From 0125bd7762e48049cfd0a1eb201480ef2b7ba03a Mon Sep 17 00:00:00 2001
From: Mateusz Pusz <mateusz.pusz@gmail.com>
Date: Fri, 8 Nov 2019 16:51:45 +0000
Subject: [PATCH] Derived unit factory helpers refactored

---
 README.md                                     |  5 +-
 doc/DESIGN.md                                 | 77 +++++++++++--------
 src/include/units/dimensions/capacitance.h    |  2 +-
 src/include/units/dimensions/current.h        |  2 +-
 .../units/dimensions/electric_charge.h        |  2 +-
 src/include/units/dimensions/energy.h         |  2 +-
 src/include/units/dimensions/force.h          |  2 +-
 src/include/units/dimensions/frequency.h      |  2 +-
 src/include/units/dimensions/length.h         | 10 +--
 .../units/dimensions/luminous_intensity.h     |  2 +-
 src/include/units/dimensions/mass.h           |  2 +-
 src/include/units/dimensions/power.h          |  2 +-
 src/include/units/dimensions/pressure.h       |  2 +-
 src/include/units/dimensions/si_prefixes.h    |  2 +-
 src/include/units/dimensions/substance.h      |  2 +-
 src/include/units/dimensions/temperature.h    |  2 +-
 src/include/units/dimensions/time.h           |  6 +-
 src/include/units/dimensions/voltage.h        |  2 +-
 src/include/units/prefix.h                    | 17 ++--
 src/include/units/unit.h                      | 43 ++++++-----
 .../runtime/digital_information_test.cpp      |  6 +-
 test/unit_test/static/cgs_test.cpp            | 10 +--
 test/unit_test/static/custom_unit_test.cpp    |  6 +-
 23 files changed, 116 insertions(+), 92 deletions(-)

diff --git a/README.md b/README.md
index 77396d3f..b2331c6c 100644
--- a/README.md
+++ b/README.md
@@ -118,9 +118,10 @@ NOTE: This library as of now compiles correctly only with gcc-9.1 and newer.
   - `units` removed from a `std::experimental` namespace
   - Downcasting facility refactored so the user does not have to write the boilerplate code anymore
   - From now on base dimensions should inherit from `base_dimension` class template
-  - Added unit symbols definitions to `base_dimension` and `derived_unit`
-  - Added `coherent_derived_unit` helper
+  - Added unit symbols definitions to `base_dimension` and derived units
   - Added support for `operator<<` on `quantity`
+  - `fmt` support added
+  - Derived unit factory helpers refactored
   - Refactored the way prefixed units are defined
 
 - 0.3.1 Sep 18, 2019
diff --git a/doc/DESIGN.md b/doc/DESIGN.md
index 46a6aef9..c1a35c85 100644
--- a/doc/DESIGN.md
+++ b/doc/DESIGN.md
@@ -246,62 +246,74 @@ concept Unit =
     detail::is_unit<downcast_base_t<T>>;  // exposition only
 ```
 
+The library provides a bunch of helpers to create the derived unit:
+- `named_coherent_derived_unit<Child, Dim, Symbol, PrefixType>`
+- `coherent_derived_unit<Child, Dim>`
+- `named_scaled_derived_unit<Child, Dim, Symbol, R, PrefixType = no_prefix>`
+- `named_deduced_derived_unit<Child, Dim, Symbol, PrefixType, U, Us...>`
+- `deduced_derived_unit<Child, Dim, U, Us..>`
+- `prefixed_derived_unit<Child, P, U>`
+
 Coherent derived units (units with `ratio<1>`) are created with a `named_coherent_derived_unit`
 or `coherent_derived_unit` class templates:
 
 ```cpp
-template<typename Child, basic_fixed_string Symbol, Dimension D, typename PrefixType = no_prefix>
-struct named_coherent_derived_unit : downcast_child<Child, unit<D, ratio<1>>> {
+template<typename Child, Dimension Dim, basic_fixed_string Symbol, PrefixType PT>
+struct named_coherent_derived_unit : downcast_child<Child, unit<Dim, ratio<1>>> {
   static constexpr auto symbol = Symbol;
-  using prefix_type = PrefixType;
+  using prefix_type = PT;
 };
 
-template<typename Child, Dimension D, typename PrefixType = no_prefix>
-struct coherent_derived_unit : downcast_child<Child, unit<D, ratio<1>>> {
+template<typename Child, Dimension Dim>
+struct coherent_derived_unit : downcast_child<Child, unit<Dim, ratio<1>>> {
   static constexpr auto symbol = /* ... */;
-  using prefix_type = PrefixType;
+  using prefix_type = no_prefix;
 };
 ```
 
 The above exposes public `prefix_type` member type and `symbol` used to print unit symbol
 names. `prefix_type` is a tag type used to identify the type of prefixes to be used (i.e. SI,
-data).
+data) and should satisfy the following concept:
+
+```cpp
+template<typename T>
+concept PrefixType = std::derived_from<T, prefix_type>;
+```
 
 For example to define the coherent unit of `length`:
 
 ```cpp
-struct metre : named_coherent_derived_unit<metre, "m", length, si_prefix> {};
+struct metre : named_coherent_derived_unit<metre, length, "m", si_prefix> {};
 ```
 
 Again, similarly to `derived_dimension`, the first class template parameter is a CRTP idiom used
 to provide downcasting facility (described below).
 
-To create the rest of derived units the following class templates can be used:
+To create scaled unit the following template should be used:
 
 ```cpp
-template<typename Child, basic_fixed_string Symbol, Dimension D, Ratio R>
-struct named_derived_unit : downcast_child<Child, unit<D, R>> {
+template<typename Child, Dimension Dim, basic_fixed_string Symbol, Ratio R, PrefixType PT = no_prefix>
+struct named_scaled_derived_unit : downcast_child<Child, unit<Dim, R>> {
   static constexpr auto symbol = Symbol;
+  using prefix_type = PT;
 };
 ```
 
-User has to provide a symbol name (in case of a named unit), dimension, and a ratio relative
-to a coherent derived unit. For example to define `minute`:
+For example to define `minute`:
 
 ```cpp
-struct minute : named_derived_unit<minute, "min", time, ratio<60>> {};
+struct minute : named_scaled_derived_unit<minute, time, "min", ratio<60>> {};
 ```
 
-The `mp-units` library provides also a few helper class templates to simplify the above process.
-
+The `mp-units` library provides also a helper class templates to simplify the above process.
 For example to create a prefixed unit the following may be used:
 
 ```cpp
 template<typename Child, Prefix P, Unit U>
-  requires requires { U::symbol; }
+  requires (!std::same_as<typename U::prefix_type, no_prefix>)
 struct prefixed_derived_unit : downcast_child<Child, unit<typename U::dimension,
-                                                           ratio_multiply<typename P::ratio,
-                                                                          typename U::ratio>>> {
+                                                          ratio_multiply<typename P::ratio,
+                                                                         typename U::ratio>>> {
   static constexpr auto symbol = P::symbol + U::symbol;
   using prefix_type = P::prefix_type;
 };
@@ -310,8 +322,8 @@ struct prefixed_derived_unit : downcast_child<Child, unit<typename U::dimension,
 where `Prefix` is a concept requiring the instantiation of the following class template:
 
 ```cpp
-template<typename Child, typename PrefixType, Ratio R, basic_fixed_string Symbol>
-struct prefix : downcast_child<Child, detail::prefix_base<PrefixType, 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;
 };
 ```
@@ -323,25 +335,28 @@ or its symbol and just has to do the following:
 struct kilometre : prefixed_derived_unit<kilometre, kilo, metre> {};
 ```
 
-SI prefixes are predefined in the library and the user may easily predefined his/her own with:
+SI prefixes are predefined in the library and the user may easily provide his/her own with:
 
 ```cpp
-struct data_prefix;
+struct data_prefix : units::prefix_type {};
 
-struct kibi : units::prefix<kibi, data_prefix, units::ratio<1'024>, "Ki"> {};
+struct kibi : units::prefix<kibi, data_prefix, units::ratio<    1'024>, "Ki"> {};
+struct mebi : units::prefix<mebi, data_prefix, units::ratio<1'048'576>, "Mi"> {};
 ```
 
 For the cases where determining the exact ratio is not trivial another helper can be used:
 
 ```cpp
-template<typename Child, basic_fixed_string Symbol, Dimension D, Unit U, Unit... Us>
-struct named_deduced_derived_unit : downcast_child<Child, detail::make_derived_unit<D, U, Us...>> {
+template<typename Child, Dimension Dim, basic_fixed_string Symbol, PrefixType PT, Unit U, Unit... Us>
+struct named_deduced_derived_unit : downcast_child<Child, /* magic to get the correct type */> {
   static constexpr auto symbol = Symbol;
+  using prefix_type = PT;
 };
 
-template<typename Child, Dimension D, Unit U, Unit... Us>
-struct deduced_derived_unit : downcast_child<Child, detail::make_derived_unit<D, U, Us...>> {
-  static constexpr auto symbol = /* ... */;
+template<typename Child, Dimension Dim, Unit U, Unit... Us>
+struct deduced_derived_unit : downcast_child<Child, /* magic to get the correct type */> {
+  static constexpr auto symbol = /* even more magic to get the correct unit symbol */;
+  using prefix_type = no_prefix;
 };
 ```
 
@@ -350,7 +365,7 @@ dimension and in case the symbol name is not explicitly provided it with create
 on the provided ingredients:
 
 ```cpp
-struct mile_per_hour : deduced_derived_unit<mile_per_hour, velocity, mile, hour> {};
+struct kilometre_per_hour : deduced_derived_unit<kilometre_per_hour, velocity, kilometre, hour> {};
 ```
 
 
@@ -590,7 +605,7 @@ aliases.
 template<typename BaseType>
 struct downcast_base {
   using base_type = BaseType;
-  friend auto downcast_guide(downcast_base);
+  friend auto downcast_guide(downcast_base);  // declaration only (no implementation)
 };
 ```
 
diff --git a/src/include/units/dimensions/capacitance.h b/src/include/units/dimensions/capacitance.h
index 42558471..736fc9a6 100644
--- a/src/include/units/dimensions/capacitance.h
+++ b/src/include/units/dimensions/capacitance.h
@@ -33,7 +33,7 @@ namespace units {
   template<typename T>
   concept Capacitance =  QuantityOf<T, capacitance>;
 
-  struct farad : named_coherent_derived_unit<farad, "F", capacitance, si_prefix> {};
+  struct farad : named_coherent_derived_unit<farad, capacitance, "F", si_prefix> {};
 
   inline namespace literals {
 
diff --git a/src/include/units/dimensions/current.h b/src/include/units/dimensions/current.h
index bf2c608f..072f9a8e 100644
--- a/src/include/units/dimensions/current.h
+++ b/src/include/units/dimensions/current.h
@@ -32,7 +32,7 @@ namespace units {
   template<typename T>
   concept Current = QuantityOf<T, current>;
 
-  struct ampere : named_coherent_derived_unit<ampere, "A", current, si_prefix> {};
+  struct ampere : named_coherent_derived_unit<ampere, current, "A", si_prefix> {};
 
   inline namespace literals {
 
diff --git a/src/include/units/dimensions/electric_charge.h b/src/include/units/dimensions/electric_charge.h
index 98c0f118..20586ef8 100644
--- a/src/include/units/dimensions/electric_charge.h
+++ b/src/include/units/dimensions/electric_charge.h
@@ -33,7 +33,7 @@ namespace units {
   template<typename T>
   concept ElectricCharge =  QuantityOf<T, electric_charge>;
 
-  struct coulomb : named_coherent_derived_unit<coulomb, "C", electric_charge, si_prefix> {};
+  struct coulomb : named_coherent_derived_unit<coulomb, electric_charge, "C", si_prefix> {};
 
   inline namespace literals {
 
diff --git a/src/include/units/dimensions/energy.h b/src/include/units/dimensions/energy.h
index 51c7fea5..709a1b14 100644
--- a/src/include/units/dimensions/energy.h
+++ b/src/include/units/dimensions/energy.h
@@ -34,7 +34,7 @@ namespace units {
   template<typename T>
   concept Energy =  QuantityOf<T, energy>;
 
-  struct joule : named_coherent_derived_unit<joule, "J", energy, si_prefix> {};
+  struct joule : named_coherent_derived_unit<joule, energy, "J", si_prefix> {};
   struct millijoule : prefixed_derived_unit<millijoule, milli, joule> {};
   struct kilojoule : prefixed_derived_unit<kilojoule, kilo, joule> {};
   struct megajoule : prefixed_derived_unit<megajoule, mega, joule> {};
diff --git a/src/include/units/dimensions/force.h b/src/include/units/dimensions/force.h
index 691f2988..798b3220 100644
--- a/src/include/units/dimensions/force.h
+++ b/src/include/units/dimensions/force.h
@@ -33,7 +33,7 @@ namespace units {
   template<typename T>
   concept Force =  QuantityOf<T, force>;
 
-  struct newton : named_coherent_derived_unit<newton, "N", force, si_prefix> {};
+  struct newton : named_coherent_derived_unit<newton, force, "N", si_prefix> {};
 
   inline namespace literals {
 
diff --git a/src/include/units/dimensions/frequency.h b/src/include/units/dimensions/frequency.h
index 39bd2ebd..ff01380f 100644
--- a/src/include/units/dimensions/frequency.h
+++ b/src/include/units/dimensions/frequency.h
@@ -33,7 +33,7 @@ namespace units {
   template<typename T>
   concept Frequency =  QuantityOf<T, frequency>;
 
-  struct hertz : named_coherent_derived_unit<hertz, "Hz", frequency, si_prefix> {};
+  struct hertz : named_coherent_derived_unit<hertz, frequency, "Hz", si_prefix> {};
   struct millihertz : prefixed_derived_unit<millihertz, milli, hertz> {};
   struct kilohertz : prefixed_derived_unit<kilohertz, kilo, hertz> {};
   struct megahertz : prefixed_derived_unit<megahertz, mega, hertz> {};
diff --git a/src/include/units/dimensions/length.h b/src/include/units/dimensions/length.h
index 26c9fa6b..608c87bb 100644
--- a/src/include/units/dimensions/length.h
+++ b/src/include/units/dimensions/length.h
@@ -34,7 +34,7 @@ namespace units {
   concept Length = QuantityOf<T, length>;
 
   // SI units
-  struct metre : named_coherent_derived_unit<metre, "m", length, si_prefix> {};
+  struct metre : named_coherent_derived_unit<metre, length, "m", si_prefix> {};
   struct millimetre : prefixed_derived_unit<millimetre, milli, metre> {};
   struct centimetre : prefixed_derived_unit<centimetre, centi, metre> {};
   struct kilometre : prefixed_derived_unit<kilometre, kilo, metre> {};
@@ -60,10 +60,10 @@ namespace units {
   } // namespace literals
 
   // US customary units
-  struct yard : named_derived_unit<yard, "yd", length, ratio<9'144, 10'000>> {};
-  struct foot : named_derived_unit<foot, "ft", length, ratio_multiply<ratio<1, 3>, yard::ratio>> {};
-  struct inch : named_derived_unit<inch, "in", length, ratio_multiply<ratio<1, 12>, foot::ratio>> {};
-  struct mile : named_derived_unit<mile, "mi", length, ratio_multiply<ratio<1'760>, yard::ratio>> {};
+  struct yard : named_scaled_derived_unit<yard, length, "yd", ratio<9'144, 10'000>> {};
+  struct foot : named_scaled_derived_unit<foot, length, "ft", ratio_multiply<ratio<1, 3>, yard::ratio>> {};
+  struct inch : named_scaled_derived_unit<inch, length, "in", ratio_multiply<ratio<1, 12>, foot::ratio>> {};
+  struct mile : named_scaled_derived_unit<mile, length, "mi", ratio_multiply<ratio<1'760>, yard::ratio>> {};
 
   inline namespace literals {
 
diff --git a/src/include/units/dimensions/luminous_intensity.h b/src/include/units/dimensions/luminous_intensity.h
index dcf8803d..736da752 100644
--- a/src/include/units/dimensions/luminous_intensity.h
+++ b/src/include/units/dimensions/luminous_intensity.h
@@ -32,7 +32,7 @@ namespace units {
   template<typename T>
   concept LuminousIntensity = QuantityOf<T, luminous_intensity>;
 
-  struct candela : named_coherent_derived_unit<candela, "cd", luminous_intensity, si_prefix> {};
+  struct candela : named_coherent_derived_unit<candela, luminous_intensity, "cd", si_prefix> {};
 
   inline namespace literals {
 
diff --git a/src/include/units/dimensions/mass.h b/src/include/units/dimensions/mass.h
index b6e72687..d5da2587 100644
--- a/src/include/units/dimensions/mass.h
+++ b/src/include/units/dimensions/mass.h
@@ -32,7 +32,7 @@ namespace units {
   template<typename T>
   concept Mass = QuantityOf<T, mass>;
 
-  struct gram : named_derived_unit<gram, "g", mass, ratio<1, 1000>> {};
+  struct gram : named_scaled_derived_unit<gram, mass, "g", ratio<1, 1000>, si_prefix> {};
   struct kilogram : prefixed_derived_unit<kilogram, kilo, gram> {};
 
   inline namespace literals {
diff --git a/src/include/units/dimensions/power.h b/src/include/units/dimensions/power.h
index 3d8ead4b..d83642da 100644
--- a/src/include/units/dimensions/power.h
+++ b/src/include/units/dimensions/power.h
@@ -33,7 +33,7 @@ namespace units {
   template<typename T>
   concept Power =  QuantityOf<T, power>;
 
-  struct watt : named_coherent_derived_unit<watt, "W", power, si_prefix> {};
+  struct watt : named_coherent_derived_unit<watt, power, "W", si_prefix> {};
   struct milliwatt : prefixed_derived_unit<milliwatt, milli, watt> {};
   struct kilowatt : prefixed_derived_unit<kilowatt, kilo, watt> {};
   struct megawatt : prefixed_derived_unit<megawatt, mega, watt> {};
diff --git a/src/include/units/dimensions/pressure.h b/src/include/units/dimensions/pressure.h
index 85479866..4302ed26 100644
--- a/src/include/units/dimensions/pressure.h
+++ b/src/include/units/dimensions/pressure.h
@@ -33,7 +33,7 @@ namespace units {
   template<typename T>
   concept Pressure =  QuantityOf<T, pressure>;
 
-  struct pascal : named_coherent_derived_unit<pascal, "Pa", pressure, si_prefix> {};
+  struct pascal : named_coherent_derived_unit<pascal, pressure, "Pa", si_prefix> {};
 
   inline namespace literals {
 
diff --git a/src/include/units/dimensions/si_prefixes.h b/src/include/units/dimensions/si_prefixes.h
index 9f43bbb3..a2d5bc5c 100644
--- a/src/include/units/dimensions/si_prefixes.h
+++ b/src/include/units/dimensions/si_prefixes.h
@@ -28,7 +28,7 @@
 namespace units {
 
   // prefix tags
-  struct si_prefix;
+  struct si_prefix : prefix_type {};
 
   // SI prefixes
 
diff --git a/src/include/units/dimensions/substance.h b/src/include/units/dimensions/substance.h
index 191422aa..54b15fa4 100644
--- a/src/include/units/dimensions/substance.h
+++ b/src/include/units/dimensions/substance.h
@@ -32,7 +32,7 @@ namespace units {
   template<typename T>
   concept Substance = QuantityOf<T, substance>;
 
-  struct mole : named_coherent_derived_unit<mole, "mol", substance, si_prefix> {};
+  struct mole : named_coherent_derived_unit<mole, substance, "mol", si_prefix> {};
 
   inline namespace literals {
 
diff --git a/src/include/units/dimensions/temperature.h b/src/include/units/dimensions/temperature.h
index d15e9b68..5a26a56a 100644
--- a/src/include/units/dimensions/temperature.h
+++ b/src/include/units/dimensions/temperature.h
@@ -32,7 +32,7 @@ namespace units {
   template<typename T>
   concept ThermodynamicTemperature = QuantityOf<T, temperature>;
 
-  struct kelvin : named_coherent_derived_unit<kelvin, "K", temperature> {};
+  struct kelvin : named_coherent_derived_unit<kelvin, temperature, "K", no_prefix> {};
 
   inline namespace literals {
 
diff --git a/src/include/units/dimensions/time.h b/src/include/units/dimensions/time.h
index 4f96c1fc..3c60e384 100644
--- a/src/include/units/dimensions/time.h
+++ b/src/include/units/dimensions/time.h
@@ -33,12 +33,12 @@ namespace units {
   template<typename T>
   concept Time = QuantityOf<T, time>;
 
-  struct second : named_coherent_derived_unit<second, "s", time, si_prefix> {};
+  struct second : named_coherent_derived_unit<second, time, "s", si_prefix> {};
   struct nanosecond : prefixed_derived_unit<nanosecond, nano, second> {};
   struct microsecond : prefixed_derived_unit<microsecond, micro, second> {};
   struct millisecond : prefixed_derived_unit<millisecond, milli, second> {};
-  struct minute : named_derived_unit<minute, "min", time, ratio<60>> {};
-  struct hour : named_derived_unit<hour, "h", time, ratio<3600>> {};
+  struct minute : named_scaled_derived_unit<minute, time, "min", ratio<60>> {};
+  struct hour : named_scaled_derived_unit<hour, time, "h", ratio<3600>> {};
 
   inline namespace literals {
 
diff --git a/src/include/units/dimensions/voltage.h b/src/include/units/dimensions/voltage.h
index 688123ef..b4581061 100644
--- a/src/include/units/dimensions/voltage.h
+++ b/src/include/units/dimensions/voltage.h
@@ -35,7 +35,7 @@ namespace units {
   template<typename T>
   concept Voltage =  QuantityOf<T, voltage>;
 
-  struct volt : named_coherent_derived_unit<volt, "V", voltage, si_prefix> {};
+  struct volt : named_coherent_derived_unit<volt, voltage, "V", si_prefix> {};
 
   inline namespace literals {
 
diff --git a/src/include/units/prefix.h b/src/include/units/prefix.h
index 6a12c288..1677644b 100644
--- a/src/include/units/prefix.h
+++ b/src/include/units/prefix.h
@@ -28,18 +28,23 @@
 
 namespace units {
 
+  struct prefix_type {};
+
+  template<typename T>
+  concept PrefixType = std::derived_from<T, prefix_type>;
+
   namespace detail {
 
-    template<typename PrefixType, Ratio R>
-    struct prefix_base : downcast_base<prefix_base<PrefixType, R>> {
-      using prefix_type = PrefixType;
+    template<PrefixType PT, Ratio R>
+    struct prefix_base : downcast_base<prefix_base<PT, R>> {
+      using prefix_type = PT;
       using ratio = R;
     };
 
   }
 
-  template<typename Child, typename PrefixType, Ratio R, basic_fixed_string Symbol>
-  struct prefix : downcast_child<Child, detail::prefix_base<PrefixType, 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;
   };
 
@@ -60,6 +65,6 @@ namespace units {
 //  concept Prefix = detail::is_prefix<T>;
   concept Prefix = true;
 
-  struct no_prefix;
+  struct no_prefix : prefix_type {};
 
 }  // namespace units
diff --git a/src/include/units/unit.h b/src/include/units/unit.h
index a4c9b117..218e80c8 100644
--- a/src/include/units/unit.h
+++ b/src/include/units/unit.h
@@ -273,38 +273,41 @@ namespace units {
 
   // derived_unit
 
-  template<typename Child, basic_fixed_string Symbol, Dimension D, typename PrefixType = no_prefix>
-  struct named_coherent_derived_unit : downcast_child<Child, unit<D, ratio<1>>> {
+  template<typename Child, Dimension Dim, basic_fixed_string Symbol, PrefixType PT>
+  struct named_coherent_derived_unit : downcast_child<Child, unit<Dim, ratio<1>>> {
     static constexpr auto symbol = Symbol;
-    using prefix_type = PrefixType;
+    using prefix_type = PT;
   };
 
-  template<typename Child, Dimension D, typename PrefixType = no_prefix>
-  struct coherent_derived_unit : downcast_child<Child, unit<D, ratio<1>>> {
-    static constexpr auto symbol = detail::symbol_text(D());
-    using prefix_type = PrefixType;
+  template<typename Child, Dimension Dim>
+  struct coherent_derived_unit : downcast_child<Child, unit<Dim, ratio<1>>> {
+    static constexpr auto symbol = detail::symbol_text(Dim());
+    using prefix_type = no_prefix;
   };
 
-  template<typename Child, basic_fixed_string Symbol, Dimension D, Ratio R>
-  struct named_derived_unit : downcast_child<Child, unit<D, R>> {
+  template<typename Child, Dimension Dim, basic_fixed_string Symbol, Ratio R, PrefixType PT = no_prefix>
+  struct named_scaled_derived_unit : downcast_child<Child, unit<Dim, R>> {
     static constexpr auto symbol = Symbol;
+    using prefix_type = PT;
+  };
+
+  template<typename Child, Dimension Dim, basic_fixed_string Symbol, PrefixType PT, Unit U, Unit... Us>
+  struct named_deduced_derived_unit : downcast_child<Child, detail::make_derived_unit<Dim, U, Us...>> {
+    static constexpr auto symbol = Symbol;
+    using prefix_type = PT;
+  };
+
+  template<typename Child, Dimension Dim, Unit U, Unit... Us>
+  struct deduced_derived_unit : downcast_child<Child, detail::make_derived_unit<Dim, U, Us...>> {
+    static constexpr auto symbol = basic_fixed_string("bbb");
+    using prefix_type = no_prefix;
   };
 
   template<typename Child, Prefix P, Unit U>
-    requires requires { U::symbol; }
+    requires (!std::same_as<typename U::prefix_type, no_prefix>)
   struct prefixed_derived_unit : downcast_child<Child, unit<typename U::dimension, ratio_multiply<typename P::ratio, typename U::ratio>>> {
     static constexpr auto symbol = P::symbol + U::symbol;
     using prefix_type = P::prefix_type;
   };
 
-  template<typename Child, basic_fixed_string Symbol, Dimension D, Unit U, Unit... Us>
-  struct named_deduced_derived_unit : downcast_child<Child, detail::make_derived_unit<D, U, Us...>> {
-    static constexpr auto symbol = Symbol;
-  };
-
-  template<typename Child, Dimension D, Unit U, Unit... Us>
-  struct deduced_derived_unit : downcast_child<Child, detail::make_derived_unit<D, U, Us...>> {
-    static constexpr auto symbol = basic_fixed_string("bbb");
-  };
-
 }  // namespace units
diff --git a/test/unit_test/runtime/digital_information_test.cpp b/test/unit_test/runtime/digital_information_test.cpp
index 822ac5c1..600c17e6 100644
--- a/test/unit_test/runtime/digital_information_test.cpp
+++ b/test/unit_test/runtime/digital_information_test.cpp
@@ -33,14 +33,14 @@ namespace data {
   template<typename T>
   concept DigitalInformation = units::QuantityOf<T, digital_information>;
 
-  struct data_prefix;
+  struct data_prefix : units::prefix_type {};
 
   struct kibi : units::prefix<kibi, data_prefix, units::ratio<    1'024>, "Ki"> {};
   struct mebi : units::prefix<mebi, data_prefix, units::ratio<1'048'576>, "Mi"> {};
 
-  struct bit : units::named_coherent_derived_unit<bit, "b", digital_information, data_prefix> {};
+  struct bit : units::named_coherent_derived_unit<bit, digital_information, "b", data_prefix> {};
   struct kilobit : units::prefixed_derived_unit<kilobit, kibi, bit> {};
-  struct byte : units::named_derived_unit<byte, "B", digital_information, units::ratio<8>> {};
+  struct byte : units::named_scaled_derived_unit<byte, digital_information, "B", units::ratio<8>, data_prefix> {};
   struct kilobyte : units::prefixed_derived_unit<kilobyte, kibi, byte> {};
 
   inline namespace literals {
diff --git a/test/unit_test/static/cgs_test.cpp b/test/unit_test/static/cgs_test.cpp
index ab8e3140..341126f2 100644
--- a/test/unit_test/static/cgs_test.cpp
+++ b/test/unit_test/static/cgs_test.cpp
@@ -31,11 +31,11 @@ namespace cgs {
   using units::gram;
   using units::second;
   struct centimetre_per_second : units::deduced_derived_unit<centimetre_per_second, units::velocity, centimetre, second> {};
-  struct gal : units::named_deduced_derived_unit<gal, "Gal", units::acceleration, centimetre, second> {};
-  struct dyne : units::named_deduced_derived_unit<dyne, "dyn", units::force, centimetre, gram, second> {};
-  struct erg : units::named_deduced_derived_unit<erg, "erg", units::energy, centimetre, gram, second> {};
-  struct ergps : units::named_deduced_derived_unit<ergps, "erg/s", units::power, centimetre, gram, second> {};   // TODO make it work for erg and non-named
-  struct barye : units::named_deduced_derived_unit<barye, "Ba", units::pressure, centimetre, gram, second> {};
+  struct gal : units::named_deduced_derived_unit<gal, units::acceleration, "Gal", units::no_prefix, centimetre, second> {};
+  struct dyne : units::named_deduced_derived_unit<dyne, units::force, "dyn", units::no_prefix, centimetre, gram, second> {};
+  struct erg : units::named_deduced_derived_unit<erg, units::energy, "erg", units::no_prefix, centimetre, gram, second> {};
+  struct ergps : units::named_deduced_derived_unit<ergps, units::power, "erg/s", units::no_prefix, centimetre, gram, second> {};   // TODO make it work for erg and non-named
+  struct barye : units::named_deduced_derived_unit<barye, units::pressure, "Ba", units::no_prefix, centimetre, gram, second> {};
 
 
   inline namespace literals {
diff --git a/test/unit_test/static/custom_unit_test.cpp b/test/unit_test/static/custom_unit_test.cpp
index 95f923ec..2df947e8 100644
--- a/test/unit_test/static/custom_unit_test.cpp
+++ b/test/unit_test/static/custom_unit_test.cpp
@@ -35,14 +35,14 @@ namespace {
   template<typename T>
   concept DigitalInformation = units::QuantityOf<T, digital_information>;
 
-  struct data_prefix {};
+  struct data_prefix : units::prefix_type {};
 
   struct kibi : units::prefix<kibi, data_prefix, units::ratio<    1'024>, "Ki"> {};
 
-  struct bit : units::named_coherent_derived_unit<bit, "b", digital_information, data_prefix> {};
+  struct bit : units::named_coherent_derived_unit<bit, digital_information, "b", data_prefix> {};
   struct kilobit : units::prefixed_derived_unit<kilobit, kibi, bit> {};
 
-  struct byte : units::named_derived_unit<byte, "B", digital_information, units::ratio<8>> {};
+  struct byte : units::named_scaled_derived_unit<byte, digital_information, "B", units::ratio<8>, data_prefix> {};
   struct kilobyte : units::prefixed_derived_unit<kilobyte, kibi, byte> {};
 
   inline namespace literals {