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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
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:
```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>;
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];
```
@ -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`.
`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.