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Scalr

Scalr is a std::chrono inspired C++ library for the representation and manipulation of scalar physical quantities and their units of measurements.

Scalr is a work in progress...

Features and goals

  • C++11/14/17 compatibility
  • Compile-time dimension analysis
  • User experience of popular std::chrono library
  • Shorter and more meaningful compilation errors --> good to have
  • User-defined literals for units and std::cout support --> good to have

Quick look

Scalr is based on the International System (SI) of Units starting with seven base dimensions. In the nutshell, Scalr provides a generic compile-time dimension/unit arithmetic as well as defines a facade for named quantities with std::chrono::duration style. Using Scalr, we define physical quantities as follows:

    scalr::duration<intmax_t, std::ratio<1>> secs(120); 
    scalr::speed<double, std::ratio<10, 36>> kphs(18.0);
    scalr::length<double, std::ratio<1000>> dist(0.6);

    CHECK(dist == (kph * secs)); // Pass

However, the compiler would complain if you make a dimensional mistake such as:

    static_assert(kms == (kph * secs)); // Fail

Such compile-time guarantees and automatic (lazy) conversions between units are the reason why you should use Scalr.

Moreover, a number of helper aliases is available for named units. For example,

using hours = duration<intmax_t, std::ratio<3600>>;
using minutes = duration<intmax_t, std::ratio<60>>;
using seconds = duration<intmax_t>;

using meters_per_second = speed<double, std::ratio<1>>;
using kilometers_per_hour = speed<double, std::ratio<5, 18>>;
using miles_per_hour = speed<double, std::ratio<1397, 3125>>;

using kilohertz = frequency<double, std::kilo>;

using grams = mass<double, std::ratio<1, 1000>>;

The source directory of named quantities and unit can be found here for the reference.

Finally the design document gives more information about the underlying structures and how it works.

Other Libraries

There are quite a number of similar libraries in existence. Some prominent ones are listed below, with my short comments.

  • boost/units - Heavy design. I think std::chrono does a better job to handle time quantities.
  • mputsz/units - New proposal for the future C++23 standard, not compatible with C++11/14/17. Besides I am not in favor of explicit systems as in Boost.
  • PhysUnits-CT-Cpp11 - Close to Scalr in many ways but eagerly converts quantities to base units (unlike the lazy way in std::chrono::duration implementations, which has certain advantages).
  • nholthaus/units - Has some very nice features but not compatible with C++11. Besides representation type cannot be selected independently.