mp-units - The 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 the following ISO C++ proposals:

• P1935: A C++ Approach to Physical Units,
• P2980: A motivation, scope, and plan for a quantities and units library,
• P3045: Quantities and units library.

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 "The Power of C++ Templates With mp-units: Lessons Learned & a New Library Design" talk at the C++ on Sea 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:

import mp_units;

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

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

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

// derived quantities
static_assert(1 * km / (1 * s) == 1000 * m / s);
static_assert(2 * km / h * (2 * h) == 4 * km);
static_assert(2 * km / (2 * km / h) == 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 <format>
#include <iomanip>
#include <iostream>
#include <print>
import mp_units;

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 << std::setw(10) << std::setfill('*') << v2 << '\n';  // ***70 mi/h
  std::cout << std::format("{:*^10}\n", v3);                      // *110 km/h*
  std::println("{:%N in %U of %D}", v4);                          // 70 in mi/h of LT⁻¹
  std::println("{::N[.2f]}", v5);                                 // 30.56 m/s
  std::println("{::N[.2f]U[dn]}", v6);                            // 31.29 m⋅s⁻¹
  std::println("{:%N}", v7);                                      // 31
}

Try it on the Compiler Explorer.