implemented a SFINAE compile-time checker for receive policies

check_policy.hpp

@@ -0,0 +1,24 @@
+#pragma once
+#include <type_traits>
+#include "chat_messages.hpp"
+
+
+
+/* SFINAE compile-type checker for receive policies (as defined in chat_networking.hpp)
+ * it checks whether PolicyCandidate supplies all of the following methods:
+ *      - void message_do_what(chat_message)                                         */
+template <class PolicyCandidate, typename = void>
+struct is_valid_policy : std::false_type {};
+
+template <class PolicyCandidate>
+struct is_valid_policy<
+        PolicyCandidate,
+        typename
+        std::enable_if<
+                std::is_same<
+                        decltype(PolicyCandidate
+                                 ::message_do_what(std::declval<chat::chat_message>())),
+                        void
+                >::value
+        >::type>
+: std::true_type {};