We're excited to announce a major enhancement to the mp-units documentation: an automated systems reference generator that extracts and documents all quantities, units, dimensions, and their relationships directly from the library's C++ source code.
A new product version can be obtained from GitHub and Conan.
I initially had different plans for this release, but during development, it became clear that the new major feature I was working on was too large and would require breaking changes. This put me off track, and unfortunately, the mp-units development slowed down recently 😞
It also turned out that I got laid off, so now I depend solely on my C++ trainer's career to earn money to live. Fortunately, I delivered many in-house C++ trainings to my customers this year, and I hope this will also be the case in the upcoming 2026. I was also busy preparing for new talks and classes at this year's C++ conferences.
Having that much on my plate, a break from mp-units was needed to not burn out on the way. But hopefully I am back now, and I plan to continue working on long-awaited features.
That said, if you care about mp-units and would like to see it grow faster, please either contribute or consider sponsoring my work.
This release contains many small patches and improvements.
This post describes the most significant changes while a much longer list of the changes introduced by the new version can be found in our Release Notes.
An absolute quantity represents an absolute amount of a physical property — measured from a true, physically meaningful zero. Examples include mass in kilograms, temperature in Kelvin, or length in meters (as a size, not a position). Such quantities live on a ratio scale and have a well-defined origin; negative values are typically meaningless.
Absolute quantities stand in contrast to:
Arithmetic on absolute quantities behaves like ordinary algebra: addition, subtraction, and scaling are well-defined and map naturally to physical reasoning. This article proposes making absolute quantities the default abstraction in mp-units V3, reflecting how scientists express equations in practice.
Note: Revised October 31 2025 for clarity, accuracy, and completeness.
All quantities and units libraries need to be unit-safe. Most of the libraries on the market do this correctly. Some of them are also dimension-safe, which adds another level of protection for their users.
mp-units is probably the only library on the market that additionally is quantity-safe. This gives a new quality and possibilities. I've described the major idea behind it, implementation details, and benefits to the users in the series of posts about the International System of Quantities.
However, this is only the beginning. We've always planned more and worked on the extensions in our free time. In this post, I will describe:
The Wrocław 2024 meeting was another efficient step in the standardization of
this library. We've spent the entire day on the joint LEWGI and SG6 discussion
and got lots of feedback. We've also introduced std::fixed_string to LEWG for
C++26.
This article might be the last one from our series. This time, we will discuss the challenges and issues with modeling of the ISQ in software.
A new product version can be obtained from GitHub and Conan.
This release was unexpected. We planned a significant new feature to happen next, but while preparing for it and writing API Reference documentation, we made so many vital fixes and improvements that we decided they deserve a dedicated release first.
This post describes the most significant improvements while a much longer list of the changes introduced by the new version can be found in our Release Notes.
In the previous articles, we introduced the International System of Quantities, described how we can model and implement it in a programming language, and presented the issues of software that does not use such abstraction to implement a units library.
Some of the issues raised in Part 2 of our series were addressed in Part 3 already. This article will present how our ISQ model elegantly addresses the remaining problems.
Up until now, we have introduced the International System of Quantities and described how we can model its main aspects. This article will present how to implement those models in a programming language, and we will point out some of the first issues that stand in our way.
The physical units libraries on the market typically only focus on modeling one or more systems of units. However, as we have learned, this is not the only system kind to model. Another, and maybe even more important, is a system of quantities. The most important example here is the International System of Quantities (ISQ) defined by ISO/IEC 80000.
This article continues our series about the International System of Quantities. This time, we will learn about the main ideas behind the ISQ and describe how it can be modelled in a programming language.