mp_units::value_cast

template<
    Quantity ToQ,
    typename FwdQP,
    QuantityPoint QP = std::remove_cvref_t<FwdQP>>
requires (ToQ::quantity_spec == QP::quantity_spec) && UnitOf<MP_UNITS_NONCONST_TYPE(ToQ::unit), QP::quantity_spec> &&
          std::constructible_from<typename ToQ::rep, typename QP::rep> &&
          detail::ExplicitlyCastable<QP::unit, ToQ::unit, typename ToQ::rep>
[[nodiscard]]
constexpr
QuantityPoint auto
value_cast(FwdQP&& qp);

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.

inline constexpr struct A : absolute_point_origin A;

using ToQ = quantity<mm, int>; auto qp = value_cast<ToQ>(quantity_point{1.23 * m});

Note that value_cast only changes the "representation aspects" (unit and representation type), but not the "meaning" (quantity type or the actual point that is being described).