[#mp_units-value_cast-0a]
= xref:mp_units.adoc[mp_units]::value_cast
:relfileprefix: ../
:mrdocs:

Explicit cast of a quantity's representation

== Synopsis

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

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
template<
    xref:mp_units/Quantity.adoc[Quantity] ToQ,
    typename FwdQ,
    xref:mp_units/Quantity.adoc[Quantity] Q = std::remove_cvref_t<FwdQ>>
requires (ToQ::quantity_spec == Q::quantity_spec) &&
          (MP_UNITS_WEAK_UNIT_OF(MP_UNITS_NONCONST_TYPE(ToQ::unit), Q::quantity_spec)) &&
          std::constructible_from<typename ToQ::rep, typename Q::rep> &&
          detail::SaneScaling<Q::unit, ToQ::unit, typename ToQ::rep>
[[nodiscard]]
constexpr
xref:mp_units/Quantity.adoc[Quantity] auto
value_cast(FwdQ&& q);
----

== Description

Implicit conversions between quantities of different types are allowed only for "safe" (e.g. non‐truncating) conversion. In truncating cases an explicit cast have to be used.

using ToQ = quantity<us, int>; auto q = value_cast<ToQ>(1.23 * ms);

Note that value_cast only changes the "representation aspects" (unit and representation type), but not the "meaning" (quantity type).

== Template Parameters

[cols=2]
|===
| Name
| Description
| *ToQ*
| a target quantity type to which to cast the representation
|===


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