A specification of a derived quantity
<mp-units/framework/quantity_spec.h>
template</* implementation-defined */... Expr>
struct derived_quantity_spec final
: /* implementation-defined */
| Name | Description |
|---|---|
/* implementation-defined */ |
Derived quantity is a quantity, in a system of quantities, defined in terms of other quantities of that system. Its dimension is an expression of the dependence of a quantity on the base quantities of a system of quantities as a product of powers of factors corresponding to the base quantities, omitting any numerical factors.
Instead of using a raw list of exponents this library decided to use symbolic expression syntax to make types more digestable for the user both for quantity specification and its dimension. The positive exponents are ordered first and all negative exponents are put as a list into the per<...> class template. If a power of exponent is different than 1 the quantity type is enclosed in power<Q, Num, Den> class template. Otherwise, it is just put directly in the list without any wrapper. In case all of the exponents are negative than the dimensionless/dimension_one is put in the front to increase the readability.
The character of those quantities is derived from ingredients or overriden with a template parameter.
For example:
{.cpp}
auto frequency = inverse(period_duration);
auto area = pow<2>(length);
auto speed = distance / duration;
auto velocity = displacement / duration;
auto acceleration = velocity / duration;
- the type of frequency is derived_quantity_spec<dimensionless, per<period_duration>> - the dimension type of frequency is derived_dimension<dimension_one, per<dim_time>> - the type of area is derived_quantity_spec<power<length, 2>> - the dimension type of area is derived_dimension<power<dim_length, 2>> - the type of speed is derived_quantity_spec<distance, per<duration>> - the dimension type of speed is derived_dimension<dim_length, per<dim_time>> - the type of velocity is derived_quantity_spec<displacement, per<duration>> - the dimension type of velocity is derived_dimension<dim_length, per<dim_time>> - the type of acceleration is derived_quantity_spec<velocity, per<duration>> - the dimension type of acceleration is derived_dimension<dim_length, per<power<dim_time, 2>>>
User should not instantiate this type! It is not exported from the C++ module. The library will instantiate this type automatically based on the dimensional arithmetic equation provided by the user.
| Name | Description |
|---|---|
| Expr | a parameter pack consisting tokens allowed in the quantity specification (named quantity specification, dimensionless, power<Q, Num, Den>, per<...>) |