Derived unit factory helpers refactored
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Mateusz Pusz committed
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@@ -246,62 +246,74 @@ concept Unit =
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detail::is_unit<downcast_base_t<T>>; // exposition only
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```
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The library provides a bunch of helpers to create the derived unit:
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- `named_coherent_derived_unit<Child, Dim, Symbol, PrefixType>`
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- `coherent_derived_unit<Child, Dim>`
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- `named_scaled_derived_unit<Child, Dim, Symbol, R, PrefixType = no_prefix>`
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- `named_deduced_derived_unit<Child, Dim, Symbol, PrefixType, U, Us...>`
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- `deduced_derived_unit<Child, Dim, U, Us..>`
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- `prefixed_derived_unit<Child, P, U>`
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Coherent derived units (units with `ratio<1>`) are created with a `named_coherent_derived_unit`
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or `coherent_derived_unit` class templates:
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```cpp
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template<typename Child, basic_fixed_string Symbol, Dimension D, typename PrefixType = no_prefix>
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struct named_coherent_derived_unit : downcast_child<Child, unit<D, ratio<1>>> {
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template<typename Child, Dimension Dim, basic_fixed_string Symbol, PrefixType PT>
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struct named_coherent_derived_unit : downcast_child<Child, unit<Dim, ratio<1>>> {
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static constexpr auto symbol = Symbol;
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using prefix_type = PrefixType;
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using prefix_type = PT;
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};
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template<typename Child, Dimension D, typename PrefixType = no_prefix>
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struct coherent_derived_unit : downcast_child<Child, unit<D, ratio<1>>> {
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template<typename Child, Dimension Dim>
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struct coherent_derived_unit : downcast_child<Child, unit<Dim, ratio<1>>> {
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static constexpr auto symbol = /* ... */;
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using prefix_type = PrefixType;
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using prefix_type = no_prefix;
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};
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```
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The above exposes public `prefix_type` member type and `symbol` used to print unit symbol
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names. `prefix_type` is a tag type used to identify the type of prefixes to be used (i.e. SI,
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data).
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data) and should satisfy the following concept:
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```cpp
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template<typename T>
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concept PrefixType = std::derived_from<T, prefix_type>;
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```
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For example to define the coherent unit of `length`:
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```cpp
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struct metre : named_coherent_derived_unit<metre, "m", length, si_prefix> {};
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struct metre : named_coherent_derived_unit<metre, length, "m", si_prefix> {};
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```
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Again, similarly to `derived_dimension`, the first class template parameter is a CRTP idiom used
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to provide downcasting facility (described below).
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To create the rest of derived units the following class templates can be used:
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To create scaled unit the following template should be used:
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```cpp
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template<typename Child, basic_fixed_string Symbol, Dimension D, Ratio R>
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struct named_derived_unit : downcast_child<Child, unit<D, R>> {
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template<typename Child, Dimension Dim, basic_fixed_string Symbol, Ratio R, PrefixType PT = no_prefix>
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struct named_scaled_derived_unit : downcast_child<Child, unit<Dim, R>> {
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static constexpr auto symbol = Symbol;
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using prefix_type = PT;
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};
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```
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User has to provide a symbol name (in case of a named unit), dimension, and a ratio relative
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to a coherent derived unit. For example to define `minute`:
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For example to define `minute`:
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```cpp
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struct minute : named_derived_unit<minute, "min", time, ratio<60>> {};
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struct minute : named_scaled_derived_unit<minute, time, "min", ratio<60>> {};
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```
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The `mp-units` library provides also a few helper class templates to simplify the above process.
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The `mp-units` library provides also a helper class templates to simplify the above process.
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For example to create a prefixed unit the following may be used:
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```cpp
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template<typename Child, Prefix P, Unit U>
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requires requires { U::symbol; }
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requires (!std::same_as<typename U::prefix_type, no_prefix>)
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struct prefixed_derived_unit : downcast_child<Child, unit<typename U::dimension,
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ratio_multiply<typename P::ratio,
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typename U::ratio>>> {
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ratio_multiply<typename P::ratio,
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typename U::ratio>>> {
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static constexpr auto symbol = P::symbol + U::symbol;
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using prefix_type = P::prefix_type;
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};
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@@ -310,8 +322,8 @@ struct prefixed_derived_unit : downcast_child<Child, unit<typename U::dimension,
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where `Prefix` is a concept requiring the instantiation of the following class template:
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```cpp
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template<typename Child, typename PrefixType, Ratio R, basic_fixed_string Symbol>
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struct prefix : downcast_child<Child, detail::prefix_base<PrefixType, R>> {
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template<typename Child, PrefixType PT, Ratio R, basic_fixed_string Symbol>
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struct prefix : downcast_child<Child, detail::prefix_base<PT, R>> {
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static constexpr auto symbol = Symbol;
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};
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```
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@@ -323,25 +335,28 @@ or its symbol and just has to do the following:
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struct kilometre : prefixed_derived_unit<kilometre, kilo, metre> {};
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```
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SI prefixes are predefined in the library and the user may easily predefined his/her own with:
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SI prefixes are predefined in the library and the user may easily provide his/her own with:
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```cpp
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struct data_prefix;
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struct data_prefix : units::prefix_type {};
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struct kibi : units::prefix<kibi, data_prefix, units::ratio<1'024>, "Ki"> {};
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struct kibi : units::prefix<kibi, data_prefix, units::ratio< 1'024>, "Ki"> {};
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struct mebi : units::prefix<mebi, data_prefix, units::ratio<1'048'576>, "Mi"> {};
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```
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For the cases where determining the exact ratio is not trivial another helper can be used:
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```cpp
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template<typename Child, basic_fixed_string Symbol, Dimension D, Unit U, Unit... Us>
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struct named_deduced_derived_unit : downcast_child<Child, detail::make_derived_unit<D, U, Us...>> {
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template<typename Child, Dimension Dim, basic_fixed_string Symbol, PrefixType PT, Unit U, Unit... Us>
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struct named_deduced_derived_unit : downcast_child<Child, /* magic to get the correct type */> {
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static constexpr auto symbol = Symbol;
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using prefix_type = PT;
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};
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template<typename Child, Dimension D, Unit U, Unit... Us>
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struct deduced_derived_unit : downcast_child<Child, detail::make_derived_unit<D, U, Us...>> {
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static constexpr auto symbol = /* ... */;
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template<typename Child, Dimension Dim, Unit U, Unit... Us>
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struct deduced_derived_unit : downcast_child<Child, /* magic to get the correct type */> {
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static constexpr auto symbol = /* even more magic to get the correct unit symbol */;
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using prefix_type = no_prefix;
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};
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```
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@@ -350,7 +365,7 @@ dimension and in case the symbol name is not explicitly provided it with create
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on the provided ingredients:
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```cpp
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struct mile_per_hour : deduced_derived_unit<mile_per_hour, velocity, mile, hour> {};
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struct kilometre_per_hour : deduced_derived_unit<kilometre_per_hour, velocity, kilometre, hour> {};
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```
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@@ -590,7 +605,7 @@ aliases.
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template<typename BaseType>
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struct downcast_base {
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using base_type = BaseType;
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friend auto downcast_guide(downcast_base);
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friend auto downcast_guide(downcast_base); // declaration only (no implementation)
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};
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```
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