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docs: a tip with unit_can_be_prefixed workaround added

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Mateusz Pusz
2024-08-21 11:17:41 +02:00
parent ab51b47328
commit aab0ef8ea0
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18
docs/users_guide/framework_basics/systems_of_units.md
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@ -190,3 +190,21 @@ 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;
```