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]

5.4.3.3.1 Named [named.qty]

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namespace mp_units { struct is_kind {}; template<BaseDimension auto Dim, QSProperty auto... Args> struct quantity_spec<Dim, Args...> : quantity-spec-interface { using base-type = quantity_spec; static constexpr BaseDimension auto dimension = Dim; static constexpr quantity_character character = see below; }; template<DerivedQuantitySpec auto Eq, QSProperty auto... Args> struct quantity_spec<Eq, Args...> : quantity-spec-interface { using base-type = quantity_spec; static constexpr auto equation = Eq; static constexpr Dimension auto dimension = Eq.dimension; static constexpr quantity_character character = see below; }; template<NamedQuantitySpec auto QS, QSProperty auto... Args> struct quantity_spec<QS, Args...> : quantity-spec-interface { using base-type = quantity_spec; static constexpr auto parent = QS; static constexpr auto equation = parent.equation; // exposition only, present only // if the qualified-id parent.equation is valid and denotes an object static constexpr Dimension auto dimension = parent.dimension; static constexpr quantity_character character = see below; }; template<NamedQuantitySpec auto QS, DerivedQuantitySpec auto Eq, QSProperty auto... Args> requires QuantitySpecExplicitlyConvertibleTo<Eq, QS> struct quantity_spec<QS, Eq, Args...> : quantity-spec-interface { using base-type = quantity_spec; static constexpr auto parent = QS; static constexpr auto equation = Eq; static constexpr Dimension auto dimension = parent.dimension; static constexpr quantity_character character = see below; }; }
1
#
A named quantity is a type that models NamedQuantitySpec.
A specialization of quantity_spec is used as a base type when defining a named quantity.
2
#
In the following descriptions, let Q be a named quantity defined with an alluded signature.
The identifier of Q represents its quantity name (IEC 60050, 112-01-02).
3
#
Let Ch be an enumerator value of quantity_character.
The possible arguments to quantity_spec are
  • (3.1)
    ,
  • (3.2)
    ,
  • (3.3)
    , and
  • (3.4)
    .
4
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If the first argument is a base quantity dimension, then Q is that base quantity (IEC 60050, 112-01-08).
If an argument is a quantity calculus (IEC 60050, 112-01-30) C, then Q is implicitly convertible from C.
If the first argument is a named quantity, then Q is of its kind (IEC 60050, 112-01-04).
5
#
The member character represents the set of the numerical value of Q ([qty.char.traits]) and is equal to
  • (5.1)
    Ch if specified,
  • (5.2)
    otherwise, quantity_character​::​real_scalar for the first signature, and
  • (5.3)
    otherwise, (BC).character, where BC is the argument preceding Ch in the signatures above.
6
#
is_kind specifies Q to start a new hierarchy tree of a kind.
7
#
Optional arguments may appear in any order.
8
#
[Example 1: // The first signature defines a base quantity. inline constexpr struct length final : quantity_spec<dim_length> { } length; // Length is a base quantity. // The second signature defines a derived quantity. inline constexpr struct area final : quantity_spec<pow<2>(length)> { } area; // An area equals length by length. // The third and fourth signatures add a leaf to a hierarchy of kinds. inline constexpr struct width final : quantity_spec<length> { } width; // Width is a kind of length. // The fourth signature also refines the calculus required for implicit conversions. inline constexpr struct angular_measure final : quantity_spec<dimensionless, arc_length / radius, is_kind> { } angular_measure; // Requires an arc length per radius, not just any quantity of dimension one. — end example]