object generator progress

include/matador/object/attribute.hpp

@@ -19,6 +19,7 @@ struct attribute_options {
   int index{-1};
 };
 class object;
+class attribute_generator;
 
 class attribute {
 public:
@@ -87,6 +88,7 @@ public:
 
 private:
   friend class object;
+  friend class attribute_generator;
 
   std::string name_;
   object *owner_{nullptr};