mp-units/docs/users_guide/framework_basics/concepts.md

Concepts

This chapter enumerates all the user-facing concepts in the mp-units library.

Dimension<T>

Dimension concept matches a dimension of either a base or derived quantity:

• Base dimensions are explicitly defined by the user by inheriting from the instantiation of a base_dimension class template. It should be instantiated with a unique symbol identifier describing this dimension in a specific system of quantities.
• Derived dimensions are implicitly created by the library's framework based on the quantity equation provided in the quantity specification.

DimensionOf<T, V>

DimensionOf concept is satisfied when both arguments satisfy a Dimension concept and when they compare equal.

QuantitySpec<T>

QuantitySpec concept matches all the quantity specifications including:

• Base quantities defined by a user by inheriting from the quantity_spec class template instantiated with a base dimension argument.
• Derived named quantities defined by a user by inheriting from the quantity_spec class template instantiated with a result of a quantity equation passed as an argument.
• Other named quantities forming a hierarchy of quantities of the same kind defined by a user by inheriting from the quantity_spec class template instantiated with another "parent" quantity specification passed as an argument.
• Quantity kinds describing a family of mutually comparable quantities.
• Intermediate derived quantity specifications being a result of a quantity equations on other specifications.

QuantitySpecOf<T, V>

QuantitySpecOf concept is satisfied when both arguments satisfy a QuantitySpec concept and when T is implicitly convertible to V.

??? info "More details"

Additionally:

- `T` should not be a [nested quantity specification of `V`](dimensionless_quantities.md/#nested-quantity-kinds)
- either `T` is quantity kind or `V` should not be a
  [nested quantity specification of `T`](dimensionless_quantities.md/#nested-quantity-kinds)

Those additional conditions are required to make the following work:

```cpp
static_assert(ReferenceOf<si::radian, isq::angular_measure>);
static_assert(!ReferenceOf<si::radian, dimensionless>);
static_assert(!ReferenceOf<isq::angular_measure[si::radian], dimensionless>);
static_assert(ReferenceOf<one, isq::angular_measure>);
static_assert(!ReferenceOf<dimensionless[one], isq::angular_measure>);
```

Unit<T>

Unit concept matches all the units in the library including:

• Base units defined by a user by inheriting from the named_unit class template instantiated with a unique symbol identifier describing this unit in a specific system of units.
• Named scaled units defined by a user by inheriting from the named_unit class template instantiated with a unique symbol identifier and a product of multiplying another unit with some magnitude.
• Prefixed units defined by a user by inheriting from the prefixed_unit class template instantiated with a prefix symbol, a magnitude, and a unit to be prefixed.
• Derived named units defined by a user by inheriting from the named_unit class template instantiated with a unique symbol identifier and a result of unit equation passed as an argument.
• Derived unnamed units being a result of a unit equations on other units.

!!! note

In the **mp-units** library, [physical constants are also implemented as units](faster_than_lightspeed_constants.md).

AssociatedUnit<T>

AssociatedUnit concept describes a unit with an associated quantity and is satisfied by:

• All units derived from a named_unit class template instantiated with a unique symbol identifier and a QuantitySpec of a quantity kind.
• All units being a result of unit equations on other associated units.

??? abstract "Examples"

All units in the [SI](../../appendix/glossary.md#si) have associated quantities. For example,
`si::second` is specified to measure `isq::time`.

Natural units typically do not have an associated quantity. For example, if we assume `c = 1`,
a `natural::second` unit can be used to measure both `time` and `length`. In such case, `speed`
would have a unit of `one`.

PrefixableUnit<T>

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;
```

UnitOf<T, V>

UnitOf concept is satisfied for all units T matching an AssociatedUnit concept with an associated quantity type implicitly convertible to V.

??? info "More details"

Additionally, the kind of `V` and the kind of quantity type associated with `T` must be the same,
or the quantity type associated with `T` may not be derived from the kind of `V`.

This condition is required to make `dimensionless[si::radian]` invalid as `si::radian` should
be only used for `isq::angular_measure`, which is a
[nested quantity kind within the dimensionless quantities tree](dimensionless_quantities.md/#nested-quantity-kinds).

UnitCompatibleWith<T, V1, V2>

UnitCompatibleWith concept is satisfied for all units T when:

• V1 is a Unit,
• V2 is a QuantitySpec,
• T and V1 are defined in terms of the same reference unit,
• if T is an AssociatedUnit it should satisfy UnitOf<V2>.

Reference<T>

Reference concept is satisfied by all quantity reference types. Such types provide all the meta-information required to create a Quantity. A Reference can either be:

• An AssociatedUnit.
• The instantiation of a reference class template with a QuantitySpec passed as the first template argument and a Unit passed as the second one.

ReferenceOf<T, V>

ReferenceOf concept is satisfied by references T which have a quantity specification that satisfies QuantitySpecOf<V> concept. |

Representation<T>

Representation concept constraints a type of a number that stores the value of a quantity.

RepresentationOf<T, Ch>

RepresentationOf concept is satisfied by all Representation types that are of a specified quantity character Ch.

A user can declare a custom representation type to be of a specific character by providing the specialization with true for one or more of the following variable templates:

• is_scalar<T>
• is_vector<T>
• is_tensor<T>

??? tip

If we want to use scalar types to also express [vector quantities](character_of_a_quantity.md#defining-vector-and-tensor-quantities)
(e.g., ignoring the "direction" of the vector) the following definition can be provided to enable such a behavior:

```cpp
template<class T>
  requires mp_units::is_scalar<T>
inline constexpr bool mp_units::is_vector<T> = true;
```

Quantity<T>

Quantity concept matches every quantity in the library and is satisfied by all types being or deriving from an instantiation of a quantity class template.

QuantityOf<T, V>

QuantityOf concept is satisfied by all the quantities for which a QuantitySpecOf<V> is true.

PointOrigin<T>

PointOrigin concept matches all quantity point origins in the library. It is satisfied by either:

• All types derived from an absolute_point_origin class template.
• All types derived from a relative_point_origin class template.

PointOriginFor<T, V>

PointOriginFor concept is satisfied by all PointOrigin types that have quantity type implicitly convertible from quantity specification V, which means that V must satisfy QuantitySpecOf<T::quantity_spec>.

??? abstract "Examples"

`si::ice_point` can serve as a point origin for _points_ of `isq::Celsius_temperature` because this
quantity type implicitly converts to `isq::thermodynamic_temperature`.

However, if we define `mean_sea_level` in the following way:

```cpp
inline constexpr struct mean_sea_level : absolute_point_origin<isq::altitude> {} mean_sea_level;
```

then it can't be used as a point origin for _points_ of `isq::length` or `isq::width` as none of them
is implicitly convertible to `isq::altitude`:

- not every _length_ is an _altitude_,
- _width_ is not compatible with _altitude_.

QuantityPoint<T>

QuantityPoint concept is satisfied by all types being either a specialization or derived from quantity_point class template.

QuantityPointOf<T, V>

QuantityPointOf concept is satisfied by all the quantity points T that match the following value V:

V	Condition
QuantitySpec	The quantity point quantity specification satisfies QuantitySpecOf<V> concept.
PointOrigin	The point and V have the same absolute point origin.

QuantityLike<T>

QuantityLike concept provides interoperability with other libraries and is satisfied by a type T for which an instantiation of quantity_like_traits type trait yields a valid type that provides:

• Static data member reference that matches the Reference concept,
• rep type that matches RepresentationOf concept with the character provided in reference.
• to_numerical_value(T) static member function returning a raw value of the quantity packed in either convert_explicitly or convert_implicitly wrapper that enables implicit conversion in the latter case.
• from_numerical_value(rep) static member function returning T packed in either convert_explicitly or convert_implicitly wrapper that enables implicit conversion in the latter case.

??? abstract "Examples"

This is how support for `std::chrono::seconds` can be provided:

```cpp
template<>
struct mp_units::quantity_like_traits<std::chrono::seconds> {
  static constexpr auto reference = si::second;
  using rep = std::chrono::seconds::rep;

  [[nodiscard]] static constexpr convert_implicitly<rep> to_numerical_value(const std::chrono::seconds& q)
  {
    return q.count();
  }

  [[nodiscard]] static constexpr convert_implicitly<std::chrono::seconds> from_numerical_value(const rep& v)
  {
    return std::chrono::seconds(v);
  }
};

quantity q = 42s;
std::chrono::seconds dur = 42 * s;
```

QuantityPointLike<T>

QuantityPointLike concept provides interoperability with other libraries and is satisfied by a type T for which an instantiation of quantity_point_like_traits type trait yields a valid type that provides:

• Static data member reference that matches the Reference concept.
• Static data member point_origin that matches the PointOrigin concept.
• rep type that matches RepresentationOf concept with the character provided in reference.
• to_quantity(T) static member function returning the quantity being the offset of the point from the origin packed in either convert_explicitly or convert_implicitly wrapper that enables implicit conversion in the latter case.
• from_quantity(quantity<reference, rep>) static member function returning T packed in either convert_explicitly or convert_implicitly wrapper that enables implicit conversion in the latter case.

??? abstract "Examples"

This is how support for a `std::chrono::time_point` of `std::chrono::seconds` can be provided:

```cpp
template<typename C>
struct mp_units::quantity_point_like_traits<std::chrono::time_point<C, std::chrono::seconds>> {
  using T = std::chrono::time_point<C, std::chrono::seconds>;
  static constexpr auto reference = si::second;
  static constexpr struct point_origin : absolute_point_origin<isq::time> {} point_origin{};
  using rep = std::chrono::seconds::rep;

  [[nodiscard]] static constexpr convert_implicitly<quantity<reference, rep>> to_quantity(const T& qp)
  {
    return quantity{qp.time_since_epoch()};
  }

  [[nodiscard]] static constexpr convert_implicitly<T> from_quantity(const quantity<reference, rep>& q)
  {
    return T(q);
  }
};

quantity_point qp = time_point_cast<std::chrono::seconds>(std::chrono::system_clock::now());
std::chrono::sys_seconds q = qp + 42 * s;
```