`hello_units`

!!! example "Try it on Compiler Explorer"

This is a really simple example showcasing the features of the mp-units library.

--8<-- "example/hello_units.cpp:28:33"

First, we include the headers for:

a system of quantities (ISQ)
symbols of SI units
symbols of international units
text and stream output support

--8<-- "example/hello_units.cpp:35:36"

Next, to shorten the definitions, we "import" mp_units namespace.

--8<-- "example/hello_units.cpp:37:40"

The above function template takes any quantities implicitly convertible to isq::length and isq::time. Those quantities can use any compatible unit and a representation type. The function returns a result of a really simple equation and ensures that its quantity type is implicitly convertible to isq::speed.

!!! note

Besides verifying the type returned from the function, constraining a generic return
type is really useful for users of such a function as it provides more information
of what to expect from a function than just using `auto`.

--8<-- "example/hello_units.cpp:42:45"

The above lines explicitly opt-in to use unit symbols from two systems of units. As this introduces a lot of short identifiers into the current scope, it is not done implicitly while including a header file.

--8<-- "example/hello_units.cpp:47:53"

Lines 16 & 17 create a quantity of kind isq::length / isq::time with the numbers and units provided. Such quantities can be converted or assigned to any other quantity with a matching kind.
Line 18 calls our function template with quantities of kind isq::length and isq::time and number and units provided.
Line 19 explicitly specifies quantity specifications of the quantities passed to a function template. This time those will not be quantity kinds anymore and will have more restrictive conversion rules.
Line 20 changes the unit of a quantity v3 to m / s in a value-preserving way (floating-point representations are considered to be value-preserving).
Line 21 does a similar operation but this time it would succeed also for non-truncating cases (if it was the case).
Line 22 does a value-truncating operation of changing the underlying representation type from double to int.

--8<-- "example/hello_units.cpp:55"

The above presents various ways to print a quantity. Both stream insertion operations and std::format are supported.

!!! note

`MP_UNITS_STD_FMT` is used for compatibility reasons. In case a specific compiler
does not support `std::format` or a user prefers to use `{fmt}` library, this macro
will resolve to `fmt` namespace. Otherwise, `std` namespace will be used.

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