Generic app constructor

include/crow/app.h

@@ -60,6 +60,12 @@ namespace crow
         Crow()
         {}
 
+        /// Construct Crow with a subset of middleware
+        template<typename... Ts>
+        Crow(Ts&&... ts):
+          middlewares_(make_middleware_tuple(std::forward<Ts>(ts)...))
+        {}
+
         /// Process an Upgrade request
 
         ///
@@ -492,6 +498,17 @@ namespace crow
             cv_started_.wait(lock);
         }
 
+    private:
+        template<typename... Ts>
+        std::tuple<Middlewares...> make_middleware_tuple(Ts&&... ts)
+        {
+            auto fwd = std::forward_as_tuple((ts)...);
+            return std::make_tuple(
+              std::forward<Middlewares>(
+                black_magic::tuple_extract<Middlewares, decltype(fwd)>(fwd))...);
+        }
+
+
     private:
         std::uint8_t timeout_{5};
         uint16_t port_ = 80;

include/crow/utility.h

@@ -281,6 +281,30 @@ namespace crow
             static const int value = 1 + tuple_index<T, std::tuple<Types...>>::value;
         };
 
+        // Extract element from forward tuple or get default
+#ifdef CROW_CAN_USE_CPP14
+        template<typename T, typename Tup>
+        typename std::enable_if<has_type<T&, Tup>::value, typename std::decay<T>::type&&>::type
+          tuple_extract(Tup& tup)
+        {
+            return std::move(std::get<T&>(tup));
+        }
+#else
+        template<typename T, typename Tup>
+        typename std::enable_if<has_type<T&, Tup>::value, T&&>::type
+          tuple_extract(Tup& tup)
+        {
+            return std::move(std::get<tuple_index<T&, Tup>::value>(tup));
+        }
+#endif
+
+        template<typename T, typename Tup>
+        typename std::enable_if<!has_type<T&, Tup>::value, T>::type
+          tuple_extract(Tup&)
+        {
+            return T{};
+        }
+
         // Kind of fold expressions in C++11
         template<bool...>
         struct bool_pack;

tests/unittest.cpp

@@ -1514,6 +1514,21 @@ TEST_CASE("local_middleware")
     app.stop();
 } // local_middleware
 
+struct OnlyMoveConstructor
+{
+    OnlyMoveConstructor(int) {}
+    OnlyMoveConstructor(const OnlyMoveConstructor&) = delete;
+    OnlyMoveConstructor(OnlyMoveConstructor&&) = default;
+};
+
+TEST_CASE("app_constructor")
+{
+    App<NullMiddleware, OnlyMoveConstructor, FirstMW<false>, SecondMW<false>>
+      app1(OnlyMoveConstructor(1), SecondMW<false>{});
+    App<NullMiddleware, OnlyMoveConstructor, FirstMW<false>, SecondMW<false>>
+      app2(FirstMW<false>{}, OnlyMoveConstructor(1));
+} // app_constructor
+
 TEST_CASE("middleware_blueprint")
 {
     // Same logic as middleware_context, but middleware is added with blueprints