Design Overview¶

The most important entities in the mp-units library are:

quantity,
quantity point,
unit,
dimension,
quantity specification
quantity reference,
quantity representation.

The graph provided below presents how those and a few other entities depend on each other:

flowchart TD
    Unit --- Reference
    Dimension --- QuantitySpec["Quantity specification"]
    quantity_character["Quantity character"] --- QuantitySpec
    QuantitySpec --- Reference["Quantity reference"]
    Reference --- Quantity
    quantity_character -.- Representation
    Representation --- Quantity
    Quantity --- QuantityPoint["Quantity point"]
    PointOrigin["Point origin"] --- QuantityPoint

    click Dimension "#dimension"
    click quantity_character "#quantity-character"
    click QuantitySpec "#quantity-specification"
    click Unit "#unit"
    click Reference "#quantity-reference"
    click Representation "#quantity-representation"
    click Quantity "#quantity"
    click PointOrigin "#point-origin"
    click QuantityPoint "#quantity-point"

Dimension¶

Dimension specifies the dependence of a quantity on the base quantities of a particular system of quantities. It is represented as a product of powers of factors corresponding to the base quantities, omitting any numerical factor.

In the mp-units library, we use the terms:

base dimension to refer to the dimension of a base quantity,
derived dimension to refer to the dimension of a derived quantity.

For example:

length (\(\mathsf{L}\)), mass (\(\mathsf{M}\)), time (\(\mathsf{T}\)), electric current (\(\mathsf{I}\)), thermodynamic temperature (\(\mathsf{Θ}\)), amount of substance (\(\mathsf{N}\)), and luminous intensity (\(\mathsf{J}\)) are the base dimensions of the ISQ.
A derived dimension of force in the ISQ is denoted by \(\textsf{dim }F = \mathsf{LMT}^{–2}\).
The implementation of IEC 80000 in this library provides iec80000::dim_traffic_intensity base dimension to extend ISQ with strong information technology quantities.

Base dimensions can be defined by the user in the following way:

inline constexpr struct dim_length : base_dimension<"L"> {} dim_length;
inline constexpr struct dim_time : base_dimension<"T"> {} dim_time;

Derived dimensions are implicitly created by the library's framework based on the quantity equation provided in the quantity specification:

C++23C++20Portable

inline constexpr struct length : quantity_spec<dim_length> {} length;
inline constexpr struct time : quantity_spec<dim_time> {} time;
inline constexpr struct speed : quantity_spec<length / time> {} speed;

static_assert(speed.dimension == dim_length / dim_time);

inline constexpr struct length : quantity_spec<length, dim_length> {} length;
inline constexpr struct time : quantity_spec<time, dim_time> {} time;
inline constexpr struct speed : quantity_spec<speed, length / time> {} speed;

static_assert(speed.dimension == dim_length / dim_time);

QUANTITY_SPEC(length, dim_length);
QUANTITY_SPEC(time, dim_time);
QUANTITY_SPEC(speed, length / time);

static_assert(speed.dimension == dim_length / dim_time);

Important

Users should not explicitly define any derived dimensions. Those should always be implicitly created by the framework.

The multiplication/division on quantity specifications also multiplies/divides their dimensions:

static_assert((length / time).dimension == dim_length / dim_time);

The dimension equation of isq::dim_length / isq::dim_time results in the derived_dimension<isq::dim_length, per<isq::dim_time>> type.

Quantity character¶

ISO 80000 explicitly states that quantities (even of the same kind) may have different characters:

scalar,
vector,
tensor.

The quantity character in the mp-units library is implemented with the quantity_character enumeration:

enum class quantity_character { scalar, vector, tensor };

Info

You can read more on quantity characters in the "Character of a Quantity" chapter.

Quantity specification¶

Dimension is not enough to describe a quantity. This is why the ISO 80000 provides hundreds of named quantity types. It turns out that there are many more quantity types in the ISQ than the named units in the SI.

This is why the mp-units library introduces a quantity specification entity that stores:

dimension,
quantity type/name,
quantity character,
the quantity equation being the recipe to create this quantity (only for derived quantities that specify such a recipe).

Note

We know that it might be sometimes confusing to talk about quantities, quantity types/names, and quantity specifications. However, it might be important to notice here that even the ISO 80000 admits that:

It is customary to use the same term, "quantity", to refer to both general quantities, such as length, mass, etc., and their instances, such as given lengths, given masses, etc. Accordingly, we are used to saying both that length is a quantity and that a given length is a quantity by maintaining the specification – "general quantity, \(Q\)" or "individual quantity, \(Q_\textsf{a}\)" – implicit and exploiting the linguistic context to remove the ambiguity.

In the mp-units library, we have a:

quantity - implemented as a quantity class template,
quantity specification - implemented with a quantity_spec class template that among others identifies a specific quantity type/name.

For example:

isq::length, isq::mass, isq::time, isq::electric_current, isq::thermodynamic_temperature, isq::amount_of_substance, and isq::luminous_intensity are the specifications of base quantities in the ISQ.
isq::width, isq::height, isq::radius, and isq::position_vector are only a few of many quantities of a kind length specified in the ISQ.
isq::area, isq::speed, isq::moment_of_force are only a few of many derived quantities provided in the ISQ.

Quantity specification can be defined by the user in one of the following ways:

C++23C++20Portable

inline constexpr struct length : quantity_spec<dim_length> {} length;
inline constexpr struct height : quantity_spec<length> {} height;
inline constexpr struct speed : quantity_spec<length / time> {} speed;

inline constexpr struct length : quantity_spec<length, dim_length> {} length;
inline constexpr struct height : quantity_spec<height, length> {} height;
inline constexpr struct speed : quantity_spec<speed, length / time> {} speed;

QUANTITY_SPEC(length, dim_length);
QUANTITY_SPEC(height, length);
QUANTITY_SPEC(speed, length / time);

The quantity equation of isq::length / isq::time results in the derived_quantity_spec<isq::length, per<isq::time>> type.

Unit¶

A unit is a concrete amount of a quantity that allows us to measure the values of quantities of the same kind and represent the result as a number being the ratio of the two quantities.

For example:

si::second, si::metre, si::kilogram, si::ampere, si::kelvin, si::mole, and si::candela are the base units of the SI.
si::kilo<si::metre> is a prefixed unit of length.
si::radian, si::newton, and si::watt are examples of named derived units within the SI.
non_si::minute is an example of a scaled unit of time.
si::si2019::speed_of_light_in_vacuum is a physical constant standardized by the SI in 2019.

Note

In the mp-units library, physical constants are also implemented as units.

A unit can be defined by the user in one of the following ways:

template<PrefixableUnit auto U> struct kilo_ : prefixed_unit<"k", mag_power<10, 3>, U> {};
template<PrefixableUnit auto U> inline constexpr kilo_<U> kilo;

inline constexpr struct second : named_unit<"s", kind_of<isq::time>> {} second;
inline constexpr struct minute : named_unit<"min", mag<60> * second> {} minute;
inline constexpr struct gram : named_unit<"g", kind_of<isq::mass>> {} gram;
inline constexpr struct kilogram : decltype(kilo<gram>) {} kilogram;
inline constexpr struct newton : named_unit<"N", kilogram * metre / square(second)> {} newton;

inline constexpr struct speed_of_light_in_vacuum : named_unit<"c", mag<299'792'458> * metre / second> {} speed_of_light_in_vacuum;

The unit equation of si::metre / si::second results in the derived_unit<si::metre, per<si::second>> type.

Quantity reference¶

ISO defines a quantity as:

Quote

property of a phenomenon, body, or substance, where the property has a magnitude that can be expressed as a number and a reference

After that, it says:

Quote

A reference can be a measurement unit, a measurement procedure, a reference material, or a combination of such.

In the mp-units library, a quantity reference provides all the domain-specific metadata for the quantity besides its numerical value:

all the data stored in the quantity specification,
unit.

Together with the value of a representation type, it forms a quantity.

In the library, we have two different ways to provide a reference:

every unit with the associated quantity kind is a valid reference,
providing a unit to an indexing operator of a quantity specification explicitly instantiates a reference class template with this quantity spec and a unit passed as arguments.

Note

All the units of the SI have associated quantity kinds and may serve as a reference.

For example:

si::metre is defined in the SI as a unit of isq::length and thus can be used as a reference to instantiate a quantity of length (e.g., 42 * m).
The expression isq::height[m] results with reference<isq::height, si::metre>, which can be used to instantiate a quantity of isq::height with a unit of si::metre (e.g., 42 * isq::height[m]).

Quantity representation¶

Quantity representation defines the type used to store the numerical value of a quantity. Such a type should be of a specific quantity character provided in the quantity specification.

Note

By default, all floating-point and integral (besides bool) types are treated as scalars.

Quantity¶

ISO defines a quantity as:

Quote

property of a phenomenon, body, or substance, where the property has a magnitude that can be expressed as a number and a reference

This is why a quantity class template is defined in the library as:

template<Reference auto R,
         RepresentationOf<get_quantity_spec(R).character> Rep = double>
class quantity;

Its value can be easily created by multiplying/dividing the numerical value and a reference.

For example:

All of 42 * m, 42 * si::metre, 42 * isq::height[m], and isq::height(42 * m) create a quantity.
A quantity type can also be specified explicitly (e.g., quantity<si::metre, int>, quantity<isq::height[m]>).

Point origin¶

In the affine space theory, the point origin specifies where the "zero" of our measurement's scale is.

In the mp-units library, we have two types of point origins:

absolute - defines an absolute "zero" for our point,
relative - defines an origin that has some "offset" relative to an absolute point.

For example:

the absolute point origin can be defined in the following way:

inline constexpr struct absolute_zero : absolute_point_origin<isq::thermodynamic_temperature> {} absolute_zero;

the relative point origin can be defined in the following way:

inline constexpr struct ice_point : relative_point_origin<absolute_zero + 273'150 * milli<kelvin>> {} ice_point;

Quantity point¶

Quantity point implements a point in the affine space theory.

In the mp-units library, the quantity point is implemented as:

template<Reference auto R,
         PointOriginFor<get_quantity_spec(R)> auto PO,
         RepresentationOf<get_quantity_spec(R).character> Rep = double>
class quantity_point;

Its value can be easily created by adding/subtracting the quantity with a point origin.

For example:

The following specifies a quantity point defined in terms of an ice_point provided in the previous example:

constexpr auto room_reference_temperature = ice_point + isq::Celsius_temperature(21 * deg_C);