# 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 quantity is created by multiplying a number with a predefined unit:

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

using namespace mp_units;

auto q = 42 * si::metre;
```

!!! note

    The above spelling of `metre` is not a typo. For motivation, please check our
    [FAQ](../faq/#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:

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

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

auto q = 42 * m;
```

!!! note

    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:

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

using namespace mp_units;

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


## User-provided unit wrappers

Sometimes it might be awkward to type some derived units:

```cpp
auto speed = 60 * (km / h);
```

!!! note

    Please note that `60 * km / h` will not compile. To read more about the rationale for such
    a design please check our [FAQ](../faq/#why-dont-we-use-udls-to-create-a-quantity).

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

```cpp
constexpr auto kmph = km / h;
auto speed = 60 * kmph;
```

or even:

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

!!! note

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