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

Absolute 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/Unit.adoc[Unit] U>
struct standard_uncertainty;
----

== Description

Wraps the standard uncertainty (GUM/CODATA `u`) of a measured constant as an exact symbolic expression of the same shape as the constant's value: a magnitude times a unit of the same dimension. Spelling the unit out makes the expression self‐contained ‐ the relative form derives as an exact ratio of two canonical magnitudes, with the units cancelling exactly. Like `relative_standard_uncertainty`, it is metadata only.

A measured constant declares exactly one of the two wrappers ‐ whichever form its source publishes. Metrology tables (e.g., CODATA) tabulate the absolute uncertainty next to the value, both rounded consistently, so a definition transcribing a table row should use this form; deriving it from a published `u_r` instead can be off by several percent in the last digit.

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,
                    standard_uncertainty{mag_ratio<15, 10> * mag_power<10, -15> * m3 / kg / s2}> {}
 newtonian_constant_of_gravitation;
----
== Data Members

[cols="1"]
|===
| Name
| xref:mp_units/standard_uncertainty/unit.adoc[`unit`] 
|===


== Template Parameters

[cols="1,4"]
|===
| Name| Description
| *U*
| a unit expression representing the standard uncertainty in the constant&apos;s dimension
|===


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