refactor: 💥 ! PrefixFamily support removed

It doesn't have much sense to restrict prefixes usage for most units and in many places we were wrong to do so already.

src/core/include/units/bits/basic_concepts.h

@@ -37,25 +37,14 @@
 
 namespace units {
 
-// PrefixFamily
-struct prefix_family;
-
-/**
- * @brief A concept matching a prefix family
- *
- * Satisfied by all types derived from `prefix_family`
- */
-template<typename T>
-concept PrefixFamily = std::derived_from<T, prefix_family>;
-
 // Prefix
 namespace detail {
 
-template<PrefixFamily PF, ratio R>
+template<ratio R>
 struct prefix_base;
 
-template<PrefixFamily PF, ratio R>
-void to_prefix_base(const volatile prefix_base<PF, R>*);
+template<ratio R>
+void to_prefix_base(const volatile prefix_base<R>*);
 
 }  // namespace detail
 

src/core/include/units/bits/unit_text.h

@@ -57,28 +57,19 @@ constexpr auto ratio_text()
   }
 }
 
-template<ratio R, typename PrefixFamily, std::size_t SymbolLen>
+template<ratio R, std::size_t SymbolLen>
 constexpr auto prefix_or_ratio_text()
 {
   if constexpr (R.num == 1 && R.den == 1 && R.exp == 0) {
     // no ratio/prefix
     return basic_fixed_string("");
   } else {
-    if constexpr (!is_same_v<PrefixFamily, no_prefix>) {
-      // try to form a prefix
-      using prefix = downcast<detail::prefix_base<PrefixFamily, R>>;
+    // try to form a prefix
+    using prefix = downcast<detail::prefix_base<R>>;
 
-      if constexpr (!is_same_v<prefix, prefix_base<PrefixFamily, R>>) {
-        // print as a prefixed unit
-        return prefix::symbol;
-      } else {
-        // print as a ratio of the coherent unit
-        constexpr auto txt = ratio_text<R>();
-        if constexpr (SymbolLen > 0 && txt.standard().size() > 0)
-          return txt + basic_fixed_string(" ");
-        else
-          return txt;
-      }
+    if constexpr (!is_same_v<prefix, prefix_base<R>>) {
+      // print as a prefixed unit
+      return prefix::symbol;
     } else {
       // print as a ratio of the coherent unit
       constexpr auto txt = ratio_text<R>();
@@ -151,15 +142,13 @@ constexpr auto unit_text()
       // use predefined coherent unit symbol
       constexpr auto symbol_text = coherent_unit::symbol;
       constexpr auto prefix_txt =
-        prefix_or_ratio_text<U::ratio / coherent_unit::ratio, typename U::reference::prefix_family,
-                             symbol_text.standard().size()>();
+        prefix_or_ratio_text<U::ratio / coherent_unit::ratio, symbol_text.standard().size()>();
       return prefix_txt + symbol_text;
     } else {
       // use derived dimension ingredients to create a unit symbol
       constexpr auto symbol_text = derived_dimension_unit_text<Dim>();
       constexpr auto prefix_txt =
-        prefix_or_ratio_text<U::ratio / coherent_unit::ratio, typename U::reference::prefix_family,
-                             symbol_text.standard().size()>();
+        prefix_or_ratio_text<U::ratio / coherent_unit::ratio, symbol_text.standard().size()>();
       return prefix_txt + symbol_text;
     }
   }

src/core/include/units/generic/angle.h

@@ -32,7 +32,7 @@
 
 namespace units {
 
-struct radian : named_unit<radian, "rad", isq::si::prefix> {};
+struct radian : named_unit<radian, "rad"> {};
 
 template<Unit U = radian>
 struct dim_angle : base_dimension<"A", U> {};

src/core/include/units/generic/dimensionless.h

@@ -30,8 +30,8 @@
 
 namespace units {
 
-struct one : named_unit<one, "", no_prefix> {};
-struct percent : named_scaled_unit<percent, "%", no_prefix, ratio(1, 100), one> {};
+struct one : named_unit<one, ""> {};
+struct percent : named_scaled_unit<percent, "%", ratio(1, 100), one> {};
 
 /**
  * @brief Dimension one

src/core/include/units/prefix.h

@@ -31,26 +31,10 @@
 
 namespace units {
 
-/**
- * @brief The base for all prefix families
- *
- * Every prefix family should inherit from this type to satisfy PrefixFamily concept.
- */
-struct prefix_family {};
-
-/**
- * @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_family {};
-
 namespace detail {
 
-template<PrefixFamily PF, ratio R>
-struct prefix_base : downcast_base<prefix_base<PF, R>> {
-  using prefix_family = PF;
+template<ratio R>
+struct prefix_base : downcast_base<prefix_base<R>> {
   static constexpr ::units::ratio ratio = R;
 };
 
@@ -63,17 +47,14 @@ struct prefix_base : downcast_base<prefix_base<PF, R>> {
  * - 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
+ *   factor matches ratio of a prefix, its symbol will be prepended to the symbol of the unit
  *
  * @tparam Child inherited class type used by the downcasting facility (CRTP Idiom)
- * @tparam PF 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, PrefixFamily PF, basic_symbol_text Symbol, ratio R>
-  requires(!std::same_as<PF, no_prefix>)
-struct prefix : downcast_dispatch<Child, detail::prefix_base<PF, R>, downcast_mode::on> {
+template<typename Child, basic_symbol_text Symbol, ratio R>
+struct prefix : downcast_dispatch<Child, detail::prefix_base<R>, downcast_mode::on> {
   static constexpr auto symbol = Symbol;
 };
 

src/core/include/units/unit.h

@@ -35,6 +35,13 @@
 
 namespace units {
 
+namespace detail {
+
+template<typename>
+inline constexpr bool can_be_prefixed = false;
+
+}  // namespace detail
+
 /**
  * @brief A common point for a hierarchy of units
  *
@@ -69,39 +76,31 @@ struct same_unit_reference : is_same<typename U1::reference, typename U2::refere
  * @brief A named unit
  *
  * Defines a named (in most cases coherent) unit that is then passed to a dimension definition.
- * A named unit may be used by other units defined with the prefix of the same type, unless
- * no_prefix is provided for PF template parameter (in such a case it is impossible to define
- * a prefix unit based on this one).
+ * A named unit may be composed with a prefix to create a prefixed_unit.
  *
  * @tparam Child inherited class type used by the downcasting facility (CRTP Idiom)
  * @tparam Symbol a short text representation of the unit
- * @tparam PF no_prefix or a type of prefix family
  */
-template<typename Child, basic_symbol_text Symbol, PrefixFamily PF>
+template<typename Child, basic_symbol_text Symbol>
 struct named_unit : downcast_dispatch<Child, scaled_unit<ratio(1), Child>> {
   static constexpr auto symbol = Symbol;
-  using prefix_family = PF;
 };
 
 /**
- * @brief A scaled unit
+ * @brief A named scaled unit
  *
- * Defines a new named unit that is a scaled version of another unit. Such unit can be used by
- * other units defined with the prefix of the same type, unless no_prefix is provided for PF
- * template parameter (in such a case it is impossible to define a prefix unit based on this
- * one).
+ * Defines a new named unit that is a scaled version of another unit.
+ * A named unit may be composed with a prefix to create a prefixed_unit.
  *
  * @tparam Child inherited class type used by the downcasting facility (CRTP Idiom)
  * @tparam Symbol a short text representation of the unit
- * @tparam PF no_prefix or a type of prefix family
  * @tparam R a scale to apply to U
  * @tparam U a reference unit to scale
  */
-template<typename Child, basic_symbol_text Symbol, PrefixFamily PF, ratio R, Unit U>
+template<typename Child, basic_symbol_text Symbol, ratio R, Unit U>
   requires UnitRatio<R>
 struct named_scaled_unit : downcast_dispatch<Child, scaled_unit<R * U::ratio, typename U::reference>> {
   static constexpr auto symbol = Symbol;
-  using prefix_family = PF;
 };
 
 /**
@@ -116,10 +115,9 @@ struct named_scaled_unit : downcast_dispatch<Child, scaled_unit<R * U::ratio, ty
  * @tparam U reference unit
  */
 template<typename Child, Prefix P, NamedUnit U>
-  requires std::same_as<typename P::prefix_family, typename U::prefix_family>
+  requires detail::can_be_prefixed<U>
 struct prefixed_unit : downcast_dispatch<Child, scaled_unit<P::ratio * U::ratio, typename U::reference>> {
   static constexpr auto symbol = P::symbol + U::symbol;
-  using prefix_family = no_prefix;
 };
 
 /**
@@ -131,9 +129,7 @@ struct prefixed_unit : downcast_dispatch<Child, scaled_unit<P::ratio * U::ratio,
  * @tparam Child inherited class type used by the downcasting facility (CRTP Idiom)
  */
 template<typename Child>
-struct derived_unit : downcast_dispatch<Child, scaled_unit<ratio(1), Child>> {
-  using prefix_family = no_prefix;
-};
+struct derived_unit : downcast_dispatch<Child, scaled_unit<ratio(1), Child>> {};
 
 /**
  * @brief A unit with a deduced ratio and symbol
@@ -152,7 +148,6 @@ template<typename Child, DerivedDimension Dim, NamedUnit U, NamedUnit... URest>
   requires detail::same_scaled_units<typename Dim::recipe, U, URest...>
 struct derived_deduced_unit : downcast_dispatch<Child, detail::derived_deduced_unit<Dim, U, URest...>> {
   static constexpr auto symbol = detail::derived_symbol_text<Dim, U, URest...>();
-  using prefix_family = no_prefix;
 };
 
 /**
@@ -160,18 +155,14 @@ struct derived_deduced_unit : downcast_dispatch<Child, detail::derived_deduced_u
  *
  * Defines a named alias for another unit. It is useful to assign alternative names and symbols
  * to the already predefined units (i.e. "tonne" for "megagram").
- * An alias unit may be used by other units defined with the prefix of the same type, unless
- * no_prefix is provided for PF template parameter (in such a case it is impossible to define
- * a prefix unit based on this one).
+ * A alias unit may be composed with a prefix to create a prefixed_alias_unit.
  *
  * @tparam U Unit for which an alias is defined
  * @tparam Symbol a short text representation of the unit
- * @tparam PF no_prefix or a type of prefix family
  */
-template<Unit U, basic_symbol_text Symbol, PrefixFamily PF>
+template<Unit U, basic_symbol_text Symbol>
 struct alias_unit : U {
   static constexpr auto symbol = Symbol;
-  using prefix_family = PF;
 };
 
 /**
@@ -188,16 +179,15 @@ struct alias_unit : U {
 // TODO gcc bug: 95015
 // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95015
 // template<Unit U, Prefix P, AliasUnit AU>
-//   requires (!AliasUnit<U>) && std::same_as<typename P::prefix_family, typename AU::prefix_family>
+//   requires (!AliasUnit<U>)
 template<Unit U, Prefix P, NamedUnit AU>
-  requires std::same_as<typename P::prefix_family, typename AU::prefix_family>
+  requires detail::can_be_prefixed<AU>
 struct prefixed_alias_unit : U {
   static constexpr auto symbol = P::symbol + AU::symbol;
-  using prefix_family = no_prefix;
 };
 
 /**
- * @brief Unknown unit
+ * @brief Unknown coherent unit
  *
  * Used as a coherent unit of an unknown dimension.
  */
@@ -205,17 +195,17 @@ struct unknown_coherent_unit : derived_unit<unknown_coherent_unit> {};
 
 namespace detail {
 
-template<typename Child, basic_symbol_text Symbol, PrefixFamily PF>
-void is_named_impl(const volatile named_unit<Child, Symbol, PF>*);
+template<typename Child, basic_symbol_text Symbol>
+void is_named_impl(const volatile named_unit<Child, Symbol>*);
 
-template<typename Child, basic_symbol_text Symbol, PrefixFamily PF, ratio R, Unit U>
-void is_named_impl(const volatile named_scaled_unit<Child, Symbol, PF, R, U>*);
+template<typename Child, basic_symbol_text Symbol, ratio R, typename U>
+void is_named_impl(const volatile named_scaled_unit<Child, Symbol, R, U>*);
 
 template<typename Child, typename P, typename U>
 void is_named_impl(const volatile prefixed_unit<Child, P, U>*);
 
-template<Unit U, basic_symbol_text Symbol, PrefixFamily PF>
-void is_named_impl(const volatile alias_unit<U, Symbol, PF>*);
+template<typename U, basic_symbol_text Symbol>
+void is_named_impl(const volatile alias_unit<U, Symbol>*);
 
 template<typename U, typename P, typename AU>
 void is_named_impl(const volatile prefixed_alias_unit<U, P, AU>*);
@@ -223,6 +213,18 @@ void is_named_impl(const volatile prefixed_alias_unit<U, P, AU>*);
 template<Unit U>
 inline constexpr bool is_named<U> = requires(U * u) { is_named_impl(u); };
 
+template<typename Child, basic_symbol_text Symbol>
+void can_be_prefixed_impl(const volatile named_unit<Child, Symbol>*);
+
+template<typename Child, basic_symbol_text Symbol, ratio R, typename U>
+void can_be_prefixed_impl(const volatile named_scaled_unit<Child, Symbol, R, U>*);
+
+template<typename U, basic_symbol_text Symbol>
+void can_be_prefixed_impl(const volatile alias_unit<U, Symbol>*);
+
+template<Unit U>
+inline constexpr bool can_be_prefixed<U> = requires(U * u) { can_be_prefixed_impl(u); };
+
 }  // namespace detail
 
 }  // namespace units