[#mp_units-common_unit]
= xref:mp_units.adoc[mp_units]::common_unit
:relfileprefix: ../
:mrdocs:

Measurement unit for an accumulation of two quantities of different units

== Synopsis

Declared in `<mp‐units/framework/unit.h>`

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
template<
    xref:mp_units/Unit.adoc[Unit] U1,
    xref:mp_units/Unit.adoc[Unit] U2,
    xref:mp_units/Unit.adoc[Unit]... Rest>
struct common_unit final
    : ::_base_type_
----

== Base Classes

[cols=2]
|===
| Name
| Description
| `::_base_type_`
| 
|===

== Types

[cols=1]
|===
| Name
| xref:mp_units/common_unit/_base_type_.adoc[`_base_type_`] 
|===

== Static Data Members

[cols=1]
|===
| Name
| xref:mp_units/common_unit/_common_unit_.adoc[`_common_unit_`] 
|===

== Description

While adding two quantities of different units we can often identify which of those unit should be used to prevent data truncation. For example, adding `1 * m + 1 * mm` will end up in a quantity expressed in millimeters. However, for some cases this is not possible. Choosing any of the units from the arguments of the addition would result in a data truncation. For example, a common unit for `1 * km + 1 * mi` is `[8/125] m`. Instead of returning such a complex unit type the library will return a `common_unit<mi, km>`. This type is convertible to both `mi` and `km` without risking data truncation, but is not equal to any of them.

[NOTE]
====
User should not instantiate this type! It is not exported from the C++ module. The library will instantiate this type automatically based on the unit arithmetic equation provided by the user.

====


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