[#mp_units-standard_uncertainty]
= xref:mp_units.adoc[mp&lowbar;units]::standard&lowbar;uncertainty
:relfileprefix: ../
:mrdocs:

Absolute standard uncertainty of a measured constant

== Synopsis

Declared in `&lt;mp&hyphen;units&sol;framework&sol;unit&lowbar;definitions&period;h&gt;`

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
template&lt;xref:mp_units/Unit.adoc[Unit] U&gt;
struct standard&lowbar;uncertainty;
----

== Description

Wraps the standard uncertainty (GUM&sol;CODATA `u`) of a measured constant as an exact symbolic expression of the same shape as the constant&apos;s value&colon; a magnitude times a unit of the same dimension&period; Spelling the unit out makes the expression self&hyphen;contained &hyphen; the relative form derives as an exact ratio of two canonical magnitudes, with the units cancelling exactly&period; Like `relative&lowbar;standard&lowbar;uncertainty`, it is metadata only&period;

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

For example&colon;

[,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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