Generic Concepts

This section discusses concepts about units libraries generally. Some may be more precise formulations of principles that most users know intuitively, such as Arithmetic Operations. Others may be novel innovations which originated with Au, but would enhance any units library, such as Zero. From A to Z, all will sharpen your thinking and help you use units libraries more effectively.

• Arithmetic Operations. Find out about the two kinds of math you can do with quantities, and learn a simple rule for reasoning about the results of products and quotients.

• Common Units. To compare, add, or subtract quantities expressed in different units, the first step is to convert them to the same unit. But which one? This page teaches you what choice we make, and what advantages it has.

• Dimensionless Units and Quantities. “Dimensionless” isn’t the same thing as “unitless”; we support dimensionless units, like Percent. Here we explain how the library handles these situations, and avoids common pitfalls.

• Overflow. Unit conversions risk overflow. The degree of risk depends on both the conversion factor, and the range of values that fit in the destination type. Learn how different units libraries have approached this problem, including Au’s novel contribution, the “overflow safety surface”.

• Quantity Point. An abstraction for “point types” that have units. Most use cases don’t need this, but for a few — including temperatures — it’s indispensable.

• Zero. Switching to a units library can make some easy computations hard. Learn how we make them easy again with a special constant, ZERO, that works with quantities of any units.