2.1 KiB
Quick Start
A quantity is a concrete amount of a unit representing a quantity type of a specified dimension with a
specific representation. It 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 two-parameter
constructor:
#include <mp-units/systems/si/si.h>
using namespace mp_units;
quantity q{42, si::metre};
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;
!!! 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).