5 Quantities and units library [quantities]

5.6 Quantity [qty]

5.6.2 Interoperability [qty.like]

The interfaces specified in this subclause and subclause [qty.pt.like] are used by quantity and quantity_point to specify conversions with other types representing quantities.

[Note 1:

[qty.chrono] implements them for std::chrono::duration and std::chrono::time_point.

— end note]

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template<typename T, template<typename> typename Traits>
concept qty-like-impl = requires(const T& qty, const Traits<T>::rep& num) {  // exposition only
  { Traits<T>::to_numerical_value(qty) } -> std::same_as<typename Traits<T>::rep>;
  { Traits<T>::from_numerical_value(num) } -> std::same_as<T>;
  requires std::same_as<decltype(Traits<T>::explicit_import), const bool>;
  requires std::same_as<decltype(Traits<T>::explicit_export), const bool>;
  typename std::bool_constant<Traits<T>::explicit_import>;
  typename std::bool_constant<Traits<T>::explicit_export>;
};

template<typename T>
concept QuantityLike = !Quantity<T> && qty-like-impl<T, quantity_like_traits> && requires {
  typename quantity<quantity_like_traits<T>::reference, typename quantity_like_traits<T>::rep>;
};

In the following descriptions, let

(2.1)
Traits be quantity_like_traits or quantity_point_like_traits,
(2.2)
Q be a type for which Traits<Q> is specialized,
(2.3)
qty be an lvalue of type const Q, and
(2.4)
num be an lvalue of type const Traits<Q>::rep.

Q models qty-like-impl<Traits> if and only if:

(3.1)
Traits<Q>::to_numerical_value(qty) returns the numerical value (IEC 60050, 112-01-29) of qty.
(3.2)
Traits<Q>::from_numerical_value(num) returns a Q with numerical value num.
(3.3)
If Traits is quantity_point_like_traits, both numerical values are offset from Traits<Q>::point_origin.

If the following expression is true, the specified conversion will be explicit.

(4.1)
Traits<Q>::explicit_import for the conversion from Q to this library's type.
(4.2)
Traits<Q>::explicit_export for the conversion from this library's type to Q.