# 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:

Important

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:

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:

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 << q1.in(J / s) << "\n";
std::cout << q1.in(N * m / s) << "\n";
std::cout << q1.in(kg * m2 / s3) << "\n";
```

prints:

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:

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:

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):