upcastable class template added to provide the base class for upcastable classes

README.md

@@ -29,9 +29,7 @@ or more base dimensions:
 
 ```cpp
 template<Exponent... Es>
-struct dimension {
-  using base_type = dimension<Es...>;
-};
+struct dimension : upcastable<dimension<Es...>> {};
 ```
 
 `units::Dimension` is a Concept that is satisfied by a type that is empty and publicly
@@ -173,8 +171,7 @@ struct merge_dimension {
 ```cpp
 template<Dimension D, Ratio R>
   requires (R::num > 0)
-struct unit {
-  using base_type = unit<D, R>;
+struct unit : upcastable<unit<D, R>> {
   using dimension = D;
   using ratio = R;
 };
@@ -333,7 +330,7 @@ and
 are not arguably much easier to understand thus provide better user experience.
 
 Upcasting capability is provided through dedicated `upcasting_traits` and by `base_type` member
-type in `dimension` and `unit` class templates.
+type in `upcastable` class template.
 
 ```cpp
 template<typename T>

src/include/units/bits/tools.h

@@ -110,6 +110,13 @@ namespace units {
   template<Ratio Ratio1, Ratio Ratio2>
   using common_ratio_t = typename common_ratio<Ratio1, Ratio2>::type;
 
+  // upcastable
+
+  template<typename BaseType>
+  struct upcastable {
+    using base_type = BaseType;
+  };
+
   // upcasting_traits
 
   template<typename T>

src/include/units/dimension.h

@@ -71,7 +71,7 @@ namespace units {
   struct exp_greater_equal : std::bool_constant<(E1::value >= E2::value)> {
   };
 
-// exp_invert
+  // exp_invert
 
   template<Exponent E>
   struct exp_invert;
@@ -87,9 +87,7 @@ namespace units {
   // dimension
 
   template<Exponent... Es>
-  struct dimension {
-    using base_type = dimension<Es...>;
-  };
+  struct dimension : upcastable<dimension<Es...>> {};
 
   // is_dimension
   namespace detail {

src/include/units/unit.h

@@ -29,8 +29,7 @@ namespace units {
 
   template<Dimension D, Ratio R>
     requires (R::num > 0)
-  struct unit {
-    using base_type = unit<D, R>;
+  struct unit : upcastable<unit<D, R>> {
     using dimension = D;
     using ratio = R;
   };