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

System‐specific reference

== Synopsis

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

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
template<
    xref:mp_units/QuantitySpec.adoc[QuantitySpec] auto Q,
    xref:mp_units/Unit.adoc[Unit] auto CoU>
requires (!MP_UNITS_ASSOCIATED_UNIT_T(decltype(CoU))) || (CoU == one)
struct system_reference;
----

== Member Functions

[cols=2]
|===
| Name
| Description
| xref:mp_units/system_reference/operator_subs.adoc[`operator[]`] 
| Subscript operator
|===

== Static Data Members

[cols=1]
|===
| Name
| xref:mp_units/system_reference/coherent_unit.adoc[`coherent_unit`] 
| xref:mp_units/system_reference/quantity_spec.adoc[`quantity_spec`] 
|===

== Description

This type is used in rare cases where more than one base quantity in a specific system of units uses the same unit. For example in a hypothetical system of natural units where constant for speed of light `c = 1`, length and time could be measured in seconds. In such cases `system_reference` has to be used to explicitly express such a binding.

For example:

[,cpp]
----
{.cpp}
 // hypothetical natural system of units for c=1

 inline constexpr struct second final : named_unit<"s"> {} second;
 inline constexpr struct minute final : named_unit<"min", mag<60> * second> {} minute;
 inline constexpr struct gram final : named_unit<"g"> {} gram;
 inline constexpr auto kilogram = si::kilo<gram>;

 inline constexpr struct time : system_reference<isq::time, second> {} time;
 inline constexpr struct length : system_reference<isq::length, second> {} length;
 inline constexpr struct speed : system_reference<isq::speed, second / second> {} speed;
 inline constexpr struct force : system_reference<isq::force, kilogram / second> {} force;
----
== Template Parameters

[cols=2]
|===
| Name
| Description
| *Q*
| quantity for which a unit is being assigned
| *CoU*
| coherent unit for a quantity in this system
|===


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