A header only c++ library that provides type safety and user defined literals for handling pyhsical values defined in the International System of Units regulated by the International Bureau of Weights and Measures (BIPM) and published in the SI brochure. For a quick start see the installation guide. Contributions and comments are welcome, please check the contribution guidelines for further information.

An illustrative example:

#include <SI/electric_charge.h>
#include <SI/mass.h>

using namespace SI::literals;

constexpr auto one_kilogramm = 1.0_kg;
constexpr auto ten_coulomb = 5.0_A * 2.0_s;
constexpr auto half_an_ampere = ten_coulomb / 20.0_s;

void calculate_mass(const SI::kilo_gram_t<long double> &kg) {
  // do something meaningful here
}

int main(int, char **) {
  calculate_mass(one_kilogramm);
  return 0;
}

SI provides conversions and arithmetic operations with values of any of the International System of Units with strong type safety at compile time. All units are special typedefs of the template struct SI::unit_t. Only the value of the unit is stored internally, the ratio (i.e. milli, micro, kilo...) is determined as a type trait to allow all units to have the same resolution across the whole implemented ratios. SI handles operations of units of the same ratios as well as when the ratios are different. See the documentation implementation details for further information. Operations of between units of the same ratio are overhead-free, else there is additional computation cost to adjust the values to the units. By passing the flag SI_DISABLE_IMPLICIT_RATIO_CONVERSION to the compiler implicit ratio conversion is not done and fails with a compiler error. See the continuous benchmark results for comparing the reference measurements and the implementation in SI.

It is possible to supply custom ratios to the built-in types and they are fully compatible for calculation with other units. However the necessary literals or typedefs have to be supplied by the user. For instance SI::velocity_t<double, std::ratio<1000, 36>> would be "kilometre per one-hundreth-of-an-hour".

For each unit the available literals are the implemented ratios prefixed with an underscore. i.e. _mm. _km. Generally the ratios follow metric prefixes of the international system of units The typedefs are prefixed (or in rare cases interfixed) with the standard metric prefixes. i.e. metre_t, milli_metre_t, kilo_metre_t. The prefix or interfix is marked with an * in the tables below. Units that have defined typedefs and literals can be converted to strings using stream operators or the SI::to_string function.

* for mass the base ratio is kg (not g) as it is defined in the SI unit table. So there is a mismatch between the literal prefix and the internal representation.

** The dimension symbol for thermodynamic temperature should be Θ (Theta) but the current implementation does not allow for non-ASCII symbols or multi-char symbols

* Angle and solid angle are simple containers, not containing any functionality to do angle/room-angle computation such as an overflow after 2*pi.

All units that can be built from other units decay to the respective units by inverting the mathematical operation. I.e if Q = I * T then Q / I = T and Q / T = I

* These dimensions do not yet have the correct symbols, because the current implementation does not allow for non-ASCII symbols or multi-char symbols. The dimension symbol for electric resistance should be Ω (Ohm) and for magnetic flux Φ (Phi)but. Illuminace should be E_b.

** luminous flux should be Φ_v which is even more less supported than Φ (Phi) itself.

Non standard units are not regulated by the BIPM but are accepted for use with the SI standard.

SI is a header-only library that uses C++17 features. Building is tested using cmake > 3.5 and verified for g++7, g++8, clang5, clang6, clang7, msvc 19, and AppleClang 10.0. It uses conan to download any dependencies for testing, but can be used without it, if the tests are not built.

to build use:

mkdir build
cd build
cmake ..
cmake --build . --config Debug -- -j $(nproc)

substitute --config Debug with --config Release for optimized builds

The default installation location for SI is /usr/local/lib/SI. SI can be installed using raw cmake, cpack (cmakes package mechanism), or as a conan.io package provided from Bintray

See the installation guide for detailed instructions

SI is available as 'raw' download from this repo but also as conan package. All releases are available from SI/stable from bintray. Getting SI from there is the preferred way.

For further information and deeper technical details refer to the implementation details document