Value Conversions¶
Value-preserving conversions¶
The second line above converts the current quantity to the one expressed in meters and prints its contents. The third line converts the quantity expressed in kilometers into the one measured in meters.
In case a user would like to perform an opposite transformation:
Both conversions will fail to compile.
There are two ways to make the above work. The first solution is to use a floating-point representation type:
or
auto q1 = 5 * m;
std::cout << value_cast<double>(q1).in(km) << '\n';
quantity<si::kilo<si::metre>> q2 = q1; // double by default
Important
The mp-units library follows std::chrono::duration
logic and treats floating-point types as value-preserving.
Value-truncating conversions¶
The second solution is to force a truncating conversion:
auto q1 = 5 * m;
std::cout << value_cast<km>(q1) << '\n';
quantity<si::kilo<si::metre>, int> q2 = q1.force_in(km);
This explicit cast makes it clear that something unsafe is going on. It is easy to spot in code reviews or while chasing a bug in the source code.
Note
q.force_in(U) is just a shortcut to run value_cast<U>(q). There is no difference in behavior
between those two interfaces. q.force_in(U) was added for consistency with q.in(U) and
q.force_numerical_value_in(U).
Another place where this cast is useful is when a user wants to convert a quantity with a floating-point representation to the one using an integral one. Again, this is a truncating conversion, so an explicit cast is needed:
Info
It is often OK to use an integral as a representation type, but in general, floating-point types provide better precision and are privileged in the library as they are considered to be value-preserving.
In some cases, a unit and a representation type should be changed simultaneously. Moreover,
sometimes, the order of doing those operations matters. In such cases, the library provides
the value_cast<U, Rep>(q) which always returns the most precise result:
inline constexpr struct dim_currency : base_dimension<"$"> {} dim_currency;
inline constexpr struct currency : quantity_spec<dim_currency> {} currency;
inline constexpr struct us_dollar : named_unit<"USD", kind_of<currency>> {} us_dollar;
inline constexpr struct scaled_us_dollar : named_unit<"USD_s", mag_power<10, -8> * us_dollar> {} scaled_us_dollar;
namespace unit_symbols {
inline constexpr auto USD = us_dollar;
inline constexpr auto USD_s = scaled_us_dollar;
} // namespace unit_symbols
using Price = quantity_point<currency[us_dollar]>;
using Scaled = quantity_point<currency[scaled_us_dollar], zeroth_point_origin<currency>, std::int64_t>;
inline constexpr struct dim_currency : base_dimension<"$"> {} dim_currency;
inline constexpr struct currency : quantity_spec<currency, dim_currency> {} currency;
inline constexpr struct us_dollar : named_unit<"USD", kind_of<currency>> {} us_dollar;
inline constexpr struct scaled_us_dollar : named_unit<"USD_s", mag_power<10, -8> * us_dollar> {} scaled_us_dollar;
namespace unit_symbols {
inline constexpr auto USD = us_dollar;
inline constexpr auto USD_s = scaled_us_dollar;
} // namespace unit_symbols
using Price = quantity_point<currency[us_dollar]>;
using Scaled = quantity_point<currency[scaled_us_dollar], zeroth_point_origin<currency>, std::int64_t>;
inline constexpr struct dim_currency : base_dimension<"$"> {} dim_currency;
QUANTITY_SPEC(currency, dim_currency);
inline constexpr struct us_dollar : named_unit<"USD", kind_of<currency>> {} us_dollar;
inline constexpr struct scaled_us_dollar : named_unit<"USD_s", mag_power<10, -8> * us_dollar> {} scaled_us_dollar;
namespace unit_symbols {
inline constexpr auto USD = us_dollar;
inline constexpr auto USD_s = scaled_us_dollar;
} // namespace unit_symbols
using Price = quantity_point<currency[us_dollar]>;
using Scaled = quantity_point<currency[scaled_us_dollar], zeroth_point_origin<currency>, std::int64_t>;