`units`

Summary

Units is a compile-time enabled Modern C++ library that provides compile-time dimensional analysis and unit/quantity manipulation. The basic idea and design heavily bases on std::chrono::duration and extends it to work properly with many dimensions.

Here is a small example of possible operations:

static_assert(1000 / 1_s == 1_kHz);
static_assert(1_h == 3600_s);
static_assert(1_km + 1_m == 1001_m);
static_assert(10_km / 5_km == 2);
static_assert(10_km / 2 == 5_km);
static_assert(1_km / 1_s == 1000_mps);
static_assert(2_kmph * 2_h == 4_km);
static_assert(2_km / 2_kmph == 1_h);

Requirements

Safety and performance

strong types
template metaprogramming
constexpr all the things

The best possible user experience

compiler errors
debugging

No macros in the user interface
Easy extensibility
No external dependencies
Possibility to be standardized as a part of the C++ Standard Library

Basic Concepts

`Dimensions`

units::dimension is a type-list like type that stores an ordered list of exponents of one or more base dimensions:

template<Exponent... Es>
struct dimension : upcast_base<dimension<Es...>> {};

units::Dimension is a Concept that is satisfied by a type that is empty and publicly derived from units::dimension class template:

template<typename T>
concept Dimension =
    std::is_empty_v<T> &&
    detail::is_dimension<typename T::base_type> &&  // exposition only
    DerivedFrom<T, typename T::base_type>;

`Exponents`

units::exp provides an information about a single base dimension and its exponent in a derived dimension:

template<typename BaseDimension, int Value>
struct exp {
  using dimension = BaseDimension;
  static constexpr int value = Value;
};

where BaseDimension is a unique sortable compile-time value and for now is implemented as:

template<int UniqueValue>
using dim_id = std::integral_constant<int, UniqueValue>;

but it is meant to be replaced with C++20 class constexpr values provided as non-type template parameters (when feature will be available in a compiler) so that for example base dimension for length will be expressed as dimension<exp<"length", 1>>.

units::Exponent concept is satisfied if provided type is an instantiation of units::exp class template:

template<typename T>
concept Exponent =
    detail::is_exp<T>;  // exposition only

`make_dimension`

Above design of dimensions is created with the ease of use for end users in mind. Compile-time errors should provide as short as possible template instantiations strings that should be easy to understand by every engineer. Also types visible in a debugger should be easy to understand. That is why units::dimension type for derived dimensions always stores information about only those base dimensions that are used to form that derived dimension.

However, such an approach have some challenges:

constexpr Velocity auto v1 = 1_m / 1_s;
constexpr Velocity auto v2 = 2 / 2_s * 1_m;

static_assert(Same<decltype(v1), decltype(v2)>);
static_assert(v1 == v2);

Above code, no matter what is the order of the base dimensions in an expression forming our result, must produce the same Velocity type so that both values can be easily compared. In order to achieve that, dimension class templates should never be instantiated manually but through a make_dimension_t template metaprogramming factory function:

template<Exponent... Es>
struct make_dimension {
  using type = /* unspecified */;
};

template<Exponent... Es>
using make_dimension_t = typename make_dimension<Es...>::type;

So for example to create a dimension_velocity type we have to do:

struct dimension_velocity : make_dimension_t<exp<base_dim_length, 1>, exp<base_dim_time, -1>> {};

In order to make make_dimension_t work as expected it has to provide unique ordering for contained base dimensions. Beside providing ordering to base dimensions it also has to:

aggregate two arguments of the same base dimension but different exponents
eliminate two arguments of the same base dimension and with opposite equal exponents

Additionally, it would be good if the final type produced by make_dimension_t would be easy to understand for the user. For example we may decide to order base dimensions with decreasing order of their exponents. That is why second sorting of a type list may be required. For example:

template<Exponent... Es>
struct make_dimension {
  using type = mp::type_list_sort_t<detail::dim_consolidate_t<mp::type_list_sort_t<dimension<Es...>, exp_dim_id_less>>, exp_greater_equal>;
};

`merge_dimension`

units::merge_dimension is similar to make_dimension but instead of sorting the whole list of base dimensions from scratch it assumes that provided input dimension types are already sorted as a result of make_dimension.

Typical use case for merge_dimension is to produce final dimension return type of multiplying two different dimensions:

template<Dimension D1, Dimension D2>
struct dimension_multiply;

template<Exponent... E1, Exponent... E2>
struct dimension_multiply<dimension<E1...>, dimension<E2...>> {
  using type = upcasting_traits_t<merge_dimension_t<dimension<E1...>, dimension<E2...>>>;
};

template<Dimension D1, Dimension D2>
using dimension_multiply_t = typename dimension_multiply<typename D1::base_type, typename D2::base_type>::type;

Example implementation of merge_dimension may look like:

template<Dimension D1, Dimension D2>
struct merge_dimension {
  using type = mp::type_list_sort_t<detail::dim_consolidate_t<mp::type_list_merge_sorted_t<D1, D2, exp_dim_id_less>>, exp_greater_equal>;
};

units::unit is a class template that expresses the unit of a specific physical dimension:

template<Dimension D, Ratio R>
  requires (R::num > 0)
struct unit : upcast_base<unit<D, R>> {
  using dimension = D;
  using ratio = R;
};

units::Unit is a Concept that is satisfied by a type that is empty and publicly derived from units::unit class template:

template<typename T>
concept Unit =
    std::is_empty_v<T> &&
    detail::is_unit<typename T::base_type> &&  // exposition only
    DerivedFrom<T, typename T::base_type>;

`Quantities`

units::quantity is a class template that expresses the quantity/amount of a specific dimension expressed in a specific unit of that dimension:

template<Dimension D, Unit U, Number Rep>
  requires Same<D, typename U::dimension>
class quantity;

units::Quantity is a Concept that is satisfied by a type that is an instantiation of units::quantity class template:

template<typename T>
concept Quantity =
    detail::is_quantity<T>;  // exposition only

units::quantity provides the interface really similar to std::chrono::duration with additional member types and functions as below:

template<Dimension D, Unit U, Number Rep>
    requires Same<D, typename U::dimension>
class quantity {
public:
  using dimension = D;
  using unit = U;

  template<Dimension D1, Unit U1, Number Rep1, Dimension D2, Unit U2, Number Rep2>
      requires treat_as_floating_point<std::common_type_t<Rep1, Rep2>> || std::ratio_multiply<typename U1::ratio, typename U2::ratio>::den == 1
  quantity<dimension_multiply_t<D1, D2>, upcasting_traits_t<unit<dimension_multiply_t<D1, D2>, std::ratio_multiply<typename U1::ratio, typename U2::ratio>>>, std::common_type_t<Rep1, Rep2>>
  constexpr operator*(const quantity<D1, U1, Rep1>& lhs,
                      const quantity<D2, U2, Rep2>& rhs);

  template<Number Rep1, Dimension D, Unit U, Number Rep2>
  quantity<dim_invert_t<D>, upcasting_traits_t<unit<dim_invert_t<D>, std::ratio<U::ratio::den, U::ratio::num>>>, std::common_type_t<Rep1, Rep2>>
  constexpr operator/(const Rep1& v,
                      const quantity<D, U, Rep2>& q);

  template<Dimension D1, Unit U1, Number Rep1, Dimension D2, Unit U2, Number Rep2>
      requires treat_as_floating_point<std::common_type_t<Rep1, Rep2>> || std::ratio_divide<typename U1::ratio, typename U2::ratio>::den == 1
  quantity<dimension_divide_t<D1, D2>, upcasting_traits_t<unit<dimension_divide_t<D1, D2>, std::ratio_divide<typename U1::ratio, typename U2::ratio>>>, std::common_type_t<Rep1, Rep2>>
  constexpr operator/(const quantity<D1, U1, Rep1>& lhs,
                      const quantity<D2, U2, Rep2>& rhs);
};

Additional functions provide the support for operations that result in a different dimension type than those of their arguments.

`quantity_cast`

To explicitly force truncating conversions quantity_cast function is provided which is a direct counterpart of std::chrono::duration_cast.

Strong types instead of aliases, and type upcasting capability

Most of the important design decisions in the library are dictated by the requirement of providing the best user experience as possible.

For example with template aliases usage the following code:

const Velocity t = 20_s;

could generate a following compile time error:

C:\repos\units\example\example.cpp:39:22: error: deduced initializer does not satisfy placeholder constraints
   const Velocity t = 20_s;
                      ^~~~
In file included from C:\repos\units\example\example.cpp:23:
C:/repos/units/src/include/units/si/velocity.h:41:16: note: within 'template<class T> concept const bool units::Velocity<T> [with T = units::quantity<units::dimension<units::exp<units::base_dim_time, 1> >, units::unit<units::dimension<units::exp<units::base_dim_time, 1> >, std::ratio<1> >, long long int>]'
   concept Velocity = Quantity<T> && Same<typename T::dimension, dimension_velocity>;
           ^~~~~~~~
In file included from C:/repos/units/src/include/units/bits/tools.h:25,
                 from C:/repos/units/src/include/units/dimension.h:25,
                 from C:/repos/units/src/include/units/si/base_dimensions.h:25,
                 from C:/repos/units/src/include/units/si/velocity.h:25,
                 from C:\repos\units\example\example.cpp:23:
C:/repos/units/src/include/units/bits/stdconcepts.h:33:18: note: within 'template<class T, class U> concept const bool mp::std_concepts::Same<T, U> [with T = units::dimension<units::exp<units::base_dim_time, 1> >; U = units::dimension<units::exp<units::base_dim_length, 1>, units::exp<units::base_dim_time, -1> >]'
     concept Same = std::is_same_v<T, U>;
             ^~~~
C:/repos/units/src/include/units/bits/stdconcepts.h:33:18: note: 'std::is_same_v' evaluated to false

Time and velocity are not that complicated dimensions and there are much more complicated dimensions out there, but even for those dimensions

[with T = units::quantity<units::dimension<units::exp<units::base_dim_time, 1> >, units::unit<units::dimension<units::exp<units::base_dim_time, 1> >, std::ratio<1> >, long long int>]

and

[with T = units::dimension<units::exp<units::base_dim_time, 1> >; U = units::dimension<units::exp<units::base_dim_length, 1>, units::exp<units::base_dim_time, -1> >]

starts to be really hard to analyze or debug.

That is why it was decided to provide automated upcasting capability when possible. With that the same code will result with such an error:

C:\repos\units\example\example.cpp:40:22: error: deduced initializer does not satisfy placeholder constraints
   const Velocity t = 20_s;
                      ^~~~
In file included from C:\repos\units\example\example.cpp:23:
C:/repos/units/src/include/units/si/velocity.h:48:16: note: within 'template<class T> concept const bool units::Velocity<T> [with T = units::quantity<units::dimension_time, units::second, long long int>]'
   concept Velocity = Quantity<T> && Same<typename T::dimension, dimension_velocity>;
           ^~~~~~~~
In file included from C:/repos/units/src/include/units/bits/tools.h:25,
                 from C:/repos/units/src/include/units/dimension.h:25,
                 from C:/repos/units/src/include/units/si/base_dimensions.h:25,
                 from C:/repos/units/src/include/units/si/velocity.h:25,
                 from C:\repos\units\example\example.cpp:23:
C:/repos/units/src/include/units/bits/stdconcepts.h:33:18: note: within 'template<class T, class U> concept const bool mp::std_concepts::Same<T, U> [with T = units::dimension_time; U = units::dimension_velocity]'
     concept Same = std::is_same_v<T, U>;
             ^~~~
C:/repos/units/src/include/units/bits/stdconcepts.h:33:18: note: 'std::is_same_v' evaluated to false

Now

[with T = units::quantity<units::dimension_time, units::second, long long int>]

and

[with T = units::dimension_time; U = units::dimension_velocity]

are not arguably much easier to understand thus provide better user experience.

Upcasting capability is provided through dedicated upcasting_traits and by base_type member type in upcast_base class template.

template<typename T>
struct upcasting_traits : std::type_identity<T> {};

template<typename T>
using upcasting_traits_t = typename upcasting_traits<T>::type;

struct dimension_length : make_dimension_t<exp<base_dim_length, 1>> {};

template<>
struct upcasting_traits<typename dimension_length::base_type> :
    std::type_identity<dimension_length> {};

struct kilometer : unit<dimension_length, std::kilo> {};

template<>
struct upcasting_traits<typename kilometer::base_type> :
    std::type_identity<kilometer> {};

Adding new dimensions

In order to extend the library with custom dimensions the user has to:

Create a new dimension type and provide upcasting trait for it:

struct dimension_velocity : make_dimension_t<exp<base_dim_length, 1>, exp<base_dim_time, -1>> {};
template<> struct upcasting_traits<typename dimension_velocity::base_type> : std::type_identity<dimension_velocity> {};

Define the base unit (std::ratio<1>) and secondary ones and provide upcasting traits for them via:

struct meter_per_second : unit<dimension_velocity, std::ratio<1>> {};
template<> struct upcasting_traits<typename meter_per_second::base_type> : std::type_identity<meter_per_second> {};

Define a concept that will match a new dimension:

template<typename T>
concept Velocity = Quantity<T> && Same<typename T::dimension, dimension_velocity>;

Provide user-defined literals for the most important units:

namespace literals {
  constexpr auto operator""_mps(unsigned long long l) { return velocity<meter_per_second, std::int64_t>(l); }
  constexpr auto operator""_mps(long double l)        { return velocity<meter_per_second, long double>(l); }
}

Adding new base dimensions

For now base dimensions are defined in terms of std::integral_constant<int, ...> and the provided values must be unique. For example:

struct base_dim_length : dim_id<0> {};
struct base_dim_mass : dim_id<1> {};
struct base_dim_time : dim_id<2> {};
struct base_dim_electric_current : dim_id<3> {};
struct base_dim_temperature : dim_id<4> {};
struct base_dim_amount_of_substance : dim_id<5> {};
struct base_dim_luminous_intensity : dim_id<6> {};

However, as soon as C++20 class type values will be supported as non-type template parameters base dimensions will be just a text values. For example:

inline constexpr base_dim base_dim_length = "length";

With that it should be really easy to add support for any new non-standard base units to the library without the risk of collision with any dimension type defined by the library itself or by other users extending the library with their own dimension types.

Additionally, it should make the error logs even shorter thus easier to understand.

Open questions

Should we ensure that dimension is always a result of make_dimension? How to do it?
Should we provide strong types and upcasting_traits for quantity type?

In such a case all the operators have to be provided to a child class. Or maybe use CRTP?
What to do with time which is ambiguous (conflict wit ANSI C)?
What to do with std::chrono::duration? Is it possible to make it derive from quantity<dimension_time, U, Rep> which will most probably an ABI break? Alternatively, should we provide specialization of quantity<dimension_time, U, Rep> to work with/covnert from/to std::duration?
Should we provide seconds<int> or stay with time<second, int>? What about CTAD problem for units::length<units::mile> d3(3);?
What is the best way to add support for temperatures?

Temperatures not only require std::ratio but also should be adjusted/shifted by some constant values (i.e. [°C] = [K] − 273.15).
Should we use units::multiply or stay with std::ratio for multiplication?
Should we consider making units::multiply and units::offset a non-class template parameters as they provide different ratio values rather than types?

In example instead:
```
struct celsius    : unit<dimension_temperature, convert<offset<-27315, 100>>> {};
```
we could think about something like:
```
struct celsius    : unit<dimension_temperature, kelvin() - 27315/100>>> {};
```
Do we need non-linear scale?
Should we provide cmath-like functions for quantities?
What should be the resulting type of auto d = 1_km + 1_ft;?
Should we require explicit casts (i.e. quantity_cast) between different systems of measurement?
Should we provide Boost-like support for a quantity_cast to a reference that allows direct access to the underlying value of a quantity variable?
What should be the default representation (integral or double)?
Provide ostream overloads to print quantity units (use std::format)?
Should we provide support for dimensionless quantities?

Because dimensionless quantities have no associated units, they behave as normal scalars, and allow implicit conversion to and from the underlying value type or types that are convertible to/from that value type.

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DESIGN.md

DESIGN.md

`units` - Physical Units Library for C++

Summary

Requirements

Basic Concepts

`Dimensions`

`Exponents`

`make_dimension`

`merge_dimension`

`Units`

`Quantities`

`quantity_cast`

Strong types instead of aliases, and type upcasting capability

Adding new dimensions

Adding new base dimensions

Open questions

Files

DESIGN.md

Latest commit

History

DESIGN.md

File metadata and controls

units - Physical Units Library for C++

Summary

Requirements

Basic Concepts

Dimensions

Exponents

make_dimension

merge_dimension

Units

Quantities

quantity_cast

Strong types instead of aliases, and type upcasting capability

Adding new dimensions

Adding new base dimensions

Open questions

`units` - Physical Units Library for C++

`Dimensions`

`Exponents`

`make_dimension`

`merge_dimension`

`Units`

`Quantities`

`quantity_cast`