Units and dimensions redesigned

src/include/units/prefix.h

@@ -28,43 +28,69 @@
 
 namespace units {
 
-  struct prefix_type {};
+/**
+ * @brief The base for all prefix types
+ * 
+ * Every prefix type should inherit from this type to satisfy PrefixType concept.
+ */
+struct prefix_type {};
 
-  template<typename T>
-  concept PrefixType = std::derived_from<T, prefix_type>;
+template<typename T>
+concept PrefixType = std::derived_from<T, prefix_type>;
 
-  namespace detail {
+/**
+ * @brief No prefix possible for the unit
+ * 
+ * This is a special prefix type tag specifying that the unit can not be scaled with any kind
+ * of the prefix.
+ */
+struct no_prefix : prefix_type {};
 
-    template<PrefixType PT, Ratio R>
-    struct prefix_base : downcast_base<prefix_base<PT, R>> {
-      using prefix_type = PT;
-      using ratio = R;
-    };
+namespace detail {
 
-  }
+template<PrefixType PT, Ratio R>
+struct prefix_base : downcast_base<prefix_base<PT, R>> {
+  using prefix_type = PT;
+  using ratio = R;
+};
 
-  template<typename Child, PrefixType PT, Ratio R, basic_fixed_string Symbol>
-  struct prefix : downcast_child<Child, detail::prefix_base<PT, R>> {
-    static constexpr auto symbol = Symbol;
-  };
+}  // namespace detail
 
+/**
+ * @brief A prefix used to scale units
+ * 
+ * Data from a prefix class is used in two cases:
+ * - when defining a prefixed_unit its ratio is used to scale the reference unit and its
+ *   symbol is used to prepend to the symbol of referenced unit
+ * - when printing the symbol of a scaled unit that was not predefined by the user but its
+ *   factor matches ratio of a prefix from the specified prefix family, its symbol will be
+ *   prepended to the symbol of the unit
+ * 
+ * @tparam Child inherited class type used by the downcasting facility (CRTP Idiom)
+ * @tparam PT a type of prefix family
+ * @tparam Symbol a text representation of the prefix
+ * @tparam R factor to be used to scale a unit
+ */
+template<typename Child, PrefixType PT, basic_fixed_string Symbol, Ratio R>
+  requires(!std::same_as<PT, no_prefix>)
+struct prefix : downcast_child<Child, detail::prefix_base<PT, ratio<R::num, R::den>>> {
+  static constexpr auto symbol = Symbol;
+};
 
-  // TODO gcc:92150
-  // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92150
-  // namespace detail {
+// TODO gcc:92150
+// https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92150
+// namespace detail {
 
-  //   template<typename T>
-  //   inline constexpr bool is_prefix = false;
+//   template<typename T>
+//   inline constexpr bool is_prefix = false;
 
-  //   template<typename PrefixType, Ratio R, basic_fixed_string Symbol>
-  //   inline constexpr bool is_prefix<prefix<PrefixType, R, Symbol>> = true;
+//   template<typename PrefixType, Ratio R, basic_fixed_string Symbol>
+//   inline constexpr bool is_prefix<prefix<PrefixType, R, Symbol>> = true;
 
-  // }  // namespace detail
+// }  // namespace detail
 
-  template<typename T>
+template<typename T>
 //  concept Prefix = detail::is_prefix<T>;
-  concept Prefix = true;
-
-  struct no_prefix : prefix_type {};
+concept Prefix = true;
 
 }  // namespace units