Welcome to mp-units¶

mp-units is a compile-time enabled feature-rich Modern C++ modular/header-only library that provides compile-time dimensional analysis and unit/quantity manipulation. Its key strengths include safety, performance, and developer experience.

It is the first library on the market that, besides being unit-safe and dimension-safe, is also quantity-safe.

The library source code is hosted on GitHub with a permissive MIT license.

Supported compilers

This library tries to provide the best user experience possible with the C++ language. To achieve that, it extensively uses the latest C++ language features.

Even though the library benefits from the latest C++ versions (if available), C++20 is enough to compile and use all of the library's functionality.

Please refer to C++ compiler support chapter for more details.

C++ modulesHeader files

#include <iostream>
#include <print>
import mp_units;

using namespace mp_units;

inline constexpr struct smoot final : named_unit<"smoot", mag<67> * usc::inch> {} smoot;

int main()
{
  constexpr quantity dist = 364.4 * smoot;
  std::println("Harvard Bridge length = {::N[.1f]} ({::N[.1f]}, {::N[.2f]}) ± 1 εar",
               dist, dist.in(usc::foot), dist.in(si::metre));
}

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

using namespace mp_units;

inline constexpr struct smoot final : named_unit<"smoot", mag<67> * usc::inch> {} smoot;

int main()
{
  constexpr quantity dist = 364.4 * smoot;
  std::println("Harvard Bridge length = {::N[.1f]} ({::N[.1f]}, {::N[.2f]}) ± 1 εar",
               dist, dist.in(usc::foot), dist.in(si::metre));
}

Output:

Harvard Bridge length = 364.4 smoot (2034.6 ft, 620.14 m) ± 1 εar

Try it live on Compiler Explorer

What is smoot?

The smoot (/ˈsmuːt/) is a nonstandard unit of length created as part of an MIT fraternity prank. It is named after Oliver R. Smoot, a fraternity pledge to Lambda Chi Alpha, who, in October 1958, lay on the Harvard Bridge (between Boston and Cambridge, Massachusetts) and was used by his fraternity brothers to measure the length of the bridge.

One smoot equals Oliver Smoot's height at the time of the prank (five feet and seven inches). The bridge's length was measured to be 364.4 smoots plus or minus one ear, with the "plus or minus" intended to express the measurement uncertainty.

Oliver Smoot graduated from MIT with the class of 1962, became a lawyer, and later became chairman of the American National Standards Institute (ANSI) and president of the International Organization for Standardization (ISO).

More on the smoot unit of length can be found at https://en.wikipedia.org/wiki/Smoot.

Important: Help shape the future of C++!

The mp-units library is a candidate for ISO standardization in C++29. To support this effort, we need your help in demonstrating its real-world value to the ISO C++ Committee.

We are seeking:

Field trial partners – Organizations interested in evaluating the library in production environments
Success stories – Real-world testimonials from production deployments, research projects, or educational use
Feedback – Practical insights from developers, academics, and organizations using the library

Your experience—whether from production systems, research applications, or teaching—provides invaluable evidence for standardization and helps other potential users evaluate the library.

For more details, see the ISO C++ proposals: P1935 · P2980 · P3045