5 Quantities and units library [quantities]

5.4 Reference [qty.ref]

5.4.4 Unit [qty.unit]

5.4.4.5 Types [qty.unit.types]

5.4.4.5.3 Named [qty.named.unit]

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namespace mp_units { template<symbol_text Symbol, QuantityKindSpec auto QS> requires(!Symbol.empty()) && BaseDimension<decltype(auto(QS.dimension))> struct named_unit<Symbol, QS> : unit-interface { using base-type = named_unit; // exposition only static constexpr auto symbol = Symbol; // exposition only static constexpr auto quantity-spec = QS; // exposition only }; template<symbol_text Symbol, QuantityKindSpec auto QS, PointOrigin auto PO> requires(!Symbol.empty()) && BaseDimension<decltype(auto(QS.dimension))> struct named_unit<Symbol, QS, PO> : unit-interface { using base-type = named_unit; // exposition only static constexpr auto symbol = Symbol; // exposition only static constexpr auto quantity-spec = QS; // exposition only static constexpr auto point-origin = PO; // exposition only }; template<symbol_text Symbol> requires(!Symbol.empty()) struct named_unit<Symbol> : unit-interface { using base-type = named_unit; // exposition only static constexpr auto symbol = Symbol; // exposition only }; template<symbol_text Symbol, Unit auto U> requires(!Symbol.empty()) struct named_unit<Symbol, U> : decltype(U)::base-type { using base-type = named_unit; // exposition only static constexpr auto symbol = Symbol; // exposition only }; template<symbol_text Symbol, Unit auto U, PointOrigin auto PO> requires(!Symbol.empty()) struct named_unit<Symbol, U, PO> : decltype(U)::base-type { using base-type = named_unit; // exposition only static constexpr auto symbol = Symbol; // exposition only static constexpr auto point-origin = PO; // exposition only }; template<symbol_text Symbol, AssociatedUnit auto U, QuantityKindSpec auto QS> requires(!Symbol.empty()) && (QS.dimension == get-associated-quantity(U).dimension) struct named_unit<Symbol, U, QS> : decltype(U)::base-type { using base-type = named_unit; // exposition only static constexpr auto symbol = Symbol; // exposition only static constexpr auto quantity-spec = QS; // exposition only }; template<symbol_text Symbol, AssociatedUnit auto U, QuantityKindSpec auto QS, PointOrigin auto PO> requires(!Symbol.empty()) && (QS.dimension == get-associated-quantity(U).dimension) struct named_unit<Symbol, U, QS, PO> : decltype(U)::base-type { using base-type = named_unit; // exposition only static constexpr auto symbol = Symbol; // exposition only static constexpr auto quantity-spec = QS; // exposition only static constexpr auto point-origin = PO; // exposition only }; }
1
#
A named unit is a type that models PrefixableUnit.
A specialization of named_unit is used as a base type when defining a named unit.
2
#
In the following descriptions, let U be a named unit defined with an alluded signature.
The identifier of U represents its unit name (IEC 60050, 112-01-15) or special unit name (IEC 60050, 112-01-16).
Symbol is its unit symbol (IEC 60050, 112-01-17).
3
#
The possible arguments to named_unit are
  • (3.1)
    ,
  • (3.2)
    ,
  • (3.3)
    , and
  • (3.4)
    .
4
#
The first signature defines the unit of a base quantity without a unit prefix (IEC 60050, 112-01-26).
The second signature defines a unit that can be reused by several base quantities.
The third and fourth signatures with a unit expression argument E define U as implicitly convertible from E.
The first and fourth signatures with a kind of quantity (IEC 60050, 112-01-04) Q also restrict U to Q.
5
#
A point origin argument specifies the default point origin of U ([qty.pt.syn]).
6
#
[Example 1: // The first signature defines a base unit restricted to a kind of base quantity. inline constexpr struct second final : named_unit<"s", kind_of<time>> { } second; // The third and fourth signatures give a name to the unit argument. inline constexpr struct minute final : named_unit<"min", mag<60> * second> { } minute; // . // The fourth signature also further restricts the kind of quantity. inline constexpr struct hertz final : named_unit<"Hz", inverse(second), kind_of<frequency>> { } hertz; // Hz can't measure becquerel, activity, // or any other quantity with dimension // that isn't a kind of frequency. — end example]