add function signature traits

CMakeLists.txt

@@ -5,6 +5,7 @@ option(BUILD_EXAMPLES "Build examples" ON)
 
 set(LIB_HEADERS
     include/snipplib/concepts/snplib_concepts.h
+    include/snipplib/concepts/snplib_traits.h
     include/snipplib/utils/snplib_hash.h
     include/snipplib/utils/snplib_string.h
     include/snipplib/utils/snplib_utils.h

include/snipplib/concepts/snplib_traits.h

@@ -0,0 +1,84 @@
+#pragma once
+
+
+#include "snplib_concepts.h"
+
+namespace snplib
+{
+
+/*                          deduce callable's signature                         */
+
+// WARNING: it does not work for generic lambdas!
+
+// helper classes
+template <typename... Ts>
+struct snplib_func_traits_helper_t;
+
+template <typename R>
+struct snplib_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 snplib_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 snplib_func_traits_t {
+    // use of an empty struct here to match std::invoke_result behaivior (at least of GCC)
+};
+
+// special case
+template <>
+struct snplib_func_traits_t<std::nullptr_t> {
+    using ret_t = std::nullptr_t;
+    using args_t = std::tuple<>;
+    using arg1_t = std::nullptr_t;
+    static constexpr size_t arity = 0;
+};
+
+template <typename R, typename... Args>
+struct snplib_func_traits_t<R (*)(Args...)> : snplib_func_traits_helper_t<R, Args...> {
+};
+
+template <typename R, typename... Args>
+struct snplib_func_traits_t<R(Args...)> : snplib_func_traits_helper_t<R, Args...> {
+};
+
+template <typename C, typename R, typename... Args>
+struct snplib_func_traits_t<R (C::*)(Args...)> : snplib_func_traits_helper_t<R, Args...> {
+};
+
+template <typename C, typename R, typename... Args>
+struct snplib_func_traits_t<R (C::*)(Args...) const> : snplib_func_traits_helper_t<R, Args...> {
+};
+
+template <typename F>
+    requires snplib_callable_c<F>
+struct snplib_func_traits_t<F> : snplib_func_traits_t<decltype(&F::operator())> {
+};
+
+template <typename F>
+struct snplib_func_traits_t<F&> : snplib_func_traits_t<F> {
+};
+
+template <typename F>
+struct snplib_func_traits_t<const F&> : snplib_func_traits_t<F> {
+};
+
+template <typename F>
+struct snplib_func_traits_t<F&&> : snplib_func_traits_t<F> {
+};
+
+template <typename T>
+using snplib_func_arg1_t = typename snplib_func_traits_t<T>::arg1_t;
+
+}  // namespace snplib