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

Returns the 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/MeasuredConstant.adoc[MeasuredConstant] U>
[[nodiscard]]
consteval
xref:mp_units/Unit.adoc[Unit] auto
get_standard_uncertainty(U u);
----

== Description

The result is a unit expression of the constant's own dimension (constants are units in this library, and so are their uncertainties). For a constant declared with `relative_standard_uncertainty`, the result is derived by scaling the constant itself (`u(x) = u_r * |x|`, so the result is positive even for a negative‐valued constant).

As for `get_relative_standard_uncertainty`, there is no valid result for constants that are exact by definition.

== Return Value

the standard uncertainty of a measured constant

[NOTE]
====
The return value https://en.cppreference.com/cpp/language/attributes/nodiscard[should not be discarded^].
====

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