...

cxx/mcc_traits.h

@@ -38,5 +38,76 @@ concept mcc_formattable =
     requires(T v, std::format_context ctx) { std::formatter<std::remove_cvref_t<T>>().format(v, ctx); };
 
 
+/*  a callable concept and its signature traits   */
+
+template <typename T>
+concept mcc_is_callable = std::is_function_v<T> || (std::is_object_v<T> && requires(T) { &T::operator(); });
+
+
+// helper classes for callable signature deducing
+template <typename... Ts>
+struct mcc_func_traits_helper_t;
+
+template <typename R>
+struct mcc_func_traits_helper_t<R> {
+    using ret_t = R;
+    using args_t = std::tuple<>;
+    using arg1_t = void;
+    static constexpr size_t arity = 0;
+};
+
+template <typename R, typename Arg, typename... Args>
+struct mcc_func_traits_helper_t<R, Arg, Args...> {
+    using ret_t = R;
+    using args_t = std::tuple<Arg, Args...>;
+    using arg1_t = Arg;
+    static constexpr size_t arity = sizeof...(Args) + 1;
+};
+
+template <typename F>
+struct mcc_func_traits {
+    // use of an empty struct here to match std::invoke_result behaivior (at least of GCC)
+};
+
+template <typename R, typename... Args>
+struct mcc_func_traits<R (*)(Args...)> : mcc_func_traits_helper_t<R, Args...> {
+};
+
+template <typename R, typename... Args>
+struct mcc_func_traits<R(Args...)> : mcc_func_traits_helper_t<R, Args...> {
+};
+
+template <typename C, typename R, typename... Args>
+struct mcc_func_traits<R (C::*)(Args...)> : mcc_func_traits_helper_t<R, Args...> {
+};
+
+template <typename C, typename R, typename... Args>
+struct mcc_func_traits<R (C::*)(Args...) const> : mcc_func_traits_helper_t<R, Args...> {
+};
+
+template <typename F>
+    requires mcc_is_callable<F>
+struct mcc_func_traits<F> : mcc_func_traits<decltype(&F::operator())> {
+};
+
+// reference/const ref and rvalue helpers
+template <typename F>
+struct mcc_func_traits<F&> : mcc_func_traits<F> {
+};
+
+template <typename F>
+struct mcc_func_traits<const F&> : mcc_func_traits<F> {
+};
+
+template <typename F>
+struct mcc_func_traits<F&&> : mcc_func_traits<F> {
+};
+
+template <typename T>
+using mcc_retval_t = typename mcc_func_traits<T>::ret_t;
+
+template <typename T>
+using mcc_func_arg1_t = typename mcc_func_traits<T>::arg1_t;
+
 
 }  // namespace mcc::traits