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feat: 💥 unit_can_be_prefixed removed - from now on all named units can be prefixed

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Resolves #604
This commit is contained in:
Mateusz Pusz
2024-08-22 08:06:00 +02:00
parent aab0ef8ea0
commit 77625d63cb
7 changed files with 15 additions and 108 deletions
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16
docs/users_guide/framework_basics/concepts.md
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@@ -117,20 +117,8 @@ and is satisfied by:
### `PrefixableUnit<T>` { #PrefixableUnit }
`PrefixableUnit` concept is satisfied by all units derived from a `named_unit` class template for
which a customization point `unit_can_be_prefixed<T{}>` was not explicitly set to `false`. Such
units can be passed as an argument to a `prefixed_unit` class template.
??? abstract "Examples"
All units in the [SI](../../appendix/glossary.md#si) can be prefixed with SI-defined prefixes.
Some [off-system units](../../appendix/glossary.md#off-system-unit) like `non_si::day`
can't be prefixed. To enforce that, the following has to be provided:
```cpp
template<> inline constexpr bool unit_can_be_prefixed<non_si::day> = false;
```
`PrefixableUnit` concept is satisfied by all units derived from a `named_unit` class template.
Such units can be passed as an argument to a `prefixed_unit` class template.
### `UnitOf<T, V>` { #UnitOf }

18
docs/users_guide/framework_basics/systems_of_units.md
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@@ -190,21 +190,3 @@ inline constexpr struct mag_pi final : magnitude<std::numbers::pi_v<long double>
```cpp
inline constexpr struct degree final : named_unit<{u8"°", "deg"}, mag_pi / mag<180> * si::radian> {} degree;
```
!!! tip
The ISO 8000 and [SI](../../appendix/glossary.md#si) standards explicitly forbid using prefixes
with some units (e.g., day, minute, hour, degree Celsius). This is why the library disallows
this as well by providing specializations of the `unit_can_be_prefixed` variable template for
such units. Thanks to it trying to create a prefixed version for them will result in
a compile-time error.
However, some projects are not aware of those limitations and use prefixed version of such units
(e.g., [linux kernel uses millidegrees Celsius](https://github.com/search?q=repo%3Atorvalds%2Flinux+millidegree&type=code)).
To enable compatibility with those projects we can workaround the limitation in **mp-units**
by providing a scaled version of the unit explicitly:
```cpp
inline constexpr struct milli_degree_celsius final : named_unit<symbol_text{u8"m℃", "m`C"}, mag_ratio<1, 1000> * si::degree_Celsius> {} milli_degree_celsius;
inline constexpr auto mdeg_C = milli_degree_celsius;
```