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

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

== Description

For a constant declared with `standard_uncertainty`, the result is derived as the exact ratio of the uncertainty's canonical magnitude to the constant's one (`u_r = u(x)/|x|`, so the result never carries a sign even for a negative‐valued constant).

There is no valid result for constants that are exact by definition. Asking for one would conflate "exact by definition" with "measured infinitely precisely", so it is a compile‐time error rather than a zero magnitude.

== Return Value

the relative 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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