[#mp_units-named_constant-02]
= xref:mp_units.adoc[mp_units]::xref:mp_units/named_constant-03.adoc[named_constant]<Symbol, U, RSU>
:relfileprefix: ../
:mrdocs:

Specialization for a measured constant declaring its relative standard uncertainty

== Synopsis

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

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
template<
    xref:mp_units/symbol_text-032.adoc[symbol_text] Symbol,
    xref:mp_units/Unit.adoc[Unit] auto U,
    typename M,
    xref:mp_units/relative_standard_uncertainty.adoc[relative_standard_uncertainty<M>] RSU>
requires (!Symbol.empty()) && detail::magnitude_is_positive<M{}>
struct xref:mp_units/named_constant-03.adoc[named_constant]<Symbol, U, RSU>
    : ::_base_type_
----

== Description

The same as the above but additionally stores the relative standard uncertainty of a constant that is measured rather than exact by definition. The value transcribes the entry published in the source metrology table (e.g., CODATA).

== Base Classes

[cols="1,4"]
|===
| Name| Description
| `::_base_type_`
| 
|===

== Type Aliases

[cols="1"]
|===
| Name
| xref:mp_units/named_constant-02/_base_type_.adoc[`_base_type_`] 
|===


== Static Data Members

[cols="1,4"]
|===
| Name| Description
| xref:mp_units/named_constant-02/_relative_standard_uncertainty_.adoc[`_relative_standard_uncertainty_`] 
| 
| xref:mp_units/named_constant-02/_symbol_.adoc[`_symbol_`] 
| Unique constant identifier
|===


== Template Parameters

[cols="1,4"]
|===
| Name| Description
| *Symbol*
| a short text representation of the constant
| *Unit*
| a unit that we use to define a constant
| *RSU*
| the relative standard uncertainty of the measured value
|===


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