mp-units/docs/getting_started/quick_start.md

Quick Start

This chapter provides a quick introduction to get you started with mp-units. Much more details can be found in our User's Guide.

Quantities

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.

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 / si::second;

!!! info

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:

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

using namespace mp_units;

quantity q{42, si::metre / si::second};
```

The above creates an instance of quantity<derived_unit<si::metre, per<si::second>>{}, int>. The same can be obtained using optional unit symbols:

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

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

quantity q = 42 * m / s;

!!! tip

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

Quantities of the same kind can be added, subtracted, and compared to each other:

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

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

static_assert(1 * km + 50 * m == 1050 * m);

Various quantities can be multiplied or divided by each other:

static_assert(140 * km / (2 * h) == 70 * km / h);

!!! 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).

Quantity points

The quantity point specifies an absolute quantity with respect to an origin. If no origin is provided explicitly, an implicit one will be provided by the library.

Together with quantities, they model The Affine Space.

Quantity points should be used in all places where adding two values is meaningless (e.g., temperature points, timestamps, altitudes, readouts from the car's odometer, etc.).

The set of operations that can be done on quantity points is limited compared to quantities. This introduces an additional type-safety.

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

int main()
{
  using namespace mp_units;
  using namespace mp_units::si::unit_symbols;
  using namespace mp_units::usc::unit_symbols;

  quantity_point temp{20. * deg_C};
  std::cout << "Temperature: "
            << temp.quantity_from_zero() << " ("
            << temp.in(deg_F).quantity_from_zero() << ")\n";
}

The above outputs:

Temperature: 20 °C (68 °F)

!!! info

Check [The Affine Space](../users_guide/framework_basics/the_affine_space.md) chapter to learn
more about quantity points.