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,
- P2981: Improving our safety with a physical quantities and units library,
- P2982:
std::quantityas a numeric type.
We are actively looking for parties interested in field-trialing the library.
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.
An extensive project documentation can be found on mp-units GitHub Pages. It includes installation instructions and a detailed user's guide.
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.
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}", v4); // 70 in mi/h
std::println("{:{%N:.2f}%?%U}", v5); // 30.56 m/s
std::println("{:{%N:.2f}%?{%U:n}}", v6); // 31.29 m s⁻¹
std::println("{:%N}", v7); // 31
}Try it on the Compiler Explorer.
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