Replaced sizeof... with variadic templates

dlib/dnn/layers.h

@@ -1844,6 +1844,7 @@ namespace dlib
         };
         template <template<typename> class TAG_TYPE>
         struct concat_helper_impl<TAG_TYPE>{
+            constexpr static size_t tag_count() {return 1;}
             template<typename SUBNET>
             static void resize_out(resizable_tensor& out, const SUBNET& sub, long sum_k)
             {
@@ -1865,6 +1866,9 @@ namespace dlib
         };
         template <template<typename> class TAG_TYPE, template<typename> class... TAG_TYPES>
         struct concat_helper_impl<TAG_TYPE, TAG_TYPES...>{
+
+            constexpr static size_t tag_count() {return 1 + concat_helper_impl<TAG_TYPES...>::tag_count();}
+
             template<typename SUBNET>
             static void resize_out(resizable_tensor& out, const SUBNET& sub, long sum_k)
             {
@@ -1896,6 +1900,8 @@ namespace dlib
     class concat_
     {
     public:
+        constexpr static size_t tag_count() {return impl::concat_helper_impl<TAG_TYPES...>::tag_count();};
+
         template <typename SUBNET>
         void setup (const SUBNET&)
         {
@@ -1924,7 +1930,8 @@ namespace dlib
         friend void serialize(const concat_& item, std::ostream& out)
         {
             serialize("concat_", out);
-            serialize(sizeof...(TAG_TYPES), out);
+            size_t count = tag_count();
+            serialize(count, out);
         }
 
         friend void deserialize(concat_& item, std::istream& in)
@@ -1935,15 +1942,16 @@ namespace dlib
                 throw serialization_error("Unexpected version '"+version+"' found while deserializing dlib::concat_.");
             size_t count_tags;
             deserialize(count_tags, in);
-            if (count_tags != sizeof...(TAG_TYPES))
+            if (count_tags != tag_count())
                 throw serialization_error("Invalid count of tags "+ std::to_string(count_tags) +", expecting " +
-                                          std::to_string(sizeof...(TAG_TYPES)) + " found while deserializing dlib::concat_.");
+                                          std::to_string(tag_count()) +
+                                                  " found while deserializing dlib::concat_.");
         }
 
         friend std::ostream& operator<<(std::ostream& out, const concat_& item)
         {
             out << "concat\t ("
-                << sizeof...(TAG_TYPES)
+                << tag_count()
                 << ")";
             return out;
         }