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

Relative standard uncertainty of a measured constant

== Synopsis

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

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
template<xref:mp_units/UnitMagnitude.adoc[UnitMagnitude] M>
struct relative_standard_uncertainty;
----

== Description

Wraps the relative standard uncertainty (GUM/CODATA `u_r`) of a measured constant as an exact symbolic magnitude so that no representation type is imposed at the definition point. It is metadata only ‐ it never participates in unit equality, conversion factors, or symbolic simplification.

A measured constant declares exactly one of `relative_standard_uncertainty` and `standard_uncertainty` ‐ whichever form its source publishes. The published pair is mutually rounded, so carrying both would embed a contradiction. This form is invariant under a change of unit, so it is the right choice for a constant whose defining unit has no published absolute uncertainty.

For example:

[,cpp]
----
{.cpp}
 inline constexpr struct newtonian_constant_of_gravitation final :
     named_constant<"G", mag_ratio<667'430, 100'000> * mag_power<10, -11> * m3 / kg / s2,
                    relative_standard_uncertainty{mag_ratio<22, 10> * mag_power<10, -5>}> {}
 newtonian_constant_of_gravitation;
----
== Data Members

[cols="1"]
|===
| Name
| xref:mp_units/relative_standard_uncertainty/magnitude.adoc[`magnitude`] 
|===


== Template Parameters

[cols="1,4"]
|===
| Name| Description
| *M*
| an exact magnitude representing the relative standard uncertainty
|===


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