`mp-units` - A Physical Quantities and Units library for C++

The mp-units library might be the subject of ISO standardization for C++29. More on this can be found in ISO C++ paper P1935. We are actively looking for parties interested in field trialing the library.

Video Introduction

A brief introduction to the library's interfaces and the rationale for changes in the version 2.0 of mp-units were provided in detail by Mateusz Pusz in the "mp-units: Lessons Learned and a New C++ Library Design" talk at the ACCU 2023 conference.

Documentation

An extensive project documentation can be found on mp-units GitHub Pages. It includes installation instructions and a detailed user's guide.

Terms and Definitions

This project uses the official metrology vocabulary defined by the ISO and BIPM. Please familiarize yourself with those terms to better understand the documentation and improve domain-related communication and discussions. You can find essential project-related definitions in our documentation's "Glossary" chapter. Even more terms are provided in the official vocabulary of the ISO and BIPM.

TL;DR

mp-units is a compile-time enabled Modern C++ library that provides compile-time dimensional analysis and unit/quantity manipulation.

Here is a small example of possible operations:

#include <mp-units/systems/si/si.h>

using namespace mp_units;
using namespace mp_units::si::unit_symbols;

// simple numeric operations
static_assert(10 * km / 2 == 5 * km);

// unit conversions
static_assert(1 * h == 3600 * s);
static_assert(1 * km + 1 * m == 1001 * m);

// derived quantities
inline constexpr auto kmph = km / h;
static_assert(1 * km / (1 * s) == 1000 * (m / s));
static_assert(2 * kmph * (2 * h) == 4 * km);
static_assert(2 * km / (2 * kmph) == 1 * h);

static_assert(2 * m * (3 * m) == 6 * m2);

static_assert(10 * km / (5 * km) == 2 * one);

static_assert(1000 / (1 * s) == 1 * kHz);

Try it on the Compiler Explorer.

This library heavily uses C++20 features (concepts, classes as NTTPs, ...). Thanks to them the user gets a powerful but still easy to use interfaces and all unit conversions and dimensional analysis can be performed without sacrificing on runtime performance or accuracy. Please see the below example for a quick preview of basic library features:

#include <mp-units/format.h>
#include <mp-units/ostream.h>
#include <mp-units/systems/international/international.h>
#include <mp-units/systems/isq/isq.h>
#include <mp-units/systems/si/si.h>
#include <iostream>

using namespace mp_units;

constexpr QuantityOf<isq::speed> auto avg_speed(QuantityOf<isq::length> auto d,
                                                QuantityOf<isq::time> auto t)
{
  return d / t;
}

int main()
{
  using namespace mp_units::si::unit_symbols;
  using namespace mp_units::international::unit_symbols;

  constexpr quantity v1 = 110 * (km / h);
  constexpr quantity v2 = 70 * mph;
  constexpr quantity v3 = avg_speed(220. * isq::distance[km], 2 * h);
  constexpr quantity v4 = avg_speed(isq::distance(140. * mi), 2 * h);
  constexpr quantity v5 = v3.in(m / s);
  constexpr quantity v6 = value_cast<m / s>(v4);
  constexpr quantity v7 = value_cast<int>(v6);

  std::cout << v1 << '\n';                                  // 110 km/h
  std::cout << v2 << '\n';                                  // 70 mi/h
  std::cout << std::format("{}", v3) << '\n';               // 110 km/h
  std::cout << std::format("{:*^14}", v4) << '\n';          // ***70 mi/h****
  std::cout << std::format("{:%Q in %q}", v5) << '\n';      // 30.5556 in m/s
  std::cout << std::format("{0:%Q} in {0:%q}", v6) << '\n'; // 31.2928 in m/s
  std::cout << std::format("{:%Q}", v7) << '\n';            // 31
}