Updated spec to say that the user defined layers must be copyable.

dlib/dnn/input_abstract.h

@@ -37,6 +37,14 @@ namespace dlib
                   layer objects be default constructable. 
         !*/
 
+        EXAMPLE_INPUT_LAYER (
+            const EXAMPLE_INPUT_LAYER& item
+        );
+        /*!
+            ensures
+                - EXAMPLE_INPUT_LAYER objects are copy constructable
+        !*/
+
         EXAMPLE_INPUT_LAYER(
             const some_other_input_layer_type& item
         );

dlib/dnn/layers_abstract.h

@@ -110,6 +110,14 @@ namespace dlib
                   layer objects be default constructable. 
         !*/
 
+        EXAMPLE_LAYER_ (
+            const EXAMPLE_LAYER_& item
+        );
+        /*!
+            ensures
+                - EXAMPLE_LAYER_ objects are copy constructable
+        !*/
+
         EXAMPLE_LAYER_(
             const some_other_layer_type& item
         );

dlib/dnn/loss_abstract.h

@@ -42,6 +42,13 @@ namespace dlib
         const static unsigned int sample_expansion_factor;
         typedef whatever_type_you_use_for_labels label_type;
 
+        EXAMPLE_LOSS_LAYER_ (
+            const EXAMPLE_LOSS_LAYER_& item
+        );
+        /*!
+            ensures
+                - EXAMPLE_LOSS_LAYER_ objects are copy constructable
+        !*/
 
         // Implementing to_label() is optional.
         template <