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Systems of Units

Modeling a system of units is probably the most important feature and a selling point of every physical units library. Thanks to that, the library can protect users from performing invalid operations on quantities and provide automated conversion factors between various compatible units.

Probably all the libraries in the wild model the SI and many of them provide support for additional units belonging to various other systems (e.g., imperial, cgs, etc).

Systems of Units are based on Systems of Quantities

Systems of quantities specify a set of quantities and equations relating to those quantities. Those equations do not take any unit or a numerical representation into account at all. To create a quantity, we need to add those missing pieces of information. This is where a system of units kicks in.

The SI is explicitly stated to be based on the ISQ. Among others, it defines 7 base units, one for each base quantity. In the mp-units this is expressed by associating a quantity kind (that we discussed in detail in the previous chapter) with a unit that is used to express it:

inline constexpr struct metre final : named_unit<"m", kind_of<isq::length>> {} metre;


The kind_of<isq::length> above states explicitly that this unit has an associated quantity kind. In other words, si::metre (and scaled units based on it) can be used to express the amount of any quantity of kind length.

Units compose

One of the most vital points of the SI system is that its units compose. This allows providing thousands of different units for hundreds of various quantities with a tiny set of predefined units and prefixes.

The same is modeled in the mp-units library, which also allows composing predefined units to create a nearly infinite number of different derived units. For example, one can write:

quantity<si::metre / si::second> q;

to express a quantity of speed. The resulting quantity type is implicitly inferred from the unit equation by repeating the same operations on the associated quantity kinds.

Many shades of the same unit

The SI provides the names for 22 common coherent units of 22 derived quantities.

Each such named derived unit is a result of a specific predefined unit equation. For example, a unit of power quantity is defined in the library as:

inline constexpr struct watt final : named_unit<"W", joule / second> {} watt;

However, a power quantity can be expressed in other units as well. For example, the following:

auto q1 = 42 * W;
std::cout << q1 << "\n";
std::cout << / s) << "\n";
std::cout << * m / s) << "\n";
std::cout << * m2 / s3) << "\n";


42 W
42 J/s
42 N m/s
42 kg m²/s³

All of the above quantities are equivalent and mean exactly the same.

Constraining a derived unit to work only with a specific derived quantity

Some derived units are valid only for specific derived quantities. For example, SI specifies both hertz and becquerel derived units with the same unit equation 1 / s. However, it also explicitly states:

SI Brochure

The hertz shall only be used for periodic phenomena and the becquerel shall only be used for stochastic processes in activity referred to a radionuclide.

The above means that the usage of becquerel as a unit of a frequency quantity is an error.

The library allows constraining such units to work only with quantities of a specific kind in the following way:

inline constexpr struct hertz final : named_unit<"Hz", one / second, kind_of<isq::frequency>> {} hertz;
inline constexpr struct becquerel final : named_unit<"Bq", one / second, kind_of<isq::activity>> {} becquerel;

With the above, hertz can only be used with frequencies, while becquerel should only be used with quantities of activity. This means that the following equation will not compile:

auto q = 1 * Hz + 1 * Bq;   // Fails to compile

This is exactly what we wanted to achieve to improve the type-safety of the library.

Prefixed units

Besides named units, the SI specifies also 24 prefixes (all being a power of 10) that can be prepended to all named units to obtain various scaled versions of them.

Implementation of std::ratio provided by all major compilers is able to express only 16 of them. This is why, in the mp-units, we had to find an alternative way to represent unit magnitude in a more flexible way.

Each prefix is implemented similarly to the following:

template<PrefixableUnit U> struct quecto_ : prefixed_unit<"q", mag_power<10, -30>, U{}> {};
template<PrefixableUnit auto U> inline constexpr quecto_<decltype(U)> quecto;

and then a PrefixableUnit can be prefixed in the following way:

inline constexpr auto qm = quecto<metre>;

The usage of mag_power not only enables providing support for SI prefixes, but it can also efficiently represent any rational magnitude. For example, IEC 80000 prefixes used in the IT industry can be implemented as:

template<PrefixableUnit U> struct yobi_ : prefixed_unit<"Yi", mag_power<2, 80>, U{}> {};
template<PrefixableUnit auto U> inline constexpr yobi_<decltype(U)> yobi;

Scaled units

In the SI, all units are either base or derived units or prefixed versions of those. However, those are only some of the options possible.

For example, there is a list of off-system units accepted for use with SI. Those are scaled versions of the SI units with ratios that can't be explicitly expressed with predefined SI prefixes. Those include units like minute, hour, or electronvolt:

inline constexpr struct minute final : named_unit<"min", mag<60> * si::second> {} minute;
inline constexpr struct hour final : named_unit<"h", mag<60> * minute> {} hour;
inline constexpr struct electronvolt final : named_unit<"eV", mag_ratio<1'602'176'634, 1'000'000'000> * mag_power<10, -19> * si::joule> {} electronvolt;

Also, units of other systems of units are often defined in terms of scaled versions of the SI units. For example, the international yard is defined as:

inline constexpr struct yard final : named_unit<"yd", mag_ratio<9'144, 10'000> * si::metre> {} yard;

For some units, a magnitude might also be irrational. The best example here is a degree which is defined using a floating-point magnitude having a factor of the number π (Pi):

inline constexpr struct mag_pi final : magnitude<std::numbers::pi_v<long double>> {} mag_pi;
inline constexpr struct degree final : named_unit<{u8"°", "deg"}, mag_pi / mag<180> * si::radian> {} degree;