mp-units - A Units Library for C++

The mp-units library is the subject of ISO standardization for C++23/26. More on this can be found in ISO C++ paper P1935 and CppCon 2020 talk. We are actively looking for parties interested in field trialing the library.

Documentation

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

TL;DR

mp-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:

#include <units/isq/si/area.h>
#include <units/isq/si/frequency.h>
#include <units/isq/si/length.h>
#include <units/isq/si/speed.h>
#include <units/isq/si/time.h>

using namespace units::isq::si::references;

// 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);

// dimension conversions
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);

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

Try it on the Compiler Explorer.

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

#include <units/format.h>
#include <units/isq/si/international/length.h>
#include <units/isq/si/international/speed.h>
#include <units/isq/si/length.h>
#include <units/isq/si/speed.h>
#include <units/isq/si/time.h>
#include <units/quantity_io.h>
#include <iostream>

using namespace units::isq;

constexpr Speed auto avg_speed(Length auto d, Time auto t)
{
  return d / t;
}

int main()
{
  using namespace units::isq::si::literals;
  using namespace units::isq::si::references;
  using namespace units::aliases::isq::si::international;

  constexpr Speed auto v1 = 110 * (km / h);
  constexpr Speed auto v2 = mi_per_h(70.);
  constexpr Speed auto v3 = avg_speed(220_q_km, 2_q_h);
  constexpr Speed auto v4 = avg_speed(si::length<si::international::mile>(140), si::time<si::hour>(2));
#if UNITS_DOWNCAST_MODE == 0
  constexpr Speed auto v5 = quantity_cast<si::speed<si::metre_per_second>>(v3);
  constexpr Speed auto v6 = quantity_cast<si::dim_speed, si::metre_per_second>(v4);
#else
  constexpr Speed auto v5 = quantity_cast<si::speed<si::metre_per_second>>(v3);
  constexpr Speed auto v6 = quantity_cast<si::metre_per_second>(v4);
#endif
  constexpr Speed auto v7 = quantity_cast<int>(v6);

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

Try it on the Compiler Explorer.