5 Quantities and units library [quantities]

5.4 Reference [qty.ref]

5.4.3 Quantity specification [qty.spec]

5.4.3.3 Types [qty.spec.types]

5.4.3.3.2 Derived [derived.qty]

namespace mp_units { template<NamedQuantitySpec Q> using to-dimension = decltype(auto(Q::dimension)); // exposition only template<typename... Expr> struct derived-quantity-spec-impl : // exposition only quantity-spec-interface, expr-fractions<struct dimensionless, Expr...> { using base-type = derived-quantity-spec-impl; using base = expr-fractions<struct dimensionless, Expr...>; static constexpr Dimension auto dimension = expr-map<to-dimension, derived_dimension, struct dimension_one>(base{}); static constexpr quantity_character character = see below; }; template<SymbolicConstant... Expr> struct derived_quantity_spec final : derived-quantity-spec-impl<Expr...> {}; }

derived_quantity_spec is used by the library to represent the result of a quantity calculus not equal to a named quantity.

[Example 1: constexpr auto area = pow<2>(isq::length); int x = area; // error: cannot construct from derived_quantity_spec<power<isq::length, 2>> — end example]

A program that instantiates a specialization of derived_quantity_spec that is not a possible result of the library specifications is ill-formed, no diagnostic required.

Let

(2.1)
Nums and Dens be packs denoting the template arguments of base::nums and base::dens, respectively,
(2.2)
QUANTITY-CHARACTER-OF(Pack) be std::max({quantity_character::real_scalar, expr-type<Pack>::character...}) and
(2.3)
num_char be QUANTITY-CHARACTER-OF(Nums) and den_char be QUANTITY-CHARACTER-OF(Dens).

The member character is equal to quantity_character::real_scalar if num_char == den_char is true, and std::max(num_char, den_char) otherwise.