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docs: old customization poits removed from the "Character of a Quantity" chapter

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Mateusz Pusz
2025-02-12 18:58:15 +01:00
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docs/users_guide/framework_basics/character_of_a_quantity.md
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@ -168,9 +168,6 @@ associated with this quantity type.
However, thanks to the provided customization points, any linear algebra library types can be used However, thanks to the provided customization points, any linear algebra library types can be used
as a vector or tensor quantity representation type. as a vector or tensor quantity representation type.
To enable the usage of a user-defined type as a representation type for vector or tensor quantities,
we need to provide a partial specialization of `is_vector` or `is_tensor` customization points.
For example, here is how it can be done for the [P1385](https://wg21.link/p1385) types: For example, here is how it can be done for the [P1385](https://wg21.link/p1385) types:
```cpp ```cpp
@ -178,13 +175,6 @@ For example, here is how it can be done for the [P1385](https://wg21.link/p1385)
using la_vector = STD_LA::fixed_size_column_vector<double, 3>; using la_vector = STD_LA::fixed_size_column_vector<double, 3>;
template<>
constexpr bool mp_units::is_vector<la_vector> = true;
```
With the above, we can use `la_vector` as a representation type for our quantity:
```cpp
Quantity auto q = la_vector{1, 2, 3} * isq::velocity[m / s]; Quantity auto q = la_vector{1, 2, 3} * isq::velocity[m / s];
``` ```
@ -219,32 +209,3 @@ either:
In all the cases above, the SI unit `m / s` has an associated scalar quantity of `isq::length / isq::time`. In all the cases above, the SI unit `m / s` has an associated scalar quantity of `isq::length / isq::time`.
`la_vector` is not a correct representation type for a scalar quantity so the construction fails. `la_vector` is not a correct representation type for a scalar quantity so the construction fails.
## Hacking the character
Sometimes we want to use a vector quantity, but we don't care about its direction. For example,
the standard gravity acceleration constant always points down, so we might not care about this
in a particular scenario. In such a case, we may want to "hack" the library to allow scalar types
to be used as a representation type for scalar quantities.
For example, we can do the following:
```cpp
template<class T>
requires mp_units::is_scalar<T>
constexpr bool mp_units::is_vector<T> = true;
```
which says that every type that can be used as a scalar representation is also allowed for vector
quantities.
Doing the above is actually not such a big "hack" as the ISO 80000 explicitly allows it:
!!! quote "ISO 80000-2"
A vector is a tensor of the first order and a scalar is a tensor of order zero.
Despite it being allowed by ISO 80000, for type-safety reasons, we do not allow such a behavior
by default, and a user has to opt into such scenarios explicitly.