argtype.h

// -*-C++-*- #pragma once #include <concepts> #include <tuple> #include <type_traits> namespace detail { template<typename F> struct ArgTypeHelper; template<typename F> requires (!std::same_as<F, std::decay_t<F>>) struct ArgTypeHelper<F> : ArgTypeHelper<std::decay_t<F>> {}; template<typename R, typename... A, bool NE> struct ArgTypeHelper<R (*)(A...) noexcept(NE)> { using type = std::tuple<A...>; }; template<typename O> requires requires { &O::operator(); } struct ArgTypeHelper<O> { private: template<typename R, typename... A> static std::tuple<A...> return_arg_type(R (O::*)(A...) const); template<typename R, typename... A> static std::tuple<A...> return_arg_type(R (O::*)(A...)); public: using type = decltype(return_arg_type(&O::operator())); }; } // namespace detail // Number of arguments that a function or callable object takes template<typename F> constexpr std::size_t kArity = std::tuple_size_v<typename detail::ArgTypeHelper<F>::type>; // Nth argument type of a function or callable object template<std::size_t N, typename F> using ArgNType = std::tuple_element_t<N, typename detail::ArgTypeHelper<F>::type>; // Argument type of a function callable object with one argument template<typename F> requires (kArity<F> == 1) using UnaryType = ArgNType<0, F>;