5 Quantities and units library [quantities]

5.7 Quantity point [qty.pt]

5.7.8 Quantity value observers [qty.pt.obs]

template<PointOrigin PO2>
  requires(PO2{} == point_origin)
constexpr quantity_type& quantity_ref_from(PO2) & noexcept;
template<PointOrigin PO2>
  requires(PO2{} == point_origin)
constexpr const quantity_type& quantity_ref_from(PO2) const & noexcept;

Returns: quantity-from-origin.

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template<PointOrigin PO2>
  requires requires(const quantity_point qp) { qp - PO2{}; }
constexpr Quantity auto quantity_from(PO2 rhs) const;

template<QuantityPointOf<absolute_point_origin> QP>
constexpr Quantity auto quantity_from(const QP& rhs) const;

Effects: Equivalent to: return *this - rhs;

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constexpr Quantity auto quantity_from_zero() const;

Effects: Equivalent to: if constexpr (requires { unit.point-origin; }) { // can lose the input unit const auto q = quantity_from(unit.point-origin); if constexpr (requires { q.in(unit); }) // restore the unit return q.in(unit); else return q; } else return quantity_from(absolute_point_origin);