mp-units/docs/getting_started/quick_start.md

Quick Start

A quantity is a concrete amount of a unit for a quantity type of a specified dimension with a specific representation, and is represented in the library with a quantity class template.

Creating a quantity

The SI Brochure says:

!!! quote "SI Brochure"

The value of the quantity is the product of the number and the unit. The space between the number
and the unit is regarded as a multiplication sign (just as a space between units implies
multiplication).

Following the above, the value of a quantity in the mp-units library is created by multiplying a number with a predefined unit:

#include <mp-units/systems/si/si.h>

using namespace mp_units;

quantity q = 42 * si::metre;

!!! note

The above spelling of `metre` is not a typo. For motivation, please check our
[FAQ](faq.md#why-do-we-spell-metre-instead-of-meter).

The above creates an instance of quantity<si::metre(), int>. The same can be obtained using an optional unit symbol:

#include <mp-units/systems/si/si.h>

using namespace mp_units;
using namespace mp_units::si::unit_symbols;

quantity q = 42 * m;

!!! tip

Unit symbols introduce a lot of short identifiers into the current namespace, and that is
why they are opt-in. A user has to explicitly "import" them from a dedicated `unit_symbols`
namespace.

In case someone doesn't like the multiply syntax or there is an ambiguity between operator* provided by this and other libraries, a quantity can also be created with a dedicated factory function:

#include <mp-units/systems/si/si.h>

using namespace mp_units;

quantity q = make_quantity<si::metre>(42);

User-provided unit wrappers

Sometimes it might be awkward to type some derived units:

quantity speed = 60 * km / h;

In case such a unit is used a lot in the project, a user can easily provide a nicely named wrapper for it with:

constexpr auto kmph = km / h;
quantity speed = 60 * kmph;

or even:

constexpr auto kilometre = si::kilo<si::metre>;
constexpr auto kilometre_per_hour = kilometre / si::hour;
constexpr auto kmph = kilometre_per_hour;
quantity speed = 60 * kmph;

!!! note

In case you wonder why this library does not use UDLs to create quantities, please check
our [FAQ](faq.md#why-dont-we-use-udls-to-create-quantities).