Add trait to test unwrappable

fluent/Continuable.h

@@ -31,8 +31,10 @@ struct Continuable
     ForwardFunction _callback_insert;
 
     Continuable<_ATy...>() { }
-    Continuable<_ATy...>(ForwardFunction&& callback_insert)
-        : _callback_insert(std::forward<ForwardFunction>(callback_insert)) { }
+
+    template<typename _FTy>
+    Continuable<_ATy...>(_FTy&& callback_insert)
+        : _callback_insert(std::forward<_FTy>(callback_insert)) { }
 
     template <typename _CTy>
     Continuable<_ATy...>& then(_CTy&&)

fluent/functional_unwrap.hpp

@@ -25,6 +25,7 @@
 #define _FUNCTIONAL_UNWRAP_HPP_
 
 #include <functional>
+#include <type_traits>
 
 namespace fu
 {
@@ -106,6 +107,20 @@ namespace fu
             typedef unwrap_function_impl<_RTy(_ATy...)> type;
         };
 
+        template <typename>
+        struct to_true
+            : std::true_type { };
+
+        /// std::true_type if unwrappable
+        template<typename... Function>
+        static auto test_unwrappable(int)
+            -> to_true<typename select_best_unwrap<Function...>::type::function_type>;
+
+        /// std::false_type if not unwrappable
+        template<typename... Function>
+        static auto test_unwrappable(long)
+            -> std::false_type;
+
      } // detail
 
     /// Trait to unwrap function parameters of various types:
@@ -140,6 +155,11 @@ namespace fu
     using function_ptr_of_t =
         typename detail::select_best_unwrap<Function...>::type::function_ptr;
 
+    /// Trait which defines if the given function is unwrappable or not.
+    template<typename... Function>
+    struct is_unwrappable
+        : decltype(detail::test_unwrappable<Function...>(0)) { };
+
 } // fu
 
 #endif // _FUNCTIONAL_UNWRAP_HPP_

test.cpp

@@ -6,6 +6,8 @@
 #include <iostream>
 #include <exception>
 #include <type_traits>
+#include <string>
+#include <vector>
 
 enum SpellCastResult
 {
@@ -28,9 +30,24 @@ Continuable<SpellCastResult> CastSpell(int id)
     });
 }
 
-int main(int argc, char** argv)
+template <typename... T>
+void test_unwrap(std::string const& msg)
 {
-    auto lam = [=](Callback<SpellCastResult>&& callback)
+    std::cout << msg << " is unwrappable: " << (fu::is_unwrappable<T...>::value ? "true" : "false") << std::endl;
+}
+
+int main(int /*argc*/, char** /*argv*/)
+{
+    test_unwrap<void()>("void()");
+    test_unwrap<std::function<void()>>("std::function<void()>");
+    test_unwrap<std::vector<std::string>>("std::vector<std::string>");
+
+    auto voidcontinue = make_continuable([=](Callback<>&& /*callback*/)
+    {
+
+    });
+
+    auto lam = [=](Callback<SpellCastResult>&& /*callback*/)
     {
         // on success call the callback with SPELL_FAILED_SUCCESS
         // callback(SPELL_FAILED_SUCCESS);