celeritas::Quantity< UnitT, ValueT > Class Template Reference

A numerical value tagged with a unit. More...

#include <Quantity.hh>

Public Types
Type aliases
using	value_type = ValueT
using	unit_type = UnitT

Public Member Functions
constexpr	Quantity ()=default
	Construct with default (zero)
CELER_CONSTEXPR_FUNCTION	Quantity (value_type value) noexcept
	Construct with value in celeritas native units.
template<detail::QConstant QC>
CELER_CONSTEXPR_FUNCTION	Quantity (detail::UnitlessQuantity< QC >) noexcept
	Construct implicitly from a unitless quantity.
CELER_CONSTEXPR_FUNCTION value_type const *	data () const
	Access the underlying data for more efficient loading from memory.
CELER_CONSTEXPR_FUNCTION value_type &	value () &noexcept
CELER_CONSTEXPR_FUNCTION value_type const &	value () const &noexcept

Detailed Description

template<class UnitT, class ValueT = decltype(UnitT::value())>
class celeritas::Quantity< UnitT, ValueT >

A numerical value tagged with a unit.

Template Parameters

UnitT	unit tag class
ValueT	value type

A quantity is a value expressed in terms of the given unit. Storing values in a different unit system can help with some calculations (e.g. operating in natural unit systems) by avoiding numerical multiplications and divisions by large constants. It can also make debugging easier (numeric values are obvious).

Example usage by physics class, where charge is in units of q_e+, and mass and momentum are expressed in atomic natural units (where m_e = 1 and c = 1).

using MevEnergy   = Quantity<Mev>;
using MevMass     = Quantity<UnitDivide<Mev, CLightSq>>;
using MevMomentum = Quantity<UnitDivide<Mev, CLight>>;

A relativistic equation that operates on these quantities can do so without unnecessary floating point operations involving the speed of light:

real_type eval = value_as<MevEnergy>(energy); // Natural units
MevMomentum momentum{std::sqrt(eval * eval
                               + 2 * value_as<MevMass>(mass) * eval)};

The resulting quantity can be converted to the native Celeritas unit system with native_value_from, which multiplies in the constant value of ElMomentumUnit:

real_type mom = native_value_from(momentum);

When using a Quantity from another part of the code, e.g. an imported unit system, use the quantity free function rather than .value() in order to guarantee consistency of units between source and destination.

An example unit class would be:

struct DozenUnit
{
    static constexpr int value() { return 12; }
    static constexpr char const* label() { return "dozen"; }
};

The label is used solely for outputting to JSON.

Note

The Quantity is designed to be a simple "strong type" class, not a complex mathematical class. To operate on quantities, you must use value_as (to operate within the Quantity's unit system) or native_value_from (to operate in the Celeritas native unit system), use the resulting numeric values in your mathematical expressions, then return a new Quantity class with the resulting value and correct type.

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The documentation for this class was generated from the following file:

• Quantity.hh