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

Controls whether conversion from `quantity<FromUnit, FromRep>` to `quantity<ToUnit, ToRep>` is implicit or explicit.

== Synopsis

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

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
template<
    auto FromUnit,
    typename FromRep,
    auto ToUnit,
    typename ToRep>
constexpr bool implicitly_scalable = std::is_convertible_v<FromRep, ToRep> &&
  (treat_as_floating_point<ToRep> || (!treat_as_floating_point<FromRep> && is_integral_scaling(FromUnit, ToUnit)));
----

== Description

The default is `true` iff `FromRep` is convertible to `ToRep` and the scaling is non‐truncating: either `ToRep` is floating‐point, or both reps are non‐floating‐point and the unit magnitude ratio is an integral factor.

Specialize this variable template to customize the implicit/explicit decision for your own representation types.

== Template Parameters

[cols="1,4"]
|===
|Name|Description

| *FromUnit*
| the source unit value (NTTP)
| *FromRep*
| the source representation type
| *ToUnit*
| the target unit value (NTTP)
| *ToRep*
| the target representation type
|===


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