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.

Documentation

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

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.h>

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

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.

Try It Out

The Compiler Explorer

As can be see in the previous chapter, the mp-units library is available in the Compiler Explorer. Use it to get a fast hands-on experience or try some ideas.

GitHub Codespaces

For more advanced development or contributions we encourage you to use our pre-configured GitHub Codespaces environment. You can either click the button below

or navigate to the main page of the repository and click "Code" button -> "Codespaces" tab -> "Create codespace on master".

You can also use the pre-configured devcontainer and Docker image manually within your IDE.

For detailed environment documentation, see .devcontainer/README.md.