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Range-Validated Quantity Points

Physical units libraries have always been very good at preventing dimensional errors and unit mismatches. But there is a category of correctness that they have universally ignored: domain constraints on quantity point values.

A latitude is not just a length divided by a radius. It is a value that lives in \([-90°, +90°]\); anything outside that range is physically meaningless. An angle used in bearing navigation wraps cyclically around a circle; treating it as an unbounded real number ignores a fundamental property of the domain. A clinical body-temperature sensor should reject a reading of \(44\ \mathrm{°C}\) at the API boundary, not silently pass it downstream.

No units library — before this work — has provided a way to attach this kind of constraint to a quantity point at the type level, have it enforced automatically, and express different flavours (clamp, wrap, reflect, check) without any runtime polymorphism.

This article describes the motivation in depth, the design we arrived at, and the open questions we would love the community's help to answer.

Report from the Croydon 2026 ISO C++ Committee meeting

It has been 1.5 years since the last major update on the ISO C++ standardization progress here. It is not that I got lazy 😉, but there was really not much to share.

This time, things were different. We achieved a nearly unprecedented success — one probably not even expected by most, definitely not by me! 🎉

Keep reading to learn more...

Understanding Safety Levels in Physical Units Libraries

Physical quantities and units libraries exist primarily to prevent errors at compile time. However, not all libraries provide the same level of safety. Some focus only on dimensional analysis and unit conversions, while others go further to prevent representation errors, semantic misuse of same-dimension quantities, and even errors in the mathematical structure of equations.

This article explores six distinct safety levels that a comprehensive quantities and units library can provide. We'll examine each level in detail with practical examples, then compare how leading C++ libraries and units libraries from other languages perform across these safety dimensions. Finally, we'll analyze the performance and memory costs associated with different approaches, helping you understand the trade-offs between safety guarantees and runtime efficiency.

We'll pay particular attention to the upper safety levels—especially quantity kind safety (distinguishing dimensionally equivalent concepts such as work vs. torque, or Hz vs. Bq) and quantity safety (enforcing correct quantity hierarchies and scalar/vector/tensor mathematical rules)—which are well-established concepts in metrology and physics, yet remain widely overlooked in the C++ ecosystem. Most units library authors and users simply do not realize these guarantees are achievable, or how much they matter in practice. These levels go well beyond dimensional analysis, preventing subtle semantic errors that unit conversions alone cannot catch, and are essential for realizing truly strongly-typed numerics in C++.

  • in Features
  • 6 min read

Interactive Learning: Tutorials and Workshops

We're thrilled to announce a major expansion of mp-units learning resources: comprehensive tutorials and hands-on workshops that make learning type-safe physical quantities and units both accessible and engaging. Whether you're taking your first steps with the library or ready to master advanced patterns, we've got you covered.

  • in Features
  • 11 min read

New Systems Documentation Generator

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.

  • in Releases
  • 9 min read

mp-units 2.5.0 released

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.

Introducing Absolute Quantities

Until now, mp-units forced users to choose between points (no arithmetic) and deltas (no physical semantics) — missing the most common case: a non-negative absolute amount.

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 are anchored at a physically meaningful zero; negative values are typically meaningless.

Absolute quantities stand in contrast to:

  • Affine points (e.g., \(20\ \mathrm{°C}\), \(100\ \mathrm{m}\ \mathrm{AMSL}\)) — values measured relative to an arbitrary or conventional origin.
  • Deltas (e.g., \(10\ \mathrm{K}\), \(–5\ \mathrm{kg}\)) — differences between two values.

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 on March 23, 2026 for clarity, accuracy, and completeness.

Bringing Quantity-Safety To The Next Level

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:

  • What a quantity character is?
  • The importance of using proper representation types for the quantities.
  • The power of providing character-specific operations for the quantities.
  • Discuss implementation challenges and possible solutions.