
    static_assert(std::is_same_v<
        function_traits<functor>::args,
        std::tuple<const int&, const double&, std::string>
    >);
}